RFC292 - Graphics Protocol: Level 0 only

Network Working Group 12 January 1972

Request for Comment: 292 Jim Michener, MAC

NIC 8302 Ira Cotton, MITRE

References: 282, 285 Karl Kelley, U. of Ill.

Updates: None Dave Liddle, Owens Ill.

Obsoletes: None Ed Meyer, MAC

GRAPHICS PROTOCOL - LEVEL 0 ONLY

INTRODUCTION

This document reflects opinions eXPressed and decisions reached at

the second meeting of the Network Graphics Group, held at the

Stanford Artificial Intelligence Laboratory in late November 1971.

It describes part of a proposed Network Standard Graphics Protocol

for transmitting graphics data within the ARPA network. The

particular ASPects of the protocol covered in this document relate to

the form and content of graphics information sent from a source of

graphical information (an application program, say, in the "Serving

Host") to a display package for output to a graphics console (at the

"Using Host"). This will take the form of a sequence of 8-bit bytes,

and will be called the graphics output byte stream. In particular,

only the simplest forms of graphics data will be covered in this, the

first version of this document. The next version, already in

preparation, will be much more complete. In any case this is not

intended to describe a finished protocol; rather it should serve as a

basis for graphics experimentation on the network.

This document does not include form or content of graphics input

(data sent from the Using Host to the Serving Host) nor does it cover

how the connection is established between the hosts. A proposal for

the former will be generated eventually by this committee; the latter

is the job of the Connection Committee (of the Network Graphics

Group).

This RFCdescribes the commands which are available in the protocol

in terms of the effect they would have at the receiving (Using Host)

end. Clearly, some subroutine package is desirable at the Serving

Host for use by applications package in transmitting graphics data,

but on this topic this RFCdoes not intend to comment.

It may be observed by the reader that no facility is specified in

this protocol allowing the Using Host to report logical errors in the

graphics output byte stream to the Serving Host. Such a facility

would have to be intergrated with the graphics _input_ byte stream,

since it involves most of the problems related to synchrony of

independent hosts.

BACKGROUND

The reader should probably peruse RFC282: "Graphics Meeting Report"

by Mike Padlipsky to oBTain some of the framework surrounding this

discussion of network graphics. Also it might be valuable to make

note of the model described in RFC285: "Network Graphics" by Donald

Huff.

LEVEL AND GROUND RULES PERTAINING THERETO

Functions within the graphics protocol will be classified into a

number of levels depending partly on how difficult it is to implement

those functions. It is intended that any host which claims to

implement the functions of level N must implement all lower levels as

well. Thus, it is envisioned that sites will implement levels

incremently. Implementations will be improved as a continuing

process to include more and more functions, and it is intended that

each implementation will be able to identify its own level to a

graphics protocol at a remote site which is requesting a graphics

interchange. A side result is that each site will be able to

determine its own priorities in committing programmers to the

graphics protocol as opposed to other efforts.

It is also our intention that implementation of level N will require

no knowledge of level N+1. Thus a site can implement a level in the

(reasonably) firm knowledge that no changes at higher levels will

alter the level implemented. At some time it may be decided by the

Network Graphics Group to redefine a level which has previously been

firmed up. It is not our intention that this shall happen but one

must recognize that the proposed Graphics Protocol is experimental

and may have to be changed.

One further ground rule: a stream of commands and data which is

valid at a given level, K, shall produce "identical" results on any

interpreter of level K or higher. By this we mean that as defined

operations, similar pictures should result. Aspects of the protocol

which are not strictly defined (at this time) include character size,

character position relative to the beam, how control characters in

text output affect the terminal and what happens when the beam is

moved or a line drawn outside of the logical screen boundary. This

rule forces upwards compatibility, so that an application written

using features of low numbered level will still work at sites which

have moved on to implement higher levels. Additionally, any aspects

of this protocol which are explicitly "left unspecified" in the

detailed operations descriptions below _shall_ be explicitly

specified in any public description of an actual implementation.

We now describe the framework which will be common to all levels.

BASIC DATA FORMS

Information in the Network Standard Graphics Protocol will be

expressed as a sequence of 8-bit bytes. A command will consist of a

command byte followed by zero or more arguments. The same command

byte will always take the same number of arguments in the same form.

The length of each argument may be fixed or variable depending on the

argument.

A simple type of argument is a "value," which is an 8-bit integer.

Another type of argument is a "string" which is a count followed by

(count)number of 8-bit bytes. If the count is between 0 and 127, it

is sent in a single byte. If the count is between 128 and 2**15-1

(** means exponentiation), it is sent in two bytes with the high

order bit of the first byte set to one. The first byte contains the

seven high order bits of the number, and the second byte contains the

eight low order bits. A string is the only type of argument of a

command which can vary in length.

Coordinate data engendered considerable discussion at the second

Network Graphics Group meeting. It was decided that a two-

dimensional Logical Coordinate System was required, and each

interpreter for the graphic command byte stream would be responsible

for mapping this coordinate system to physical device coordinates.

