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RFC 0178

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Network graphic attention handling


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Network Working Group                                      Ira W. Cotton
Request for Comments: 178                                          MITRE
NIC: 7118                                                  June 27, 1971



   Discussions of network graphic protocols have thus far primarily
   dealt with protocols for the description of graphic data to be
   displayed.  RFC 86 proposed a Network Standard Graphic Data Stream
   (NGDS) which would serve to convey graphic images expressed in the
   Network Standard Display List (NGDL).  RFC 94 expanded on this
   proposal, and pointed out some shortcomings of the original scheme.
   RFC 125 also replied to RFC 86 with comments and extensions, but also
   recognized that a protocol for graphic display alone is insufficient
   to support an interactive graphic system.


   The present paper addresses itself to this requirement.  The process
   of attention handling is briefly described, various graphic
   configurations are discussed, input devices are surveyed to identify
   the types of data which they produce, and an attention protocol is


   It should be made clear at the onset that the discussion which follow
   will be from the viewpoint of a graphics user or a graphic
   application program serving one or more users.  Our concern is with
   third-level protocols only.  We assume the network is capable of
   delivering arbitrary bit streams from terminal to graphic application
   program, but don't care how this is accomplished.


   In order to demonstrate the need for an attention protocol, we must
   first define what is meant by "attention" and "attention-handling."
   We therefore begin by borrowing the definitions given in a recent
   survey of this area(1).

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   Graphic attention handling refers to the processes and techniques
   whereby human inputs to a computer graphic system are serviced.  An
   attention event, or simply "attention," is a stimulus to the graphic
   system, such as that resulting from a keystroke or light pen usage,
   which presents information to the system.  Servicing includes
   accepting or detecting the hardware input, processing it to determine
   its intended meaning, and either passing this information to a user
   routine or taking some _immediate_ action related to the display
   and/or its underlying data structure, or both.  The emphasis is on
   "immediate."  Attention-handling is not intended to include any
   detailed, application-oriented processing which the attention
   information may indicate is to be performed.  Thus, attention
   handling may be considered separately from any particular


   Not only may attention handling be considered separately from any
   application, but attention generating hardware may be considered
   separately from display hardware.  Oftentimes, it is only
   coincidental that they come in the same package.  Indeed, in some
   configurations an input be processed locally (by the terminal) to
   provide the appropriate response.  For example, a keystroke may or
   may not cause a character to be displayed on a terminal, and the
   logic causing the display may or may not be local (within the
   terminal).  The keystroke might be immediately displayed locally, as
   in the case of an alphanumeric display terminal which buffers
   keystrokes and transmits messages of many characters or it might be
   transmitted to the host computer and "echoed" back for display as in
   teletype-like terminals.

   The question is not limited to such simple input devices as
   keyboards.  So-called "intelligent terminals" with integrated
   programmable logic or even complete small computers can process more
   sophisticated attentions locally, and even alter a local distillate
   of the central (host) data structure without central knowledge.  This
   raises the problem of insuring that the display and the graphic
   application program do not get "out of sync," and requires a more
   expressive protocol from terminal to host processor.

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   We now turn to a consideration of the evolution of system
   configurations for computer graphics.  Our intent is to demonstrate
   that just as display generation has evolved from the output of device
   dependent codes to a generalized protocol, so too should attention
   generation evolve.


   Figure 1 illustrates the stand-alone graphic configuration which was
   the first and is still the most common.  As we have stressed, input
   and output are entirely independent, and are shown as separate
   devices.  In this configuration, display code generation and
   interrupt processing are both done within the graphic application
   program in the host processor.  The graphic application is very


   The significant conceptual change occurs when the input and output
   processors are removed from the graphic application program.  The
   graphic application program then generates output and accepts input
   in a generalized form, as illustrated in Figure 2.  The important
   fact to note is that in order to accommodate additional (different)
   input and/or output devices, only these input/output processing
   routines must be replaced or altered.  Graphic application programs
   may be designed without regard to which particular processing routine
   will be used.  So far as the application program is concerned,
   device-independence has been achieved.

