[Docs] [txt|pdf] [Tracker]
Network Working Group C. Irby
Request for Comments: 553 K. Victor
NIC: 17810 SRI-ARC
14 July 1973
Draft design for a text/graphics protocol
DRAFT DESIGN FOR A TEXT/GRAPHICS PROTOCOL
This proposal should be seen as a synthesis of existing ideas rather
than an attempt to put forth new ones. It is based on work by the
NGG, Elaine Thomas, Peter Deutsch, Charles Irby, Ken Victor, Bill
Duvall, Bob Sproull, and others at ARC, PARC, and BBN.
We are concerned about the lack of text-handling capabilities of the
protocol suggested in RFC 493. Also, we feel that the protocol will
have a significant influence on the interface provided to writers of
future graphics application programs, and consequently that such
things as "beam twiddling" should not be part of the protocol.
Things of this nature address the problem at too low a level for a
facility which is intended to service the needs of a wide range of
graphics devices.
We feel that, although the breakdown into "levels" as proposed in
RFC 493 may be expedient for initial experimentation, it is
inappropriate for a Network Standard Protocol. Instead, we
propose that the protocol allow for two levels, segmented and
structured. This allows the writers of graphics application
programs to deal with a very simple display facility (segments
consisting of lines, dots, or character strings) or with a
powerful structure of display subroutines.
We propose an experimental implementation of such a scheme on the
ARC, BBN, and PARC systems to test these ideas using several
application programs (including NLS) and at least an IMLAC, ARDS, and
an E&S LDS.
Environment
We are trying to design a protocol used to communicate with a
"virtual display" to operate at the other end of a wire (ARPANET
connection) from a "host" which is running some kind of display
application program.
Irby, et. al. [Page 1]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
We will adopt the terminology that the human user, sitting at the
display, is the "user" and the application program is the
"server".
We wish to stress the fact that within a single application, a single
terminal should be useable both as an "interactive graphics" terminal
AND as an "interactive control" terminal. Thus, the graphics
protocol must allow for teletype-like operations.
The need for two sets of connections for running graphics programs
over the Net (according to our understanding) is centered about the
issue of handling (being able to recover gracefully from) berserk
programs (and perhaps achieving greater bandwidth through the net).
We recognize this problem but also think one should be able to run
graphics programs using only one set of telnet connections. Also, it
seems obvious that even though one is running a graphics program, one
must expect to be able to handle "unescorted" characters (not
embedded in a command or response message) being sent to his
terminal.
Consequently, we are proposing that the graphics protocol be
implemented within telnet using 8-bit BEGIN-GRAPHICS-COMMAND and
END-GRAPHICS-COMMAND characters or the 8-bit transparent mode of the
new telnet. This means that one will be able to run graphics
programs with one, two, or more sets of telnet connections.
We also strongly propose that any site which is interested in
supporting display terminals for use in network graphics would be
prudent to implement local control over the display (such as IGNORE-
GRAPHICS-COMMANDS, RESET-TO-TTY-MODE commands from the user to the
using host). Failure to take such precautions may very well lead to
burned out tubes!
Basic concepts
The model
The model we are adopting consists of an application program
manipulating a (remote) display file. This file may be
"segmented" or "structured", in which case it may be manipulated
independently from the display itself.
For structured display files an "update display" command causes
the display file to get mapped onto the display in whatever
fashion is appropriate for the display.
Irby, et. al. [Page 2]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
Part of this protocol deals with commands issued to the (remote)
display file editor. This editor creates and changes the display
file at the user host.
Structured Display Files
A structured display file consists of named subpictures, each
containing any number of named units. There are two types of
units, primitive units and call units. The effect of a unit is
independent of its name or creation within the subpicture.
Primitive units contain drawing instructions and associated
coordinates that may generate visible information on the
display screen. Drawing instructions and coordinates can occur
only in primitive units.
Call units give the display structure a subroutine capability.
A call unit invokes the display of another subpicture. In
other words, a call unit allows one subpicture to contain
instances of other subpictures. As well as providing for
subroutine-style control transfer, call units can be used to
establish display parameters and maintain parameter
transparency. For example, a call unit can be used to call a
subpicture with a translation and relative intensity setting.
