1. RFC 0809
INDRA Note 1185                                            INDRA

Feb. 1982                                                 Working

RFC 809

                      UCL FACSIMILE SYSTEM

                           Tawei Chang

     ABSTRACT:  This note describes the features  of
                the  computerised  facsimile  system
                developed  in  the   Department   of
                Computer  Science at UCL.  First its
                functions  are  considered  and  the
                related    experimental   work   are
                reported. Then the  disciplines  for
                system    design    are   discussed.
                Finally, the implementation  of  the
                system are described, while detailed
                description are given as appendices.

                 Department of Computer Science

                   University College, London
NOTE: Figures 5 and 6 may be obtained by sending a request to
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      Way, Marina del Rey, California, 90291 (or WESTINE@ISIF) including
      your name and postal mailing address.  Please mention that you are
      requesting figures 5 and 6 from RFC 809.

      OR: You can obtain these two figures online from the files

          <NETINFO>RFC809a.FAX   and   <NETINFO>RFC809b.FAX

      from the SRI-NIC online library.  These files are in the format
      described in RFC 769.
UCL FACSIMILE SYSTEM                              INDRA Note 1185


  1. INTRODUCTION...........................................1

  2. SYSTEM FUNCTIONS.......................................2

     2.1 Communication......................................4
     2.2 Interworking with Other Equipment..................8
        2.2.1 Facsimile machines............................8
        2.2.2 Output Devices................................9
     2.3 Image Enhancement..................................11
     2.4 Image Editing......................................15
     2.5 Integration with Other Data Types..................16

  3. SYSTEM ARCHITECTURE....................................17

     3.1 System Requirements................................17
     3.2 Hierarchical Model.................................19
     3.3 Clean and Simple Interface.........................20
        3.3.1 Principles....................................21
        3.3.2 Synchronisation and Desynchronisation.........21
        3.3.3 Data Transfer.................................22
     3.4 Control and Organisation of the Tasks..............22
        3.4.1 Command Language..............................23
        3.4.2 Task Controller...............................23
     3.5 Interface Routines.................................26
        3.5.1 Sharable Control Structure....................26
        3.5.2 Buffer Management.............................27

  4. UCL FACSIMILE SYSTEM...................................28

     4.1 Multi-Task Structure...............................29
     4.2 The Devices........................................29
     4.3 The Networks.......................................30
     4.4 File System........................................31
     4.5 Data Structure.....................................32
     4.6 Data Conversion....................................34
     4.7 Image Manipulation.................................35
     4.8 Data Transmission..................................39

  5. CONCLUSION.............................................41

     5.1 Summary............................................41
     5.2 Problems...........................................42
     5.3 Future Study.......................................46
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     Appendix I:   Devices

     Appendix II:  Task Controller and Task Processes

     Appendix III: Utility and Data Formats


       The object of a  facsimile  system  is  to  reproduce
     faithfully  a document or image from one piece of paper
     onto another piece of paper  sited  remotely  from  the
     first  one.  Up  to  now,  the main method of facsimile
     communication has been via the telephone network.  Most
     facsimile  machines permit neither the storage of image
     page nor their modification before  transmission.  With
     such  machines,  it is almost impossible to communicate
     between different makes of facsimile machines. In  this
     respect,   facsimile   machines   fall   behind   other
     electronic communication services.

       Integration of  a  facsimile  service  with  computer
     communication  techniques  can bring great improvements
     in service. Not only is the reliability and  efficiency
     improved   but,  more  important,  the  system  can  be
     integrated with  other  forms  of  data  communication.
     Moreover, the computer enables the facsimile machine to
     fit into a complete message and information  processing
     environment.   The  storage  facilities provided by the
     computer system make it possible to store large amounts
     of  facsimile  data  and  retrieve  them  rapidly. Data
     conversion allows facsimile machines of different types
     to   communicate  with  each  other.  Furthermore,  the
     facsimile image is edited and/or  combined  with  other
     forms  of  data,  such  as text, voice and graphics, to
     construct a multi-media message, which  can  be  widely
     distributed over computer networks.

       In the Department  of  Computer  Science  at  UCL,  a
     computerised  facsimile  system  has  been developed in
     order to fully apply  computer  technology,  especially
     communication,  to  the facsimile field.  Some work has
     been done to improve the facsimile service  in  several

      (1) Adaptation of the facsimile machine for  use  with
          computer networks.  This permits more reliable and
          accurate  document  transmission,   as   well   as
          improving the normal point-to-point transfers.

      (2) Storage  of  facsimile  pages.  This  permits  the
          queueing  of pages, so saving operator time. Also,
          standard documents can  be  kept  permanently  and
          transmitted at any time.

      (3) Interworking with other facsimile  machines.  This
          permits  different  makes of facsimile machines to

                              - 1 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

          exchange images.

      (4) Compression of the facsimile images.  This  allows
          more   efficient   transmission  to  be  achieved.
          Different compression schemes are investigated.

      (5) Display of images  on  other  devices.   A  colour
          display  is  used  so  that  the  result  of image
          processing can be shown very vividly.

      (6) Improvement of the images. The ability to  'clean'
          the  facsimile  images  not  only  allows for even
          higher  compression  ratio,  but  also  provide  a
          better result at the destination.

      (7) Editing of  facsimile  pages.  This  includes  the
          ability  to  change  pictures,  alter  the size of
          images  and  merge  two  or   more   images,   all

      (8) Integration of the facsimile  service  with  other
          data  types.   For the time being, coded character
          text can be converted into  facsimile  format  and
          mixed  pages  containing  pictures and text can be

       This  note  first  considers  the  functions  of  the
     facsimile  system,  the related experimental work being
     reported.  Then the discipline for the system design is
     discussed.  Finally,  the  implementation  of  the  UCL
     facsimile system is described. As appendices,  detailed
     description of the system are given, namely

             I.   Devices
             II.  Task controller and task processes
             III. Utility routines and Data format


       The computerised facsimile system we  have  developed
     is composed of an LSI-11 micro-computer running the MOS
     operating system [14] with two AED62 floppy disk drives
     [17], a Grinnell colour display [18], a DACOM facsimile
     machine [16], and a VDU as  the  system  console.  This
     LSI-11  is also attached to several networks, including
     the ARPANET/SATNET [21], [22]  and  the  UCL  Cambridge
     Ring. A schematic of the system is shown in Fig. 1.

                              - 2 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

              facsimile machine  bit-map display
                     +------+    +------+
                     !      !    !      !
                     +------+    +------+
           +------+        \      /        VDU
           ! disk !      +----------+    +-----+
           +------+ ---- !  LSI-11  ! -- !     !
           ! disk !      +----------+    +-----+
           +------+           |
                           !  NI  !
                       Network Interface

            Fig. 1  Schematic of UCL facsimile system

       In this system, a  page  is  read  on  the  facsimile
     machine  and  the  image data produced is stored on the
     floppy disk. This data can be processed locally in  the
     micro-computer  and  then  sent  to  a  file store of a
     remote computer across the  computer  network.  At  the
     remote  site,  the  image  data  may  be  processed and
     printed on a facsimile machine.

       On the other hand, we can receive image data which is
     sent  by a remote host on the network. This data can be
     manipulated in the same way, including being printed on
     the local machine.

       Section 2.1  dicusses  the  problems  concerned  with
     transmission  of  facsimile  image data over a network,
     while the following sections deal with those  of  local
     manipulation of image data.

       In order to interwork with other  facsimile  machine,
     we   have   to   convert   the   image  data  from  one
     representation format  to  another.  Interworking  with
     other  output devices requires that the image be scaled
     to fit the dimension of the destination  device.  These
     are described in section 2.2.

       Being able to process the image by computer opens the
     door  to  many  possibilities.  First, as considered in
     section 2.3, an image can  be  enhanced,  so  that  the
     quality of the image may be improved and more efficient
     storage and transmission can be achieved.  Secondly,  a
     facsimile  editing  system  can  be supported whereby a
     picture can  be  changed  and/or  combined  with  other

                              - 3 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     pictures. This is described in section 2.4.

       In our system, coded character text can be  converted
     into  its  bit-map representation format so that it can
     be  handled  as  a  facsimile  image  and  merged  with
     pictures. This provides an environment where multi-type
     information can be dealt with.  This  is  discussed  in
     section 2.5.

     2.1 Communication

       The first goal of our computerised  facsimile  system
     is  to  use a computer network to transmit data between
     facsimile machines which are geographically separated.

       Normally, facsimile machines are used in  association
     with  telephone  equipment,  the  data being sent along
     telephone lines.  Placing the facsimile machines  on  a
     computer  network  presents  a problem as the facsimile
     machine does not have the ability  to  use  a  computer
     network  directly.   To  perform  the  network  tasks a
     computer is required, and so the  first  phase  was  to
     attach the facsimile machine to a computer.

       The facsimile machine is not like a standard piece of
     computer  equipment.  We  required  a  special hardware
     interface to enable communication between the facsimile
     machine  and  a small computer. This interface was made
     to appear exactly like  the  telephone  system  to  the
     facsimile   machine.   Furthermore,  the  computer  was
     programmed  to  act  exactly  as  if  it  were  another
     facsimile  machine on the end of a telephone line. Thus
     the local facsimile machine could transmit data to  the
     computer  quite happily, believing that it was actually
     talking to a remote facsimile machine on the other  end
     of  a  telephone  wire.  Because of the property of the
     DACOM 6450 used in the experiment [16],  the  interface
     could  be  identical to one developed for connecting to
     an X25 network. The binary synchronous mode of the chip
     used  (SMC  COM5025) was appropriate to drive the DACOM

       At the other side of the computer network there was a
     similar  computer  with an identical facsimile machine.
     The problem of transmitting  a  facsimile  picture  now
     appeared  simple:  data  was  taken  from the facsimile
     machine into the computer, transmitted over the network
     as  if  it was normal computer data, and then sent from
     the computer to the facsimile  machine  at  the  remote
     end.  The  data  being  sent  over  the network appears

                              - 4 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     exactly as any other computer data;  there  is  nothing
     special  about  it  to  signify  that  it  came  from a
     facsimile machine.  The  schematic  of  such  facsimile
     transfer system is shown in Fig. 2.

      +---+  interface
      !   !    +--+    +-----+
      !   ! == !  ! == !     ! computer
      +---+    +--+    +-----+
                           - - - - - -    computer
                         /             \  network

                         \             /             facsimile
                           - - - - - -               machine
                                      |    interface  +---+
                                   +-----+    +--+    !   !
                          computer !     ! == !  ! == !   !
                                   +-----+    +--+    +---+

                Fig. 2  Facsimile transfer system

       The experimental system was used to perform  a  joint
     experiment  between  UCL  and  two groups in the United
     States. Pictures were exchanged via the  ARPANET/SATNET
     [21],  [22]  between UCL in London, ISI in Los Angeles,
     and  COMSAT  in  Washington   D.C.   (Fig.   3).   This
     environment  was chosen because no equivalent group was
     available in the UK.

       One  problem   concerned   with   such   image   data
     transmission  is  the  quantity of data. Even with data
     compression,  a  single  page  of  facsimile  data  can
     produce  as  much  computer  data  as would normally be
     sufficient   for   sending   over   20,000   alphabetic
     characters  -  or  over a dozen typed pages. Thus for a
     given number of pages put into the system,  an  immense
     amount  of  computer  data is produced. This means that
     the transmission will be slower than for sending  text,
     and  that far more storage will be required to hold the

       Another problem was encountered which became only too
     apparent  when we implemented this system.  The network
     we were using was often unable  to  keep  up  with  the
     speed of the facsimile machine.  When this happened the

                              - 5 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

                      US               UK
     COMSAT                   __
     +---+    +--+           /  \
     !   ! -- !  !           /  \
     +---+    +--+          /    \
       |          \        /      \
     +---+         \      /        \           UCL
     !fax!          \+--+/          \+--+    +---+
     +---+  ARPANET  !  !   SATNET   !  ! -- !   !
                    /+--+            +--+    +---+
                   /                           |
     ISI         /                          +---+
     +---+    +--+                           !fax!
     !   ! -- !  !                           +---+
     +---+    +--+

     Fig. 3. The three participants of the facsimile experiments

     computer tried to slow down the facsimile machine.  The
     facsimile  machine  would  detect  this 'slowness' as a
     communication problem (as a telephone line would  never
     act  in  this  manner),  and would abandon the transfer
     mid-way through the page.

       This is because the the  facsimile  machine  we  were
     using  was never intended for use on a computer; it was
     designed and built for use on telephone lines.  Indeed,
     being  unaware that it was connected to a computer, the
     facsimile machine transmitted data at a constant  rate,
     which exceeded the limit that the network could accept.
     In other words, the computer network we were using  was
     not  designed for the transfer rate that we were trying
     to use over it.

       Both  these  problems  are  surmountable.   Facsimile
     machines are coming on the market that are designed for
     direct communication with a computer. These machines do
     not  mind  the delays on the computer interface and are
     tolerant of the stops and re-starts. On the other hand,
     if  there were a serious use of facsimile machines on a
     computer network, the network could be designed for the
     high  data rate required. Our problem was aggravated by

                              - 6 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     using a network that was never designed  for  the  data
     rates required in our mode of usage.

       Despite the problems we encountered being a result of
     the  experimental  equipment  we  were working with, we
     still had to  improve  the  situation  to  permit  more
     extensive communications to take place. The easiest way
     to do this was to introduce a local storage area in our
     computer   where  the  data  could  be  held  prior  to
     transmission.  The transfer of a page is  now  done  in
     three  stages.   First, the facsimile data is read from
     the facsimile machine and stored on a local disk.  This
     takes  place  at  high  speed  as  this is just a local
     operation.  When this is complete,  the  data  is  sent
     over  the  network  to  a  disk on the remote computer.
     Finally, the data from  that  disk  is  output  to  the
     remote  facsimile  machine.   This  improved  system is
     shown in Fig. 4.

                     computer network
      fax    computer    - - - -     computer   fax
     +---+   +-----+   /         \   +-----+   +---+
     !   ! = !     ! =     ==>     = !     ! = !   !
     +---+   +-----+   \         /   +-----+   +---+
        - - - + |        - - - -        | + - - >
              | | + - - - - - - - - - + | |
              | | |                   | | |
              V | |                   V | |
              +---+                   +---+
              !   !                   !   !
              !   !                   !   !
              +---+                   +---+
              disk                    disk

         Fig. 4.  The improved facsimile transfer system

       The idea  behind  this  method  is  to  decouple  the
     facsimile  machine from the network communications. The
     data is read from the facsimile machine at full  speed,
     without  the  delays  caused  by  the computer network.
     This also has the effect of being  more  acceptable  to
     the human operators: each page is now read in less than
     a minute.  The transmission over the network then takes
     place  at  whatever speed the network can sustain. This
     does not affect the facsimile machines at all; they are
     not involved in the sending or receiving. Only when all
     the data has been received at the remote  disk  is  the
     remote  facsimile  machine told that the data is ready.

                              - 7 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     The facsimile machine is then given the data as fast as
     it will accept it.

