Classifications

• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T15/00—3D [Three Dimensional] image rendering
  • G06T15/10—Geometric effects
  • G06T15/40—Hidden part removal
  • G06T15/405—Hidden part removal using Z-buffer
• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T15/00—3D [Three Dimensional] image rendering
  • G06T15/10—Geometric effects
  • G06T15/40—Hidden part removal
• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T2210/00—Indexing scheme for image generation or computer graphics
  • G06T2210/62—Semi-transparency

Definitions

• This invention relates to a 3 -dimensional computer graphics system and in particular to methods and apparatus which reduce the number of times the data assigned to each pixel have to be modified when rendering an image in such a system.
• Tile based rendering systems are known. These break down an image to be rendered into a plurality of rectangular blocks or tiles. The way in which this is done and the subsequent texturing and shading performed is shown schematically in figure 1.
• This shows a geometry processing unit 2 which receives the image data from an application and transforms it into screen space using a well-known method. The data is then supplied to a tiling unit 4, which inserts the screen space geometry into object lists for a set of defined rectangular regions, or tiles, 6.
• Each list contains primitives (surfaces) that exist wholly or partially in a sub-region of a screen (i.e. a tile) .
• a list exists for every tile on the screen, although it should be borne in mind that some lists may have no data in them.
• Data then passes tile by tile to a hidden surface removal unit 8 (HSR) and from there to a texturing and shading unit 10 (TSU) .
• HSR hidden surface removal unit 8
• TSU texturing and shading unit 10
• the HSR unit In order to correctly render such an image, the HSR unit must pass "layers" of pixels which need to be shaded to the TSU. This is because more than one object will contribute to the image data to be applied to a particular pixel. For example the view from the inside of a building looking through a pane of dirty glass requires both the geometry visible through the glass, and then the pane of glass itself to be passed to the TSU. This process is referred to as "pass spawning".
• a tile based rendering device of the type shown in figure 1 will use a buffer to hold a tag for the front most object for each pixel in the tile currently being processed.
• a pass is typically spawned whenever the HSR unit 8 processes a translucent object, before the visibility test is performed. This results in all currently visible tags stored in the buffer followed by the visible pixels of the translucent object being sent to the TSU, i.e. more than one set of pixel data being passed for each pixel.
• the flow diagram of figure 2 illustrates this approach.
• a determination is made at 12 as to whether or not a primitive being processed is opaque. If it is not, then the buffer of tags is sent at 14 to the TSU 10. All visible tags for the non-opaque primitives are then also passed to the TSU at 15, the HSR unit 8 will then move onto the next primitive at 18. If the primitive is determined to be opaque at step 12 then its tags are written into the buffer at 16 before moving onto the next primitive at 18.
• the tag is a piece of data indicating which object is visible at a pixel. More than one tag per pixel is required when translucent objects cover opaque objects.
• tag buffer in the above description enables modifications to be made to the pass spawning rules (also described above) that allow the removal of some unnecessary passes.
• the translucent tags are rendered into the tag buffer in the same manner as opaque objects and a pass only spawned at the point a visible translucent pixel is required to be written to a location that is already occupied. Further, as the translucent object tags are now being rendered into the tag buffer there is no need to pass them immediately to the TSU. Therefore, in the event of them being subsequently obscured they may be discarded.
• Figure 1 shows a block diagram of a tile based rendering system discussed above
• Figure 2 shows a flow chart of a known pass spawning system
• Figure 3 (a) - (c) shows a sequence of three triangles being rendered using modified pass spawning rules embodying the invention
• Figure 4 is a flow diagram of the embodiment of the invention
• Figure 5 is an enhancement to figure 4
• Figure 6 is a block diagram of an embodiment of the invention.
• FIG 3 a sequence of three triangles is shown being rendered using a set of modified pass spawning rules.
• an opaque triangle Tl is rendered into a tag buffer. If a translucent triangle T2 is rendered on top of the visible opaque pixels then the HSR unit must pass those pixels to the TSU before it can continue to rasterise the translucent tags. The opaque triangle is encountered in scan line order as T2 is rasterised. Thus all the previously rasterised pixels of Tl and the pixels of T2 are passed to the TSU. This leaves the remainder of the translucent object as shown in figure 3b. An opaque triangle T3 is then rendered into the tag buffer as shown in figure 3c. This triangle T3 obscures all of Tl and T2.
