Network Working Group Richard Stallman
Request for Comments 746 MIT-AI
NIC 43976 17 March 1978 The SUPDUP Graphics Extension
... extends SUPDUP to permit the display of drawings on the screen of
the terminal, as well as text. We refer constantly to the
documentation of the SUPDUP protocol, described by Crispin in RFC 734
Since this extension has never been implemented, it presumably has
some problems. It is being published to ask for suggestions, and to
encourage someone to try to bring it up.
The major accomplishments are these:
* It is easy to do simple things.
* Any program on the server host can at any time begin outputting
pictures. No special preparations are needed.
* No additional network connections are needed. Graphics commands
go through the normal text output connection.
* It has nothing really to do with the network. It is suitable
for use with locally connected intelligent display terminals in
a terminal-independent manner, by programs which need not know
whether they are being used locally or remotely. It can be used
as the universal means of expression of graphics output, for
whatever destination. Programs can be written to use it for
non-network terminals, with little loss of convenience, and
automatically be usable over the ARPA network.
* Loss of output (due, perhaps, to a "silence" command typed by
the user) does not leave the user host confused.
* The terminal does not need to be able to remember the internal
"semantic" structure of the picture being displayed, but just
the lines and points, or even just bits in a bit matrix.
* The server host need not be able to invoke arbitrary
terminal-dependent software to convert a standard language into
one that a terminal can use. Instead, a standard language is
defined which all programmable terminals can interpret easily.
Major differences between terminals are catered to by
conventions for including enough redundant information in the
output stream that all types of terminals will have the
necessary information available when it is needed, even if they
are not able to remember it in usable form from one command to
Those interested in network graphics should read about the Multics
Graphics System, whose fundamental purpose is the same, but whose
particular assumptions are very different (although it did inspire a few
of the features of this proposal).
SUPDUP Initial Negotiation:
One new optional variable, the SMARTS variable, is defined. It
should follow the other variables sent by the SUPDUP user process to
the SUPDUP server process. Bits and fields in the left half-word of
this variable are given names starting with "%TQ". Bits and fields
in the right half are given names starting with "%TR". Not all of
the SMARTS variable has to do with the graphics protocol, but most of
it does. The %TQGRF bit should be 1 if the terminal supports
graphics output at all.
Invoking the Graphics Protocol:
Graphics mode is entered by a %TDGRF (octal 231) code in the output
stream. Following characters in the range 0 - 177 are interpreted
according to the graphics protocol. Any character 200 or larger (a
%TD code) leaves graphics mode, and then has its normal
interpretation. Thus, if the server forgets that the terminal in
graphics mode, the terminal will not long remain confused.
Once in graphics mode, the output stream should contain a sequence of
graphics protocol commands, each followed by its arguments. A zero
as a command is a no-op. To leave graphics mode deliberately, it is
best to use a %TDNOP.
Graphics mode uses a cursor position which is remembered from one
graphics command to the next while in graphics mode. The graphics
mode cursor is not the same one used by normal type-out: Graphics
protocol commands have no effect on the normal type-out cursor, and
normal type-out has no effect on the graphics mode cursor. In
addition, the graphics cursor's position is measured in dots rather
than in characters. The relationship between the two units (dots,
and characters) is recorded by the %TQHGT and %TQWID fields of the
SMARTS variable of the terminal, which contain the height and width
in dots of the box occupied by a character. The size of the screen
in either dimension is assumed to be the length of a character box
times the number of characters in that direction on the screen. If
the screen is actually bigger than that, the excess is may or may not
be part of the visible area; the program will not know that it
exists, in any case.
Each co-ordinate of the cursor position is a 14-bit signed number,
where zero is at the center of the screen (if the screen dimension is
an even number of dots, then the visible negative points extend one
unit farther that the positive ones, in proper two's complement
fashion). Excessively large values of the co-ordinates will be off
the screen, but are still meaningful.
