Hidden culling in tile-based computer generated images

• US 9,805,500 B2
• Filed: 12/19/2013
• Issued: 10/31/2017
• Est. Priority Date: 12/20/2012
• Status: Active Grant

• Current Assignee: Imagination Technologies Limited (Imagination Technologies Group Ltd.)
• Original Assignee: Imagination Technologies Limited (Imagination Technologies Group Ltd.)
• Inventors: Yang, Xile, Howson, John W., Fenney, Simon
• Primary Examiner(s): Yang, Andrew G
• Application Number:
  US14/135,278
• Publication Number:
  US 20140267258A1
• Time in Patent Office: 1,412 Days
• Field of Search: None
• CPC Class Codes: G06T 15/005 General purpose rendering architectures
• CPC Class Codes: G06T 15/405 using Z-buffer

Abstract

A method and system is provided for culling hidden objects in a tile-based graphics system before they are indicated in a display list for a tile. A rendering space is divided into a plurality of regions which may for example be a plurality of tiles or a plurality of areas into which one or more tiles are divided. Depth thresholds for the regions, which are used to identify hidden objects for culling, are updated when an object entirely covers a region and in dependence on a comparison between a depth value for the object and the depth threshold for the region. For example, if the depth threshold is a maximum depth threshold, the depth threshold may be updated if an object entirely covers the tile and the maximum depth value of the object is less than the maximum depth threshold.

Claims

1. A method for culling non-visible objects in a tiling unit of a tile-based rendering system configured to use a rendering space which is subdivided into a plurality of tiles, wherein each of the tiles comprises one or more regions for which depth comparisons are to be performed, each of the tiles comprising a plurality of pixels, the method comprising, for each of the tiles of the rendering space:

compiling a display list for the tile, the display list comprising object data for potentially visible objects in the tile, the compiling of the display list for the tile comprising, for each region of the tile and before rendering of the tile begins;

deriving depth values for an object in the region;

reading a depth threshold for the region from a memory;

performing comparisons between the depth values for the object and the depth threshold for the region;

determining whether the object entirely covers the region; and

when it is determined that the object entirely covers the region and in dependence on the results of at least one of the comparisons, updating the depth threshold for the region;

in dependence on the result of at least one of the comparisons for each of the one or more regions of the tile, determining whether to cull the object for the tile, such that the object is not indicated in the display list for the tile; and

subsequently beginning rendering the tile using the display list for the tile.

2. A method according to claim 1 wherein the object comprises a single primitive, a plurality of primitives or a patch of surface data.

3. A method according to claim 1 wherein the step of performing comparisons between the depth values for the object and the depth threshold for the region comprises performing a comparison between a maximum depth value for the object and a maximum depth threshold for the region;

and wherein the step of updating the depth threshold for the region in dependence on the result of at least one of the comparisons comprises updating the maximum depth threshold for the region if the maximum depth value for the object is less than the maximum depth threshold for the region.

4. A method according to claim 3, wherein the step of determining whether to cull the object in dependence on the result of at least one of the comparisons for each of the one or more regions of the tile such that the subsequent object is not indicated in the display list for the tile comprises comparing a minimum depth value for the object to the maximum depth threshold for the region.

5. A method according to claim 3, further comprising the steps of:

performing a comparison between a minimum depth value for the object and a minimum depth threshold for the region; and

updating the minimum depth threshold for the region if the minimum depth value for the object is less than the minimum depth threshold for the region to a value equal to or less than the minimum depth value for the object.

6. A method according to claim 1 wherein the step of performing comparisons between the depth values for the object and the depth threshold for the region comprises performing a comparison between a minimum depth value for the object and a minimum depth threshold for the region;

and wherein the step of updating the depth threshold for the region in dependence on the result of at least one of the comparisons comprises updating the minimum depth threshold for the region if the minimum depth value for the object is greater than the minimum depth threshold for the region.

7. A method according to claim 6, wherein the step of determining whether to cull the object in dependence on the result of at least one of the comparisons for each of the one or more regions of the tile such that the subsequent object is not indicated in the display list for the tile comprises comparing a maximum depth value for the object to the minimum depth threshold for the region.