On return from the called subpicture, these parameters are
restored to their original values.
A subpicture is an ordered list of units which can be any
mixture of primitive and call units. Each subpicture begins
with a header and terminates with the subpicture end unit. The
subpicture end unit is a single unique unit in a display
structure linked to the end of each subpicture.
In order to understand how control passes through a structure,
one can think of the display elements as follows: subpictures
are subroutines and units are linked blocks of in-line code.
When all of the units contained in a subpicture have been
executed, the subpicture end unit returns control to wherever
the subpicture was called from. A primitive unit contains
display code and transfer to the next unit. A call unit
contains a subroutine call to a subpicture and a transfer to
the next unit in line.
Irby, et. al. [Page 3]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
Segmented Display Files
A segmented display file consists of named segments, each
containing any number of primitive units. The only operations
available for segmented display file is to add new, delete old, or
replace old segments (updating the actual display happens
automatically). The effect of a unit is independent of its name
or creation order within the subpicture.
Hosts
Since a given terminal may be under partial control of several
different hosts, all further discussion of names, coordinates,
display files, etc. should be taken as relative to each individual
host.
That is, each host believes it has a display file, naming, and
coordinate space and a set of state parameters entirely under its
control; its only evidence of resource sharing is that the
terminal may appear to be of different sizes at different times.
(We feel that in principle it should be processes within hosts,
rather than hosts, that enjoy these properties, but it does not
seem feasible to construct a process identification scheme that
all hosts will find acceptable.)
Subpictures
A subpicture has a name and zero or more units.
Subpicture names are arbitrary, globally unique, fixed-length
identifiers (subpicture names are chosen by the host).
Each unit (displayable component) has a name, which is local to
the subpicture.
A unit may be a "primitive unit", such as a string or a vector, or
a "call unit", which implies displaying a (possibly transformed)
copy of another subpicture.
The display data are organized into a re-entrant tree (acyclic
graph) by the call units.
A unit may be "visible" or "invisible".
Irby, et. al. [Page 4]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
A particular instance of a subpicture (the result of a call-
unit) appears on the screen precisely if it and all subpictures
on the logical path to it from the root of the tree are
"visible".
Primitive units
Strings
A string is a horizontal line of characters with its own mode
and X,Y origin relative to the origin of the subpicture.
Note: intensity is always relative.
Font and mode (e.g. blinking) information logically accompanies
each character. This is accomplished by means of embedded mode
and font specification characters and a "restore original
string mode and font" character.
Note: Mode modifiers are non-displayable characters and do
not take up character positions on the screen.
Determining the space occupied on the screen by a string
requires knowledge of the font(s) being used; this is a
separate question which is dealt with later.
TTY units
A tty unit is a rectangle that consists of a number of lines.
Within this unit the display acts as if it were an alpha-numeric
display, e.g.,
characters which would write beyond the right hand margin of
the rectangle cause an automatic line folding to take place
ascii control characters CarriageReturn, LineFeed, FormFeed,
and BackSpaceCharacter, (HorizontalTab and VerticalTab?), are
interpreted appropriately
other control characters are displayed in a terminal specific
manner, e.g. ^F, <^F>, etc.
display of the characters in the range 200-377 is left
unspecified at this point (truncated to 7 bits?, alternate
fonts?, alternate modes?)
Irby, et. al. [Page 5]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
It is hoped that we can agree on a standardization of some
of the characters in this range to allow for such things as
greek letters, common mathematical symbols, super-scripting,
and sub-scripting.
linefolding that would cause characters to be written below the
rectangle (whether performed automatically or by a LineFeed
character, etc.) cause the text within the unit to be scrolled
upwards one line (storage tube may adopt a different scheme).
Characters are displayed in a teletype unit in one of two ways:
Characters sent to the terminal that are not part of any
command (unescorted characters) are appended to appropriate
tty-units (see below --- USE-TTY-UNITS, TTY)
By use of the APPEND-STRING-TO-UNITS command for structured
display files
The first character sent to a tty-unit appears as the first
character (at the left margin) of the top line. This is necessary
for a number of reasons, the most convincing of which is the
behavior characteristics of storage tubes and most real alpha-
numeric displays.
