Some thoughts on Network Graphics

Network Working Group                                         E. Harslem
Request for Comments: 94                                      J. Heafner
NIC: 5725                                                3 February 1971


                   Some Thoughts on Network Graphics

Purpose

   This note states some of our initial reactions to NWG/RFC #86, whose
   purpose was to provide a basis for discussion and development of
   Network graphics.

   The method of operation described in Note 86 was to interpret data
   structures to produce graphic order codes for display.  This method
   has proven satisfactory in the past and we favor this approach.  The
   Note 86 proposal is directed toward a particular concept of operation
   (i.e., minimal graphics terminal connected to computational
   facilities at remote sites); our remarks embrace extended operations
   that include smart programs at each end of the connection as well as
   the minimal terminal.

   The proposal in Note 86 should be broadened to include the
   description of more complex entities and it should be raised to a
   level of describing more general things.  In this note, we first
   criticize the limitations imposed by the details of Note 86; then
   suggest some supplementary ingredients to extend its scope; and
   lastly, we suggest an alternate approach that reduces Network
   conversations (where possible) to symbol manipulation rather than
   gross detail.

Comments on the Detailed Restrictions of Note 86

   The detailed constraints enumerated in Note 86 restrict many
   interesting features of the Rand display hardware that we consider
   necessary (from a human factors standpoint) to some current
   applications.  They likewise restrict other nodes whose ARPA-
   sponsored research is dependent upon the use of sophisticated
   hardware.  For example, the point, vector, and character capability
   of Note 86 excludes line type mode, intensity control, and many other
   attractive control operations; the maximum symbol sizes are too small
   for our large character size; the origin of all of our symbols is
   specified as the "centroid" of the symbol rather than the lower left
   corner of a virtual rectangle encompassing the symbol; under mode
   control for plotting purposes, the beam may not be advanced to the
   next character position; a 7-bit ASCII is insufficient; etc.  In
   short, the five list items of Note 86 are not expressive enough; for
   example, there is nothing to allow one to position and open a graphic
   compare "window".  The problem was not treated of supplying
   parameters identifying structure for match, etc. that are not actual
   display commands.

   Perhaps some necessary information gathering (i.e., the display
   hardware descriptions and the characteristics of every node) is
   preliminary to the generation of a detailed specification.  It is
   important that, without delay, a mechanism be defined for gathering
   and collating this information in such a way that it doesn't deter
   progress on Network graphics development.

Some General Extensions to the Note 86 Proposal

   1. DISPLAY LANGUAGE CAPABILITIES SHOULD ENCOMPASS THE UNION OF
      CURRENT AND ANTICIPATED NETWORK GRAPHICS HARDWARE.  Our experience
      in exploring interactive graphics communication techniques for use
      by researchers and non-programmers indicates that this is not just
      a "motherhood".  The utility of such applications programs depends
      highly upon incorporating sophisticated graphics hardware.  In
      absence of those features, some programs simply won't be used.

   2. THE DATA STRUCTURE SHOULD ALLOW LOGICAL AS WELL AS PICTORIAL
      REPRESENTATION OF THE USER'S PROBLEM.  This close coupling of the
      meaning of a picture with the actual picture is desirable from a
      processing program's point of view, especially if a user is to
      interact with the picture.  We have found this an efficient way to
      operate with the GRAIL Project and its derivatives here at Rand.
      This technique is included in a recently proposed graphics
      language generated by Bob Anderson (Rand) and Ben Wegbreit
      (Harvard).

   3. TRANSMIT DEFINITIONS OF GRAPHICS AND THEN INSTANCES OF THEIR USE.
      The attempt here is to raise the level of "conversation" between
      programs (where possible) and to reduce processing overhead.  For
      example, if one wishes to draw lots of resistors, why not
      graphically define a resistor once and then transmit instances by
      giving the definition name accompanied by attributes? A typical
      form of an instance is shown below.

         Item Name (position, size, intensity, scaling, labeling,
                    rotation, etc.)

      There are many examples of this approach such as the recent work
      by William Newman (Utah) and many earlier studies at MIT.

   4. PARTITION THE DISPLAY STRUCTURE FOR 1) STATIC VS. DYNAMIC
      INFORMATION, AND 2) CONTEXT.  As opposed to refreshing an entire
      picture whose domain is the entire screen, we have found it useful
      to give the processing routine (that wishes to draw a picture)
      knowledge of only of a named rectangular portion of the CRT and an
      accompanying display structure.  With our particular hardware we
      can then update only the dynamic part of a picture rather than
      regenerating the entire display structure.  Just as important, we
      can logically assign areas of the CRT to different concurrent
      processing routines.  Coupled with the logical/pictorial
      representation noted in 2) above, this is a powerful technique.
      Named partitions also naturally accommodate those applications
      requiring multiple CRTs.