An alternate mode is defined, which some terminals may support, in
which virtual co-ordinates are used. The specified co-ordinates are
still 14-bit signed numbers, but instead of being in units of
physical dots on the terminal, it is assumed that +4000 octal is the
top of the screen or the right edge, while -4000 octal is the bottom
of the screen or the left edge. The terminal is responsible for
scaling these virtual co-ordinates into units of screen dots. Not
all terminals need have this capability; the %TQVIR bit in the SMARTS
variable indicates that it exists. To use virtual co-ordinates, the
server should send a %GOVIR; to use physical co-ordinates again, it
should send a %GOPHY. These should be repeated at intervals, such as
when graphics mode is entered, even though the terminal must attempt
to remember the state of the switch anyway. This repetition is so
that a loss of some output will not cause unbounded confusion.
The virtual co-ordinates are based on a square. If the visible area
on the terminal is not a square, then the standard virtual range
should correspond to a square around the center of the screen, and
the rest of the visible area should correspond to virtual
co-ordinates just beyond the normally visible range.
Graphics protocol commands take two types of cursor position
arguments, absolute ones and relative ones. Commands that take
address arguments generally have two forms, one for each type of
address. A relative address consists of two offsets, delta-X and
delta-Y, from the old cursor position. Each offset is a 7-bit two's
complement number occupying one character. An absolute address
consists of two co-ordinates, each 14 bits long, occupying two
characters, each of which conveys 7 bits. The X co-ordinate or
offset precedes the Y. Both types of address set the running cursor
position which will be used by the next address, if it is relative.
It is perfectly legitimate for parts of objects to go off the screen.
What happens to them is not terribly important, as long as it is not
disastrous, does not interfere with the reckoning of the cursor
position, and does not cause later objects, drawn after the cursor
moves back onto the screen, to be misdrawn.
Whether a particular spot on the screen is specified with an absolute
or a relative address is of no consequence. The sequence in which
they are drawn is of no consequence. Each object is independent of
all others, and exists at the place which was specified, in one way
or other, by the command that created it. Relative addresses are
provided for the sake of data compression. They are not an attempt
to spare programs the need for the meagre intelligence required to
convert between absolute and relative addresses; more intelligence
than that will surely be required for other aspects of the graphics
protocol. Nor are relative addresses intended to cause several
objects to relocate together if one is "moved" or erased. Terminals
are not expected to remember any relation between objects once they
are drawn. Most will not be able to.
Although the cursor position on entry to graphics mode remains set
from the last exit, it is wise to reinitialize it with a %GOMVA
command before any long transfer, to limit the effects of lost
Commands to draw an object always have counterparts which erase the
same object. On a bit matrix terminal, erasure and drawing are
almost identical operations. On a display list terminal, erasure
involves searching the display list for an object with the specified
characteristics and deleting it from the list. It is assumed that
any terminal whose %TOERS bit is set can erase graphic objects.
The commands to draw objects run from 100 to 137, while those to
erase run in a parallel sequence from 140 to 177. Other sorts of
operations have command codes below 100. Meanwhile, the 20 bit in
the command code says which type of addresses are used as arguments:
if the 20 bit is set, absolute addresses are used. Graphics commands
are given names starting with "%GO".
Graphics often uses characters. The %GODCH command is followed by a
string of characters to be output, terminated by a zero. The
characters must be single-position printing characters. On most
terminals, this limits them to ASCII graphic characters. Terminals
with %TOSAI set in the TTYOPT variable allow all characters 0-177.
The characters are output at the current graphics cursor position
(the lower left hand corner of the first character's rectangle being
placed there), which is moved as the characters are drawn. The
normal type-out cursor is not relevant and its position is not
changed. The cursor position at which the characters are drawn may
be in between the lines and columns used for normal type-out. The
%GOECH command is similar to %GODCH but erases the characters
specified in it. To clear out a row of character positions on a bit
matrix terminal without having to respecify the text, a rectangle
command may be used.
The way to send a simple line drawing is this:
%TDRST ;Reset all graphics modes.
%TDGRF ;Enter graphics.
%GOCLR ;Clear the screen.
%GOMVA xx yy ;Set cursor.
%GODLA xx yy ;Draw line from there.
<< repeat last two commands for each line >>
%TDNOP ;Exit graphics.
The %TRGIN bit in the right half of the SMARTS variable indicates
that the terminal can supply a graphic input in the form of a cursor
position on request. Sending a %GOGIN command to the terminal asks
to read the cursor position. It should be followed by an argument
character that will be included in the reply, and serve to associate
the reply with the particular request for input that elicited it.