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Figure 1 Stand-Alone Graphic Configuration

   |                            |                _______
   | +---------+-----------+    |               /       \
   | |         |OUTPUT     |    |              /         \
   | |     /-->|PROCESSOR  |----|------------>|           |
   | |    /    +-----------+    |              \         /
   | |    |                |    |               \_______/
   | |    |                |    |             OUTPUT DEVICE
   | |    |    +-----------+    |              ______
   | |    \    |INPUT      |    |             |      \
   | |     \---|PROCESSOR  |<-- |-------------|_______\
   | +---------+-----------+    |
   |     Graphic Application    |             INPUT DEVICE
   |         Program            |
   \USING  /

Figure 2 Stand-Alone Configuration with Standardized Input and Output

+-------------------------------------+                        ______
|                                     |                 /---->/      \
|                      +-----------+  |DEVICE-DEPENDENT/  ___/___     \
|                    +-----------+ |--|---------------/  /       \    |
|        STANDARD    | OUTPUT    | |  |DISPLAY LIST     /         \   /
| +-----+DISPLAY LIST|PROCESSOR  |-+  |                 |         |__/
| |  ---|----------->|           |----|---------------->\         /
| |  |  |            +-----------+    |                  \_______/
| |  |  |                             |                 OUTPUT DEVICE(S)
| |  |  |                             |
| |  |  |              +-----------+  |DEVICE-DEPENDENT       ______
| |  |  |  STANDARD  +-----------+ |<-|----------------------|      \
| |  |--|<-----------|INPUT      | |  |INPUT DATA         ___|___    \
| +-----+  ATTENTION |PROCESSOR  |-+  |                  |       \____\
|                    |           |<---|------------------|        \
|                    +-----------+    |                  |_________\
|    Graphic Application Program      |                  INPUT DEVICE(S)
|                                     |

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   When the stand-alone configuration with standardized formats is
   implemented on a network, the organization illustrated in Figure 3
   results.  In the network configuration, the graphic application
   program and the input and output processors may be in different
   hosts.  The standardized formats become network standards, and now
   any using host with input/output processors conforming to the
   standard can access the graphic application program in the serving
   host.  The network is transparent to the graphic configuration.


   The case of an intelligent graphics terminal configured in the
   network is illustrated in Figure 4.  Here, input and output
   processors are located within the terminal itself.  The using host
   serves only to connect the terminal to the network, and in the case
   of a terminal IMP, is dispensed with altogether.  Any type of
   intelligent terminal may access any graphic application program if
   its (the terminals) input and output processing routines conform to
   the network standard.  As before, the network is transparent to the
   graphic configuration.

   Figure 3 Network Configuration (Omitted due to complexity)

   Figure 4 Network Configuration with Intelligent Terminal (Omitted due
   to complexity)


   We now turn to a survey of graphic input devices as suggested in RFC
   87.  The survey will concern itself with the characteristics of the
   attention information presented when the device is used (rather than,
   for example, human factors considerations).

   We wish to stress at the onset that we consider all devices
   equivalent in capability.  By this we mean that with appropriate
   programming, any device can simulate any other device.  Throughout
   the survey we will illustrate typical data conversions which might be
   performed, and at times discuss how various devices may be simulated.

   It is convenient to consider the characteristics of classes of
   devices.  Information about particular commercial devices may be
   found in reference 5 and elsewhere.  Table I presents a device class

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   Perhaps the first and most primitive class of input devices is the
   pushbutton, which presents some unique code to the system when
   depressed.  In the simplest case, the code is equivalent to the
   knowledge that the button has been pushed, and may be just a flag.

   Beyond the basic pushbutton, more advanced key devices have been
   designed in a variety of ways.  For example, each key may be
   associated with a single bit in a word or with a complex pattern
   (character or byte), multiple keys may or may not be able to be
   struck simultaneously (if so, their codes being combined in some
   defined way).

   The salient feature of the function key is that it presents two
   pieces of information to the system: the fact that a keystroke has
   occurred (which may be implicit), and some unique code related to it.

   More elaborate keyboards, be they teletypes or solid state devices
   with elaborate "overlays", are merely special cases of function keys.
   They present the same information, attention source plus a unique
   code.  The fact that such a code may be associated with a displayable
   character is at this stage only incidental.

   Since keyboards permit the entry of arbitrary codes, particular
   sequences of codes may easily be defined to simulate other devices.
   If local logic permits, codes may be accumulated until a complete
   sequence is entered and then be reformatted to exactly the same
   format as the device being simulated would have produced.