       The disadvantage of such a system is that the  person
     sending  the  pages  does  not know how long it will be
     before they are actually printed at the other side.  If
     several  pages  are  input  in  quick succession by the
     operator, they will be stored on disk; it may  then  be
     some time before the last page is actually delivered to
     the destination. This is  not  always  a  disadvantage;
     where  many  operators  are  sending  data  to the same
     destination, it is a definite advantage to be  able  to
     input  the  pages and have the system deliver them when
     the  destination  becomes  free.  Such  a   system   is
     preferable to use of the current telephone system where
     the  operator  has  to  keep  re-dialing   the   remote
     facsimile machine until the call is answered.

     2.2 Interworking with Other Equipment

     2.2.1 Facsimile machines

       As was mentioned earlier, facsimile machines  produce
     a large amount of data per page due to the way in which
     the pages are encoded.  To reduce the data that has  to
     be  transmitted,  various  compression  techniques  are
     employed.  The manufacturers of facsimile machines have
     developed   proprietary  ways  in  which  the  data  is
     compressed and encoded.  Unfortunately this  has  meant
     that  interworking  of different facsimile machines has
     been impossible.  In the system described in  the  last
     section, exchange of pictures was only possible between
     sites that had identical facsimile  machines.  The  new
     set  of CCITT recommendations will reduce the extent to
     which differences in equipment persist.

       Having  the  data  on  a  computer   gives   us   the
     opportunity  to manipulate data in any way we wish.  In
     particular we could convert the data from the form used
     in  one  facsimile machine to that required by another.
     This means that interworking between different types of
     facsimile machines can be achieved.

       The development of this  system  took  place  in  two
     stages:  the  decompression  of the facsimile data from
     the coded form used in our  machine  into  an  internal
     data  form  and  the  recompression  of the data in the
     internal form into the encoded form  required  for  the
     destination  machine.  Two  programs  were developed to
     perform these two operations.

                              - 8 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       At the same time we were developing  compression  and
     decompression  programs  for  machines  that  use other
     techniques.  In particular, we  developed  programs  to
     handle  the  recently approved CCITT recommendation for
     facsimile compression [15]. The CCITT came up with  two
     varieties of compression, depending upon the resolution
     being used.

       Unfortunately there were no facsimile machines on the
     network  that  use  the  CCITT  compression  technique.
     However, the programming of the  new  methods  achieved
     two  goals:  it proved that the data could be converted
     inside a small computer, so that machines of  different
     types could be supported on the network, and it enabled
     us  to  compare  the  compression  results.  These  are
     described  in  more detail in [13].  Essentially, these
     show that the DACOM technique  used  by  our  facsimile
     machine  is  comparatively  poor, and that considerably
     less data need be transmitted if some other  method  is
     used.  This  brings  up  another  possibility: we could
     change the compression of the data to reduce the volume
     for transmission and then change the data back again at
     the   destination.   This   may    save    considerable
     transmission  time,  especially  if  fast  computers or
     special hardware was easily available.   This  has  not
     been  tried  yet  in  our  system, as none of the other
     users on the network have the  capability  of  changing
     the  data  format  back  into  that  required  by their

       There  are  many  other  more  efficient  compression
     schemes,  e.g.   block  compression  [7] and predictive
     compression [8], but we have not yet incorporated  them
     into our system.

     2.2.2 Output Devices

       One area that we have explored is the use of  devices
     other  than facsimile machines for outputting the data.
     Facsimile  machines  are  both  expensive  to  buy  and
     relatively  slow  to  operate. We have investigated the
     use of a TV-like screen to display the  data,  just  as
     character VDUs are commonly used to display text.  This
     activity requires bit-map displays, with an address  in
     memory  for each postion on the screen. Full colour and
     multiple shades can be used  with  appropriately  large
     bit-map  storage.   Although  simple  in principle, the
     implementation  of   the   relevant   techniques   took
     considerable effort.

                              - 9 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       The problems arise in  the  way  that  the  facsimile
     image  is encoded. Raw facsimile images consist of rows
     of small dots, each dot recorded as a  black  or  white
     space. When these dots are arranged together they build
     up a picture in a similar manner to the way in which  a
     newspaper  picture is made up. Unfortunately the number
     of dots used in a facsimile page is not the same as the
     number  used  on  most screens. For instance, the DACOM
     facsimile machine uses 1726 dots across each page,  but
     across  a  screen there are usually just 512 dots. Thus
     to show the picture on the screen the 1726 dots must be
     'squeezed' into just 512 dots; stated another way, 1214
     dots must be thrown away without losing the picture!

       It is in reducing the number of picture elements that
     the  problem  arises.  We could just every third dot or
     so from the facsimile  page  and  just  display  those.
     Alternatively,  we  could  take three or more at a time
     and try to convert the group  of  them  into  a  single
     black  or  white  dot.   Unfortunately,  in  both these
     cases, data can get  lost  that  is  necessary  to  the
     picture.   For  instance,  a  facsimile  encoding of an
     architect drawing could easily end up with  a  complete
     line  removed,  radically  changing the presentation of
     the image.

       After much experimentation, we developed a method  of
     reducing  the  number  of  dots  without destroying the
     picture. This is  a  thinning  technique,  whereby  key
     elements  of  the picture are thinned, but not removed.
     Occasionally, when  the  detail  gets  too  fine,  some
     elements  are merged, but under these circumstances the
     eye would not have been able to see the detail  anyway.
     The  details of this technique are described in [3] and

       It may also be required that a picture  be  enlarged.
     This enlargement can be done by simply duplicating each
     pixel in the picture.  For a  non-integral  ratio,  the
     picture  can  be expanded up to the nearest integer and
     then shrunk to the correct size.  However, this  method
     may degrade the image quality, e.g. the oblique contour
     may become stepped,  especially  when  the  picture  is
     enlarged  too much. This problem can be solved by using
     an iterative enlargement algorithm. Each time  a  pixel
     is  replaced  with a 2x2 array of pixels, whose pattern
     depends  on  the  original   pixel   and   the   pixels
     surrounding  it.  This  procedure is repeated until the
     requested ratio is reached. If  the  ration  is  not  a
     power  of 2's, the same method as that for non-integral
     ratios is used.

                             - 10 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       As a side effect of  developing  this  technique,  we
     could  freely  change  the  size and shape of an image.
     The picture can be expanded or shrunk,  or  it  can  be
     distorted.   Distortion,  whereby  the  horizontal  and
     vertical dimensions of the  image  may  be  changed  by
     different amounts, is often useful in image editing.

       The immediate consequence of this ability  to  change
     the image size meant that we could display the image on
     a screen as well as output the  image  on  a  facsimile
     machine.  To  a user of a computerised facsimile system
     this could be a very  useful  feature:  images  can  be
     displayed  on  screen  much  faster than on a facsimile
     machine, and displays are  significantly  cheaper  than
     the  facsimile machines as well. It is possible that an
     installation could have many screen displays where  the
     image  could  be viewed, but perhaps only one facsimile
     machine would be available for hard copy. This would be
     similar to many computer configurations today where the
     number of printers is limited due to  their  cost,  and
     display screens are far more numerous.

     2.3 Image Enhancement

       One aspect of computer processing that we  wanted  to
     investigate  was  that  of image enhancement. Enhancing
     the image is a  very  tricky  operation;  as  the  name
     implies  it  means  that  the image is improved in some
     sense.  Under program  control  this  is  difficult  to
     achieve: what the program thinks is an improvement, the
     human might judge to be distinctly worse.

       Our enhancement attempts were aimed  particularly  at
     printed  documents  and  other forms of typed text. The
     experiment was double pronged: we  hoped  to  make  the
     image  easier  to  read by humans while also making the
     image easier for the computer to handle.

       In our earlier experiments we had  noticed  that  the
     encoding  of  printed  matter was often very poor. This
     was especially noticeable when we  enlarged  an  image.
     Rather  than  each  character having smooth edges as on
     the original  document,  the  edges  were  very  rough,
     unexpected notches and excrescences being caused by the
     facsimile scanner.  They not  only  degrade  the  image
     quality but also decrease the compression efficiency. A
     typical enlargement of several characters is  shown  in
     Fig. 5.

                             - 11 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

             Fig 5.  An enlargement of an typed text

       The enhancement method we adopted was first  employed
     at  Loughborough  University  [5].  This method has the
     effect of smoothing the edges of the dark areas on  the
     image.  The  technique consists of considering each dot
     in the image in turn. The dot is either left as  it  is

                             - 12 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     or changed to the opposite colour (white  to  black  or
     black  to  white)  depending  upon  the eight dots that
     surround it. The particular pattern of surrounding dots
     that  are  required to change the inner dot's colour is
     used to control the harshness  of  the  algorithm  [6],

       In our  first  set  of  experiments  the  result  was
     definitely  worse  than  the original. Although square-
     like characters such as H, L, and T came out very well,
     anything  with slope (M, V, W, or S) became so bad that
     the oblique  contours  were  stepped.  The  method  was
     subsequently  modified to produce a result that was far
     more acceptable; the image looked a  lot  cleaner  than
     the  original.  Fig.  6  shows the same text as that in
     Fig. 5, but after it has been cleaned.

                             - 13 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

                     Fig. 6  A cleaned text

       The effect of these can be difficult to see  clearly.
     We have used the colour on our Grinnell display to show
     the original picture and the outcome of various picture
     processing  operations superposed in different colours.
     This brings out  the  effect  of  the  operations  very

                             - 14 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185


       It was mentioned above that the enhancement was  done
     not  only to improve the image for reading but also for
     easier  processing  by  the  computer.   As   described
     earlier,  the  image  from  the  facsimile  machine  is
     compressed in order to reduce the amount of data.   The
     cleaning  allows a higher compression rate so that more
     efficient transmission and/or storage can be achieved.

       We  learned   some   important   lessons   from   the
     enhancement  exercise.   Originally we thought that the
     main attraction in enhancement would be to improve  the
     readability.  In  the  end, we found that improving the
     readability was very difficult, especially because  the
     facsimile  image was so poor. Instead we found that the
     effect of  reducing  the  compressed  output  was  more
     important.  By reducing the data to be transmitted by a
     quarter, significant savings could be made. But  before
     such  a  technique  could be used in a live system, the
     time it  takes  to  produce  the  enhancement  must  be
     weighed  against  the  time  that  would  be  saved  in

     2.4 Image Editing

       By editing we mean that the facsimile picture can  be
     changed,  or  combined with other pictures, while it is
     stored inside the computer.  In  previous  sections  it
     was  mentioned  that we could change the size and shape
     of a facsimile image. This technique was later combined
     with  an  overlaying method that enabled one picture to
     be combined with another [12].

       In order to perform any editing it  is  necessary  to
     have  the picture displayed for the user to see. In our
     case we displayed the picture on  the  bit-map  screen.
     The image took up the left-hand side of the screen, the
     right side being reserved  for  the  picture  that  was
     being  built.   The  user  could  select an area of the
     left-hand screen and move  it  to  a  position  on  the
     right-hand  screen.   Several images could be displayed
     in succession on the left, and areas selected and moved
     to  the right.  Finally, the right-hand screen could be
     printed on the facsimile machine.

       The selection of an area of the picture was  done  by
     the   use   of   a   coloured  rectangular  subsection,
     controlled by a program in the computer, that could  be
     moved  around on the screen. The rectangular subsection

                             - 15 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     was moved with instructions typed in by  the  operator;
     it  could  be  moved  up  or  down,  and  increased  or
     decreased in size. When the  appropriate  area  of  the
     screen  had  been  selected, the program remembered the
     coordinates  and   moved   the   coloured   rectangular
     subsection  to  the  right-hand side of the screen. The
     user then selected an area again, in a similar  manner.
     When the user finished the editing, the program removed
     the part of the picture  selected  from  the  left-hand
     screen  and  converted  it  to  fit  the  shape  of the
     rectangular subsection on the  right-hand  screen.  The
     result was then displayed for the user to see.

       When an image was being edited,  the  editor  had  to
     keep  another  scaled  copy for display. This is due to
     the fact that the screen had a different  dimension  to
     that  of the facsimile machine. The editing operations,
     e.g.  chopping  and  merging,  were  performed  on  the
     original  image  data  files  with  the full resolution
     available on the facsimile machine.

     2.5 Integration with Other Data Types

       The facsimile  machine  can  be  viewed  in  a  wider
     context than merely a facsimile input/output device. It
     can work as a printer  for  other  data  representation
     types,  such  as  coded  character  text  and geometric
     graphics.  At  present,  text  can  be  converted  into
     facsimile  format and printed on the facsimile machine.
     Moreover, mixed pages containing pictures and text  can
     be  manipulated  by  our  system.  The  integration  of
     facsimile images with geometric graphics is a topic  of
     future research.

       In order to  convert  a  character  string  into  its
     facsimile  format,  the  system maintains a translation
     table whereby the patterns of the characters  available
     in  the  system  can  be retrieved. The input character
     string is translated into a set of scan lines, each  of
     which  is  created  by  concatenating the corresponding
     patterns of the characters in the string.

       The translation table is in  fact  a  software  font,
     which  can be edited and modified. Even though only one
     font is available in our system for the time being,  it
     is  quite  easy  to  introduce  other  character fonts.
     Furthermore, it is also  possible  for  a  font  to  be
     remotely  loaded  from a database via the communication

                             - 16 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       This allows for more interesting applications of  the
     facsimile  machine.  For  example,  it could serve as a
     Teletex printer, provided that  the  Teletex  character
     font  is included in our system. In this case, the text
     images may be distorted to fit the presentation  format
     requested  by  the Teletex service.  Similarly, Prestel
     viewdata pages  could  be  displayed  on  the  Grinnell

       Moreover,  pictures  can  be  mixed  with   text   by
     combining   this   text  conversion  with  the  editing
     described in  the  previous  section.  This  should  be
     regarded   as   a   notable   step  towards  multi-type

       Not  only  does  this  support  a  local   multi-type
     environment   but   multi-type   information   can   be
     transmitted over a network. So far  as  this  facsimile
     system  is  concerned, a mixed page containing text and
     pictures can be sent only when it has been  represented
     in  a  bit-map  format.  However,  much  more efficient
     transmission would be achieved if  one  could  transmit
     the text and pictures separately and reproduce the page
     at the destination site. This requires  that  a  multi-
     type  data structure be designed which is understood by
     the two communication sites.


       Now let us discuss the general disciplines for design
     and  implementation  of a computerised facsimile system
     which  carries  out  the  functions  described  in  the
     previous  sections.   Having discussed the requirements
     of the system, a hierarchical model  is  introduced  in
     which  the  modules of different layers are implemented
     as separate processes.  The Clean and Simple interface,
     which  is  adopted  for inter-process communication, is
     then  described.   The  task   controller,   which   is
     responsible  for  organising  the  tasks  involved in a
     requested job, is discussed in  detail.   Some  efforts
     have  been  made  in our experimental work to provide a
     more convenient user programming environment and a more
     efficient   data   transfer  method.  This  is  finally

     3.1 System Requirements

       In a computerised facsimile system,  the  images  are
     represented  in  a  digital  form.  To  carry  out this

                             - 17 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     conversion, a page is scanned by the optical scanner of
     the  facsimile machine, a digital number being produced
     to represent  the  darkness  of  each  pixel.  As  high
     resolution  has to be adopted to keep the detail of the
     image, the facsimile  data  files  are  usually  rather
     large.  In  order  to  achieve  efficient  storage  and
     transmission, the facsimile data must be compressed  as
     much as possible.

