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Network Working Group D. Huff
Request for Comments: 285 CWRU (Case)
NIC: 8271 December 15, 1971
Updates: None
Obsoletes: None
Network Graphics
Not much has been written about graphics on the ARPANET when the
volume of the NIC collection is considered. Presently it contains some
8000 entries of which only about 20 are on the subject of graphics. The
reason is probably similar to that given by L. G. Roberts in A FORWARD
LOOK (NIC 7542) as the reason that data base sharing or software sharing
will not be important topics for several more years: the NET hasn't been
up long enough for interested people to have enough of the facts to know
if it is feasible and to think creatively.
This paper is therefore aimed at bringing together the present
state of graphics on the NET for the newcomer and attempting to add a
little more distance to the ground covered so far. I will start with an
overview, then proceed to briefly describe past work, and finally add
some of my own thoughts.
Since the NET represents a wealth of data processors, any or all
of which may be used at one time, we are not restricted to the
configurations most generally found in private installations where there
is a main processor and a somewhat less capable machine or perhaps none
at all doing the honors as display processor. Indeed when using the NET
it might occur that one has a more powerful machine as the display
processor than the machine which is running the main job. Graphics on
the NET need not be anything like what we know it as now.
There is of course a greater more diversified mix of graphics
equipment that must be considered when designing a standard graphics
language and its processor. If we wish to drive an aribitrary display
from a program such an output language must be quite general, but the
processor which constructs the actual display list for the target
display need not and in fact will not be general, rather its only job
will be to translate a well defined general language to meet the
requirements of one specific graphics terminal.
Attention handling, a lately discussed and much worried about
topic, presents an entirely different problem. This time the NET may
cause more harm than good for the simple reason that now there may be
several, instead of one (in some cases none at all), mappings defined to
get from the initial display list that the main job process is creating
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to the final display list which interactive devices such as the light
pen actually refer to. This is a problem which has to be faced and has
been solved at many different sites in as many different ways. It is
likely to give as much trouble as the final concept.
Local processing is in many cases a very simple thing to
accomplish when the display terminal is "intelligent" or even has its
own medium or large scale processor which has little or nothing else to
do aside from refreshing the display. Such processing can be simple
additions or deletions to the picture which certainly do not require the
main job process to accomplish. The local processor need only notify the
main process of what changes have been made to the display list so that
the main copy may be updated. The allocation of abilities poses the last
problem. The lower limit is reached when the local processor is unable
to do anything beyond keeping the picture displayed, and the upper limit
applies to the case when the local processor is more powerful than the
main processor and handles all attentions itself. Now such questions as,
just which copy of the display list is the master copy, who is
responsible for seeing that all copies of the list contain the same
information, and what kind of mappings between display lists are
required, become the important ones we all seek to answer.
Proposals for Network Standard Graphics started with the idea of
a simple interpretable language containing only commands to erase the
screen, display a string of text, move the beam or draw a line or point
within a virtual rectangle which is the generalized display screen,
execute a previously defined subroutine, and replace the contents of a
subroutine with what follows in the command stream. Movements within the
screen area were defined in terms of fractions of the screen dimensions
instead of absolute lengths. This proposal was responded to with the
suggestion that a graphics standard could not be so restrictive and find
wide acceptance. The proposal was not expressive enough to handle
sophisticated picture manipulations. It was recognized that a standard
must be able to make use of all graphics hardware, present and within
the forseeable future. The data structure should represent both logical
and pictorial structure, allow for the definition and manipulation of
subpictures, and division of the display screen into logical units. The
proposed standard has now become a general high-level language rather
than a low-level language. It was pointed out that all sites need not be
able to handle the interpretation of this graphic language, but because
of the existence of the rest of the NET, one of the other machines could
run the interpreter, this is equivalent to a data reconfiguration
service. Such drawing modes as intensity, blinking, dashed, color, or
stereo should also be expressable by means of a command to set the mode.
The canonical definition of a character string should be defined since
everyone has their own way of displaying text and most of them are
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different. It is suggested that the Multics convention be used as
described by Osanna, J., Sahzer, J., Remote Terminal Character Stream
Processing of Multics, Proceedings SJCC, 1970, p. 671.
