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).

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.

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 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
   output.

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.  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.

   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.

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.

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.

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
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.

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.

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.

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.

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.

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.