• the object is opaque it is processed from 100 on a pixel by pixel basis. For each pixel within the object the system first determines its visibility at 102. If a pixel is not visible then the system skips to 108 to determine if there any more pixels left in the object. If a pixel is visible then its tag is written into the current tag buffer at 104, the tags in all other buffers are cleared at 106. The system then determines at 108 if any more pixels are left to process from the current object, if there are it moves to the next pixel at 110 and continues processing from 102. If there are no more pixels to process in the object then the system moves to the next object at 112 and returns to 20.
• 3D computer graphics often use what is termed as "punch through” objects. These objects use a back end test to determine if a pixel should be drawn. For example, before a pixel is written to the frame buffer its alpha value can be compared against a reference using one of several compare modes, if the result of this comparison is true then the pixel is determined to be visible, if false then it is not. Pixels that are determined to not be visible do not update the depth buffer. It should be noted that this test can be applied to both opaque and partially translucent objects. This technique is common in 3D game applications because it allows complex scenes such as forests to be modelled using relatively few polygons and because a traditional Z buffer can correctly render punch through translucency irrespective of the order in which polygons are presented to the system.
• TBR back end test tile based rendering
• punch through objects can be optimised. If punch through is applied to an opaque object it will either be fully visible or not visible at all, this allows them to be treated as opaque with respect to the flushing of the tag buffers. Specifically at the point an opaque punch through object is received any pixels that are determined to be visible by the hidden surface removal (HSR) unit are passed directly to the TSU. The TSU then feeds back pixels that are valid to the HSR unit which will, for the fed back pixels, update the depth buffer as appropriate and invalidate the same pixels in the tag buffer. The later is possible as the act of passing the punch through pixels to the TSU means that they have already been drawn so any valid tags at the same locations in the tag buffer are no longer needed. If multiple tags buffers are present then the tags are invalidated across all buffers.
• HSR hidden surface removal
• Partially transparent punch through data requires all tag buffers up to and including the current tag buffer to be flushed to the TSU. This is because the transparency may need to be blended with any overlapped tags currently contained in the tag buffer.
• the punch through tags may be passed to the TSU with their states modified such that they will not update the frame buffer image and their tags are written to the next buffer as dictated by the punch through test. This allows objects that lie under the punch through object that are subsequently obscured not to be rendered. However, this is at the cost of potentially rendering the punch through object twice, once to determine pixel visibility and once to render the final image if it is not obscured. The impact of rendering the punch through object twice could be reduced by splitting the TSU pixel's shading state into that required to determine punch through state and that required to render the final image.
• the system jumps to Process Punch Through 300.
• the object is then processed pixel by pixel, first determining visibility at 302. If a pixel is not visible the system skips to 324. If a pixel is visible a further test is made at 304 to determine if the object pixel is also transparent, and if this is determined to be the first visible pixel within the object at 306 then all tag buffers up to and including the current buffer are flushed at 308, i.e. sent to the TSU.
• the test for translucency is performed per pixel so that the tag buffers do not get flushed in the event of the object not being visible.
• the pixels for the object itself are then sent to the TSU at 310 where texturing and shading are applied at 312 using well- known methods.
• a punch through test is then applied at 314 and the validity of the pixel determined at 316. If the pixel is found to be invalid at 316, e.g. it fails the alpha test, the system skips to 324. If the pixel is valid its coordinates are passed back to the HSR unit at 318, which will then store the pixels depth value to the depth buffer at 320 and invalidate the corresponding tag in all tag buffers at 322.
• the system determines if there are any more pixels to be processed in the object at 324, if there are it moves to the next pixel at 326 and jumps back to 302 to continue processing.
• the system moves to the next object and returns to 20.
• the punch through determination at 24 is negative then the object must be translucent and the system jumps to Process Translucent 200.