   Pointing devices such as light pens and tablets may be simulated by
   associating particular keys with screen directions (up, down, right,
   left) and using them to position a pointer on the screen face.  This
   facilitated on terminals with a hardware connection between keys and
   cursor symbol.


   The next most basic class of input devices are those which consist of
   analog to digital converters.  These include simple shaft encoders,
   mouse and trackball.  These devices all produce a digital output
   proportional to an analog input, in this case, the rotation of a
   shaft.  Several of these inputs may be presented together, as in the
   case of the mouse and trackball.

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   These devices all present as input a device identification (which may
   be implicit depending on the hardware method of input) together with
   a number of digital codes from the same number of analog devices.
   The length of the code is arbitrary and may or may not relate to such
   measures as the maximum raster count of the display screen.

   Analog devices are often used as pointing devices by using the input
   to control the movement of a cursor on the screen face.  This method
   is superior to the use of a keyboard, since very smooth and rapid
   movement may be obtained.


   A tablet consists of a flat surface on which (X,Y) position may be
   indicated with a stylus.  If position changes can be registered
   rapidly enough, arbitrary curves may be digitized by tracing them.

   There are a variety of devices utilizing a variety of techniques
   comprising this class.  Included are such diverse techniques as
   variable resistance, variable capacitance, and ultrasonics, to
   mention a few of the devices on the market.  The surface may be
   horizontal or vertical and may even be superimposed on the screen
   itself.  A variety of styli have been used, including the operator's
   finger.  A device (the Lincoln Wand) has also been demonstrated which
   may be manipulated in space to yield a position in three dimensions

   These devices all present a device identification (which may be
   implicit), and a position value, which is most often a coordinate
   pair, but which may be a triple.


   Light pens are devices which relate the occurrence of an attention to
   the time in the refresh cycle when a particular point is illuminated
   on the screen.  The display generators are generally stopped when the
   attention occurs, to permit either the display list "P" register or
   the (X,Y) beam position registers, or both to be presented as
   attention data.  Often times this is not enough, as what is desired
   is some value which serves to identify the image which the pen
   detected.  In such cases local hardware and/or software is utilized
   to obtain this information, which may be as simple as a single
   identification code or as elaborate as a genealogical push down list.

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   Light pens present as input a device identification (which may be
   implicit) and at least one of the following:  memory address, (X,Y)
   position, item identification.

   Light pens may be used to simulate keyboards by displaying "light
   buttons" on the screen associated with particular physical buttons.
   Detecting on a light button is logically equivalent to pushing the
   related key.


   Internal attentions are stimuli arising not from operator action, but
   from various sources within the terminal such as a screen edge
   violation (overflow) or a programmed trap.  Such attentions present
   information in much the same way as the operator input devices
   already discussed.  This information consists of an attention source
   identification (equivalent to device identification, and which may
   again, be implicit) and appropriate data, which, for the two examples
   given, will generally be a memory address.

   Programmed traps are often used to permit mode changes (e.g., enable
   or disable light pen operation) during the execution of the display
   list.  Edge violation might occur when an image is being relocated in
   response to operator input.  We must provide for describing such
   attentions, since then cannot always be handled locally in the


   We may generalize the concept of an attention from a stimulus from a
   physical source to a logically generated stimulus resulting from some
   program condition which may or may not cause an interrupt.
   (Programmed traps were classified as internal attentions because, by
   definition, they cause an interrupt or set a hardware flag).  Logical
   attentions are generally "input" by setting a software flag which
   some control program can periodically inspect.  For example, logical
   attentions may be designed to detect when a software-defined edge
   violation occurs (of a region less than full screen) or when a light
   button is picked.  There is no general form for the information
   generated by logical attentions, since they are programmable, rather
   than hardware-bound.  The best we can do is to say they consist of an
   identification and appropriate data.  Actually, logical attentions
   most often simulate physical attentions, and so each may be placed in
   one of the classes already described.