8. A method according to claim 6, further comprising the steps of:

performing a comparison between a maximum depth value for the object and a maximum depth threshold for the region; and

updating the maximum depth threshold for the region if the maximum depth value for the object is greater than the maximum depth threshold for the region to a value equal to or greater than the maximum depth value for the object.

9. A method according to claim 1, wherein deriving depth values for the object comprises, for each region within which the object is located, deriving depth values for the respective part of the object within that region.

10. A method according to claim 1, wherein the plurality of regions comprise a plurality of tiles.

11. A method according to claim 1, wherein the plurality of regions comprises a plurality of areas into which a tile is subdivided.

12. A method according to claim 11, wherein the step of determining whether to cull the object for the tile in dependence on the result of at least one of the comparisons such that the object is not located in a display list for the tile, comprises:

for each of the areas of the tile within which the object is located, comparing a depth value for the object to the depth threshold for that area.

13. A method according to claim 11, further comprising the steps of:

determining a depth threshold for the tile based on the depth threshold for each of the areas of the tile; and

writing the depth threshold for the tile to a memory.

14. A method according to claim 13, wherein the step of determining a depth threshold for the tile based on the depth threshold for each of the areas of the tile comprises:

determining the greatest of the depth thresholds for the areas of the tile; and

setting the depth threshold for the tile to a value equal to or greater than the greatest of the depth thresholds for the areas of the tile.

15. A method according to claim 13, wherein the step of determining a depth threshold for the tile based on the depth threshold for each of the areas of the tile comprises:

determining the smallest of the depth thresholds for the areas of the tile; and

setting the depth threshold for the tile to a value equal to or smaller than the smallest of the depth thresholds for the areas of the tile.

16. A method according to claim 13, wherein the depth thresholds for the areas are written to a memory device for temporarily storing data.

17. A method according to claim 16, wherein the depth thresholds for the areas are evicted from the memory device in dependence on how many of the depth thresholds for the areas have been updated and or on the difference between the depth thresholds for the areas.

18. A method according to claim 11, wherein a first plurality of the plurality of areas is a plurality of sub areas into which a first portion of the tile is subdivided;

the method further comprising;

determining a depth threshold for the first portion of the tile based on the depth threshold for each of the sub areas; and

writing the depth threshold for the first portion of the tile to a memory.

19. A system for culling non-visible objects in a tile-based rendering system having a rendering space sub-divided into a plurality of tiles, wherein each of the tiles comprises one or more regions for which depth comparisons are to be performed, each of the tiles comprising a plurality of pixels, the system comprising a processor configured to compile a display list for each of the tiles of the rendering space, the display list for a tile comprising object data for potentially visible objects in the tile;

wherein the processor is configured to compile the display list for a tile by actions comprising, for each region of the tile and before rendering of the tile begins;

deriving depth values for an object in the region;

reading a depth threshold for the region from a memory;

performing comparisons between the depth values for the object and the depth threshold for the region;

determining whether the object entirely covers the region; and

when it is determined that the object entirely covers the region and in dependence on the result of at least one of the comparisons, updating the depth threshold for the region;

wherein the processor is further configured to, in dependence on the result of at least one of the comparisons for each of the one or more regions of the tile, determine whether to cull the object for the tile, such that the object is not indicated in the display list for the tile; and

subsequently begin rendering the tile using the display list for the tile.

20. A system according to claim 19 wherein the object comprises a single primitive, a plurality of primitives or a patch of surface data.

21. A system according to claim 19 wherein the comparisons between the depth values for the object and the depth threshold for the region comprise a comparison between a maximum depth value for the object and a maximum depth threshold for the region;

and

wherein the step of updating the depth threshold for the region in dependence on the result of at least one of the comparisons comprises updating the maximum depth threshold for the region if the maximum depth value for the object is less than the maximum depth threshold for the region.

22. A system according to claim 21, wherein determining whether to cull the object in dependence on the result of at least one of the comparisons for each of the one or more regions of the tile such that the subsequent object is not indicated in the display list for the tile comprises comparing a minimum depth value for the object to the maximum depth threshold for the region.