       Currently, the facsimile machines made  by  different
     manufacturers   h different  properties,  such  as
     different compression methods and different resolution.
     There   are   also  some  international  standards  for
     facsimile data compression, which are employed for  the
     facsimile  data  to be transferred over the public data
     network. These  require  that  the  facsimile  data  be
     converted  from  one representation form to another, so
     that users who are  separated  geographically  and  use
     different  machines  can  communicate  with each other.
     More sophisticated applications,  e.g.  image  editing,
     request processing facilities of the system as well.

       When being processed, the facsimile image  should  be
     represented   in  a  common  format  or  internal  data
     structure,  which  is  used  to  pass  the  information
     between  different processing routines. For the sake of
     convenience and efficiency, the internal data structure
     should  be fairly well compressed and its format should
     be  easy  for  the  computer  to  manipulate.  In   our
     experimental  work,  the  line  vector  is  chosen as a
     standard unit, a simple  run-length  compression  being
     employed  [3].  Some  processing routines may use other
     data   formats,   e.g.   bit-map,   but   it   is   the
     responsibility   of   such   routines  to  perform  the
     conversion between those formats and the standard one.

       The  system   should   contain   several   processing
     routines,  each  of  which performs one primitive task,
     such  as  chopping,  merging,  and  scale-changing.  An
     immense variety of processing operations can be carried
     out as long as those  task  modules  can  be  organised
     flexibly. The capability for flexible task organisation
     should be thought of  as  one  of  the  most  important
     requirements of the system.

       One  possibility  is  for  the  processing   routines
     involved  to  be  executed  separately, temporary files
     being used as communication media. Though very  simple,
     this method is far too inefficient.

                             - 18 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       As described above,  the  information  unit  for  the
     communication  between  the  processing routines is the
     line vector, so that the routines can be  organised  as
     embedded  loops,  where  a processing routine takes the
     input line from its source routine located in the inner
     loop,  and  passes  the  output line to the destination
     routine located in the outer loop [3].  Obviously  this
     method  is quite efficient. But it is not realistic for
     our system, because it is very difficult  to  build  up
     different  processing  loops  at  run-time and flexible
     task organisation is impossible.

       In a  real-time  operating  system  environment,  the
     primitive   tasks   can   be  implemented  as  separate
     processes. This method, which is discussed in detail in
     the   following   sections,   provides   the   required

     3.2 Hierarchical Model

       As shown in Fig. 7, the modules in a single  computer
     fall into three layers.

                       !         ! task controller

                +---+  +---+  +---+  +---+  +---+
                !   !  ! !   !  !   !  !   !
                +---+  +---+  +---+  +---+  +---+
                  |      |                    |
                +---+  +---+                +---+
                !   !  !   ! device drivers !   !
                +---+  +---+                +---+
            - - - | - -  |  - - - - - - - - - | - - - -
                +---+  +---+                +---+
                !   !  !   !    physical    |   !
                !   !  !   !    devices     !   !
                +---+  +---+                +---+

                 Fig. 7  The hierarchical model

       These are:

      (1) Device Drivers, which constitute the lowest  layer
          in the model.  The modules in this layer deal with
          I/O activities of the physical  devices,  such  as

                             - 19 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

          facsimile machine, display and floppy  disk.  This
          layer  frees  the task modules of upper layer from
          the burden of I/O programming.

      (2) Tasks, which perform all processing primitives and
          handle different data structures. Above the driver
          of each physical device, there  are  one  or  more
          such  device-independent  modules,  which  work as
          information source or sink in the task chain  (see
          below).  A file system module allows other modules
          to store and retrieve information on the secondary
          storage  device such as floppy disk. Decompression
          and recompression routines convert data structures
          of   facsimile   image  information  so  that  the
          facsimile machines can communicate with  the  rest
          of   the   system.   Processing  primitives,  e.g.
          chopping, merging,  scaling,  are  implemented  as
          task modules in this layer. They are designed such
          that they can be concatenated to  carry  out  more
          complex  jobs.  So far as the system is concerned,
          the protocols for data transmission over  computer
          networks are also regarded as task modules in this

      (3)  Task  Controller,  which   organises   the   task
          processes   to   perform  the  specified  job.  It
          provides the users of the application layer with a
          procedure-oriented  language whereby the requested
          job can be defined as a  chain  of  task  modules.
          Literally, the chain is represented by a character


            According to such a command, the task controller
          selects the relevant task modules and concatenates
          them in proper order by means  of  logical  links.
          Then the tasks on the chain are executed under its
          control, so that the data taken  from  the  source
          are processed and the result is put into the sink.

     3.3 Clean and Simple Interface

       It is important, in this application, to develop  the
     software  in  a  modular  way.  It  is desirable to put
     together a set of modules to carry  out  the  different
     image   processing  tasks.  Another  set  of  transport
     modules must be developed for shipping  data  over  the

                             - 20 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     different networks to which the UCL system is attached.
     In   our  computerised  facsimile  system,  these  task
     modules are  implemented  as  separate  processes.  The
     operation  of  the  system  relies on the communication
     between these processes.  The interface which  is  used
     for   such   communication  has  been  designed  to  be
     universal; it is independent of these modules, and  has
     been  termed  the Clean and Simple interface [20]. This
     interface is discussed in this section.

     3.3.1 Principles

       The Clean and Simple interface is concerned with  the
     synchronisation   and   transfer  of  full-duplex  data
     streams between two communicating processes.  Thus  the
     interface   has   three  major  components:  connection
     synchronisation,   data   transfer    and    connection
     desynchronisation.   These   components  are  discussed

       The connection between two processes is initiated  by
     one  of  them,  which, generally speaking, belongs to a
     higher  layer.  For  example,  the  interface   between
     protocols  of  different  layers is always initiated by
     the higher layer, though, sometimes, the connection  is
     initiated  passively by the primitive 'listen'. It will
     be seen in the next section  that  task  processes  can
     communicate  with each other via the connections to the
     higher  layer  (task  controller)  and  this  makes  it
     possible to achieve flexible task organisation.

       The process initiating the connection is  called  the
     'master' process, while the other is called the 'slave'
     process. The 'master' process is also  responsible  for
     resource   allocation   for   the   two   communicating
     processes. Here 'resource' refers mainly to the  memory
     areas  for  the message structure and data buffer. This
     asymmetric definition of the interface  eliminates  any
     possible confusion in resource allocation.

       The interface is implemented by using the signal-wait
     mechanism  provided  by  the  operating  system. A data
     structure called CSB (Clean and  Simple  Block),  which
     contains  function, data buffer, and other information,
     is sent as the event message, when one process  signals
     another [20].

                             - 21 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     3.3.2 Synchronisation and Desynchronisation

       The  procedure  for  connection  synchronisation   is
     composed   of  two  steps.  First,  the  two  processes
     exchange their identifiers for the specific  connection
     by  means  of a getcid primitive.  Usually, the pointer
     to the task control structure of the process is used as
     the connection identifier.

       Then, the 'master' sends an open CSB with appropriate
     parameter    string    passing    the    initialisation
     information. This information, which can also be called
     open   parameter,   is   process   dependent,  or  more
     accurately, task dependent. For example, the parameters
     for  the  file  system  should be the file name and the
     access mode. Provided the 'slave' accepts the  request,
     the connection is established successfully and data can
     be transferred via the interface.

       In  order  to  desynchronise  the   connection,   the
     'master' initiates a 'close' action. On the other hand,
     an error state or  EOF  (end  of  file)  state  can  be
     reported   by  the  'slave'  to  request  a  connection

       The listen primitive in our system  is  reserved  for
     the  processes  that  receive a request from the remote
     hosts on the networks.

     3.3.3 Data Transfer

       While the Clean and Simple interface is asymmetric in
     relation  to  connection synchronisation, data transfer
     is completely symmetric so long as the  connection  has
     been  established.  Data  flows  in both directions are
     permitted, though the operations are quite different.

       The  interface  provides  two  primitives  for   data
     transfer  --  read  and write. To transfer some data to
     the  'slave',  the  'master'  signals  it  with  a  CSB
     containing  the write function and a buffer filled with
     the data to be transferred.  Having consumed the  data,
     the 'slave' returns the CSB to report the result status
     of the transmission.

       On the other hand, in order to receive some data from
     the 'slave', the 'master' uses a read CSB with an empty
     buffer. Having received the CSB, the 'slave' fills  the
     buffer  with  the data requested and, then, returns the

                             - 22 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     3.4 Control and Organisation of the Tasks

       Another  important  aspect   of   the   multi-process
     architecture  of  the UCL facsimile system, is the need
     to systematise the  control  and  organisation  of  the
     tasks.  This  activity  is  the  function  of  the task
     controller, whose  operations  are  discussed  in  this

     3.4.1 Command Language

       As mentioned earlier, the task controller supports  a
     procedure-oriented  language by means of which the user
     or the routines of the upper layers can define the jobs
     requested.  A  command  should  contain  the  following

       1. the names of the task processes which are involved
          in the job.
       2. the open parameters for these task processes.
       3. the order in which the tasks are to be linked.

       The last item is quite  important,  though,  usually,
     the same order as that given in the command is used.

       A command in this language is presented  as  a  zero-
     ended  character  string.  In the task name strings and
     the attribute strings of the open parameters, '|', '"',
     and  ','  must  be  excluded as they will be treated as
     separators. The definition is shown below,  where  '|',
     which  is  the  separator of the command strings in the
     language, does not mean 'OR'.

     <command_string> ::= <task_string>
     <command_string> ::= <task_string>|<command_string>
     <task_string> ::= <task_name>
     <task_string> ::= <task_name>"<open_parameter>
     <open_parameter> ::= <attribute>
     <open_parameter> ::= <attribute>,<open_parameter>

     3.4.2 Task Controller

       In our experimental work, the task controller  module
     is  called  fitter.   This  name which is borrowed from
     UNIX hints how the  module  works.   According  to  the
     command  string,  it  links  the specified tasks into a
     chain, along which the data is processed to fulfil  the

                             - 23 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     job requested (Fig. 8).

                +-----+    +-----+    +-----+
                !  a  ! -> !  b  ! -> !  c  !
                +-----+    +-----+    +-----+

                     Fig. 8  The task chain

       Since  all  modules,  including  fitter  itself,  are
     implemented   as  processes,  the  connections  between
     modules should be via the Clean and Simple  interfaces.
     Upon  receiving  the  command string, the fitter parses
     the string to find each task process involved and opens
     a  connection  to  it. Formally, the task processes are
     chained directly, but, logically, there  is  no  direct
     connection  between  them. All of them are connected to
     the fitter (Fig. 9).

                   +-- !             ! --+
                   |   +-------------+   |
                   |          |          |
                   V          V          V
                +-----+    +-----+    +-----+
                !  a  !    !  b  !    !  c  !
                +-----+    +-----+    +-----+

          Fig. 9 The connection initiated by the fitter

       For each of the processes  it  connects,  the  fitter
     keeps  a  table called pipe. When the command string is
     parsed, the pipe tables are double-linked to  represent
     the specified order of data flow. So far as one process
     is concerned, its pipe table contains two  pointers:  a
     forward  one pointing to its destination and a backward
     one pointing to its sources. Besides the  pointers,  it
     also  maintains  the  information  to identify the task
     process and the corresponding connection.

                             - 24 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       Fig. 10 illustrates the chain of the pipe tables  for
     the  job "a|b|c".  Note that the forward (output) chain
     ends at the sink, while the backward (input) chain ends
     at  the  source.  In this sense, the task processes are
     chained in the specified order  via  the  fitter  (Fig.
     11). The data transfer along the chain is initiated and
     controlled by the  fitter,  each  process  getting  the
     input  from  its  source  and putting the output to its

               +-----+    +-----+    +-----+
               !  * -+--> !  * -+--> !  0  !
               +-----+    +-----+    +-----+
               !  0  ! <--+- *  ! <--+- *  !
               +-----+    +-----+    +-----+
               !  a  !    !  b  !    !  c  !
               +-----+    +-----+    +-----+
               !     !    !     !    !     !
               !     !    !     !    !     !
               +-----+    +-----+    +-----+

                     Fig. 10  The pipe chain

                   +-> ! * -> * -> * ! --+
                   |   +-------------+   |
                   |         | A         |
                   |         V |         V
                +-----+    +-----+    +-----+
                !  a  !    !  b  !    !  c  !
                +-----+    +-----+    +-----+

                     Fig. 11  The data flow

       This strategy makes the task organisation so flexible
     that  only the links have to be changed when a new task
     chain is to be built up. In such an  environment,  each
     task process can be implemented independently, provided
     the Clean and Simple interface is supported. This  also
     makes the system extension quite easy.

                             - 25 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       The fitter manipulates one job at a time. But it must
     maintain  a  command  queue  to cope with the requests,
     which come simultaneously from either the  upper  level
     processes or other hosts on the network.

     3.5 Interface Routines

       In a modular, multi-process system such  as  the  UCL
     facsimile   system,  the  structure  of  the  interface
     routines is very important. The CSI of section  3.3  is
     fundamental  to the modular interface; a common control
     structure is also essential. This  section  gives  some
     details  both  about the sharable control structure and
     the buffer management.

     3.5.1 Sharable Control Structure

       Though the CSI specification is straightforward,  the
     implementation   of   the  inter-process  communication
     interface may be  rather  tedious,  especially  in  our
     system,  where  there  are  many  task  processes to be
     written. Not only does each process have  to  implement
     the  same  control  structure  for signal handling, but
     also the buffer management routines must be included in
     all the processes.

       For the sake of simplicity and efficiency, a  package
     of  standard  interface  routines is provided which are
     shared by the  task  processes  in  the  system.  These
     routines  are re-entrant, so that they can be shared by
     all processes.

       The 'csinit' primitive is called for a  task  process
     to check in.  An information table is allocated and the
     pointer to the table is returned to the caller  as  the
     task  identifier,  which is to be used for each call of
     these interface routines.

       Then,  each  task  process  waits  by  invoking   the
     'csopen'  primitive  which  does  not  return until the
     calling process  is  scheduled.   When  the  connection
     between  the process and the fitter is established, the
     call returns the pointer to the open  parameter  string
     of  the  task,  the corresponding task being started. A
     typical structure of the task process (written in c) is
     shown  below.  After  the task program is executed, the
     process calls the 'csopen' and waits again. It  can  be
     seen  that  the  portability  of  the  task routines is
     improved to a great extent. Only the interface routines

                             - 26 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     should be changed if  the  system  were  to  run  in  a
     different operating environment.

     static int mytid;       /* task identifier */

             char *op;       /* open parameter */

             mytid = csinit();
             for(;;) {
                     op = csopen(mytid);
                     ...     /* the body of the task */

     3.5.2 Buffer Management

       The package of the interface routines also provides a
     universal buffer management, so that the task processes
     are freed from this burden. The allocation of the  data
     buffers  is  the  responsibility  of  the  higher level
     process, the fitter. If the  task  processes  allocated
     their own buffers, some redundant copying would have to
     be  done.  Thus,  the  primitives  for  data  transfer,
     'csread' and 'cswrite', are designed as:

             char *csread(tid, need);
             char *cswrite(tid, need);

     where 'tid' is the identifier of the task and 'need' is
     the  number  of  data  bytes  to  be  transferred.  The
     primitives return the pointer to  the  area  satisfying
     the  caller's requirement. The 'csread' returns an area
     containing  the  data  required  by  the  caller.   The
     'cswrite'  returns  an  area  into which the caller can
     copy the data to be transferred. The copied  data  will
     be  written to its destination at a proper time without
     the caller's interference.  Obviously  the  unnecessary
     copy  operations can be avoided. It is recommended that
     the data buffer returned  by  the  primitives  be  used
     directly to attain higher performance.

