Conservative triage of polygon status using low precision edge evaluation and high precision edge evaluation

A GPU (graphics processor unit), comprising:

• a set-up unit for generating polygon descriptions;
• a rasterizer unit coupled to the set-up unit for rasterizing the polygon descriptions, wherein the rasterizer unit is configured to receive a polygon for rasterization and rasterize the polygon to generate a plurality of tiles of pixels related to the polygon, rasterize the tiles at a first level precision to generate a plurality of sub-tiles related to the polygon, wherein the sub-tiles are evaluated against the polygon at each of their respective corners, discard each of the sub-tiles not related to the polygon, and rasterize the sub-tiles related to the polygon at a second level precision wherein if a sub-tile is partially covered the sub-tile is rasterized at the second level precision to identify pixels of the sub-tile that are covered and pixels of the sub-tile that are not covered wherein a sub-tile rasterized at the second level of precision comprises pixels that are a part of a tile rasterized at said first level of precision;
• wherein the first level precision is a low precision edge evaluation level and the second level precision is a high precision edge evaluation level and wherein the low level precision is truncated to ensure a conservative evaluation of the sub-tiles with respect to coverage by the graphics primitive;
• wherein the low precision edge evaluation level utilizes less than half the number of bits of the high precision edge evaluation level.

A computer system, comprising:

• a system memory;
• a central processor unit coupled to the system memory;
• and
• a graphics processor unit communicatively coupled to the central processor unit;
• a set-up unit within the graphics processor unit for generating polygon descriptions;
• a rasterizer unit within the graphics processor unit that is coupled to the set-up unit for rasterizing the polygon descriptions;
• and
• a fine raster component within the raster unit, wherein the fine rasterizer component is configured to rasterize a graphics primitive to generate a plurality of tiles of pixels related to the graphics primitive, rasterize the tiles at a low precision to generate a plurality of sub-tiles related to the graphics primitive, wherein the sub-tiles are evaluated against the graphics primitive at each of their respective corners, discard each of the sub-tiles not related to the graphics primitive, and rasterize the sub-tiles related to the graphics primitive at a high precision wherein if a sub-tile is partially covered the sub-tile is rasterized at the second level precision to identify pixels of the sub-tile that are covered and pixels of the sub-tile that are not covered,
• wherein the fine raster component is further configured to determine whether a completely covered sub-tile exists upon the rasterizing at the low precision and output the completely covered sub-tile for rendering operations in a subsequent stage of the graphics processor wherein a sub-tile rasterized at the second level of precision comprises pixels that are a part of a tile rasterized at the first level of precision;
• wherein the first level precision is a low precision edge evaluation level and the second level precision is a high precision edge evaluation level and wherein the low level precision is truncated to ensure a conservative evaluation of the sub-tiles with respect to coverage by the graphics primitive;
• wherein the low precision edge evaluation level utilizes less than half the number of bits of the high precision edge evaluation level.