23. A system according to claim 21, wherein the processor is further configured to perform a comparison between a minimum depth value for the object and a minimum depth threshold for the region;

and update the minimum depth threshold for the region if the minimum depth value for the object is less than the minimum depth threshold for the region to a value equal to or less than the minimum depth value for the object.

24. A system according to claim 19 wherein performing comparisons between the depth values for the object and the depth threshold for the region comprises performing a comparison between a minimum depth value for the object and a minimum depth threshold for the region;

and wherein updating the depth threshold for the region in dependence on the result of at least one of the comparisons comprises updating the minimum depth threshold for the region if the minimum depth value for the object is greater than the minimum depth threshold for the region.

25. A system according to claim 24, wherein determining whether to cull the object in dependence on the result of at least one of the comparisons for each of the one or more regions of the tile with the depth threshold for the region, such that the subsequent object is not indicated in the display list for the tile comprises comparing a maximum depth value for the object to the minimum depth threshold for the region.

26. A system according to claim 24, wherein the processor is further configured to perform a comparison between a maximum depth value for the object and a maximum depth threshold for the region;

and updating the maximum depth threshold for the region if the maximum depth value for the object is greater than the maximum depth threshold for the region to a value equal to or greater than the maximum depth value for the object.

27. A system according to claim 19, wherein deriving a depth value for the object comprises, for each region within which the object is located, deriving a depth value for the respective part of the object within that region.

28. A system according to claim 19, wherein the plurality of regions is a plurality of tiles.

29. A system according to claim 19, wherein the plurality of regions comprises a plurality of areas into which a tile is subdivided.

30. A system according to claim 29, wherein determining whether to cull the object for the tile in dependence on the result of at least one of the comparisons for each of the one or more regions of the tile such that the object is not located in the display list for the tile comprises, for each of the areas of the tile within which the object is located, comparing a depth value for the object to the depth threshold for that area.

31. A system according to claim 29, wherein the processor is further configured to determine a depth threshold for the tile based on the depth threshold for each of the areas of the tile;

and writing the depth threshold for the tile to a memory.

32. A system according to claim 31, wherein determining a depth threshold for the tile based on the depth threshold for each of the areas of the tile comprises determining the greatest of the depth thresholds for the areas of the tile;

and setting the depth threshold for the tile to a value equal to or greater than the greatest of the depth thresholds for the areas of the tile.

33. A system according to claim 31, wherein determining a depth threshold for the tile based on the depth threshold for each of the areas of the tile comprises determining the smallest of the depth thresholds for the areas of the tile and setting the depth threshold for the tile to a value equal to or smaller than the smallest of the depth thresholds for the areas of the tile.

34. A system according to claim 29, wherein a first plurality of the plurality of areas is a plurality of sub areas into which a first portion of the tile is subdivided, the processor is further configured to determine a depth threshold for the first portion of the tile based on the depth threshold for each of the sub areas;

and write the depth threshold for the first portion of the tile to a memory.

35. A system according to claim 31, wherein the depth thresholds for the areas are written to a memory device for temporarily storing data.

36. A system according to claim 35, wherein the processor is configured to determine depth thresholds for the areas to be evicted from the memory device based on at least one of (1) a count of how many of the depth thresholds for the areas have been updated and (2) on the difference between the depth thresholds for the areas.

37. A circuit for culling non-visible objects in a graphics system having a rendering space subdivided into a plurality of tiles, wherein each of the tiles comprises one or more regions for which depth comparisons are to be performed, each of the tiles comprising a plurality of pixels, wherein the circuit is configured to compile, for each of the tiles and before rendering of the tile begins, a display list comprising object data for potentially visible objects in the tile based, at least in part, on actions of different logics comprising:

depth calculation logic configured to derive depth values for an object;

comparator logic configured to read a depth threshold for a region of the tile from a memory and perform comparisons between the depth values for the object and the depth threshold for the region;

depth update logic configured to determine whether the object entirely covers the region, and, if so and in dependence on the result of at least one of the comparisons, update the depth threshold for the region;

object culling logic configured to, based on the result of at least one of the comparisons for each of the one or more regions of the tile, determine whether to cull the object from the tile, such that the object is not indicated in the display list for the tile; and

rendering logic configured to subsequently begin rendering the tile using the display list for the tile.