It was decided that data in the logical coordinate system would be in

twos-complement notation, that it would be fractional, that each edge

of the screen would have unit length, and that the origin would

correspond to the center of the screen on the output device. The

vertical (horizontal) edges of the screen of the output device

correspond to the lines X (Y) = -1/2 or X=+1/2-e where e is a small

positive number determined by the precision of the fractional data.

Particularly the points (-1/2,-1/2) (-1/2,1/2-e), (1/2-e, -1/2) and

(1/2-e, 1/2-e) shall be visible points at the corners of the logical

screen. (In the case of a non-square display surface, the

implementer may make his own decision, but it is recommended that the

largest possible _square_ area be utilized.) Thus we shall say that

the Logical Coordinate System contains points whose coordinates range

from -1/2 to a little less than +1/2.

Commands which take coordinate data will be available in various

modes. In absolute mode, a position is specified by giving its

coordinates in the Logical Coordinate System. In relative mode, the

_difference_ between the coordinates of the position and the

coordinates of the current position must be specified. Thus a

coordinate datum which is an argument for an absolute mode operation

should be in the range -1/2 to +1/2-e, while one for a relative mode

operation should be in the range -1+e to +1-e.

Interest was expressed at the second Graphics Group Meeting in

eventually allowing a very large coordinate space (many bits of

precision in each fractional coordinate). This is to be done by

permitting the length, in 8-bit bytes, of each coordinate datum to be

set (as a mode). It was decided at the meeting that two bytes per

coordinate would suffice for now. Thus "e" in the above discussion

is 2**(15) (one in the least significant bit of a 15-bit plus sign

fractional coordinate).

Text data will be transmitted as an argument of various commands for

display on the output device. Network ASCII will be used to

represent characters. At the lowest-levels of the protocol only one

character size will be available -- whatever is "normal" on the

display device. If the device had no "normal" size, 72 characters

per line would be desirable. Later, variable character size may be

introduced.

Also, at the lowest levels, control characters will be passed along

to the device for it to do the best it can. However, the consensus

of the graphics meeting was that at some reasonably low (but non-

zero) level carriage return, line feed, and backspace should be

interpreted to do the right thing.

COMMAND CODES

Each command in the graphics protocol will be assigned a non-negative

value which will represent this command in the byte stream. The

algorithm whereby values and commands are associated is, it turns

out, a very touchy subject. There are five or ten different criteria

for a "best" algorithm, each criterion different in emphasis. This

Gordian knot will be cut, in this proposal, by ordering the commands

approximately according to level, and then just numbering them. In

addition, if several closely related commands occur at the same

level, some attempt will be made to encode variations of meanings in

terms of bit configurations. Even if some later consideration causes

a change in ordering to be proposed, it is this committee's feeling

that the numbering should not be altered. However, until this matter

is firmly settled, it is strongly advised that any implementation

take into account the possibility of reassignment of command codes.

PARTICULAR PROPOSAL FOR LEVEL 0 PROTOCOL

It is proposed that level 0 be kept very simple. This is so that

implementation can be quickly accomplished and experimentation with

the protocol begun. Another reason is that the least powerful hosts

and even programmable terminals should be able to implement it. In

accordance with this, the "rule" was made that a command be

implemented only if the output is a function solely of the current

command and the "beam position" current at the start of the command.

In other Words the interpreter for level 0 need have no internal

storage for "modes" or pushdown stacks. With this restriction it is

hoped that a very simple implementation will be possible for level 0.

In particular, perhaps one could eventually build a hardware

translator from level 0 code to one's own particular terminal's code.

Note that in the opcode assignment for level 0, bits 4, 2, and 1 have

special meaning for the move, line and dot commands. In particular,

the 1 bit encodes absolute versus relative data mode, the 4 bit

encodes whether any visible output occurs, and the 2 bit determines

whether the visible output is a line or a dot.

LEVEL 0: COMMAND SUMMARY

The following is a list of commands (and their syntax)in level zero.

Detailed descriptions of these commands follow in the next section.

Commands dealing with protocol may be added by the Connection

Committee. (They currently request opcodes in the range 128 to 255.)

(As described in Basic Data Forms, above, <x>, <y>, <x delta> and <y

delta> are two-byte coordinate values, <string> is a count followed

by (count) many bytes and <value> is an eight bit number.)

Decimal Octal Binary Format

0 0 00000000 Null

1 1 00000001 Erase screen and reset beam

2 2 00000010 Move Absolute <x> <y>

3 3 00000011 Move Relative <x> <y>

4 4 00000100 Draw Absolute <x> <y>

5 5 00000101 Draw Relative <x delta> <y delta>

6 8 00000110 Dot Absolute <x> <y>

7 7 00000111 Dot Relative <x delta> <y delta>

8 10 00001000 Text <string>

9 11 00001001 TextR <string>

10 12 00001010 End of Picture

11 13 00001011 Escape <value> <string>

LEVEL 0: COMMAND DESCRIPTIONS

0 Null Statement ("null")

This statement has no arguments and no effect, either.

1 Erase screen and reset beam to origin ("Erase").

This command indicates that a new picture is about to be drawn.

It should always be (eventually) paired with a following End of

Picture command.