The reply should have the form of a Top-Y character (code 4131),
followed by the reply code character as just described, followed by
an absolute cursor position. Since Top-Y is not normally meaningful
as input, %GOGIN replies can be distinguished reliably from keyboard
input. Unsolicited graphic input should be sent using a Top-X instead
of a Top-Y, so that the program can distinguish them. Instead of a
reply code, for which there is no need, the terminal should send an
encoding of the buttons pressed by the user on his input device, if
it has more than one.
Terminals may define the concept of a "set" of objects. There are up
to 200 different sets, each of which can contain arbitrarily many
objects. At any time, one set is selected; objects drawn become part
of that set, and objects erased are removed from it. Objects in a
set other than the selected one cannot be erased without switching to
the sets that contain them. A set can be made temporarily invisible,
as a whole, without being erased or its contents forgotten; and it
can then be made instantly visible again. Also, a whole set can be
moved. A set has at all times a point identified as its "center",
and all objects in it are actually remembered relative to that
center, which can be moved arbitrarily, thus moving all the objects
in the set at once. Before beginning to use a set, therefore, one
should "move" its center to some absolute location. Set center
motion can easily cause objects in the set to move off screen. When
this happens, it does not matter what happens temporarily to those
objects, but their "positions" must not be forgotten, so that undoing
the set center motion will restore them to visibility in their
previous positions. Sets are not easily implemented on bit matrix
terminals, which should therefore ignore all set operations (except,
for a degenerate interpretation in connection with blinking, if that
is implemented). The %TQSET bit in the SMARTS variable of the
terminal indicates that the terminal implements multiple sets of
On a terminal which supports multiple sets, the %GOCLR command should
empty all sets and mark all sets "visible" (perform a %GOVIS on each
one). So should a %TDCLR SUPDUP command. Thus, any program which
starts by clearing the screen will not have to worry about
initializing the states of all sets.
Some terminals have the ability to blink objects on the screen. The
command %GOBNK meaning make the current set blink. All objects in it
already begin blinking, and any new objects also blink. %GOVIS or
%TOINV cancels the effect of a %GOBNK, making the objects of the set
permanently visible or invisible. %TQBNK indicates that the terminal
supports blinking on the screen.
However, there is a problem: some intelligent bit matrix terminals
may be able to implement blinking a few objects, if they are told in
advance, before the objects are drawn. They will be unable to
support arbitrary use of %GOBNK, however.
The solution to the problem is a convention for the use of %TOBNK
which, together with degenerate definitions for set operations, makes
it possible to give commands which reliably work on any terminal
which supports blinking.
On a terminal which sets %TQBNK but not %TQSET, %GOBNK is defined to
cause objects which are drawn after it to be drawn blinking. %GOSET
cancels this, so following objects will be drawn unblinking. This is
regardless of the argument to the %GOSET.
Thus, the way for a program to work on all terminals with %TQBNK,
whether they know about sets or not, is: to write a bliniking
picture, select some set other than your normal one (set 1 will do),
do %GOBNK, output the picture, and reselect set 0. The picture will
blink, while you draw things in set 0. To draw more blinking
objects, you must reselect set 1 and do another %GOBNK. Simply
reselecting set 1 will not work on terminals which don't really
support sets, since they don't remember that the blinking objects are
"in set 1" and not "in set 0".
Erasing a blinking object should make it disappear, on any terminal
which implements blinking. On bit matrix terminals, blinking MUST
always be done by XORing, so that the non-blinking background is not
%GOCLS, on a terminal which supports blinking but not sets, should
delete all blinking objects. Then, the convention for deleting all
blinking objects is to select set 1, do a %GOCLS, and reselect set 0.
This has the desired effect on all terminals. This definition of
%GOCLS causes no trouble on non-set terminals, since %GOCLS would
otherwise be meaningless to them.
To make blinking objects stop blinking but remain visible is possible
with a %GOVIS on a terminal which supports sets. But in general the
only way to do it is to delete them and redraw them as permanent.
Rectangles and XOR
Bit matrix terminals have their own operations that display list
terminals cannot duplicate. First of all, they have XOR mode, in
which objects drawn cancel existing objects when they overlap. In
this mode, drawing an object and erasing it are identical operations.
