RFC 177 (rfc177) - Page 2 of 9
Device independent graphical display description
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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