If in addition to simply displaying graphic information, if one
wishes to to interact with the picture directly, the protocol must
include a standard for feedback, attention handling as it is being
called. Attentions may not always refer directly to the picture however,
as in the case of keyboard input which can be handled as any other
standard message on the NET. Some graphics processors may also have the
capability of handling attentions locally and only need to report the
end result to the main process. This is the problem of which data
structure is most up to date, which is considered the master copy, and
how can the processes be kept in sync? The observation is also made that
as long as the graphics application program, the main process,
communicates with a pair of graphic device handling routines in a
network standard language, the system configuration can be arbitrary and
any terminal may be attached to any main process. The same is of course
true of attention handling, a set of standards for the transmission of
an attention generated by a particular device when developed will allow
any graphics terminal to be understood by any other main process. A
summary of input devices has been given along with typical outputs and
the suggestion that each attention message identify the device causing
the attention, the data which is being supplied, and of course, the data
itself.
The proposed graphic protocol has become much richer in display
types. The following list was suggested as basic: points, lines,
vectors, character strings, viewport and window, transformations of
instances, hardware-dependent byte streams, attention commands. The
point was also made that special considerations for grey-scale devices
are needed and four alternate display modes are discussed (NIC 7128).
An example of hardware sharing is described in NIC 7130. It is a
protocol for the use of the LDS-1 processor at M.I.T. by anyone on the
net who has a program for the LDS-1. This Graphics Loader, as it is
called, provides for the execution of programs that have been sent to
the PDP-10 at M.I.T. and the return of the data generated when the
program is executed. The picture is not drawn on a display, but since
the LDS-1 processor can be instructed as to what to do with the
coordinates that it generates, the Graphics Loader sets up the processor
to write back into core the computed display coordinates. These
coordinates may now be sent back to the originating site for display or
as a debugging aid.
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In NIC 7137 many of these previously discussed points are again
brought up, but this time under the supposition that a graphics terminal
should be just another terminal with minimal special privileges.
Suggestions were also made pertaining to the design of a graphics
protocol with particular emphasis on display structure, attention
handling, coordinate systems, and the difference between storage tube
and refreshed display requirements.
A specific solution for the handling of tablet input data has
been presented, (NIC 7151), along with the expression that the graphics
protocol should be designed so that non-interactive graphics should not
be complicated with the requirements imposed by the interactive aspects
of the protocol. It is pointed out that there are several types of
tablet data that can be sent as input to a graphics process. Four types
of data are described. They are single-shot, raw asynchronous, raw
synchronous, and preprocessed data. Preprocessed data can be smoothed or
filtered or thinned using various techniques to make the data more
uniform and workable. Velocities can also be calculated for each point
to aid in the interpretation of the data.
The description of NETCRT (NIC 7172) is the first encounter with
local processing, or lack of it. NETCRT is a protocol between a central
processor and a character display. The character display is completely
slaved to the central processor and can do no local processing, however
it can interrupt the processor thus signalling that the user is done
typing or wishes to begin typing. NETCRT tries to maintain good man-
machine interaction by controlling the state of the terminal.
I have refrained from commenting on the various proposals as I
summarized them because I don't think that it would have been in line
with what I am trying to do in this paper. I think that there is a need
to consider an overall model of the graphic system we are trying to
design. Previous proposals have dealt with some set of details without
identifying with a general model, producing good ideas for
implementation of details but not considering how the whole will fit
together. Thus I would like to propose a model for our graphics system.
It will contain many protocols and leave many areas to be discussed
further, but it will provide a starting point from which work can be
done along simple lines, and yet not exclude the later inclusion of more
sophisticated abilities.
