Network Working Group Ira W. Cotton
Request for Comments: 178 MITRE
NIC: 7118 June 27, 1971
NETWORK GRAPHIC ATTENTION HANDLING
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.
1.1 TOPICS COVERED
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).
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
2.2 INDEPENDENT FROM DISPLAY CONSIDERATIONS
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
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.
3.0 SYSTEM CONFIGURATIONS
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
3.1 STAND-ALONE CONFIGURATION
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
3.2 STAND-ALONE CONFIGURATION WITH STANDARDIZED FORMATS
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.
3.3 NETWORK CONFIGURATION
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.
3.4 NETWORK CONFIGURATION WITH INTELLIGENT TERMINAL
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
Figure 3 Network Configuration (Omitted due to complexity)
Figure 4 Network Configuration with Intelligent Terminal (Omitted due
4.0 INPUT DEVICES
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
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
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
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.
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.
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.
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
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
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.
INPUT DEVICE SUMMARY
DEVICE CLASS DEVICE EXAMPLES TYPICAL OUTPUT
Button Teletype 1 Character
Function Key with Overlay 1 Character and
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
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
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
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.
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.
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