Successive characters appear as successive characters within
the top line until either an explicit (e.g., linefeed) or
implicit (line overflow) line break occurs.
When a line break occurs, the next character appears on the
second from the top line of the unit.
This continues until the bottom line of the tty-unit is
reached.
At this point, a line break causes the lines within the unit
to scroll up one line.
Note: Storage scopes may use a different technique for
scrolling.
Dots
A dot unit consists of an initial X0,Y0 followed by a series of
points X,Y which describe a series of dots.
Each dot unit logically carries mode information such as
blinking, relative intensity, etc.
Irby, et. al. [Page 6]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
Lines
A line unit consists of an origin X0,Y0 followed by a series of
points X,Y which describes a series of straight lines connected
tail-to-head (i.e. a polygon).
Each line unit logically carries mode information such as
blinking, dotted vs. solid, invisible.
Other kinds of lines, such as conic sections, may belong in the
primitive set.
Special points
This primitive unit consists of a series of points, which will
be displayed joined by lines in the best available manner.
The intent is to use Flegal's algorithms to produce a smooth
curve.
Device-specific
This primitive unit consists of any number of device specific
commands. The device type may be obtained through an
interrogation command.
Call units
In addition to the name of the referenced subpicture, a call unit
may include the following transformations:
Master/instance rectangle: specifies a rectangle in the
caller's space into which a specified rectangle of the callee's
space is to be imaged. This provides independent scaling in
each coordinate as well as translation and clipping.
Rotation. It may be desirable to combine this with scaling
using the familiar idea of homogeneous transformation.
Intensity and color control. In principle, a call could
specify intensity increments (positive or negative) for each
color.
It is assumed that best effort will be used in scaling and
rotation of text. We recommend replacing it by a line when all
else fails.
Irby, et. al. [Page 7]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
Initial state
After the initial telnet connection is established, the first
graphics command issued by the applications program should be a
request for either a structured display file or for a segmented
display file.
The response to this request should be whether or not the
requested display file was allocated and other parameters about
the virtual display, e.g. screen size, character sizes, whether
or not color is available, etc.
Before the display file is allocated, the terminal should appear
as, and simulate to the best of its ability, a Network Virtual
Terminal (NVT).
Any graphic commands issued before the allocation of a display
file will be ignored.
After requesting commands and receiving a structured display file,
the following structure will exist:
There will exist a subpicture, referred to as the ICP
SUBPICTURE, whose rectangular extent corresponds to the extent
of the virtual display allocated to this host.
There will exist a tty-unit, referred to as the ICP TTY-UNIT,
in the ICP SUBPICTURE, where rectangular extent corresponds to
the extent of the virtual display allocated to this host.
This tty-unit will consist of n lines, where n is terminal
dependent and available through a query command.
This tty-unit will be instituted for the display of
unescorted characters.
There will be in effect an implicit call on the ICP SUBPICTURE.
This call is not accessible to the applications program.
The applications program causes the display of information by:
1) creating primitive units in the ICP SUBPICTURE
2) creating call units, to created subpictures, in the ICP
SUBPICTURE
Irby, et. al. [Page 8]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
3) using the TTY command to make visible/invisible the ICP
TTY-UNIT (or change its location or size)
After requesting and receiving a segmented display file, the
following structure will exist:
There will exist a segment, referred to as the ICP SEGMENT.
There will exist a tty-unit, referred to as the ICP TTY-
UNIT, in the ICP SEGMENT, whose rectangular extent
corresponds to the extent of the virtual display allocated
to this host.
This tty-unit will consist of n lines, where n is terminal
dependent and available through a query command.
This tty-unit will be instituted for the display unescorted
characters.
The applications program causes the display of information by:
1) creating primitive units in the ICP SEGMENT
2) creating new segments
3) using the TTY command to make visible/invisible the ICP
TTY-UNIT (or to relocate it or change its size)
Display editing primitives
General editing primitives
REQUEST-DISPLAY-FILE (file-type)
file-type is either structured or segmented.
This command requires a response.
Segmented display file editing
SEGMENT (Segment)
If the segment Segment already exists, then it is cleared; if
it did not exist then it is created.
Pictures are displayed within segments by the use of the
primitive unit command listed below.
