Device independent graphical display description
RFC 177
Document | Type |
RFC - Unknown
(June 1971; No errata)
Updated by RFC 181
Updates RFC 125
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Authors | |||
Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text html pdf htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 177 (Unknown) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
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 afterShow full document text