                             - 27 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       In order to implement  this  strategy,  each  time  a
     piece  of  data  is  required,  the  size of the buffer
     needed is compared with that of the unused buffer  area
     in  the current CSB. If the latter is not less than the
     former,  the  current  buffer  pointer   is   returned.
     Otherwise,  a  temporary buffer has to be employed. The
     data is copied into the buffer until the requested size
     is  reached.  In  this  case,  instead of a part of the
     current buffer, the temporary buffer will be returned.

       A 'cswrite' call with the 'need' field  set  to  zero
     tells  the  interface routine that no more data will be
     sent. It causes  a  'close'  CSB  to  be  sent  to  the
     destination routine.

       If there  is  not  enough  data  available,  'csread'
     returns zero to indicate the end of data.


       Now we discuss the implementation of the computerised
     facsimile   system   developed  in  the  Department  of
     Computer Science at UCL.

       This system has several components. Since  the  total
     system  is  a modular and multi-process one, a specific
     system must be built up for a specific application. The
     way  that this is done is discussed in section 4.1. The
     specific devices and their  drivers  are  described  in
     section  4.2. The system can be attached to a number of
     networks.  In  the  UCL  configuration,   the   network
     interface  can be direct to SATNET [22], SERC NET [23],
     PSS [24], and the Cambridge Ring. The form  of  network
     connection  is  discussed  further  in section 4.3. The
     system must transfer data between the facsimile devices
     and  the disks, and between the networks and the disks.
     For this a filing system is required which is discussed
     in section 4.4.

       A key aspect of the  UCL  system  is  flexibility  of
     devices, networks, and data formats. The flexibility of
     device is achieved by the modular nature of the  device
     drivers  (section  4.2).  The flexibility of network is
     discussed in section 4.8. The additional flexibility of
     data   structure  is  described  in  section  4.5.  The
     flexibility can be utilised by incorporating conversion
     routines  as in section 4.6. An important aspect of the
     UCL system is the ability to provide local manipulation
     facilities  for  the  graphics  files.   The facilities
     implemented for the local manipulation are discussed in

                             - 28 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     section 4.7.  In  order  to  transfer  files  over  the
     different  networks  of  section 4.3. a high level data
     transmission protocol must be defined.  The  procedures
     used in the UCL system are discussed in section 4.8.

     4.1 Multi-Task Structure

       The  task  controller  and   processing   tasks   are
     implemented  as  MOS  processes.  A  number  of utility
     routines are provided  for  users  to  build  new  task
     processes and modules at application level.

       In the environment of MOS, a process is included in a
     system  by  specifying a Process Control Table when the
     system is built up. The macro  'setpcte'  is  used  for
     this  purpose,  the  meaning  of  its  parameters being
     defined in [14].

     #define setpcte(name,entry,pridev,prodev,stklen,

       A Device Control Table (DCT) has to be specified  for
     each  device  when the system is built up. A DCT can be
     defined anywhere as devices are referenced by  the  DCT
     address.  The  macro  'setdcte'  is designed to declare
     devices, the meanings of its parameters being specified
     in   [14].    This   method   is  used  in  the  device

     #define setdcte(name,intvec,devcsr,devbuf,devinit,

     4.2 The Devices

       As mentioned in section 2,  apart  from  the  general
     purpose  system console, there are three devices in the
     system to support the facsimile service. These are:

      (1) AED62 Floppy Disk, which is used as the  secondary
          memory storing the facsimile image data. Above its
          driver, a file system is implemented to manage the
          data  stored  on  the disks, so that an image data

                             - 29 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

          file can be accessed through the Clean and  Simple
          interface.  This file system is dicussed in detail
          in the next section. For some processing jobs, the
          image  data  has  to  buffered on a temporary file
          lest time-out occurs on the facsimile machine.

      (2) DACOM Facsimile Machine, which is  used  to  input
          and  output  image  data.  It  reads  an image and
          creates the corresponding data  stream.  On  other
          hand, it accepts the image data and reproduces the
          corresponding image. Above its driver, there is  a
          interface  task  to fit the facsimile machine into
          the system, the Clean and Simple  interface  being
          supported.   The  encoding algorithm for the DACOM
          machine is described in [19].

      (3) Grinnell Colour Display,  which  is  used  as  the
          monitor  of  the  system.  Above  its  driver,  an
          interface task is implemented so  that  the  image
          data  in  standard  format can be accepted through
          the Clean and Simple interface.

       The detailed description  of  these  devices  can  be
     found  in  Appendix  1.  The  interface  task  and  the
     description for each device are listed in the following
     table. The interface tasks can be directly used as data
     source or sink in a task string.

           Device       Interface Task  Description

     AED62 Floppy Disk        fs()      aed62(device)
     DACOM fax Machine       fax()      dacom(device)
     Grinnell Display   grinnell()      grinnell(device)

       Note that the DCTs  for  the  facsimile  machine  and
     Grinnell    display   have   been   included   in   the
     corresponding interface tasks, so that there is no need
     to declare them if these tasks are used.

     4.3 The Networks

       There   are   three   relevant   wide-area   networks
     terminating  in  the  Department of Computer Science at
     the end of 1981. These are:

      (1) A British Telecom X25 network (PSS, [24]).

      (2) A private X25 network (SERC NET, [23])

                             - 30 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

      (3) A Defence network (ARPANET/SATNET, [21], [22])

       In addition there is a  Cambridge  Ring  as  a  local

       For the time  being,  the  UCL  facsimile  system  is
     directly  attached to the various networks at the point
     NI (Network Interface) of Fig. 1.

       As mentioned earlier, pictures can be  exchanged  via
     the  SATNET/ARPANET,  between UCL in London, ISI in Los
     Angeles, and COMSAT in  Washington  D.C..  The  Network
     Independent File Transfer Protocol (NIFTP, [9]) is used
     to transfer the image data.   This  protocol  has  been
     implemented  on LSI under MOS [10].  In addition, we at
     UCL have put NIFTP on an ARPANET  TOPS-20  host,  which
     can  act  as  an Internet File Forwader (IFF).  In this
     case, TCP/IP ([28], [29]) is employed as the underlying
     transport   service.   Since   TCP   provides  reliable
     communication channels, the  provision  of  checkpoints
     and  error-recovery  procedures are not included in our
     NIFTP implementations.

       In  the  X25  network,  the  transport  procedure  is
     NITS/X25   ([25],   [26]).    Though  pictures  can  be
     transferred to the X25 networks, no  experimental  work
     has been done, because:

      (1) There is at present no  collaborative  partner  on
          these networks.

      (2) The LSI-11, on which our  system  is  implemented,
          has no direct connection to these networks.

       Locally,  image  data  can  be  transmitted  to   the
     PDP11-44s   running  the  UNIX  time-sharing  operating
     system. At present, the SCP ring-driver  software  uses
     permanent   virtual  circuits  (PVCs)  to  connect  the
     various computers on the ring.

     4.4 File System

       A file system has been designed, based on  the  AED62
     double  density  floppy  disk, for use under MOS. It is
     itself implemented as  a  MOS  process  supporting  the
     Clean  and  Simple  interface.  The description of this
     task, fs(fax), can be found in Appendix 2.

                             - 31 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       In a command string, the file system  task  can  only
     serve  as  either  data  source  or data sink. In other
     words, it can only appear at the first or last position
     on  a  command  string.  In  the  former case, the file
     specified is to be  read,  while  the  file  is  to  be
     written in the latter case.

       Three access modes are allowed which are:

       * Read a file
       * Create a file
       * Append a file

       The file name and access mode are  specified  as  the
     open parameters.

       Let us consider an example.  If a document is  to  be
     read  on  the  facsimile  machine  and  the data stream
     created is to be stored on the file system, the command
     string required is:


     where:  fax - interface task for facsimile machine
             r   - read from facsimile machine
             fs  - file system task
             c   - create a new file
             doc - the name of the file to be created.

       In order to dump a  file,  a  task  process  od()  is
     provided  which  works  as  a  data  sink  in a command

     4.5 Data Structure

       Facsimile  image  data  is  created  using  a   high-
     resolution raster scanner, so that the original picture
     can  be  reproduced  faithfully.  The  facsimile   data
     represents  binary  images,  in  monochrome,  with  two
     levels of intensity, belonging  to  the  data  type  of
     bit-mapped graphics.

       The simplest representation is  the  bit-map  itself.
     The bits, each of which corresponds to a single picture
     element, are arranged in the  same  order  as  that  in
     which  the original picture is scanned, 1s standing for

                             - 32 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     black pixels and 0s for white ones. Operations  on  the
     picture are easily carried out. For example, two images
     represented  in  the  bit-map  format  can  be   merged
     together  by  using  a  simple  logic OR operation. Any
     specific  pixel  can   be   retrieved   by   a   simple
     calculation. However, its size is usually large because
     of  the  high  resolution.   This   makes   it   almost
     unrealistic for storage or transmission.

       Facsimile image data should therefore  be  compressed
     to reduce its redundancy, so that the efficient storage
     and transmission can be achieved.

       Run-length encoding is a useful  compression  scheme.
     Instead of the pattern, the counts of consecutive black
     and white runs are used to represent the image.

       Vector representation, in which the  run-lengths  are
     coded  as  integers  or  bytes,  is  a  useful internal
     representation of images. Not  only  is  it  reasonably
     compressed,  but  it is also quite easy for processing.
     Chopping, scaling and mask-scanning are examples of the
     processing   operations   which   may   be   performed.
     Furthermore, a conversion between different compression
     schemes  may  have to be carried out in such a way that
     the data is first decompressed into the  vector  format
     and  then recompressed. The difficulty in retrieval can
     be overcome by means of line  index,  which  gives  the
     pointers to each lines of the image.

       A higher compression rate leads to a  more  efficient
     transmission.  But  this  is  at the expense of ease of
     processing.  An example of this is the use  of  Huffman
     Code  in  the  CCITT  1-dimensional compression scheme.
     While the data can be compressed more  efficiently,  it
     is rather difficult to manipulate the data direcltly.

       Taking the correlation between  adjacent  lines  into
     account,  2-dimensional compression can achieve an even
     higher   compression    rate.    CCITT    2-dimensional
     compression  and  the  DACOM facsimile machine use this

       It is desirable to integrate  facsimile  images  with
     other  data types, such as text and geometric graphics;
     the  structure  of  these  other  types  must  then  be
     incorporated  in  the  system.  At  present,  only text
     structure  is  available,  while  the   structure   for
     geometric graphics is a topic for the further study.

                             - 33 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       In  the  facsimile   system,   the   following   data
     structures    are    supported.    The    corresponding
     descriptions, if any, are listed as well and  they  can
     be found in Appendix 3 (except of dacom(device)).

     type    structure       compression     description

     bit-map  bit-map               -              -
             vector          1D run-length   vector(fax)
             dacom block     2D run-length   dacom(device)
             CCITT T4        1D run-length   t4(fax)
                             2D run-length   t4(fax)

     text    text                  -         text(fax)

       As an  internal  data  structure,  vector  format  is
     widely  used  for data transfer between task processes.
     The set of interface  routines  has  been  extended  by
     introducing  two subroutines, namely getl() and putl(),
     which read and write line vectors directly through  the
     Clean  and  Simple interface. These two routines can be
     found in Appendix 3 (getl(fax) and putl(fax))

       In order to check the validity of a  vector  file,  a
     check task process check() is provided which works as a
     data sink in a command string. It  can  also  dump  the
     vector elements of the specific lines.

     4.6 Data Conversion

       In order to convert one data structure into  another,
     several conversion modules are provided in this system.
     These modules fall into two categories, task  processes
     and  subroutines.  The task processes are MOS processes
     which can only be used in the environment described  in
     this note, while the subroutines which are written in c
     and compatible under UNIX are more generally usable.

       Character strings  or  text  can  be  converted  into
     vector  format,  so  that an integrated image combining
     picture and text can be formed.

       The following table lists these  conversion  modules,
     including  their  functions and descriptions (which can
     be found in Appendix 3).

                             - 34 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     module  type          from          to      description

     decomp  process       dacom         vector   decomp(fax)
     recomp  process       vector        dacom    recomp(fax)

     ccitt   process       vector        t4       ccitt(fax)
                           t4            vector

     bitmap  subroutine    vector        bitmap    bit-map(fax)
     tovec   subroutine    bitmap        vector    tovec(fax)

     ts      subroutine    ASCII string  vector   ts(fax)
     string  process       ASCII string  vector   string(fax)
     tf      process       text          vector   tf(fax)

       Since each DACOM block contains a  Cyclic  Redundancy
     Check  (CRC)  field,  the  system supplies a subroutine
     crc()  to  calculate  or  check  the  CRC  code.   (see

       If a vector file  is  to  be  printed  on  the  DACOM
     facsimile   machine,  the  image  data  should  be  re-
     compressed into the DACOM-block  format,  the  required
     command string being shown below.


     where   fs     - file system task
             e      - read an existing file
         ic    - file name
             recomp - re-compression task
             fax    - interface task for facsimile machine
             w      - print an image on facsimile machine

     4.7 Image Manipulation

       Four processing task processes are  provided  in  the
     system.  These are:

      (1) Chop, which applies a defined window to the  input

      (2) Scale, which enlarges or shrinks the  input  image
          to the defined dimensions.

      (3) Merge, which puts the input image on the specified
          area of a background image.

                             - 35 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

      (4) Clean, which removes the noise on the input image.

       The Clean and  Simple  interfaces  are  supported  in
     these processing tasks so that the tasks can be used in
     command strings.  However, these tasks can  be  neither
     source  nor  sink in a command string.  The data format
     of their input and output is vector.

       For example, a facsimile page can be cleaned and then
     printed  on  the facsimile machine. Note that the image
     data must be recompressed  before  being  sent  to  the
     facsimile  machine. If the original data is the form of
     DACOM  block,  it  has  to  be  decompressed   as   the
     processing   tasks   only  accept  line  vectors.   The
     required command string is shown below.


     where   fs     - file system task
             e      - read an existing file
             page   - file name
             clean  - cleaning task
             recomp - re-compression task
             fax    - interface task for facsimile machine
             w      - print an image on facsimile machine

       The descriptions of these  processing  tasks  can  be
     found in Appendix 2 (chop(fax), scale(fax), merge(fax),
     and clean(fax)).

       In tasks 'chop' and  'merge',  a  window  is  set  by
     giving  the coordinates of its vertices. However, it is
     usually rather difficult for a human user to decide the
     exact  coordinates.  The  system  supplies a subroutine
     choice() which specifies a rectangular subsection of an
     image  by  interactive  manipulations  of a rectangular
     subsection  on  the  screen  of  the  Grinnell  display
     displaying the image.  It provides a set of interactive
     commands whereby a user can intuitively choose an  area
     he  is interested in. Note that this subroutine must be
     called by a MOS process and the Grinnell  display  must
     be included in the system.

