Visibility Culling

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Visibility Culling
David Luebke Computer Science Department Universit
y of Virginia ltluebke_at_cs.virginia.edugt
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Recap General Occlusion Culling

• When cells and portals dont work
• Trees in a forest
• A crowded train station
• Need general occlusion culling algorithms
• Aggregate occlusion
• Dynamic scenes
• Non-polygonal scenes

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RecapGeneral Occlusion Culling

• Ill discuss two algorithms
• Hierarchical Z-Buffer
• Ned Greene, SIGGRAPH 93
• Hierarchical Occlusion Maps
• Hansong Zhang, SIGGRAPH 97

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Recap Hierarchical Z-Buffer

• Replace Z-buffer with a Z-pyramid
• Lowest level full-resolution Z-buffer
• Higher levels each pixel represents the maximum
  depth of the four pixels underneath it
• Basic idea hierarchical rasterization of the
  polygon, with early termination where polygon is
  occluded

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Hierarchical Z-Buffer

• Idea test polygon against highest level first
• If polygon is further than distance recorded in
  pixel, stopits occluded
• If polygon is closer, recursively check against
  next lower level
• If polygon is visible at lowest level, set new
  distance value and propagate up

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Hierarchical Z-Buffer

• Z-pyramid exploits image-space coherence
• Polygon occluded in a pixel is probably occluded
  in nearby pixels
• HZB also exploits object-space coherence
• Polygons near an occluded polygon are probably
  occluded

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Hierarchical Z-Buffer

• Exploiting object-space coherence
• Subdivide scene with an octree
• All geometry in an octree node is contained by a
  cube
• Before rendering the contents of a node, render
  the faces of its cube (i.e., query the Z-pyramid)
• If cube faces are occluded, ignore the entire node

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Hierarchical Z-Buffer

• HZB can exploit temporal coherence
• Most polygons affecting the Z-buffer last frame
  will affect Z-buffer this frame
• HZB also operates at max efficiency when
  Z-pyramid already built
• So start each frame by rendering octree nodes
  visible last frame

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Hierarchical Z-BufferDiscussion

• HZB needs hardware support to be really
  competitive
• To date, hardware vendors havent bought in
• Z-pyramid (and hierarchies in general) unfriendly
  to hardware
• Unpredictable Z-query times generate bubbles in
  rendering pipe
• But there is a promising trend

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Hierarchical Z-Buffer

• Hardware beginning to support Z-query operation
• Allows systems to exploit
• Object-space coherence (bounding boxes)
• Temporal coherence (last-rendered list)
• Systems Im aware of
• HP Visualize-fx graphics
• SGI Visual Workstation products
• An aside applies to cell-portal culling!

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Hierarchical Occlusion Maps

• A more hardware-friendly general occlusion
  culling algorithm
• Two major differences from HZB
• Separates occluders from occludees
• Decouples occlusion test into an depth test and a
  overlap test

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HierarchicalOcclusion Maps

• Occluders versus occludees

Blue parts occluders Red parts occludees
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Hierarchical Occlusion Maps

• Depth versus overlap

Depth Overlap
Occlusion
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Hierarchical Occlusion Maps

• Representation of projection for overlap test
  occlusion map
• Corresponds to a screen subdivision
• Records average opacity per partition
• Generate by rendering occluders
• Record pixel opacities (i.e., coverage)

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Occlusion Maps
Rendered Image
Occlusion Map
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Occlusion Map Pyramid
64 x 64
32 x 32
16 x 16
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Occlusion Map Pyramid
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Occlusion Map Pyramid

• Analyzing cumulative projection
• A hierarchical occlusion map (HOM)
• Generate by recursive averaging (once per frame)
• Records average opacities for blocks of multiple
  pixels, representing occlusion at multiple
  resolutions
• Construction can be accelerated by texture
  hardware

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Overlap Tests

• Query is projection of occludee inside
  cumulative projection of occluders?
• Cumulative projection occlusion pyramid
• Ocludee projection expensive in general
• Overestimate ocludee with 3-D bounding box
• Overestimate projection of 3-D bounding box with
  2-D bounding rectangle in screen-space

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Overlap Tests

• Hierarchical structure enables some
  optimizations
• Predictive rejection
• Terminate test when it must fail later
• Conservative rejection
• The transparency threshold
• Aggressive Approximate Culling
• Ignore objects barely visible through holes
• The opacity threshold

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Aggressive Approximate Culling
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Hierarchical Occlusion Maps

• Not discussed here
• Depth test
• Depth estimation buffer
• Modified Z-buffer
• Selecting occluders
• For more details, see attached excerpt from
  Hansong Zhangs dissertation

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HOM Discussion

• Provides a robust, general, hardware-friendly
  occlusion culling algorithm
• Supports dynamic scenes
• Supports non-polygonal geometry
• Not many hardware assumptions

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HOM Discussion

• Efficient coding, careful tuning a must
• Fairly high per-frame overhead
• Needs high depth complexity, good occluder
  selection to be worthwhile
• UNCs MMR system

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Visibility Culling Discussion

• When is visibility culling worthwhile?
• When scene has high depth complexity
• Examples architectural walkthroughs, complex CAD
  assemblies, dense forest
• Non-examples terrain, single highly-tesselated
  object (e.g., a radiositized room)

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Visibility CullingDiscussion

• How does visibility culling compare to
• Level-of-detail
• Reduces geometry processing
• Helps transform-bound apps
• Visibility culling
• Reduces geometry and pixel processing
• Helps transform- and fill rate-bound apps
• Texture / Image representations
• Reduces geometry and pixel processing
• Incurs texture/image processing costs

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Visibility Culling Discussion

• How does visibility culling interact with level
  of detail?
• Fairly seamless integration generally a win
• One issue visibility of simplified model may
  differ from original model requires some care
• LODs can speed up occluder selection and rendering

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Visibility Culling Discussion

• How does visibility culling interact with texture
  and image-based representations?
• Texture/image reps generally replace far-field
  geometry
• Involves an implicit occlusion culling step
• Reduces scene depth complexity, decreasing the
  utility of visibility culling
• If near-field geometry still includes complex
  heavily-occlusive assemblies, still a win

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Visibility CullingDiscussion

• How much culling effort is appropriate?
• Cells and portals relatively cheap, with large
  potential speedups
• Hierarchical occlusion maps relatively costly,
  carefully weigh potential gains
• Multiple processors allow much more aggressive
  culling calculation
• Pipelining culling calculations, Performer-style,
  allows cull time render time
• Tradeoff one frame increased latency

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Summary

• The basic, very powerful idea
• Rapidly compute a potentially visible set
• Let hardware handle the rest
• For many scenes, visibility culling is a simple
  way to get huge speedups
• View-frustum culling always a must
• For scenes with high depth complexity, occlusion
  culling can be a big win

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Summary

• Architectural models visibility is practically a
  solved problem
• Cells and portals work well
• Cull-box portal culling simple, fast
• Line-stabbing elegant, powerful

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Summary

• Occlusion culling of general models still a
  largely open problem
• Important issues
• Dynamic scenes
• Aggregate occlusion effects

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Summary

• General occlusion culling algorithms
• Hierarchical Z-buffer
• A simple, truly elegant algorithm
• But doesnt seem amenable to hardware
• Hierarchical occlusion maps
• More complicated, difficult to code tune
• Better suited to current hardware
• Lends itself well to aggressive culling
• Fairly high overhead, only worthwhile with high
  depth complexity
• Promising trend in hardware