Irby, et. al. [Page 9]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
DELETE-SEGMENT(Segment)
If the segment exists, then it is deleted.
Primitive Units
All unit operations cause immediate display on the screen.
STRING-UNIT(Segment,Mode,X-Origin,Y-Origin,Text)
Writes the specified string unit.
Mode refers to relative intensity, blinking, reverse video,
color, etc.
Errors: Segment does not exist.
LINE-UNIT(Segment,Type,Mode,X0,Y0,X1,Y1, ..., Xn,Yn)
Draws the specified line segments.
Type refers to solid, dashed, dotted, etc.
Errors: Segment does not exist; illegal mode.
DOT-UNIT(Segment,Mode,X0,Y0,X1,Y1, ..., Xn,Yn)
Draws the specified dots.
Errors: Segment does not exist; illegal mode.
SPECIAL-POINTS-UNIT(Segment,Mode,X1,Y1, ..., Xn,Yn)
Draws the special-points curve.
The terminal should attempt to connect the specified points in
the nicest way possible (e.g. Flegal's spline curve algorithm,
straight line segments).
Errors: Segment does not exist; illegal mode.
TTY-UNIT(Segment,Mode,Rectangle,Lines)
Creates a unit which will behave as a tty-simulation area with
"lines" lines distributed within the specified rectangle.
Unescorted characters will be echoed in this unit in addition
to any other units they are being sent to.
Irby, et. al. [Page 10]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
Errors: Segment does not exist.
DEVICE-SPECIFIC-UNIT(Segment,device commands)
Creates a unit of device specific commands.
TTY(parameters)
parameters are:
position rectangle, visible/invisible, number of lines, mode
of characters
This refers to the ICP TTY simulation.
RESET()
delete all segments, except ICP SEGMENT, and all units of ICP
SEGMENT, except ICP TTY-UNIT
resets all nodes to their initial state (i.e., the state that
existed immediately after a REQUEST-DISPLAY-FILE command)
Structured display file editing
SUBPICTURE(Subpicture, rectangle)
Creates a new subpicture with name "Subpicture". "rectangle"
is the coordinates of a diagonal of the subpicture's virtual
screen (i.e. its coordinate system.)
If a subpicture named "Subpicture" already exists, it is
cleared and the new coordinate rectangle takes precedence.
DELETE-SUBPICTURE(Subpicture)
Deletes the subpicture named "Subpicture". Call units
referring to Subpicture are also deleted.
CLEAR-SUBPICTURE(Subpicture)
Deletes all units of the subpicture Subpicture, but does not
delete the subpicture.
Irby, et. al. [Page 11]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
Primitive Units
All the operations for creating units are transparent to the
prior existence of the designated unit, i.e. they function as
"replace" as well as "create".
STRING-UNIT(Subpicture,Unit,Target-Key,Mode,X-Origin,Y-
origin,Text)
Replaces the unit by a string unit.
Mode specifies the mode of the characters (e.g. blinking,
underlined, etc).
Target-Key is used in conjunction with the TARGET-SENSITIVE
command and target input. It may also be sent via the SET-
TARGET-KEY COMMAND.
Errors: Subpicture does not exist; X-Origin or Y-Origin is
outside the subpicture's virtual coordinate system.
We explicitly do not require an error if the string
extends beyond the right-hand edge of the subpicture;
however, the results are not defined.
LINE-UNIT(Subpicture,Unit,Target-Key,Type,Mode,X0,Y0,X1,Y1,
..., Xn,Yn)
Replaces the unit by a line unit.
Errors: Subpicture does not exist illegal mode; some X or
Y is outside the subpicture.
DOT-UNIT(Subpicture,Unit,Target-Key,Type,Mode,X0,Y0,X1,Y1, ...,
Xn,Yn)
Replaces the unit by a dot unit.
Errors: Subpicture does not exist; illegal mode; some X or Y
is outside the subpicture.
SPECIAL-POINTS-UNIT(Subpicture,Unit,Target-Key,Type,Mode,X1,Y1,
..., Xn,Yn)
Replaces the unit by a special-points unit.
Errors: Subpicture does not exist; illegal mode; some X or Y
is outside the subpicture.
