Texture range controls for improved texture mapping

Abstract

A graphical display system and process for specifying and controlling a display range in which a specified form of texture mapping is applied or suppressed. Object data from a host computer is processed by four pipelined graphics subsystems before being displayed on a display screen. These graphics subsystems include: 1) a Geometry Subsystem, 2) a Scan Conversion Subsystem, 3) a Raster Subsystem, and 4) a Display Subsystem. Span Processors within the Scan Conversion Subsystem manipulate pixel coordinates in order to handle sitations when coordinates are located out of range of a texture map. Processing logic and hardware registers located within each Span Processor implement two texture modes for handling out-of-range coordinates. First, a mask and comparison register is provided to hold a value specifying a selected range in which texture is applied to a pixel. If a pixel is outside the specified range, texture application is suppressed. Secondly, logic is provided for determining if the input pixel coordinate is within range of the texture map. If the input coordinate is out of range and positive, the coordinate is set to a value corresponding to the most positive border of the texture map. If the input coordinate is out of range and negative, the coordinate is set to a value corresponding to the least positive border of the texture map. The processed coordinate is then output to one of a plurality of Image Engines.