RFC 177 (rfc177) - Page 2 of 9

Device independent graphical display description

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