2 Move beam invisibly to absolute position

("Move Absolute") <x coordinate> <y coordinate>.

Nothing is drawn; the beam is positioned to the specified absolute

x,y position.

3 Move beam invisibly by relative amount

("Move Relative") <x delta> <y delta>.

Nothing is drawn; the beam is shifted by the specified amount in x

and y.

4 Draw line to absolute position

("Draw Absolute") <x coordinate> <y coordinate>.

A line is drawn from the current beam position to the specified

absolute x, y position.

5 Draw line to relative position

("Draw Relative") <x delta> <y delta>.

A line is drawn from the current beam position to the position

delta x and delta y away.

6 Display a Dot at absolute position

("Dot Absolute") <x coordinate> <y coordinate>.

The beam is moved invisibly to absolute position x, y and a dot is

displayed there.

7 Display a Dot at Relative position

("Dot Relative") <x delta> <y delta>.

The beam is moved invisibly by the specified amount in x and y and

a dot is displayed there.

8 Display text ("Text") <string>.

At the current beam position, display some characters at the

normal size for the device being operated. <string> consists of a

<count> followed by count many characters. If there is no "normal

size," choose the size so that seventy-two characters are

displayed per line. The characters in the string are coded in

network ASCII: all codes between 0 and 127 (decimal) inclusive are

permitted. (At level zero, what happens to control characters is

left unspecified.) Where the beam is, following execution of this

command, is left unspecified, except that another Display Text

command immediately following will append its text to the previous

string. (The use of the TEXT command is _discouraged_; use TextR

instead.) The position of the first character relative to the

initial beam position is left unspecified.

9 Display text and restore beam ("TextR") <string>.

At the current beam position, display a string of characters at

the normal size for the device being operated then reposition the

beam to where it was before the command. <string> consists of a

<count> followed by count many characters. If there is no "normal

size," choose the size so that seventy-two characters are

displayed per line. The characters in the string are coded in

network ASCII; all codes between 0 and 127 (decimal) inclusive are

permitted. (At level zero, what happens to control characters is

left unspecified.) The position of the first character relative

to the initial beam position is left unspecified.

10 End of Picture ("Endpic").

This command denotes the end of a new picture. It must be paired

with a preceding Erase command.

11 Escape to device specifics ("Escdev") <value> <string>.

If "value" is the code assigned (by the Protocol Committee) to the

device being operated, then transmit the eight-bit bytes in

<string> (which starts with a <count> indicating the number of

bytes) to the device without examining them. Otherwise ignore

this command. If the device does not accept 8-bit information,

reformat the data in some device specific way; an example would be

throwing away the high order bit for a seven bit device, or

gathering 5 8-bit bytes into one 36-bit word, again discarding the

high order bits, perhaps. The action of the bytes in the string

should leave alone (or at least restore) any hardware beam

position registers in the device which the interpreter might

conceivably depend on.

This command really should not be used; it was included at level 0

so that specific applications can do mode setting and other device

specific manipulations. For example ARDS terminals may optionally

have several, independently addressable output scopes. The

selection mechanism changes state only when a particular sequence

of ASCII characters reaches the terminal. Thus ESCDEV would be

used to select which scopes(s) is/are to be affected by following

commands. (The current state is invisible to the graphics package

at the Using Host.)

Further, suppose that another make of terminal has a similar

option, which responds to a different code sequence. This

possibility is the motivation for conditionally ignoring the

ESCDEV command based on the "<value>" specified. Given that a

particular application will only be used to output to either an

ARDS or this second make (with the multiple scope option), then

the application could always send two ESCDEV commands, one

applicable only to ARDS terminals, and the other applicable only

to the second make.

APPENDIX 1: BNF FOR THE GRAPHICS PROTOCOL BYTE STREAM

Key to below:

Non-terminals are represented in <>.

Terminals which are keywords standing for particular eight-bit values

are in capitals.

Terminals whose meaning should be clear to the reader are in lower

case. Note that "empty_string" means "zero bytes," and not "a

<string> whose <count> is zero".

<graphics output byte stream> ::= empty_string

<stt group> ::= empty_string <stt> <stt group>.

<new picture stt> ::= ERASE

<escape to device stt> ::=ESCDEV <device code> <string>

<null stt>::= NULL

<end stt>::= ENDPIC

<move absolute stt> ::= MOVEA <x coordinate> <y coordinate>

<move relative stt> ::= MOVER <x delta> <y delta>

<draw absolute stt> ::= DRAWA <x coordinate> <y coordinate>

<draw relative stt> ::= DRAWR <x delta> <y delta>

<text and restore beam stt> ::= TEXTR <string>

<coordinate> ::= singed,_two's-complement,_fraction_in_range

-1/2_to_less_than_+1/2

<double coordinate> ::= signed,_two's_complement,_fraction,

range_strictly_between_-1_and_+1

<count ::= 7-bit_non-negative_integer

15-bit_non-negative_integer_represented_in

"excess_2**15"_notation

<string> ::= <count> count_8-bit_bytes

<value> ::= 8-bit_integer

[This RFCwas put into machine readable form for entry]

[into the online RFCarchives by Kelly Tardif, Viagie 10/99]