rfc292.txt

RFC 的详细文档！


RFC 292                Graphics Protocol Level 0            January 1972


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



Michener                                                        [Page 6]

RFC 292                Graphics Protocol Level 0            January 1972


      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



Michener                                                        [Page 7]

RFC 292                Graphics Protocol Level 0            January 1972


      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.











































Michener                                                        [Page 8]

RFC 292                Graphics Protocol Level 0            January 1972


            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
                      | <picture> <graphics output byte stream>
   <picture> ::= <new picture stt> <sttgroup> <end stt>
   <stt group> ::= empty_string | <stt> <stt group>.
   <stt> ::= <control stt> | <display stt>
   <control stt> ::= <escape to device stt>
                      | <null stt>
   <display stt> ::= <move absolute stt>
                      | <move relative stt>
                      | <draw absolute stt>
                      | <draw relative stt>
                      | <dot absolute stt>
                      | <dot relative stt>
                      | <text and restore beam stt>
                      | <text stt>

   <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>
   <dot absolute stt> ::= DOTA <x coordinate> <y coordinate>
   <dot relative stt> ::= DOTR<x delta> <y delta>
   <text and restore beam stt> ::= TEXTR <string>
   <text stt> ::= TEXT <string>
   <x coordinate> ::= <coordinate>
   <y coordinate> ::= <coordinate>
   <x delta> ::= <double coordinate>
   <y delta> ::= <double coordinate>
   <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





Michener                                                        [Page 9]

RFC 292                Graphics Protocol Level 0            January 1972


   <count ::= 7-bit_non-negative_integer
                     | 15-bit_non-negative_integer_represented_in
                     "excess_2**15"_notation
   <string> ::= <count> count_8-bit_bytes
   <device code> ::= <value>
   <value> ::= 8-bit_integer











          [This RFC was put into machine readable form for entry]
      [into the online RFC archives by Kelly Tardif, Viag閚ie 10/99]
































Michener                                                       [Page 10]