Irby, et. al. [Page 12]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
CALL-UNIT(Subpicture,Unit,Target-Key,Called-
Subpicture,Parameters)
Replaces the unit by a call unit.
Parameters:
Master-Instance rectangles
rotation
mode
Errors: Subpicture does not exist; Called-Subpicture does
not exist; parameter errors.
TTY-UNIT(Subpicture, unit, mode, rectangle, lines)
Creates a unit which will behave as a tty-simulation area
with "lines" lines distributed within the specified
rectangle.
Errors: Subpicture does not exist.
DEVICE-SPECIFIC-UNIT(Subpicture, Unit, Target-Key, device,
commands)
Creates a unit of device specific commands. The action of
the commands should leave alone (or at least restore) any
global modes, e.g., the standout mode (see below).
APPEND-STRING-TO-UNIT(Subpicture, Unit, Text)
Appends the specified text to the specific commands. This only
makes sense if the specified unit is a string or tty unit.
Errors: Subpicture does not exist, unit does not exist, not a
string or tty unit.
DELETE-UNIT(Subpicture, Unit)
Deletes a unit.
Irby, et. al. [Page 13]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
VISIBLE-UNIT(Subpicture, Unit, Flag)
Makes the Unit visible or invisible as specified by Flag. If a
unit which is target sensitive is made invisible, it is no
longer target sensitive. However, in the absence of a
subsequent modifying target sensitive command, the unit becomes
target sensitive again if it should be made visible.
Errors: Subpicture does not exist, unit does not exist.
SET-TARGET-KEY(Subpicture, Unit, Target-Key)
Sets the target key for the specified unit to the specified
value.
SET-STANDOUT-MODE(mode)
Sets the mode that will be used to make text and/or units stand
out to blinking, underlining, etc.
If the terminal does not support the specified mode, the
terminal should make a best effort or use another method to
make things stand out.
STANDOUT-UNIT(Subpicture, unit, yesno)
makes the specified unit stand out (according to the mode set
by SET-STANDOUT-MODE) or not, according to "yesno". If the
unit which is to stand out is a call-unit, the instance of the
subpicture which is the result of the call (all the way to the
terminal nodes) is made to stand out.
STANDOUT-TEXT(Subpicture, unit, begin-char-count, end-char-count,
yesno)
Unit must refer to a string unit.
Makes the specified text stand out (according to the mode set
by SET-STANDOUT-MODE) or not, according to "yesno".
UPDATE-STRUCTURED-DISPLAY()
This causes any changes that have been made to the display
file, since the last update or since ICP, to be reflected on
the screen.
Irby, et. al. [Page 14]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
TTY(parameters)
parameters are:
position rectangle, visible/invisible, number of lines, mode
of characters
This refers to the ICP TTY simulation
USE-TTY-UNITS(Subpicture1, unit1, ..., Subpicturen, unitn)
Unescorted characters are to be appended only to the specified
tty units.
Errors: Subpicture, unit does not exist.
RESET(How)
Case How Of
= Permanent
Immediately resets the terminal to its initial ICP state
= Temporary
Immediately resets the terminal to its initial ICP state
without destroying the previous state.
= Restore state saved from last RESET(Temporary).
Direct Feedback
It seems extremely desirable, given network speeds, to allow the
using host to perform direct feedback to the user without
intervention from the application program in the serving host. This
is already done in telnet with local echoing. We propose extending
this capability to graphics by allowing "dragging" (attaching a
subpicture's origin to the position of the cursor), "tracking"
(following the movement of the mouse, stylus, or light pen with a
distinctive mark on the screen), "inking" (plotting the trail of the
cursor on the screen) and "rubber banding" (a straight line attached
to a fixed point on one end the cursor location on the other).
These should be seen as allowable extensions of the protocol rather
than as requirements. There should, however, be commands available
in the protocol for determining their existence and controlling them.
Irby, et. al. [Page 15]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
Data input primitives
Input Control
TARGET-SENSATIVE(key1, ..., keyn)
Arms the units which have the specified keys for target
selection.
SET-INPUT-MODE(Device, parameters)
Selects the mode in which a logical device shall produce input
and under what conditions.
the logical devices are specified below as well as their
possible input formats and conditions.
Errors: no such device.
