rfc177.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

NETWORK WORKING GROUP                                       J. McConnell
Request for Comment: 177           Ames Research Center Moffet Field, CA
Obsoletes: none                                             15 June 1971
Updates: 125
NIC: 7102

           A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION

   As more nodes are connected to the ARPA network, the types of
   graphical display processors available to users is quite varied.  To
   attempt to facilitate the transmission of graphical information over
   the network, a device independent description of a display is
   described.  The using host may make any conversions necessary to
   realize the picture on a specific device.  It is also possible to
   interpose a form machine as proposed by Heafner and Harslem in RFC
   #94.  Some of the items and entities described herein were first
   discussed by Steve Crocker in RFC #86.  It is intended that this
   description is to be oriented to a refresh display with point,
   vector, and character drawing capability, these types of devices
   include the IBM 2250, IMLAC PDS-1, the DEC 338, and DEC 340, as well
   as the Evans and Sutherland LDS-1.  However, direct video storage
   tube devices represented by the ARDS, Tektronix and Computer devices
   can also serve as clumsy interactive devices.  Hard copy devices,
   such as microfilm or plotters, can also be used at the using host's
   discretion.

   There are several items and constructs which will be defined before
   discussing the specifics of the description:

      1.  The network standard graphics description stream (NGDS)
      contains the description, data, and operators necessary to effect
      a display.

      2.  The network standard stream interpreter (NGSI) parses the NGDS
      into its components.

      3.   The network standard display list (NGDL) is the basic entity
      which, when executed, causes graphical information to be
      displayed.

      4.  The network standard list interpreter (NGLI) is the entity
      which executes the NGDL and controls the beam movement to effect
      generation of graphical information.

      5.  The network standard screen (NGS) is the entity on which the
      information is to be displayed.  It may be divided into image
      areas to be defined later.




McConnell                                                       [Page 1]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971


      6.  The network standard graphics list pointer (NGLP) and the
      network standard graphics execution stack (NGES) are entities
      which describe the state of the NGLI at any given time and provide
      the facilities to allow the NGLI to be a recursive interpreter.

   Figure 1. summarizes the preceding discussion graphically, which is
   appropriate.

   The network standard graphics screen is rectangular and positions on
   it are specified by an ordered pair of fractions representing the
   horizontal distance from the left edge and the vertical distance from
   the bottom edges respectively.  These shall be termed the (x,y)
   coordinates as is standard.  There is no specification of resolution,
   so that the quality of the picture will degrade with the decreasing
   resolution of a poor display device, or be improved if it is
   displayed on a higher resolution device.  Coordinate values are
   specified by a 16 bit unsigned fractions with the binary point to the
   left of the most significant bit - this provides a normalized screen
   with coordinates between 0.0 and .999...9.

   The NGS may be subdivided into rectangular image areas with
   possibility unique displays in each space. An image area has three
   attributes:  a 16 bit integer name, an x,y pair specifying the lower
   left corner and an x,y pair to specify the upper right corner of the
   area.  Image spaces may overlap, but may not be completely contained
   within each other.  The main image space has the name 0 and is
   coincident with the NGS.  The use of image spaces allows for
   manipulation of part of the NGS's contents, as well as redefining the
   coordinate space. Within an image area, coordinate values are
   fractional displacements from the lower left hand corner of the image
   space.  Thus an image area defined from (0.0, 0.0) to (0.5, 0.5)
   would contain an image one-fourth of the size of what it would be on
   the full NGS.

   Character strings may be displayed at arbitrary points in an image
   area.  Because many display devices have hardware character
   generators capable of producing one, or a few sizes, character
   scaling within an image area will not be expected.  Characters shall
   be assumed to be .014 screen width wide, and .025 screen height high
   including spacing.  This gives a screen capacity of 72 characters and
   40 lines.  When the beam is moved to be a screen position prior to
   drawing a character, it is assumed to be in the center of the
   rectangle defining the character space.  The beam position after
   drawing a character, or a string of characters, is undefined.

   The format of the NGDS can now be specified.  The NGSI parses the
   NGDS into commands.  The commands are in a prefix format with an
   eight bit command followed by the necessary parameters.  Their



McConnell                                                       [Page 2]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971


   descriptions follow the numerical order of the command codes.

   ERASE (commands = 0) consists of a command byte and no parameters.
   Its effect is to erase the full NGS.