• the object is then processed pixel by pixel, first determining visibility at 202. If a pixel is not visible the system skips to 222. If the pixel is visible the system determines if location in the current tag buffer is occupied at 204. If the current tag buffer location is determined to be occupied the system will move to the next tag buffer at 206. A determination is then made as to whether or not there are any valid tags in the buffer at 208 and if there are, they are sent to the TSU at 210 and the tag buffer reset at 212. If there are no valid tags then a tag is written at 220 and the system goes on to the next pixel or object as described for opaque objects.
• the pass spawning rules can be further extended such that passes only spawn when no tag buffer can be found into which a translucent pixel can be written.
• Figure 5 illustrates these updated rules which can be used to replace the portion of the flow diagram of figure 4 surrounded by a dotted line. Instead of the determination at 208 as to whether there are any valid tags in the buffer, a determination is made as to whether or not this buffer has been looked at before. If it has, then the flow moves onto 210 and 212. If it has not, flow passes back to 204 where a determination is made as to whether or not the tag location is occupied. If it is, then the diagram moves to the next tag buffer at 240 before again determining whether or not that buffer has been looked at at 242.
• a further enhancement can be made to single and multiple buffer implementations. Rather than flushing the whole tag buffer at the point that no unoccupied pixel can be found for a translucent object, only those tags that would be overwritten by the translucent pixel are flushed to the TSU.
• the main disadvantage of this approach is that it can result in the partial submission of an object to a TSU which can result in it being submitted many times. This leads to additional state fetch and set up costs in the TSU. This could be alleviated by submitting all pixels with the same tag value to the TSU rather than only those that are overlapped by the translucent object.
• the tag buffer could be subdivided into square/rectangular sections such that when the above condition occurs only the section of the tag buffer containing the conflict would be flushed. This approach also potentially results in multiple submissions of tags to the TSU but to a lesser extent.
• FIG. 6 A block diagram of a preferred embodiment of the invention is shown in figure 6.
• This comprises a parameter fetch unit 50 which reads in per tile lists of triangles and rasterisation state from a per triangle list 52. These are then passed to a scan converter 54 and to a pass spawn control unit 56 respectively.
• the scan converter 54 generates position, depth and stencil values for each pixel within each triangle and passes them to an HSR unit 58.
• the HSR unit determines the visibility of each pixel and passes this information onto the pass spawning control unit (PSC) 56.
• PSC pass spawning control unit
• This unit has access to two or more tag buffers 60 which are cycled through in a circular manner. For opaque pixels the PSC unit writes the tag of the triangle to the current tag buffer and invalidates the corresponding location in the other buffer.
• the PSC unit checks to see if the location in the current tag buffer is valid. If it is, then it switches to the other tag buffer. If a tag location is not valid it writes the translucent tag and moves onto the next pixel. If the tag location is valid then the tag buffer is flushed to the texturing and shading unit 62. At this point all locations in the tag buffer are marked as invalid and the translucent tag is written to the buffer.
• the parameter fetch unit 50 determines that there are no more triangles in the current tile it signals to the pass spawning unit 56 to flush any remaining valid tags from the tag buffers to the TSU 62.
• the parameter fetch unit then proceeds to read the parameter list for the next tile and repeats the process until all tiles that make up the final image have been rendered. It should be noted that all of the units, with the exception of the parameter fetch, can be modified to operate on multiple pixels in parallel, thereby speeding up the process.
• the HSR unit 58 has access to a depth and stencil buffer 64 in which the depth and stencil values for each pixel within each triangle are stored.

Landscapes

• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Geometry (AREA)
• Computer Graphics (AREA)
• General Physics & Mathematics (AREA)
• Theoretical Computer Science (AREA)
• Image Generation (AREA)

Priority Applications (2)

Applications Claiming Priority (2)

Publications (2)

Family

ID=27772710

Family Applications (1)

Country Status (5)

Cited By (2)

Families Citing this family (15)

Family Cites Families (10)

• 2003
  • 2003-07-25 GB GB0317479A patent/GB2404316B/en not_active Expired - Lifetime
• 2004
  • 2004-03-05 US US10/795,561 patent/US20050017970A1/en not_active Abandoned
  • 2004-06-23 JP JP2006521641A patent/JP4602334B2/ja not_active Expired - Lifetime
  • 2004-06-23 EP EP04743068.1A patent/EP1649428B1/en not_active Expired - Lifetime
  • 2004-06-23 WO PCT/GB2004/002717 patent/WO2005015503A2/en not_active Ceased
• 2007
  • 2007-04-18 US US11/787,893 patent/US8446409B2/en active Active
• 2013
  • 2013-02-11 US US13/763,974 patent/US20130207977A1/en not_active Abandoned

Also Published As

Similar Documents

Legal Events