Visibility in Games

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Visibility in Games

• Harald Riegler

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Visibility in Games

• What do we need it for?
• Increase of rendering speed by removing unseen
  scene data from the rendering pipeline as early
  as possible
• Reduction of data transfers to the graphics
  hardware
• Current games would not be possible without
  visibility calculations

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Visibility methods

• 2 very different categories
• Visibility from a region (Portals, PVS)
• (Quake, Unreal, Severance and co.)
• Visibility from a point (Z-Buffer, BFC,...)
• Racing games, outdoor scenes, sports games etc.
• We will focus on Point-Visibility here

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Traditional methods

• Traditionally used
• Back-Face culling
• Z-Buffering
• View frustum culling (Quad or Octtree)
• New methods are slowly breaking into games

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New scientific methods

• Image-space occlusion culling
• Hierarchical Z-Buffering
• Hierarchical Occlusion Maps
• Object-space occlusion culling
• Hierarchical View Frustum culling
• Hierarchical Back-Face culling
• More methods in the paper

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Hierarchical Z-Buffer

• Z-Buffer is arranged in an image pyramid
• Scene is partitioned in an oct-tree
• Oct-tree nodes are tested against the Z-Pyramid
  where pixels have the same size
• Visible nodes serve as input for the next frame
• Relies on HW visibility query

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HZB/Hierarchical occlusion maps
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Hierarchical occlusion maps

• Potential occluders are pre-selected
• These occluders are rendered to the occlusion
  map. The hierarchy can be built with MIP-Mapping
  HW
• Depth test after occlusion test
• Seperate depth estimation buffer

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Hierarchical View Frustum Culling

• Speeds up VFC by testing only 2 box corners of a
  bounding box first.
• Plane coherency during frame advancing
• Test against VF-octants.
• BB-Child masking

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Hierarchical Back-Face Culling

• Partitions each model into clusters
• Primitives in one cluster are
• Facing into similar directions
• Lie close to each other
• If the cluster fails the visibility test, all
  primitives in this cluster are culled

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Hierarchical Back-Face Culling
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The Real World (1)

• Scientific approaches often too complicated
• Science often uses models with hundreds of
  thousands of vertices, games dont. (LOD)
• Game developers pick ideas from different
  algorithms
• Research has impact on hardware design!

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The Real World (2)

• Parts of the HZB are used sometimes
• Runtime-LOD is used as input for a simple HZB
• VFC is almost always used.
• HOM introduce too much overhead for games, and
  the z-buffer is there anyway
• A simplified HBFC can be used

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The Real World (3)

• PSX-One doesnt even have a z-buffer
• ATIs Radeon has parts of a HZB (Called Hyper-Z)
• GForce2 only has a z-buffer
• GForce3 similar to Radeon, but supports HZB
  visibility query
• Dreamcasts Power-VR2 works pretty different
  (Infinite planes)

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A PSX Example

• Iron Soldier 3 on PSX
• VFC based on a quad-tree
• BFC
• Painters algorithm

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Conclusions (1)

• Visibility algorithms are used in many different
  applications
• Occlusion culling
• Shadow calculations
• Radiosity
• Volumetric lights
• All these fields benefit from advances in
  visibility techniques

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Conclusion (2)

• Occlusion culling will gain importance
• More OC will be implemented in hardware
• Discrepancies between science and gaming industry
  will alway exist due to different goals

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Hope you enjoyed the ride!