   5. THE INTERPRETER COULD BE CONTEXT-DRIVEN THUS NOT RESTRICTING ITS
      OUTPUT TO A SINGLE SET OF CRT ORDER CODES.  By providing cataloged
      descriptions such as the "forms" discussed in Note #83, the
      interpreter could reconfigure data destined for files, etc., as
      well as a display.  The gain here in terms of adapting to a users'
      Network needs is large; the price paid in terms of implementing
      this increment of the interpreter is probably small.

An Alternate Proposal

   Note 86 mentions the case of a terminal at a node with a minimal HOST
   connected to a remote computationally-oriented node.  The data
   standard, which Note 86 suggests transmitting over the Network is
   rather gross detail.  Also, the standard language is rather
   inexpressive -- encompassing only a few simple notions.

   An alternative approach is to consider the situation of communication
   between non-minimal nodes (nodes with substantial memory and
   computing power).  Here the Network standard data should be a high-
   level macro form representing the instances of gross detail with the
   power to deal with sophisticated graphics devices.  That is, the
   standard language would be rich enough to express all the special
   features of Network display devices.

   This suggestion presents two problems.  First, how can a terminal
   handle commands from a remote program of which its hardware is
   incapable? The answer is that the remote program to which it is
   connected is too sophisticated for the terminal -- the connection is
   invalid.  A terminal should NORMALLY only connect to a program that
   addresses no more than its hardware capabilities.  This concept
   allows a standard under which a simple terminal and a simple program
   can communicate (exactly the proposal of Note 86), yet a
   sophisticated terminal can talk to a sophisticated program in a
   high-level language, or it can talk to a simple program, all within
   the same Network standard.

   The second problem is that a minimal host might not have sufficient
   facilities to translate from a powerful Network standard language
   into the simple, detailed order codes of its terminals.

   When required, the needs of a minimal site would be handled by
   another Network node providing data reconfiguration services, AN
   ESSENTIAL PART OF THIS PROPOSAL.  The reconfiguration would be done
   on the basis of "forms" specifying translation form the Network
   standard to the specific non-standard data format required by the
   minimal node (i.e., tailored specifically to its hardware).  Whether
   it would be graphic order codes or some intermediate form would
   depend on the processing power and requirements of the minimal node.

   Fig. 1 shows a schematic diagram of the key elements of such a
   reconfiguration facility.  Fig. 2 shows the use of that facility by a
   local display handler and its use as an intermediary by two remote
   nodes requiring different degrees of external data reconfiguration.

              Network
                | ^
                | |
                | |
                v |
          +--------------+
          | A Network    |     Local
          | Process      |---> Files, Programs,
          | Invoking the |<--- CRTs, etc.
          | Interpreter  |
          +--------------+
                | ^
                | |
                | |
                v |
          +--------------+      +--------------+ (A user can access
          |              |      |  User's      | the logical
      |-->| Interpreter  |      |  Semantic    | representation of
      |   |              |      |  Routines    | his problem.)
      |   +--------------+      +--------------+
      |             | ^           | ^
      |             | |           | |
      |             | |           | |
      |             v |           v |
      |           +-------------------+
      |           |                   |
      |           |   Primitive       |
      |           |   Data Structure  |
      |           |   Operators       |
      |           |                   |
      |           +-------------------+
      |                           | ^
      |                           | |
   +--------------+               | |
   | Data Base of |               v |
   | "Forms" for  |         +------------------+
   | Reconfigu-   |         |  Data Structure  |
   | ration       |         |  Base:           |
   +--------------+         |  1 - Pictorial   |
                            |  2 - Logical     |
                            +------------------+

                   Fig. 1. Data Reconfiguration Service

       Host Providing                        Host Providing
   Computational Facility                Reconfiguration Service
   +--------------------+  STANDARD  +-----------------------------+
   |                    |   FORMAT   |  +----------+ +-----------+ |
   |                    |------------|--|  Inter-  |-|  Display  | |
   |                    | (of Macro  | /|  preter  | |  Handler  | |
   |                    | Form Data) |//+----------+ +-----------+ |
   +--------------------+            //--------------------|-------+
                                    //                     |
                                   /(                +-----------+
                                  /  \               | Terminal  |
                                 /    \              +-----------+
                                /      \
                               /        \
                              /          \
                   NON-STD.  /            \  NON-STD.
     (Terminal Order Codes) /              \ (Detailed Data)
                           /                \
                          /                  \
                         /                    \
                        /                      \
                       /                        \
                      /                          \
                     |                            |
             +-------|-------+            +-------|-------+
             |       |       |            | +-----------+ |
     Minimum |       |       |            | |  Display  | | Minimum
      Host   |       |       |            | |  Handler  | |  Host
             |       |       |            | +-----------+ |
             +-------|-------+            +-------|-------+
                     |                            |
               +-----------+                +-----------+
               | Terminal  |                | Terminal  |
               +-----------+                +-----------+

                Fig. 2. Use of Data Reconfiguration Service


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