rfc178.txt

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   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.

4.2 ANALOG DEVICES

   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.







Cotton                                                          [Page 6]

RFC 178            NETWORK GRAPHIC ATTENTION HANDLING          June 1971


   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.

4.3 TABLETS

   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
   (X,Y,Z).

   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.

4.4 LIGHT PEN

   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.








Cotton                                                          [Page 7]

RFC 178            NETWORK GRAPHIC ATTENTION HANDLING          June 1971


   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.

4.5 INTERNAL ATTENTIONS

   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
   terminal.

4.6 LOGICAL ATTENTIONS

   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.







Cotton                                                          [Page 8]

RFC 178            NETWORK GRAPHIC ATTENTION HANDLING          June 1971


                                TABLE I

                          INPUT DEVICE SUMMARY

DEVICE CLASS       DEVICE EXAMPLES               TYPICAL OUTPUT

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


5.0 INTELLIGENT TERMINALS

   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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RFC 178            NETWORK GRAPHIC ATTENTION HANDLING          June 1971


   program.


6.0 NETWORK PROTOCOL GUIDELINES

   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.

6.1 DEVICE IDENTIFICATION

   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

6.2 DATA IDENTIFICATION

   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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RFC 178            NETWORK GRAPHIC ATTENTION HANDLING          June 1971


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
   organization.


BIBLIOGRAPHY

      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.






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Cotton                                                         [Page 11]