   DEFINE LIST (commands = 1) has three parameters.  A 16 bit integer
   for the list name is first.  If the name has already been used, then
   this command redefines the list.  The count of the items in the list
   is also a 16 bit integer defining the length of the list in items.
   The list items follow the count terminating with an end list item.

   The items themselves are used to control the NGLI and the movement of
   the beam when drawing pictures.  They are also in prefix format, with
   an eight bit item code followed by the necessary parameters.  All
   names are 16 bit integers, coordinate data are 16 bit unsigned
   fractions as previously described.  Any other parameters will have
   their type and size specified in individual descriptions.

      a.  OPEN IMAGE AREA (name) (item type = 0) instructs the NGLI that
      list commands which follow are to be adjusted to be contained
      within the named area.  A nonexistent area name is treated as a
      NOP.  If no image area is specified for the NGLI the default area
      is area 0, the NGS.  This directive remains in effect until
      another type 0 item is encountered by the NGLI.

      b.  RESET IMAGE AREA (name) (item type = 1) causes the NGLI to
      move the beam to the lower left corner of the image area.  A
      nonexistent name is again treated as a NOP.

      c.  CLEAR IMAGE AREA (name) (item type = 2) causes the NGLI to
      erase all graphical information in the specified area, and then to
      perform a reset command.

   The following items are display items which actually cause beam
   movement by the NGLI.  Coordinate data are relative to the origin of
   the current image area and are taken as fractional displacements
   within this area.

      d.  MOVE BEAM (x,y) (item type = 3) causes the NGLI to move the
      beam to (x,y) relative to the current origin.

      e.  DISPLAY CURRENT POSITION (item type = 4) causes the NGLI to
      display the current point on the NGS.








McConnell                                                       [Page 3]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971


      f.  DISPLAY VECTOR (x,y) (item type = 5) causes a vector to be
      drawn from the current point to (x,y).  After the operation, (x,y)
      becomes the current point.

      g.  DISPLAY CHARACTERS (n, 'C1, C2...Cn') (item type = 6) causes
      the n characters to be displayed starting from the current point.
      N is the character count and is an eight bit byte.

   The character set recognized by the NGLI is composed of eight bit
   bytes interpreted in the following ways.  Any character with the most
   significant bit a zero is interpreted as an ASCII character.  Some
   non-graphic ASCII characters may have special functions within a
   string:

      1.  Line feed (LF) move the beam one line spacing downward.

      2.  Carriage return (CR) move the beam to the left most character
      position in the current line.

      3.  Backspace (BS) move the beam one character position to the
      left in the current line.

      4.  Tab move the beam a predetermined number of character
      positions to the right in the current line.  The number will
      follow the tab character in the string, but may not exceed the
      capacity of the current line.

      5.  Vertical tab (VT) move the beam down a predetermined number of
      line spacings.  The number of spacings follow the VT character and
      may not exceed the line capacity of the screen.  This is expanded
      as multiple line feeds.

      6.  Any other non-graphic character - could cause a space
      character to appear in its place on the display, or it may be used
      for any purpose two processes choose.

   Characters with the most significant bit a one are used for an
   extended character set where a device can support this.  These may
   also be used as an implicit stroke table, calls being the names of
   special graphic lists that may be defined in the NGDS.  If no eight
   bit list name exists, then the character is a space.

      h.  EXECUTE LIST (name, x, y) (item type =7) causes the NGLI to
      suspend interpretation of the list and interpret the named list.
      It is equivalent to a subroutine call.  The (x,y) pair specifies
      the origin of the new list relative to the current origin.  The
      actions taken by the NGLI are specified in the next section




McConnell                                                       [Page 4]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971


      i.  COPY LIST (name, x,y) (item type = 8) cause the items
      contained in the named list to be copied into the place of the
      item with the origin of the list at (x,y) relative to the current
      origin.  This is analogous to item (h) as a subroutine is to a
      macro.

      j.  EXECUTE TABLE (name, mode, length) (item = 9) causes the NGLI
      to treat the named list as a special entity.  It is constrained to
      contain only coordinate pairs.  It may be executed in either point
      or vector mode as specified by the mode parameter 1= vector, 0=
      point.  The length of the list is specified by a 16 bit integer
      parameter.

      k.  END LIST (item type = 10) has no parameters and informs the
      NGLI that the end of a list has been reached.

      l.  TABLE (n) (item type = 11) specifies that there are (x,y)
      pairs in this list.  N is a 16 bit integer.

      m.  Set vector mode (mode) (item type = 12)

      n.  Set character mode (mode) (item type 13)

   In addition to the above display items, some items exist which