All %GOD.. commands act IDENTICALLY to the corresponding %GOE..'s.
XOR mode is entered with a %GOXOR and left with a %GOIOR. Display
list terminals will ignore both commands. For that reason, the
program should continue to distinguish draw commands from erase
commands even in XOR mode. %TQXOR indicates a terminal which
implements XOR mode. XOR mode, when set, remains set even if
graphics mode is left and re-entered. However, it is wise to
re-specify it from time to time, in case output is lost.
Bit matrix terminals can also draw solid rectangles. They can thus
implement the commands %GODRR, %GODRA, %GOERR, and %GOERA. A
rectangle is specified by taking the current cursor position to be
one corner, and providing the address of the opposite corner. That
can be done with either a relative address or an absolute one. The
%TQREC bit indicates that the terminal implements rectangle commands.
Of course, a sufficiently intelligent bit matrix terminal can provide
all the features of a display list terminal by remembering display
lists which are redundant with the bit matrix, and using them to
update the matrix when a %GOMSR or %GOVIS is done. However, most bit
matrix terminals are not expected to go to such lengths.
How Several Process Can Draw On One Terminal Without Interfering With
If we define "input-stream state" information to be whatever
information which can affect the action of any command, other than
what is contained in the command, then each of the several processes
must have its own set of input-stream state variables.
This is accomplished by providing the %GOPSH command. The %GOPSH
command saves all such input-stream information, to be restored when
graphics mode is exited. If the processes can arrange to output
blocks of characters uninterruptibly, they can begin each block with
a %GOPSH followed by commands to initialize the input-stream state
information as they desire. Each block of graphics output should be
ended by a %TDNOP, leaving the terminal in its "normal" state for all
the other processes, and at the same time popping the what the %GOPSH
The input-stream state information consists of:
The cursor position
the state of XOR mode (default is OFF)
the selected set (default is 0)
the co-ordinate unit in use (physical dots, or virtual)
(default is physical)
whether output is going to the display screen or to a hardcopy
device (default is to the screen)
what portion of the screen is in use
(see "Using Only Part of the Screen")
(default is all)
Each unit of input-stream status has a default value for the sake of
programs that do not know that the information exists; the exception
is the cursor position, since all programs must know that it exists.
A %TDINI or %TDRST command should set all of the variables to their
The state of the current set (whether it is visible, and where its
center is) is not part of the input-stream state information, since
it would be hard to say what it would mean if it were. Besides, the
current set number is part of the input-stream state information, so
different processes can use different sets. The allocation of sets
to processes is the server host's own business.
Using Only Part of the Screen:
It is sometimes desirable to use part of the screen for picture and
part for text. Then one may wish to clear the picture without
clearing the text. On display list terminals, %GOCLR should do this.
On bit matrix terminals, however, %GOCLR can't tell which bits were
set by graphics and which by text display. For their sake, the
%GOLMT command is provided. This command takes two cursor positions
as arguments, specifying a rectangle. It declares that graphics will
be limited to that rectangle, so %GOCLR should clear only that part
of the screen. %GOLMT need not do anything on a terminal which can
remember graphics output as distinct from text output and clear the
former selectively, although it would be a desirable feature to
process it even on those terminals.
%GOLMT can be used to enable one of several processes which divide up
the screen among themselves to clear only the picture that it has
drawn, on a bit matrix terminal. By using both %GOLMT and distinct
sets, it is possible to deal successfully with almost any terminal,
since bit matrix terminals will implement %GOLMT and display list
terminals almost always implement sets.
The %TDCLR command should clear the whole screen, including graphics
output, ignoring %GOLMT.
In general, errors in graphics commands should be ignored.
Since the output and input streams are not synchronized unless
trouble is taken, there is no simple way to report an error well
enough for the program that caused it to identify just which command
was invalid. So it is better not to try.
Errors which are not the fault of any individual command, such as
running out of memory for display lists, should also be ignored as
much as possible. This does NOT mean completely ignoring the
commands that cannot be followed; it means following them as much as
possible: moving the cursor, selecting sets, etc. as they specify, so
that any subsequent commands which can be executed are executed as
This protocol does not attempt to specify commands for dealing with
every imaginable feature which a picture-drawing device can have.