       By means of these image processing modules, the image
     editing  described  in  section 2.4 can be carried out.
     Let us consider an example. An image abstracted from  a
     picture  'a'  is  to be merged onto a specified area of
     another picture 'b'. First of all, the two pictures 'a'

                             - 36 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     and 'b' should be displayed on the left half and  right
     half  of  the screen, respectively. Assume that the two
     pictures are standard DACOM pages whose dimensions  are
     1726x1200.  They have to be shrunk to fit the dimension
     of the half screen (256x512).  Note that  if  the  data
     format  is not vector, conversion should be carried out
     first.  the required command strings are:


     where   fs            - file system task
             e             - read an existing file
             a             - file name
             b             - file name
             scale         - scale task
             1726,1200     - old dimension
             256,512       - new dimension
             grinnell       - grinnell display interface task
             0,511,255,0   - presentation area (the left half)
             256,511,511,0 - presentation area (the right half)
             z             - zero write mode
             g             - green
             b             - blue

       In an application process, the subroutine choice() is
     called in the following ways for the user to choose the
     areas on both pictures.

                             - 37 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     choice(r, 1726, 1200, 1, 0, 0);
             /* choice the area on 'a' */
             /* r    - red
                1726 - width of the original picture
                1200 - height of the original picture
                1    - left half of the screen
                0    - the subsection can be of any width
                0    - the subsection can be of any height
     choice(r, 1726, 1200, 2, 0, 0);
             /* choice the area on 'b' */
             /* r    - red
                1726 - width of the original picture
                1200 - height of the original picture
                2    - right half of the screen
                0    - the subsection can be of any width
                0    - the subsection can be of any height

       When the user finishes editing,  the  coordinates  of
     the  chosen  rectangular areas are returned. An example
     is given in the table below.  The  widths  and  heights
     listed  in  the  table are actually calculated from the
     coordinates returned and they indicate that the  source
     image has to be enlarged to fit its destination.

              (0, 0)
                +-------------------------------> x
                |  (x0, y0)     w
                |     +--------------------+
                |     !                    !
                |     !                    !
                |     !                    ! h
                |     !                    !
                |     !                    !
                |     +--------------------+
                |                       (x1, y1)

     original   x0      y0      x1      y1      w       h

        a       30      40     100     120      70      80
        b      100     100    1100    1100    1000    1000

                             - 38 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       At this stage, our final  goal  can  be  achieved  by
     performing  a  job  specified below. It is assumed that
     the result image is to be stored as a new file 'c'.


     where   fs                - file system task
             e                 - read an existing file
             a                 - file name
             chop              - chop task
             30,40,100,120     - the area to be abstracted
             scale             - scale task
             70,80             - old dimension
             1000,1000         - new dimension
             merge             - merge task
             b                 - file name of the background image
             0                 - to be overlaid
             100,100,1100,1100 - the area to be overlaid
             fs                - file system task
             c                 - create a new file
             c                 - the name of the file to be

     4.8 Data Transmission

       In  order  to  transmit  facsimile  image  data  over
     computer  networks,  using the configuration of Fig. 1,
     the Network Independent File Transfer Protocol  [9]  is
     implemented as a MOS task process, the Clean and Simple
     interface of section 3.3  being  supported  [10].  Thus
     this  module  can be used in a command string directly.
     In this case, the module always works in the  initiator
     mode,  though the server mode is supported as well. Its
     description can be found in Appendix 2 (ftp(fax)).

       As  a  network-independent  protocol,  it  employs  a
     transport  service  to communicate across the networks.
     The Clean and Simple interface is  also  used  for  the
     communication  between the module and transport service

       Suppose that an image file stored in  a  remote  file
     system is to be printed on the local facsimile machine.
     Assume that the data is  transmitted  via  the  ARPANET
     [21],  Transport Control Protocol (TCP) [28] being used
     as the underlying transport service. As  was  described

                             - 39 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     before, since the  delay  caused  by  the  network  may
     result  in  a  time-out on the local facsimile machine,
     the job should be divided into two subjobs.

      (1) The remote file  is  transmitted  by  using  NIFTP
          module.   However,  instead  of  being  put on the
          facsimile machine directly, the received  data  is
          store in a temporary file.


          where   ftp - NIFTP task
                  t   - receive
                  b   - binary
                  ucl - remote user name
                  fax - remote password
                  pic - remote file name
                  tcp - transport service process

                  parameters for the transport service:

                      1234      - local channel number
                      10,3,3,42 - remote address
                      4521      - channel reserved for the
                                  remote server

                  fs  - local file system task
                  c   - create a new file
                  tmp - the name of the file to be created

      (2) The temporary file is read and the image  is  sent
          to  the facsimile machine for printing. Here it is
          assumed the data received is in the form of  DACOM
          block so that no conversion is needed.


          where   fs     - file system task
                  e      - read an existing file
                  tmp    - file name
                  fax    - interface task for facsimile machine
                  w      - print an image on facsimile machine

       We are able to  exchange  image  data  with  ISI  and
     COMSAT.  At present DACOM block is the only format that
     can be used as  all  the  three  participants  in  this
     experiment  possess  DACOM  facsimile  machines  and no

                             - 40 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     other data format is available in both ISI and  COMSAT.
     However,  it  is  the  intention  of the ARPA-Facsimile
     community to adopt the CCITT standard for future  work.
     As mentioned earlier, UCL already has this facility.

       Above NIFTP, a simple protocol was  used  to  control
     the  transmission  of facsimile data. In this protocol,
     the format of a facsimile  data  file  was  defined  as
     follows:  Each  DACOM  block was recorded with a 2-byte
     header at the front. This  header  was  composed  of  a
     length-byte   indicating   the   length  of  the  block
     (including the header) and a code-byte  indicating  the
     type  of  the  block.  This  is  shown in the following

             |<--- header ---->|<------ 74 bytes ------->|
             ! length !  code  !       DACOM block       !

       The Length-byte is 76 (decimal) for all DACOM blocks.
     The  code-byte for a setup block is 071 (octal) and 072
     for a data block. A  special  EOP  block  was  used  to
     indicate  the  end  of  a page. This block had only the
     header with the length-byte set to 2 and the  code-byte
     undefined.  A facsimile data file could contain several
     pages, which were separated by EOP blocks.


     5.1 Summary

       Though techniques  for  facsimile  transmission  were
     invented  in  1843,  it  was not until the recent years
     that integration with  computer  communication  systems
     gave rise to "great expectation".  The system described
     in  this  note   incarnates   the   compatibility   and
     flexibility of computerised facsimile systems.

       In this system, facsimile no longer refers simply  to
     the  transmission device, but rather to the function of
     transferring hard copy from one place to another.   Not
     only  does  the  system  allow  for  more  reliable and
     accurate document transmission over  computer  networks
     but  images  can  also  be  manipulated electronically.
     Image is converted from one  representation  format  to
     another,  so that different makes of facsimile machines
     can communicate with each other.  It is possible for  a

                             - 41 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     picture to be presented on different  bit-map  devices,
     e.g.  TV-like  screen,  as it can be scaled to overcome
     the incompatibilities.  Moreover, the  system  provides
     windowing   and   overlaying   facilities   whereby   a
     sophisticated editor can be supported.

       One of the most important aspects of this  system  is
     that   text   can  be  converted  into  its  bit-mapped
     representation format  and  integrated  with  pictures.
     Geometric  graphics  could  also  be  included  in  the
     system. Thus, the facsimile  machine  may  serve  as  a
     printer  for  multi-type  documents.  It  is clear that
     facsimile  will  play  an  important  role  in   future
     information processing system.

       As far  as  the  system  per  se  is  concerned,  the
     following  advantages  can  be  recognised.  Though our
     discussion is concentrated  on  the  facsimile  system,
     many  features  developed  here  apply  equally well to
     other information-processing systems.

      (1)  Flexibility:  The  user  jobs   can   be   easily
          organised.  The  only  thing  to  be done for this
          purpose is to  make  the  logical  links  for  the
          appropriate task processes.

      (2) Simplicity: The interface routines are responsible
          for  the  operations  such  as signal handling and
          buffer management.  By avoiding this  burden,  the
          implementation  of the task processes becomes very
          "clean and simple".

      (3) Portability: The interface routines also makes the
          task   processes   totally   independent   of  the
          operating environment.  Only these routines should
          be modified if the environment were changed.

      (4) Ease of extension: The power of the system can  be
          simply  and infinitely extended by adding new task

      (5) Distributed  Environment:  This  approach  can  be
          easily  extended  to  a  distributed  environment,
          where limitless hardware  and  software  resources
          can be provided.

     5.2 Problems

       As discussed earlier, the network we were  using  for
     the  experimental  work was not designed for image data

                             - 42 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     transmission.  The data transfer  is  so  slow  that  a
     time-out may be caused on the facsimile machine. Though
     this problem was solved by means of local buffering and
     pictures  were successfully exchanged over the network,
     the slowness is rather  disappointing  because  of  the
     quantity of image data. The measurement showed that the
     throughput was around 500 bits/sec. In other words,  it
     took  at  least  5 minutes to transfer a page. This was
     caused by the network but not our system. The situation
     has been improved recently. However, It is nevertheless
     required that more  efficient  compression  schemes  be

       At present, the system must be directly  attached  to
     the  network to be accessed. However, the network ports
     are much demanded, so that frequent reconfiguration  is

       The facsimile system can be  connected  only  to  the
     local  network,  the  Cambridge Ring, while the foreign
     networks are connected via gateways to the  ring.  This
     is shown in Fig. 12. Now the X25 network is attached to
     the Ring via an X25 gateway, XG [25], while  SATNET  is
     connected by another gateway, SG [25]. Both network are
     at the transport level; XG and SG support the  relevant
     transport  procedures.  In  the  case  of  XG,  this is
     NITS/X25 ([26], [27]); in the case  of  SATNET,  it  is
     TCP/IP ([28], [29]).

     UCL facsimile
       system          - - - - - - - -
     +--------+      /                 \      +------+
     !        ! ----    Cambridge Ring   ---- !  PE  !
     +--------+      \                 /      +------+
                       - - - - - - - -            |
                         /         \              |
                   +------+       +------+        |
                   !  XG  !       !  SG  ! --- SATNET
                   +------+       +------+
                   /       \
                 PSS    SERC NET

          Fig. 12  Schematic of UCL network connection

       When the network software runs in the same machine as
     the   application   software,   the  Clean  and  Simple
     interface of section  3.5  was  used  as  an  interface
     between  the  modules.  When  the  gateway software was
     removed to a separate machine, an Inter-Processor Clean

                             - 43 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     and  Simple  [30]  was   required.    The   appropriate
     transport   process  is  transferred  to  the  relevant
     gateway, and appropriate facilities are implemented for
     addressing   the   relevant   gateway.  Otherwise,  the
     software has to be little  altered  to  cater  for  the
     distributed case.

       In our experimental work, the following problems were
     also encountered.

      (1) The primary memory of the LSI-11 is so small  that
          we  cannot  build  up  a system to include all the
          modules we have developed.  In order  to  transfer
          an  edited picture using the NIFTP module, we have
          to first  load  an  editor  system  to  input  and
          process  the  picture, and then an NIFTP system is
          then loaded to transmit it.

      (2) The execution of  an  image  processing  procedure
          becomes  very  slow. For example, it takes several
          minutes to shrink a picture to fit the  screen  of
          the  Grinnell  display.  This  prevents the system
          from being widely used in its present form.

      (3) As secondary storage, floppy disks  are  far  from
          adequate  to keep image data files. At present, we
          have two double-density floppy  disk  drives,  the
          capacity  of  each  disk  being  about 630K bytes.
          However, an image page contains at least 50K bytes
          and,  sometimes,  this number may be doubled for a
          rather complex picture.  Only a limited number  of
          pages can be stored.

       On the other hand, in our  department,  we  have  two
     PDP11-44s   running  UNIX  together  with  large  disks
     supplying abundant file storage. Their processing speed
     is  much  higher  than  that of the LSIs. The UNIX file
     system  supports   a   very   convenient   information-
     management environment. This inspired the idea that the
     UNIX file system could pretend  to  be  a  file  server
     responsible for storing and managing the image data, so
     that all the processing tasks may  be  carried  out  on
     UNIX. Not only does this immediately solve the problems
     listed above, but the following  additional  advantages
     immediately accrue.

      (1) UNIX provides a  far  better  software-development
          environment than LSI MOS ever can or will.

      (2) The facsimile service can be enhanced to  be  able

                             - 44 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

          to support many users at a time.

      (3) The UNIX file system is so sophisticated that more
          complex data entities can be handled.

       In  fact  the  44s  and  the  LSI-11,  to  which  the
     facsimile  machine  and  Grinnell display are attached,
     are  all  connected  to  the  UCL  Cambridge  Ring.   A
     distributed  processing  environment  can  be  built up
     where a job in one computer can be initiated by another
     and  then the job will be carried out by cooperation of
     both computers.

       In such  a  distributed  system,  the  LSI-11  micro-
     computer,   together   with   the   facsimile  machine,
     constitutes  a   totally   passive   facsimile   server
     controlled  by  a  UNIX  user.  A  page  is read on the
     facsimile machine and the image data stream produced is
     transmitted to the UNIX via the ring. The image data is
     stored  as  a  UNIX  file  and  may  be  processed   if
     necessary.  It  can  also  be  sent via the ring to the
     facsimile server where it  will  be  reprinted  on  the
     facsimile machine.

       In order to build up such a distributed  environment,
     IPCS  [30] is far from adequate for this purpose, as it
     does not provide any facility for a remote  job  to  be
     organised.  In  our  system, the task controller can be
     modified so that the command strings  can  be  supplied
     from  a remote host on the network. Having accepted the
     request, the task  controller  organises  the  relevant
     task  chain and the requested job is executed under its
     control.  The execution  of  the  distributed  job  may
     require  synchronisation  between  the  two  computers.
     These problems are discussed in detail in [31].

       Generally speaking, a distributed system based  on  a
     local network, which supplies cheap, fast, and reliable
     communication, could be the ultimate  solution  of  the
     operational problems discussed in this section. In such
     a system, different system operations are  carried  out
     in the most suitable places.

       For the time being, only a  procedure-oriented  task-
     control  language  is  available  in  this system.  The
     command string of the fitter  can  be  typed  from  the
     system  console  directly,  the corresponding job being
     organised and executed.  Theoretically, this  is  quite
     enough   to  cope  with  any  requirement  of  a  user.
     However,  when  the  job  is  complex,  command  typing
     becomes very tedious and prone to error.

                             - 45 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

       Above the task-controller, a job-controller layer  is
     required  which  provides  a  problem-oriented language
     whereby the user can easily put forward his requirement
     to  the  system.  On receipt of such a command, the job
     controller translates it into a command string  of  the
     task  controller  and  passes  the  string  to the task
     controller so  that  operation  request  can  be  done.
     Sometimes,  one  job  has  to  be  divided into several
     subjobs, which are to be dealt  with  separately.   The
     job  controller  should  be  also  responsible for high
     level calculation and management, so that the user need
     not be concerned with system details.

       In the  system  supporting  facsimile  service  under
     UNIX,  a  set  of high-level command is provided, while
     the command  strings  for  the  facsimile  station  are
     arranged automatically and they are totally hidden from
     a UNIX user.

