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Copyright (c) 1999 Massachusetts Institute of Technology

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------------------------------

               This file was moved to MIT-MC from
              MIT-AI by David C. Plummer on 6 June
                  1981, and may be up to date.


NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension



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
"SUPDUP Protocol".

     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 another.

     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).
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


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.
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


Co-ordinates:

     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 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 than 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 output.
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


Commands:

     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.  Thus, on such
terminals you can erase an object only if precisely that object was
drawn, and was specified the same way when drawn as when erased.
(presumably;  a very hairy program might allow more).  Any terminal
whose %TOERS bit is set must be able to erase to at least that extent.

     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.


Example:

     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.
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


Graphics Input:

     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.


Sets:

     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
objects.

     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.
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


Blinking:

     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 destroyed.

     %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.
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


Rectangles and XOR Mode:

     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.
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


How Several Process Can Draw On One Terminal Without Interfering With
Each Other:

     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
pushed.

    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 default values.

     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.
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


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.


Errors:

     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 intended.
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


Extensions:

     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.
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


Note: the values of these commands are represented as 8.-bit octal
bytes.  Arguments to the commands are in lower case inside angle
brackets.

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.
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


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.
 002    %GOXOR
                Turn on XOR mode.  Bit matrix terminals only.
 022    %GOIOR
                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.
 006    %GOINV
                Make current set invisible.
 026    %GOVIS
                Make current set visible.
 007    %GOBNK
                Make current set blink.  Canceled by %GOINV or %GOVIS.
 010    %GOCLR
                Erase whole screen.
 030    %GOCLS
                Erase entire current set (display list terminals).
 011    %GOPSH
                Push all input-stream status information, to be
                restored when graphics mode is exited.
 012    %GOVIR
                Start using virtual co-ordinates
 032    %GOPHY
                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.
\f
NWG/RFC# 746                                      RMS 17-MAR-78  43976
SUPDUP Graphics Extension


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 be diverted.
\f
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