Additional features should be left until they are needed and well
understood, so that they can be done right.
Storage of Graphics Commands in Files:
This can certainly be done. Since graphics commands are composed
exclusively of the ASCII characters 0 - 177, any file that can hold
ASCII text can hold the commands to draw a picture. This is less
useful than you might think, however. Any program for editing, in
whatever loose sense, a picture, will have its own internal data
which determine the relationships between the objects depicted, and
control the interpretation of the programs commands, and this data
will all be lost in the SUPDUP graphics commands for displaying the
picture. Thus, each such program will need to have its own format for
storing pictures in files, suitable for that program's internal data
structure. Inclusion of actual graphics commands in a file will be
useful only when the sole purpose of the file is to be displayed.
Note: the values of these commands are represented as 8.-bit octal
bytes. Arguments to the commands are in lower case inside angle
The Draw commands are:
Value Name Arguments
101 %GODLR <p>
Draw line relative, from the cursor to <p>.
102 %GODPR <p>
Draw point relative, at <p>.
103 %GODRR <p>
Draw rectangle relative, corners at <p> and at the
current cursor position.
104 %GODCH <string> <0>
Display the chars of <string> starting at the current
graphics cursor position.
121 %GODLA <p>
Draw line absolute, from the cursor to <p>. The same
effect as %GODLR, but the arg is an absolute address.
122 %GODPA <p>
Draw point absolute, at <p>.
123 %GODRA <p>
Draw rectangle absolute, corners at <p> and at the
current cursor position.
The Erase commands are:
Value Name Arguments
141 %GOELR <p>
Erase line relative, from the cursor to <p>.
142 %GOEPR <p>
Erase point relative, at <p>.
143 %GOERR <p>
Erase rectangle relative, corners at <p> and at the
current cursor position.
144 %GOECH <string> <0>
Erase the chars of <string> starting at the current
graphics cursor position.
161 %GOELA <p>
Erase line absolute, from the cursor to <p>.
162 %GOEPA <p>
Erase point absolute, at <p>.
163 %GOERA <p>
Erase rectangle absolute, corners at <p> and at the
current cursor position.
The miscellaneous commands are:
Value Name Arguments
001 %GOMVR <p>
Move cursor to point <p>
021 %GOMVA <p>
Move cursor to point <p>, absolute address.
Turn on XOR mode. Bit matrix terminals only.
Turn off XOR mode.
003 %GOSET <n>
Select set. <n> is a 1-character set number, 0 - 177.
004 %GOMSR <p>
Move set origin to <p>. Display list terminals only.
024 %GOMSA <p>
Move set origin to <p>, absolute address.
Make current set invisible.
Make current set visible.
Make current set blink. Canceled by %GOINV or %GOVIS.
Erase whole screen.
Erase entire current set (display list terminals).
Push all input-stream status information, to be restored
when graphics mode is exited.
Start using virtual co-ordinates
Resume giving co-ordinates in units of dots.
013 %GOHRD <n>
Divert output to output subdevice <n>. <n>=0 reselects
the main display screen.
014 %GOGIN <n>
Request graphics input (mouse, tablet, etc). <n> is the
reply code to include in the answer.
015 %GOLMT <p1> <p2>
Limits graphics to a subrectangle of the screen. %GOCLR
will clear only that area. This is for those who would
use the rest for text.
Bits in the SMARTS Variable Related to Graphics:
Note: the values of these bits are represented as octal 36.-bit words,
with the left and right 18.-bit halfword separated by two commas as in
the normal PDP-10 convention.
Name Value Description
%TQGRF 000001,,0 terminal understands graphics protocol.
%TQSET 000002,,0 terminal supports multiple sets.
%TQREC 000004,,0 terminal implements rectangle commands.
%TQXOR 000010,,0 terminal implements XOR mode.
%TQBNK 000020,,0 terminal implements blinking.
%TQVIR 000040,,0 terminal implements virtual co-ordinates.
%TQWID 001700,,0 character width, in dots.
%TQHGT 076000,,0 character height, in dots.
%TRGIN 0,,400000 terminal can provide graphics input.
%TRGHC 0,,200000 terminal has a hard-copy device to which output can