     5.3 Future Study

       At the next stage, our attention should be moved to a
     higher-level,  more sophisticated system which supports
     a multi-type environment. In such a  system,  not  only
     does   the  facsimile  machine  work  as  an  facsimile
     input/output device, but it should also play  the  role
     of  a  printer  for  the  multi-type  document. This is
     because other data types, e.g. coded character text and
     geometric  graphics  can  be easily converted into bit-
     mapped graphics format which the facsimile  machine  is
     able to accept.

       First of all, a data structure should be designed  to
     represent  multi-type  information.  In  a  distributed
     environment, such a structure should be understood  all
     over  the  system,  so  that multi-media message can be

       In a future  system,  different  services  should  be
     supported,   including  viewdata,  Teletex,  facsimile,
     graphics,  slow-scan  TV  and  speech.  The  techniques
     developed  for facsimile will be generalised for use of
     other bit-mapped image representations, such  as  slow-
     scan TV.

       To improve the performance of the  facsimile  system,
     we  are  investigating  how  we  could use an auxiliary
     special purpose processor to perform some of the  image
     processing   operations.   Such  a  processor  will  be
     essential for the higher data rate  involved  in  slow-

                             - 46 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     scan TV.

                             - 47 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185


      [1] P. T. Kirstein, "The Role of Facsimile in Business
          Communication", INDRA Note 1047, Jan. 1981.

      [2]  T.  Chang,  "A  Proposed  Configuration  of   the
          Facsimile station", INDRA Note 922, May, 1980.

      [3] T.  Chang,  "Data  Structure  and  Procedures  for
          Facsimile Signal Processing", INDRA Note 923, May,

      [4] S. Treadwell,  "On  Distorting  Facsimile  Image",
          INDRA Note No 762, June, 1979.

      [5] M. G. B. Ismail and R.  J.  Clarke,  "A  New  Pre-
          Processing   Techniques   for   Digital  Facsimile
          Transmission", Dept.  of  Electronic  Engineering,
          University of Technology, Loughborough.

      [6]  T.  Chang,  "Mask  Scanning  Algorithm  and   Its
          Application", INDRA Note 924, June, 1980.

      [7] M. Kunt and O. Johnsen, "Block Coding of Graphics:
          A  Tutorial  Review",  Proceedings  of  the  IEEE,
          special issue on  digital  encoding  of  graphics,
          Vol. 68, No 7, July, 1980.

      [8]  T.  Chang,   "Facsimile   Data   Compression   by
          Predictive  Encoding",  INDRA  Note  No  978, May.

      [9] High Level Protocol Group, "A Network  Independent
          File  Transfer  Protocol",  HLP/CP(78)1, alos INWG
          Protocol Note 86, Dec. 1978.

     [10] T. Chang, "The Implementation of NIFTP on LSI-11",
          INDRA Note 1056, Mar. 1981.

     [11] T. Chang, "The  Design  and  Implementation  of  a
          Computerised  Facsimile  System",  INDRA  Note No.
          1184, Apr. 1981.

     [12] T. Chang, "The Facsimile Editor", INDRA Note 1085,
          Apr. 1981.

     [13]  K.  Jackson,  "Facsimile   Compression",  Project
          Report,  Dept.  of  Computer  Science,  UCL, June,

                             - 48 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

     [14] R. Cole and S. Treadwell, "MOS User Guide",  INDRA
          Note 1042, Jan. 1981.

     [15] CCITT,  "Recommendation  T.4,  Standardisation  of
          Group   3   Facsimile   Apparatus   for   Document
          Transmission", Geneva, 1980.

     [16]  "DACOM  6450  Computerfax  Transceiver   Operator
          Instructions", DACOM, Mar. 1977.

     [17] "AED 6200LP Floppy Disk Storage System", Technical
          Manual,  105499-01A,  Advanced Electronics Design,
          Inc. Feb. 1977.

     [18] "The User Manual for Grinnelll Colour Display".

     [19] D. R. Weber,  "An  Adaptive  Run  Length  Encoding
          Algorithm", ICC-75.

     [20] R. Braden and P. L. Higginson, "Clean  and  Simple
          Interface  under  MOS",  INDRA Note No. 1054, Feb.

     [21] L. G. Roberts et al, "The ARPA Computer  Network",
          Computer  Communication  Networks,  Prentice Hall,
          Englewood, pp485-500, 1973.

     [22] I. M. Jacobs et  al:  "General  Purpose  Satellite
          Network",   Proc.   IEEE,   Vol.   66,   No.   11,
          pp1448-1467, 1978.

     [23] J.  W.  Burren  et  al,  "Design  fo  an  SRC/NERC
          Computer   Network",   RL   77-0371A,   Rutherford
          Laboratory, 1977.

     [24] P. T. F.  Kelly,  "Non-Voice  Network  Services  -
          Future     Plans",     Proc.     Conf.    Business
          Telecommunications, Online, pp62-82, 1980.

     [25] P. T. Kirstein, "UK-US  Collaborative  Computing",
          INDRA Note No. 972, Aug. 1980.

     [26] "A Network  Independent  Transport  Service",  PSS
          User   Forum,  Study  Group  3,  British  Telecom,
          London, 1980.

     [27] CCITT, Recommendation X3,  X25,  X28  and  X29  on
          Packet Switched Data Services", Geneva 1978.

     [28]  "DoD  Standard  Transmission  Control  Protocol",
          RFC761,  Information  Sciences  Inst.,  Marina del

                             - 49 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

          Rey, 1979.

     [29]  "DoD   Standard   Internet   Protocol",   RFC760,
          Information Sciences Inst., Marina del Rey, 1979.

     [30] P. L. Higginson, "The Orgainisation of the Current
          IPCS System", INDRA Note No. 1163, Oct. 1981.

     [31] T. Chang, "Distributed Processing for  LSIs  under
          MOS", INDRA Note No. 1199, Jan. 1982.

                             - 50 -
UCL FACSIMILE SYSTEM                              INDRA Note 1185

                     Appendix I: Devices
AED62(DEV)                                             AED62(DEV)


     aed62 - double density floppy disk


     DCT aed62
     setdct("aed62", 0170, 0170450, 0170450,
             aedini, aedsio, aedint, 0);


     The Double Density disks contain 77 tracks numbered  from  0
     to  76.  There  are 16 sectors (sometimes called blocks) per
     track, for a total of 1232 sectors on each side of the disk.
     These  are  numbered  0  to  1231.  Each sector contains 512
     bytes, for a total of 630,784 bytes  on  each  side  of  the

     Only one side of the floppy can be accessed at a time. There
     is  only one head per drive, and it is located on the under-
     side of the disk. To access the other side, the disk must be
     manually removed and inserted the other way up.

     Each block is actually two blocks on the disk:  an  adddress
     ID  block  and the data block.  The address ID block is used
     by the hardware and contains the  track  number,  the  block
     number and the size of the data block that follows.  When an
     operation is to take place, the seek mechanism first locates
     the  block  by  reading  the address ID blocks and literally
     'hunting' for the correct one. It will  hunt  for  up  to  2
     seconds before reporting a failure.

     Both the address ID and the data blocks are  followed  by  a
     checksum word that is maintained by the hardware and is hid-
     den from the user. On writing, the  checksum  is  calculated
     and  appended  to the block. On reading it is verified (both
     on reading the ID and data blocks) and any error is reported
     as  a  Data Check. No checking on the data block takes place
     on a write, and the hardware has no idea if it  was  written
     correctly. The only way to verify it is to read it.

     Although there are two drives in the unit,  they  cannot  be
     used  simultaneously. If an operation is in progress on one,
     no access can be made to the other until the first operation
     is  complete. The driver will queue requests for both drives
     however, and ensure that are performed in order.

     The MOS driver is called aed62.obj. It operates on the  fol-
     lowing IORB entries:
AED62(DEV)                                             AED62(DEV)


          The operation to be performed, as follows:

                          0 - Read
                          1 - Write
                          2 - Verify
                          3 - Seek

          Read and Write cause data to be transferred to and from
          disk.  Verify does a hardware read without transferring
          the data to memory and is used for verifying  that  the
          data  can be successfully read. The checksum at the end
          of  the  block  of  each  sector  is  verified  by  the
          hardware.  The  seek  command  is used to move the disk
          heads to a specified track.


          The drive number. Only Zero or One is accepted. This is
          matched  against the number dialed on the drive. If the
          number is specified  on  both  drives,  or  neither,  a
          hardware error will be reported.


          The Sector or Block Number. Must be in the range  0  to
          1231 inclusive.  irusr2 specifies the block number that
          the transfer is to begin at for Read and Write, the be-
          ginning  of  the  verified area for the Verify command,
          and the position of the head for the Seek  command.  In
          the  latter  case  the  head  will be positioned to the
          track that contains the block.


          This specifies the data  adress,  which  must  be  even
          (word  boundary).   If an odd address is given, the low
          order bit is set to zero to make it even. Not  required
          for the Seek or Verify commands.


          Transfer length as a positive number of bytes. Not  re-
          quired for the seek command, bit IS used by Verify com-
          mand so that the correct number of blocks may be  veri-
          fied.  The disk is only capable of transferring an even
          number of bytes. If an odd length is given the low ord-
          er  bit  is made zero to reduce the length to the lower
          even value.  The length is NOT restricted to the sector
          size  of  512 bytes. If the length is greater than 512,
          successive blocks are read/written until  the  required
AED62(DEV)                                             AED62(DEV)

          length has been satisfied. If the length is not an  ex-
          act  multiple  of  512 bytes, only the specified length
          will be read/written. Note  that  the  hardware  always
          reads  and  writes  a  complete sector, so specifying a
          shorter length on a read will cause  the  remainder  of
          the  block to be skipped. On a write, the hardware will
          repeat the last specified  word  until  the  sector  is

     The driver will attempt to recover  from  all  soft  errors.
     There  is no automatic write/read verify as on mag tapes, so
     that data that is incorrectly written will not  be  detected
     as such until a read is attempted. For this reason, the ver-
     ify feature can be used (see above) to force the checking of
     written  data.  When an error is detected while performing a
     read, the offending block will be re-read up to 16 times and
     disk  resets  will be attempted during this time too. If all
     fails a hardware error indication is returned to  the  user.
     Other errors possible are Protection Error (attempt to write
     to a read-only disk) and User Error,  which  indicates  that
     the  parameters  in  the IORB were incorrect. Errors such as
     there being no disk loaded, or the drive door being open are
     NOT  detectable  by the program. The interface sees these as
     Seek Errors (i.e. soft errors), and thus the driver will re-
     try  several times before returning a Hardware Error indica-
     tion to the user. It should be noted that error recovery can
     take  a  long  time. As mentioned above, there is a 2 second
     delay before a seek error is reported by the  hardware,  for
GRINNELL(DEV)                                       GRINNELL(DEV)


     grinnell - colour display


     DCT grndout
     setdct("grndout", 03000, 0172520, 0172522,
             grnoi, grnot, grnoti, &grndin);
     DCT grndin
     setdct("grndin", 03000, 0172524, 0172526,
             grnoi, grnot, grnoti, &grndout);


     The Grinnell colour display has a screen  of  512x512  pels.
     Three colours (red, green and blue) can be used, but no grey
     scale is supported.  Three  graphics  modes  are  available.
     These are:

      (1) Alphanumeric: The input ASCII characters are  displayed
          at the selected positions on the screen.

      (2) Graphic: Basic geometric elements,  such  as  line  and
          rectangle, are drawn by means of graphics commands.

      (3) Image: The input data is interpreted as  bit  patterns,
          the corresponding images being illustrated.

     The values used to construct commands are described  in  the
     Grinnell User Manual. They are also listed below.

      #define LDC     0100000   /* Load Display Channels */

      #define LSM     0010000   /* Load Subchannel Mask */
      #define   RED   0000010   /* Read Subchannel */
      #define   GREEN 0000020   /* Green subchannel */
      #define   BLUE  0000040   /* Blue subchannel */

      #define WID     0000000   /* Write Image Data */
      #define WGD     0020000   /* Write Graphic Data */
      #define WAC     0022000   /* Write AlphanumCh */

      #define LWM     0024000   /* Load Write Mode */
      #define   REVERSE  0200   /* Reverse Background */
      #define   ADDITIVE 0100   /* Additive (not Replace) */
      #define   ZEROWRITE 040   /* Dark Write */
      #define   VECTOR    020   /* Select Vector Graph */
      #define   DBLEHITE  010   /* Double Height write */
      #define   DBLEWIDTH 004   /* Double Width write */
      #define   CURSORAB  002   /* Cursor (La+Lb,Ea+Eb) */
GRINNELL(DEV)                                       GRINNELL(DEV)

      #define   CURSORON  001   /* Cursor On */

      #define LUM     0026000   /* Load Update Mode */
      #define   Ec        001   /* Load Ea with Ec */
      #define   Ea_Eb     002   /* Load Ea with Ea + Eb */
      #define   Ea_Ec     003   /* load Ea with Ea + Ec */
      #define   Lc        004   /* Load La with Lc */
      #define   La_Lb     010   /* Load La with La + Lb */
      #define   La_Lc     014   /* Load La with La + Lc */
      #define   SRCL_HOME 020   /* Scroll dsiplay to HOME */
      #define   SRCL_DOWN 040   /* Scroll down one line */
      #define   SCRL_UP   060   /* Scroll up one line */

      #define ERS     0030000   /* Erase */
      #define ERL     0032000   /* Erase Line */
      #define SLU     0034000   /* Special Location Update */
      #define   SCRL_ZAP 0100   /* unlimited scroll speed */

      #define EGW     0036000   /* Execute Graphic Write */
      #define LER     0040000   /* Load Ea relative */
      #define LEA     0044000   /* Load Ea */
      #define LEB     0050000   /* Load Eb */
      #define LEC     0054000   /* Load Ec */
      #define LLR     0060000   /* Load La Relative */
      #define LLA     0064000   /* Load La */
      #define LLB     0070000   /* Load Lb */
      #define LLC     0074000   /* Load Lc */
      #define   LGW     02000   /* perform write */

      #define NOP     0110000   /* No-Operation */

      #define SPD     0120000   /* Select Special Device */
      #define LPA     0130000   /* Load Peripheral Address */
      #define LPR     0140000   /* Load Peripheral Register */
      #define LPD     0150000   /* Load Peripheral Data */
      #define RPD     0160000   /* ReadBack Peripheral Data */
      #define MEMRB     00400   /* SPD - Memory Read-Back */
      #define DATA      01000   /* SPD - Byte Unpacking */
      #define   ALPHA   06000   /* LPR - Alphanumeric data */
      #define   GRAPH   04000   /* LPR - Graphic data */
      #define   IMAGE   02000   /* LPR - Image data */
      #define   LTHENH  01000   /* take lo byte then hi byte */
      #define   DROPBYTE 0400   /* drop last byte */
      #define INTERR    02000   /* SPD - Interrupt Enable */
      #define TEST      04000   /* SPD - Diagnostic Test */

     The MOS driver is called grin.obj. It operates on  the  fol-
     lowing IORB entries.


          This is a pointer to  the  buffer  where  the  data  is
GRINNELL(DEV)                                       GRINNELL(DEV)

          This data must be ready formtatted  for  the  Grinnell,
          since no conversion is performed by the driver.