Figure 1 shows a block diagram of information flow. The PROCESS
indicates a graphics application program which is running on a computer
in the net. Its associated INPUT and OUTPUT routines can be thought of
as being a set of subroutines loaded with the main PROCESS or as
separate and running elsewhere serving many users. At the other end of
the loop are a set of INPUT and OUTPUT drivers for the DISPLAY which is
being used to display the graphics information. The information flowing
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from the PROCESS to the DISPLAY is drawing information for the building
and manipulation of pictures. The information flowing from the DISPLAY
to the PROCESS is attention information. The Graphic Data Base
associated with the main PROCESS is that which is constructed when the
picture is being drawn by the PROCESS or when the picture is being drawn
by local processing and attention messages tell the PROCESS what has
been done to the picture. This data base need not contain more
information that the PROCESS is willing to work with, and in fact need
not contain anything if no picture interaction is to be done. The
Graphic Data Base associated with the DISPLAY drivers is built by
themselves so that the OUTPUT driver can handle attentions from the
DISPLAY without requiring the main PROCESS to be able to do this and for
the INPUT driver to use when modifying the picture based on what is
actually being displayed. The information flowing to and from the main
PROCESS is the sort which is passed or received as parameters to
procedures. The INPUT and OUTPUT routines translate to and from
respectively a network standard graphics protocol which is sent out over
the net to the INPUT and OUTPUT display drivers whose responsibility it
is to translate the standard message into the appropriate byte stream to
drive the DISPLAY or translate the attention from the DISPLAY into a
network standard message. The DISPLAY is assumed to handle its own
refreshing if it requires any, so that there will be as little apparent
difference between refreshed and non-refreshed displays as is possible.
This model provides for both simplicity of use for those doing
simple things and power which is needed for those doing sophisticated
interactive graphics. It can be used with a minimum of effort and
overhead by setting runtime conditions to indicate that no interactive
graphics will be done and all associated processing should be skipped,
while still enabling other PROCESSes to do high powered graphics without
going to a completely different set of routines and rules.
Due to the existence of two separate data bases, which must be
kept up-to-date with each other there are two modes of operating this
model. For lack of better names let us call them PROGRAM graphics and
LOCAL graphics. The former indicates that the picture being displayed is
constructed by the main PROCESS and all input from the user at the
display is solicited, thus the DISPLAY data base is only updated after
and as a result of action by the main PROCESS. The latter indicates that
the user at the display is directing the construction of a picture by
means of function buttons and drawing tools, the DISPLAY data base is
updated immediately and the main PROCESS is notified of the change so
that it may keep up, but it does not perform manipulations of the
picture unless requested to do so by the DISPLAY OUTPUT driver; this can
be as a result of a request to perform some function that the DISPLAY
INPUT/OUTPUT drivers can do by themselves or a request by the user to
have the main PROCESS perform a non-standard function on the picture.
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The main purpose of this design is to allow greatest generality
of graphic configurations rather than minimum response time. The design
for an optimum requires more exact specification of the hardware
configuration and the proposed usage. Since neither of these variables
can be known, and in fact our attempt at generality keeps us from even
guessing very closely at them, we must provide intelligent INPUT/OUTPUT
drivers that will know how to split the processing load between
themselves and the main PROCESS as a function of what kind of DISPLAY
they are driving, rather than attempting to design in an optimum
breakpoint.
The Graphics Protocol should specify the format of the messages
which are transmitted between the INPUT and OUTPUT routines and drivers.
These messages can be divided as previously mentioned according to their
direction and content, i.e. drawing messages and attention messages.
Since it is often desired to intermix graphics and text there should be
a distinguishing message header for all graphics messages. Then a byte
to specify the type of information contained in the body of the message,
a count of the bytes in the body, and finally the body itself. Virtually
all of the necessary message types have been indicated in the previous
RFCs and I will not list them here, except to note that attentions now
include requests for processing that the drivers could not do.
To summarize, I believe that a simple model is enough to enable
the design of both sophisticated interactive graphics and low effort
non-interactive graphics. The primary reason for this is that our major
interest is not minimum response, but rather maximum configuration
mixes. There are opportunities to use software sharing and data
reconfiguration services when building INPUT/OUTPUT routines and
drivers. Much detailed work remains to be done, but with a basic model
in sight providing a framework to hang proposed ideas on for evaluation,
work should be able to proceed more smoothly.
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+---------+ +--------+
! INPUT ! ! OUTPUT !
+--! routine !<------||---------! driver !<--+
! +---------+ +--------+ !
! ^ !
V ! !
+---------+---------+ +---------+ ! +---------+
! ! Graphic ! ! Graphic ! ! ! !
! PROCESS ! Data ! ! Data !<->! ! DISPLAY !
! ! Base ! ! Base ! ! ! !
+---------+---------+ +---------+ ! +---------+
! ! ^
! V !
! +---------+ +--------+ !
! ! OUTPUT ! ! INPUT ! !
+->! routine !-------||-------->! driver !---+
+---------+ +--------+
Figure 1
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