Keyboard input
The keyboard has only one input mode, in which it sends a
character whenever a key is struck.
Binary devices
Unless otherwise specified, binary devices act as an extension of
the keyboard and produce 8-bit characters which are not
distinguishable from keyboard characters by the serving host.
The algorithm for translating binary devices into characters is
not specified, but something like the NLS accumulation
algorithm for mouse-keyset chords is intended.
Binary devices may also input binary data (according to their
up/down states), which is transmitted on state changes. Examples
of this type of device are function keys and overlay cards, mouse
and keyset (used independently or together), pen-up/pen/down,
light pen buttons, etc.
Coordinate input
Coordinates may be sent according to any subset of the following
criteria: with every character in some designated set (e.g.
control characters, or all characters); with every binary device
state change input; after some time interval has elapsed; after a
position change P > (y1-y0) ^2+(x1-x0)^2, etc.
Irby, et. al. [Page 16]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
Coordinates may be sent in either or both of "X-Y" or "target"
format.
X-Y format is just the location of the cursor relative to the
screen region assigned to the host.
Target format is the "call stack" (logical path from the root
unit - the ICP SUBPICTURE - to the closest unit) plus the
target-key of that unit plus the count of the closest character
within the string or the closest line segment or dot or special
point if appropriate.
Target input is unavailable for segmented display files.
In the event of overlapping target sensitive units, it is
not specified which of the units selected will be returned
as the hit unit.
Time input
Since hosts may wish to consider two events happening sufficiently
close together to be simultaneous, or to keep detailed interaction
statistics, it must be possible to request time information to be
sent with some reasonable subnet of other types of input.
Interrogations
It must be possible for the serving host to discover its environment
(e.g. screen size, available devices) and to read back state
information (display file).
This is very desirable both for debugging and for redirecting a
displayed image to another device (e.g. a plotter).
Environment
Terminal parameters: screen size and resolution, available input
devices, terminal type (for device specific control), number of
lines in the ICP TTY-UNIT.
Character parameters: available character sizes, special (non-
ASCII) characters, font characteristics, sub- and super-scripting
facilities.
State
Display file or display file components.
Irby, et. al. [Page 17]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
Cursor Position
It should be possible for the application program to read the
cursor position at any time.
Display File Support
It should be possible to find out if this user process supports
only segmented or structured display files, or both.
Command support
It should be possible to get a matrix from the user process
which indicates which commands are implemented. This is a
necessity to find out which, if any, of the direct feedback
features are supported, and might be nice to allow for, e.g.,
the possibility of a text only or graphics only subset of the
protocol to be implemented.
Encoding Principles
Commands will have the format : BGC OPCODE DATA EGC where:
BGC (Begin Graphics Command) places the telnet connection into a
"read graphics command" mode,
OPCODE DATA is the specific graphics command and data, and
EGC (End Graphics Command) restores the telnet connection to its
normal state.
Note: This may all have to be bracketed by telnet Begin-8-bit-
transparent-mode and End-8-bit-transparent-mode commands.
Numbers in general will have have 7-bits of significance in each byte
-- if the high order of a byte is on, then the significant bits from
the next byte should be concatenated onto the low-order end of the
bits collected so far, etc..
Subpicture names - shall be 14-bit numbers, assigned by the serving
host.
Unit names - shall be 14-bit numbers, assigned by the serving host.
Strings - shall be 8-bit characters, with an escape convention to
represent changes of font and mode.
Irby, et. al. [Page 18]
RFC 553 Draft design for a text/graphics protocol 14 July 1973
Since the channel is 8-bits wide, there is room for many more than
128 displayable characters. However, the interpretation of codes
200B and above is not standardized!
Coordinates should be as described in RFC 493.
Rectangles - shall be specified by the coordinates of the endpoints
of one of the diagonal.
Encoding
The actual encoding of this protocol is forthcoming. Since we expect
some changes to come about because of the upcoming Network Graphics
Group Meeting, we have postponed the actual encoding until after this
meeting.
[This RFC was put into machine readable form for entry]
[into the online RFC archives by Via Genie, 12/1999]
Irby, et. al. [Page 19]
Html markup produced by rfcmarkup 1.129b, available from
https://tools.ietf.org/tools/rfcmarkup/