          This transfer length as a positive number of bytes.

     Addressing the grinnell. Rows consist of elments numbered  0
     to 511 running left to right. The lines are number from 0 to
     511 running from bottom to top. It is thus  addressed  as  a
     conventional  X-Y  coordinate system. Note that this coordi-
   e system is different the one used for the image.

        X A
          |                                 (511, 511)
      511 +-------------------------------+
          |                               |
          |                               |
          |                               |
          |                               |
          |             (x, y)            |
          |            +                  |
          |                               |
          |                               |
          |                               |
          |                               |
          |                               |
         0                               511    Y


DACOM(DEV)                                             DACOM(DEV)


     dacom - facsimile machine


     DCT faxinput
     setdct("faxin", 0350, 0174750, 0174740,
             faxii, faxin, faxini, &faxoutput);
     DCT faxoutput
     setdct("faxout", 0354, 0174752, 0174742,
             faxoi, faxot, faxoti, &faxinput);


     The DACOM facsimile machine can read  a  document,  creating
     the  corresponding image data blocks. It can also accept the
     data of relevant format, printing the correponding image.

     Each data block consists of 585 bits, and  is  stored  in  a
     block  of  74 bytes starting on a byte boundary. The final 7
     bits of the last byte are not used and they  are  undefined.
     The  585 bits in each block need to be read as a bit stream:
     the bits in each byte run from the high  orger  end  of  the
     byte  to the low order end. The last 12 bits of the 585 bits
     in each block consistute the CRC field whereby the block can
     be validated.

     There are two kinds of blocks: SETUP blocks and DATA blocks.
     The  first of block of an image data file should be a single
     SETUP block. All following blocks in the file must  be  DATA
     blocks. Note that the second block is a DATA block that con-
     tains ZERO samples, i.e. a dummy data blocks. Form the third
     block, the DATA blocks store the reall image data.

     A standard dacom page contains about 1200 scan  lines,  each
     of which has 1726 pels. One can choose
UCL FACSIMILE SYSTEM                              INDRA Note 1185

          Appendix II: Task Controller and Task Processes
CCITT(FAX)                                             CCITT(FAX)


     ccitt - conversion between vector and CCITT T4 format


     ccitt() - a MOS task

     command string (task name is defined as ccitt):


     This routine operates as a MOS pipe task to convert the vec-
     tors to CCITT T4 format or inversely.

     The parameter function specifies what the task is to do.

      value           function

       1c             one-dimensional compression
       1d             one-dimensional decompression

       2c[<k>]        two-dimensional compression
       2d             two-dimensional decompression

     Note k is the maximun number  of  lines  to  be  coded  two-
     dimensionally  before  a one-dimensionally coded line is in-
     serted. If k is omitted, the default value 2 is adopted.


     vector(fax), t4(fax), fitter(fax)
CHECK(FAX)                                             CHECK(FAX)


     check - check the validity of a vector file.


     check() - a MOS task

     command string (the task name is defined as check):


     This routine operates as a MOS pipe task checking the  vali-
     dity of the input vector file.

     The number of lines to be checked is specified by the param-
     eter  height.   If  the height of the image is less than the
     parameter, the actual height is printed. Thus, one  can  set
     the  parameter  height to a big number in order to count the
     number of lines of the input image.

     The run lengths in each of these lines are  accumulated  and
     the sum is compared with the parameter width.

     These are the basic functions which are  performed  whenever
     the  task is invoked. However, there are several options one
     can choose by setting the one-character parameter function.

      value         function

       'n'          basic function only
       'c'          print the count of each line
       'l'          print all lines
       's'          print the lines in the interval
                    specified by parameter from and to


     A bad line will be reported and it will cause the job abort-


     vector(fax), getl(fax), fitter(fax)
CHOP(FAX)                                               CHOP(FAX)


     chop - extract a designated rectangular area from an image


     chop() - a MOS task

     command string (task name is defined as chop):


     This routine operates as a MOS pipe task extracting a desig-
     nated  rectangular area from an input image.  Input and out-
     put are image data files in the form of vectors.

     The following diagram  shows  the  coordinate  system  being
     used.  Note that the lengths are measured in number of pels.

          (0, 0)                     width  X
             |                         |
             |                         |
             |   (x0, y0)              |
             |     +---------+         |
             |     |         |         |
             |     |         |         |
             |     |         |         |
             |     |         |         |
             |     |         |         |
             |     |         |         |
             |     |         |         |
             |     +---------+         |
             |            (x1, y1)     |
             |                         |
             |                         |
             |                         |
             |                         |
      height +-------------------------+
           Y V

     As can be seen in the diagram, the rectangular  area  to  be
     extracted  is  specified  by  the parameters x0, x1, y0, y1,
     which are decimal strings.


     One has to make sure that
CHOP(FAX)                                               CHOP(FAX)

             0 < x0 < width
             0 < y0 < height
             0 < x1 < width
             0 < y1 < height


     vector(fax), getl(fax), putl(fax), fitter(fax)
CLEAN(FAX)                                             CLEAN(FAX)


     clean - clean an image.


     clean() - a MOS task

     command string (task name is defined as clean):


     This routine operates as a MOS pipe task cleaning  an  image
     by  means of mask scanning.  Input and output are image data
     files in the form of vectors.

     The width and height should be given as the parameters.


     vector(fax), getl(fax), putl(fax), fitter(fax)
DECOMP(FAX)                                           DECOMP(FAX)


     decomp - decompress DACOM blocks


     decomp() - a MOS task

     command string (task name is defined as decomp):


     This task takes DACOM blocks from the Clean and  Simple  in-
     terface,  and  decompresses them into vector format. Then it
     writes the vectors to the Clean and Simple interface.


     dacom(dev), vector(fax), fitter(fax)
FAX(FAX)                                                 FAX(FAX)


     fax - interface process for DACOM facsimile machine


     fax() - a MOS task

     command string (task name is defined as fax):


     This task uses the Clean and Simple  interface  to  read  or
     write facsimile image data.

     The one character parameter function specifies  whether  the
     data  is  to be read or written. Character w is for writing.
     In this case, 74 byte DACOM  blocks  contaning  correct  CRC
     fields  are  expected. On the other hand, character r is for
     reading. In this case, a document is read on  the  facsimile
     machine, the DACOM blocks being created.


     dacom(dev), fitter(fax)
FITTER(FAX)                                           FITTER(FAX)


     fitter - fit processes together to form a data pipe


     fitter() - the MOS task controller


     According to the command string typed on the console, fitter
     links the specified processes together to form a task chain.
     The name of the processes is the name given in the PCB.  The
     processes must communicate using the C+S interface. Only one
     C+S interface is opened per process - data is pushed in with
     a cswrite and pulled out with a csread.  The fitter does not
     inspect the data in any way but merely passes  it  from  one
     process to another.

     The format of command string is:

             A | B | C.

     The fitter takes data from the process called A, write it to
     the  process  called  B,  reads  data from the process B and
     write that data to the process  C.   Note  that  all  middle
     processes  are both read and written, while the first one in
     the list is only read from and the last in the list is  only
     written to.

     A double quote is used as the  separator  between  the  task
     name and the open parameter string, e.g.

             A"500 | B"n,xyz | C,

     where the strings '500' and 'n,xyz' are the  open  parameter
     stings  for  tasks  A  and  B,  respectively.  The parameter
     stirng is passed to the corresponding task routine when  the
     csopen call returns.


     The command string containing undefined task will be reject-


     csinit(fax), csopen(fax), csread(fax), cswrite(fax)
FS(FAX)                                                   FS(FAX)


     fs - file system for use under MOS


     fs() - a MOS task

     command string (task name is defined as fs):


     This is a file system, based on the  Double  Density  floppy
     disk,  for use under MOS. The fs task is used for manipulate
     the files, managed by the file system. This  task  can  only
     appear at the first or last position on a command string. In
     the former case, the file specified is to be read, while the
     file is to be written in the latter case.

     The <function> field contains only one character  indicating
     the function to be performed. The possible values are:

             e - open an existing file (for reading).
             c - open an existing file, and set the length
                       to zero (for rewriting).
             a - append to an existing file.

     If the capitals A, C, and E are used, the functions are  the
     same as described above but the specified file is created if
     it does not exist.


     This task is for reading and writing only. As for the  other
     facilities,  e.g.  seek, delete, status and sync, one has to
     use C+S interface directly.

     Note that only 15 files are permitted per disk, only drive 0
     is  supported  at  present, and no hierarchical directory is


     aed62(dev), fitter(fax)
FTP(FAX)                                                 FTP(FAX)


     ftp, pftp - NIFTP task processes


     ftp(), pftp() - MOS tasks

     command string (task name is defined as ftp):


     These tasks are implementation of Network  Independent  File
     Transfer  Protocol (NIFTP) for LSIs under MOS. They employ a
     transport service for communication with a  remote  host  on
     the network, where the same protocol must be supported. They
     communicate with the  user  process  and  transport  service
     processes  thourgh  the  Clean and Simple interface, so that
     they can be used in a fitter command chain directly.

     The code is available in two versions: ftp which  is  a  P+Q
     version supporting both server and intitiator and pftp which
     is a P version working only as an initiator.  Both  of  them
     are capable of sending and receiving.

     This implementation of NIFTP is just a subset of the  proto-
     col  as its main purpose is to provided the facsimile system
     with a data transmission mechanism. For the sake of  simpli-
     city,  only  the  necessary  facilities  are included in the
     module, while more complex facilities, such as data compres-
     sion  and  error recovery are not implemented. The following
     table shows the transfer control parameters being used.

      Attribute       Value Mod. Remarks

      Mode of access  0001  EQ   Creating a new file
                      8002  EQ   Retrieving file
      Codes            -    -    Text file, any parity
                      1002  EQ   Binary file
      Format effector 0000  EQ   No interpretation
      Binary mapping  0008  EQ   Default byte size
      Max record size 00FC  EQ   Default record size
      Transfer size   0400  LE   Default transfer size
      Facilities      0000  EQ   Minimum service

     The meanings of the parameters in  the  command  string  are
     listed below:

     function is the NIFTP function of our site. Any ASCII string
FTP(FAX)                                                 FTP(FAX)

     beginning with 't' means the file is to  be  transmitted  to
     the remote site.  Otherwise, the file will be retrieved from
     the remote site.

     code specifies the type of the file to be  transferred.  Any
     ASCII  string  beginning with 'b' means it is a binary file,
     while others mean text file.

     user_name is the login name of the server site.

     password is the password of the server site.

     file_name is the name of the file to be transmitted.

     transport_service_process is the process name of  the  tran-
     sport service to be used.

     transport_service_parameters are the  parameter  string  re-
     quired by the transport service.  They are network dependent
     and specified by the corresponding transport service.


GRINNELL(FAX)                                       GRINNELL(FAX)


     grinnell - task to convert and display fax vector data


     grinnell() - a MOS task

     command string (task name is defined as string):


     This task takes the vector data from a Clean and Simple  in-
     terface and displays it on the Grinnell screen. The Grinnell
     screen is viewed as an X-Y plane with (0,0) being the  lower
     left  hand  corner,  (512,  0)  being  the  lower right hand
     corner, etc.

     The parameters x0, y0, x1, y1 are decimal  strings  defining
     the rectangular space on the screen where the image is to be
     displayed. If the image is smaller than this area, it is ar-
     tificially  expanded  to the size of this area. If the image
     is larger than this area it is truncated to the size of  the

     The colour field consists of any combination of the  charac-
     ters  r,g  or  b  to  define the colours red, green and blue
     respectively. For instance "gb" would  write  the  image  as

     The mode defines how the image is to be displayed. Any  com-
     bination  of  the  characters  r,a and z may be used, to the
     following effect:

             r = reverse image
             a = additive image
             z = zerowrite image.

     There are three bit planes to define the three colours. Nor-
     mally  the  bit planes corresponding to the selected colours
     have either zero bits or one bits written to them  depending
     upon  whether  the image or the background is being written.
     For zerowrite, all non-selected bit planes  (i.e.   colours)
     are  always set to zero, thus erasing any unselected colours
     in the area. Additive mode means that in the selected colour
     planes  the  new bits are ORed in, rather than just written.
     Thus the image is added to. In reverse mode, the image writ-
     ten as one bits is written as zero bits and the bits written
     as zero bits are written as one  bits,  i.e.  the  bits  are
     flipped before being used.
GRINNELL(FAX)                                       GRINNELL(FAX)


     grinnell(dev), vector(fax), fitter(fax)
MERGE(FAX)                                             MERGE(FAX)


     merge - merge two images together


     merge() - a MOS task

     command string (task name is defined as merge):


     This routine operates as a MOS pipe task merging two  images
     together to form the result image.  Input and output are im-
     age data files in the form of vectors.

     One of the two input images is called background which is to
     be  copied  directly.  This  is  specified  by the parameter
     file_name.  The image data of the back ground is read via  a
     'tunnel',  maintained  by  this task. Another input image is
     taken form the Clean and Simple  interface  managed  by  the
     fitter.   As  shown  in  the following diagram, the position
     where it is to be put on the background image  is  specified
     by the parameters x0, y0, x1, y1, which are decimal strings.
     This implies that the dimension of the image is x1 - x0  and
     y1 -y0.

          (0, 0)                     width  X
             |                         |
             |   (x0, y0)              |
             |     +---------+         |
             |     |         |         |
             |     |         |         |
             |     |         |         |
             |     |         |         |
             |     |         |         |
             |     +---------+         |
             |            (x1, y1)     |
             |                         |
             |                         |
             |       (back ground)     |
      height +-------------------------+
           Y V

     The parameter  action  indicates  how  the  two  images  are
     merged.  If it set to 0, The second image is simply overlaid
     on the back ground image. On the  other  hand  any  non-zero
MERGE(FAX)                                             MERGE(FAX)

     causes the second image to replace the specified area of the
     back ground image.


     One has to make sure that

             0 < x0 < width_of_back_ground
             0 < y0 < height_of_back_ground
             0 < x1 < width_of_back_ground
             0 < y1 < height_of_back_ground

     In addition, x0, y0, x1, y1 must be consistent with the  di-
     mension of the image


     vector(fax), getl(fax), putl(fax), chop(fax), fitter(fax)
OD(FAX)                                                   OD(FAX)


     od - dump the input data


     od() - a MOS task

     command string (task name is defined as od):


     This routine operates as a MOS pipe task dumping  the  input
     data in a selected format.  The input data is taken from the
     Clean and Simple interface.

     The meanings of the one character parameter format are:

            value          format

             'd'           words in decimal
             'o'           words in octal
             'c'           bytes in ASCII
             'b'           bytes in octal


RECOMP(FAX)                                           RECOMP(FAX)


     recomp - compress the vectors to form the DACOM blocks


     recomp() - a MOS task

     command string (task name is defined as recomp):


     This task takes vectors from the Clean and Simple interface,
     and  recompresses them into DACOM blocks. Then it writes the
     blocks to the Clean and Simple interface.


     dacom(dev), vector(fax), fitter(fax)
SCALE(FAX)                                             SCALE(FAX)


     scale - scale an image to a specified dimension


     scale() - a MOS task

     command string (task name is defined as scale):


     This routine operates as a MOS pipe task scaling  the  input
     image  to the specified dimension.  Input and output are im-
     age data files in the form of vectors.