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                                TABLE I

                          INPUT DEVICE SUMMARY


Button             Teletype                      1 Character
                   Function Key with Overlay     1 Character and
                                                 overlay code
                   Buffered Keyboard             n Characters

A/D Converter      Shaft Encoder                 delta a
                   Mouse                         (delta a, delta b)
Tablet             Rand Tables and               (X,Y)
                   Lincoln Word                  (X,Y,Z)

Light Pen          Light Pen                     P (memory address)
                   Light Pen                     (X,Y)
                   Light Pen and Local Software  Item Name
                   Light Pen and Local Software  Item name stack

Internal           Program Trap                  P (memory address)
                   Screen Overflow               P (memory address)

Logical Attention  Any of the above              Any of the above


   As has been indicated, the question of what data results from which
   inputs is complicated when "intelligent terminals" are considered.
   An intelligent terminal has the ability to modify the data presented
   by the input device hardware.  In a sense then, all of the outputs of
   an intelligent terminal may be considered as logical attentions.  The
   logical complexity of such attentions may be very great indeed.  For
   example, such a terminal might be programmed to perform sketching
   functions, so that the net effect of a keystroke and a light pen hit
   might be the deletion of a portion of the picture together with some
   coded message to the effect.  A technique has even been developed
   which permits the light pen operator to simulate the use of a shaft
   encoder by twisting his wrist which holding the pen over a tracking
   symbol (7).

   Some intelligent terminal systems have been developed which permit
   the terminal operator to modify the picture and the local data
   structure independently.(2)  Thus, the need for a very expressive
   protocol from terminal to central computer becomes apparent, so that
   notice of such local processing may be communicated to the central

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   We now suggest a format for a (third level) network protocol from
   terminal to serving host which is sufficiently open-ended to permit
   any type of attention to be communicated.  It is not the intent here
   to formally propose such a protocol down to the level of "this bit
   means that."  When such details are decided, a Network Standard
   Attention will have been defined.

   The attention protocol has three basic elements:  device
   identification, data identification, and data.


   The device identification field must be sufficiently large to permit
   the unique identification of any TYPE OF DEVICE in the network.  If
   two or more identical input devices exist at different nodes in the
   network, it is not necessary to distinguish among them in this field.
   However, if a keyboard, for example, has keys which are logically
   different, such as typewriter keys and function keys, the distinction
   should be made in the identification field, rather than requiring an
   analysis of the data.  Further, if two different devices are
   logically equivalent, as a physical keyboard and light buttons, they
   may be so treated by NOT having identification codes different from
   each other.

   Somewhere in the network (and possibly at each host supporting a
   graphic application) a table should be kept of the input device types
   and their characteristics.  It may be convenient to organize the
   device identification field so that a subfield identifies the device
   CLASS, as discussed previously


   The device identification field is intended to contain a description
   of the data field which follows.  Information which might be provided
   here includes number of units (bits, words, bytes) of data which
   follow, qualitative description of the data (character code, memory
   address, cartesian coordinates, item name, etc.), and data format
   information.  It may be desirable, for the sake of uniformity, to
   include this information even when it is somewhat redundant.

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6.3 DATA

   Lastly comes the data itself (perhaps an anticlimax at this point!)
   which, as should be clear by now, may be of arbitrary length and


      1. Cotton, I. "Languages for Graphic Attention-Handling." Proc.
      Computer Graphics 70 Symposium, Brunel University, 197.

      2. Cotton, I. and F. Greatorex "Data Structures and Techniques for
      Remote Computer Graphics," Proc. FJCC, 1968, pp. 533-544.

      3. Crocker, S. "Proposal for a Network Standard Format for a Data
      Stream to Control Graphics Display." ARPA Network Working Group,
      RFC # 86, 1971.

      4. Harslem, E. and J. Heafner "Some Thoughts on Network Graphics,"
      ARPA Network Working Group, RFC # 94, 1971.

      5. Keast, D. "Survey of Graphic Input Devices," MACHINE DESIGN.
      August 3, 1967, pp. 114-120.

      6. McConnell, J. "Response to RFC #86," ARPA Network Working
      Group, RFC #125, 1971.

      7. Newman, W. "A Graphical Technique for Numerical Input,"
      COMPUTER J., May 1968, pp. 63-64.

      8. Vezza, A. "Topic for Discussion at the Next Network Working
      Group Meeting."  ARPA Network Working Group, RFC #87, 1971.

           [This RFC was put into machine readable form for entry]
        [into the online RFC archives by Kelly Tardif,Viagénie 11/99]