     The dimension of the input image is given by the  parameters
     old_width  and old_height, while the dimension of the output
     is specified by the parameters new_width and new_height.


     vector(fax), getl(fax), putl(fax), fitter(fax)
STRING(FAX)                                           STRING(FAX)


     string - convert an ASCII string to the vector format


     string() - a MOS task

     command string (task name is defined as string):


     This routine operates as a  MOS  pipe  task  converting  the
     parameter string s to the corresponding vectors.


     vector(fax), ts(fax)
TF(FAX)                                                   TF(FAX)


     tf - convert a text to the vector format.


     tf() - a MOS task

     command string (task name is defined as tf):


     This routine operates as a MOS pipe task converting the  in-
     put text to the corresponding vectors. The input text, taken
     from the Clean and Simple interface should be in the  format
     defined in text(fax).

             |                         |
             |            upper        |
             |                         |
             |         XXXXXXXXXXXX    |
             |         XXXXXXXXXXXX    |
             |         XXXXXXXXXXXX    |
             |         XXXXXXXXXXXX    |
             |  left   XXXXXXXXXXXX    |
             |         XXXXXXXXXXXX    |
             |         XXXXXXXXXXXX    |
             |         XXXXXXXXXXXX    |
             |         XXXXXXXXXXXX    |
             |            width        |
             |                         |

     As shown in the diagram, the parameters give the information
     for  the formating. The parameter width is the maximum width
     of the text lines.

     Every vector will be padded to fit this  width.  White  pels
     may be padded to the left of each vectors, and the number of
     pel to be padded is specified by the parameter left.

     Empty lines may also be inserted. They are defined by param-
     eters  upper  and  line_sp, the number of pels being used as
     the unit.


     vector(fax), text(fax), ts(fax), fitter(fax)
UCL FACSIMILE SYSTEM                              INDRA Note 1185

          Appendix III: Utility Routines and Data Formats
BITMAP(FAX)                                           BITMAP(FAX)


     bitmap - convert vector format to core bit map


     int  bitmap(ivec, cnt, buff);

     int  *ivec;
     int  cnt;
     char *buff;


     Bitmap converts the fax vector format into a bit map,  using
     each bit of the area pointed to by buff.  The number of ele-
     ments in ivec is given by cnt, and the first element of ivec
     is  taken  as  a  white pel count, the second as a black pel
     count, etc. The resultant bit map  is  placed  in  the  area
     pointed  to by buff. The actual number of bits stored is re-
     turned from the function.  The bits in buff  are  stored  in
     byte  order, with the highest value bit of the byte taken as
     the first bit of the byte.


     You have to make sure that buff is big enough  for  all  the


     vector(fax), tovec(fax)
TOVEC(FAX)                                             TOVEC(FAX)


     tovec - convert bitmap to vector format


     int  *tovec(buff, nbits);

     char *buff;
     int  nbits;


     The bitmap in the buffer pointed to by buff is converted  to
     vector format. The length of the bitmap in bits is passed in
     nbits.  As the caller would normally not know how many  vec-
     tor elements are going to be needed, the tovec routine allo-
     cates this area for the user.

     Buff is assumed to be  organised  in  byte  order  with  the
     highest  value  bit  of each byte being the first bit of the
     byte. The counts of white and black pels are placed into  an
     integer  vector, the first element of which is the length of
     the rest of the vector. The vector information proper starts
     in  the  second  element which is the count of the number of
     leading white pels.  This is followed by the  count  of  the
     numbr of black pels, etc.

     The routine goes to great lengths to make sure  only  enough
     vector  storage is allocated. Temporary storage is allocated
     in small chunks and then, when the length of the whole  vec-
     tor  is known, the chunks are contacenated into a contiguous
     vector.  The pointer to this vector is returned to the user.


     vector(fax), bitmap(fax)
CHOICE(FAX)                                           CHOICE(FAX)


     choice - specify a rectangular area on Grinnell


     struct  square  {
             int  x0, y0;
             int  x1, y1;
     struct  square  *choice(colour, height, width, area, fw, fh)

     char colour;
     int  height, width, area, fw, fh;


     This subroutine is called by a MOS task.  to specify a  rec-
     tangular  area  of  an image by manipulating a square on the
     Grinnel display being illustrating the image. The  dimension
     of  the  original image is defined as height and width.  The
     area on which the original image is shown  is  specified  by
     the parameter area.

      value       area           dimension    coordinates

        0     the whole screen    512x512     0,511,511,0
        1     the left half       256x512     0,511,255,0
        2     the right half      256x512     256,511,511,0

     The square will be drwan in a colour defined by the  parame-
     ter colour, which can only be:

             value   colour

              'r'     red
              'g'     green
              'b'     blue

     There are two modes being supported:

      (1) Fixed: The square will have a fixed dimension specified
          by the parameters fw and fh.  The operator can move the
          square around as a whole within the predetermined  area
          by  using  following commands, each of which is invoked
          by typing the corresponding characer on the keyboard of
          the system console.
CHOICE(FAX)                                           CHOICE(FAX)

           command         function

             'u'           move the square up one step
             'd'           move the square down one step
             'l'           move the square one step left
             'r'           move the square one step right
             'f'           move fast - set the step to 8 pel
             'o'           move slowly - set the step to 1 pel
             <CR>          ok - the area has been chosen, and
                          return its coordinates

      (2) Arbitrary: This mode is set up when the  subroutine  is
          called  with  the  parameters  fw and fh set to 0.  Any
          edge of the square can be selected to be moved  on  its
          own  by  using  the  same commands described above. The
          following commands are required to select the  relevant
          edge as well as switching the operation mode.

           command         function

             'e'           select the right ('east') edge.
             'w'           select the left ('west') edge.
             'n'           select the upper ('north') edge.
             's'           select the lower ('south') edge.
             'a'           move the square as a whole

     As soon as the user  types  <CR>,  the  coordinates  of  the
     current  square,  which  are accommodated in a square struc-
     ture, are returned. Note these are concerned with the  coor-
     dinate  system  defined  for the image but not for the grin-


     Currently, only three working areas can be used.


     vector(fax), grinnell(dev), grinnell(fax)
CRC(FAX)                                                 CRC(FAX)


     crc - calculate or check the DACOM CRC code


     int  crc(buff, insert);

     char *buff;
     int  insert;


     This routine will check/insert the 12-bit  CRC  code  for  a
     DACOM  block,  pointed  to  by buff.  The block contains 585
     bits, the last 12 bits being the  CRC  code.  The  block  is
     checked  only  when the parameter insert is set to 0, other-
     wise the CRC code is created and inserted  into  the  block.
     When the block is checked, the routine returns the result: 0
     means OK and any non-zero value means the block is  bad.  On
     the  other  hand, when the CRC code is inserted, the routine
     returns the CRC code it has created.

     This routine uses a tabular approach to  determine  the  CRC
     code,  processing  a whole byte at a time and resulting in a
     high throughput.


     Do not forget to supply enough space  when  the  12-bit  CRC
     code is to be inserted.


CSINIT(FAX)                                           CSINIT(FAX)


     csinit - initiate the Clean and Simple interface


     int  csinit();


     This routine is called to initiate the Clean and Simple  in-
     terface for the calling process.  Its code is re-entrant, so
     that only one copy is needed for all processes in a system.

     This routine returns the task identifier, which must be used
     on all subsequent interface calls.


     csopen(fax), csread(fax), cswrite(fax), fitter(fax)
CSOPEN(FAX)                                           CSOPEN(FAX)


     csopen - establish the Clean and Simple connection


     char *csopen(tid);

     int  tid;


     A process calls this routine, waiting to be scheduled.   Its
     code  is re-entrant, so that only one copy is needed for all
     processes in a system.

     The task identifier tid is the word returned from the csinit
     call.  When the fitter process has established the Clean and
     Simple connection for the process, this routine returns  the
     pointer  to  the  parameter string of the corresponding task


     csinit(fax), csread(fax), cswrite(fax), fitter(fax)
CSREAD(FAX)                                           CSREAD(FAX)


     csread - read data from the Clean and Simple interface


     char *csread(tid, need);

     int  tid, need;


     This routine is called to read data from the Clean and  Sim-
     ple interface. Its code is re-entrant, so that only one copy
     is needed for all processes in a system.

     The task identifier tid is the word returned from the csinit
     call.  The need parameter indicates the number of bytes that
     are required. This routine returns a  pointer  to  a  buffer
     with this much data in it. This is usually more efficient as
     it means that the data does not have to be reblocked.


     If the returned value is 0, the end of data is reached.


     Funnies happen at the end of data to be read.  The  csread()
     call  has  no  way of saying that the final buffer is partly
     filled.  Thus if you ask for more data,  you  hang  forever.
     But  if  the  data  structures  are  working correctly, this
     should never happen.


     csinit(fax), cswrite(fax), fitter(fax)
CSWRITE(FAX)                                         CSWRITE(FAX)


     cswrite - write data to the Clean and Simple interface


     char *cswrite(tid, need);

     int  tid, need;


     This routine is call to write data to the Clean  and  Simple
     interface.  Its code is re-entrant, so that only one copy is
     needed for all processes in a system.

     The task identifier tid is the word returned from the csinit
     call.  The need parameter indicates the number of bytes that
     are to be written. This routine returns a  write  buffer  of
     the  required  length, to which the user data can be copied.
     The subsequent cswrite()  call  automatically  releases  the
     previous write buffer.

     The cswrite() call with need set to 0 indicates the  end  of
     data, closing the current Clean and Simple connection.


     As indicated, the write buffer must be filled up before  the
     next cswrite() call.


     csinit(fax), csread(fax), fitter(fax)
GETL(FAX)                                          GETL(FAX)


     getl - get a line vector from the Clean and Simple interface


     int  *getl(tid);

     int  tid, need;


     This routine is called to read a line vector from the  Clean
     and  Simple  interface. Its code is re-entrant, so that only
     one copy is needed for all processes in a system.

     The task identifier tid is the word returned from the csinit
     call.  The  routine  returns the pointer to the buffer where
     the line vector is stored.


     0 will be returned when end of file is reached.


     Any memory violation causes  the  whole  task  chain  to  be


     vector(fax), putl(fax), fitter(fax)
PUTL(FAX)                                               PUTL(FAX)


     putl - put a line vector to the Clean and Simple Interface


     putl(tid, buf);

     int  tid, *buf;


     This routine is called to write a line vector to  the  Clean
     and  Simple  interface. Its code is re-entrant, so that only
     one copy is needed for all processes in a system.

     The task identifier tid is the word returned from the csinit
     call. The line vector is stored in a buffer pointed by buf.


     vector(fax), getl(fax), fitter(fax)
T4(FAX)                                                   T4(FAX)


     t4 - the data format defined in CCITT recommendation T4


     Dimension and Resolution: In vertical direction the  resolu-
     tion is defined below.

             Standard resolution:            3.85 line/mm
             Optional higher resolution:     7.70 line/mm

     In horizontal direction, the standard resolution is  defined
     as  1728 black and white picture elements along the standard
     line length of 215 mm.  Optionally, there  can  be  2048  or
     2432 picture elements along a scan line length of 255 or 303
     mm, respectively. The input documents up to a minimum of ISO
     A4 size should be accepted.

     One-Dimensional Coding: The one-dimensional run length  data
     compression  is accomplished by the popular modified Huffman
     coding scheme. In this scheme, black and white runs are  re-
     placed  by  a  base  64 codes representation. Compression is
     achieved since the code word lengths are invertly related to
     the  probability  of  the  occurrence of a particular run. A
     special code (000000000001), known as  EOL  (End  of  Line),
     follows  each  line  of data. This code starts the facsimile
     message phase, while the control phase is restored by a com-
     bination  of six contiguous EOLs (RTC). The data format of a
     facsimile message is shown below.

      start of the facsimile data
      !EOL! DATA !EOL! DATA !

                    end of the facsimile data
                    |<------   RTC  ------->|

     Two-Dimensional Coding: The two-dimensional coding scheme is
     labeled  as  the  Modified READ Code. It codes one line with
     reference to the line above,correlation  between  adja-
     cent lines allowing for more efficient compression. In order
     to limit the disturbed area in the event of transmission er-
T4(FAX)                                                   T4(FAX)

     a one-dimensionally coded line is transmitted after  one  or
     more  two-dimensionally  coded  lines.  A bit, following the
     EOL, indicates whether one-  or  two-dimensional  coding  is
     used for the next line:

             EOL1: one-dimensional coding;
             EOL0: two-dimensional coding.

      start of the facsimile data

                             end of the facsimile data
                       |<---------   RTC   --------->|
TEXT(FAX)                                               TEXT(FAX)


     text - the text format for use in the facsimile system


     This is the representation  structure  for  coded  character
     text.  It is used in the facsimile system.

     The  text  structure  consists  of  a  series  of  character
     strings,  each  of  which represents a text line. However no
     control characters, e.g. <CR> and  <LF>,  are  used  in  the
     structure. Each text line is proeeded by a count byte, indi-
     cating the number of characters on the line.  The  character
     sting  follows  after the the count byte. A zero count indi-
     cates the end of file.


     Here is an example text shown below:

             This is a text.
             This is a picture.

     It can be represented as:

      <017> T  h  i  s <040> i  s <040> a <040> t  e  x  t  .
      <022> T  h  i  s <040> i  s <040> a <040> p  i  c  t  u
      r e  . <0>
TS(FAX)                                                   TS(FAX)


     ts - translate an ASCII string into vector format


     ts(ar_in, left, right, tid)

     char *ar_in;
     int  left, right, tid;


     This routine will convert a zero-ended ASCII string  pointed
     to  by  ar_in  into  the corresponding vecter format. As the
     character font being used is a set of 12x20 matrices,  there
     will  be  20 line vectors created. These vectors are written
     to the Cleans and Simple interface by calling cswrite.   The
     callers task identifier tid has to be provided.

     At the two ends of the text line, blanks can be padded  that
     are  specified  as left and right.  Note that they are meas-
     ured in pels.

     Consequently, the result should be a image, whose  dimension

             width  = left + 12*length + right;
             height = 20;

     where length is  the  number  of  characters  in  the  input

     As an intermediate result the bitmap is first created  which
     is then converted into the vector format, by calling tovec.


     The input string must be ended with a zero field.


     vector(fax),    tovec(fax),    csinit(fax),    cswrite(fax),
VECTOR(FAX)                                           VECTOR(FAX)


     vector - the internal data structure for a facsimile image


     This is the representation structure for  binary  images,  a
     simple  run length compression algorithm being used. Most of
     the image files are kept in vector format for ease  of  pro-

     The vector format consists of a series of  integer  vectors,
     one vector for each row of pels in the image. Each vector is
     proceeded by a count word which indicates the number of  in-
     teger  words  in the vector.  The next element of the vector
     after the count field is the number of  white  pels  in  the
     first  run  of  the  line.   The  second word then gives the
     number of pels that follow the initial white run, and so  on
     t  the  end of the vector. Note the first run length element
     must refer to a white run. It should be  set  to  0  if  the
     first run is black.


     A line consists of 20 pels as follows:


     It can be represented as:

             5, 3, 8, 1, 3, 5

     The inverse of the line:


     should be represented as:

             6, 0, 3, 8, 1, 3, 5
  1. RFC 0809