Adaptive Shadow Maps

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Adaptive Shadow Maps

• R. Frenando, S. Fernandez,
• K. Bala, Donald P. Greenberg

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Introduction

• Shadow map
• A depth image generated from the light
• Fast but can introduce aliasing
• Adaptive shadow map
• Use a hierarchical grid structure for depth
• View-driven
• Progressive improvement

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Previous Work

• To remove aliasing
• Percentage-closer filtering
• Light buffer for non-interactive ray tracer
• Deep shadow map using jittered samples
• But do not deal with aliasing caused by
  insufficient shadow map resolution

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Adaptive Shadow Map

• Idea
• Only regions that contain shadow boundaries need
  to be samples densely
• Function
• Input a set of transformed eye view points
• Output true if a point is lit, false otherwise

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Adaptive Shadow Map

• Organization
• Tree structure
• Each node has a shadow map and a partitioning of
  shadow map into a fixed number of cells
• Each cell may contain another tree node

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Adaptive Shadow Map

• Creating nodes
• Use a cost-benefit metric to determine which
  cells to be assigned new nodes
• Quantifying benefit counting number of
  transformed eye pixels and whose shadow map
  resolution is lower than the eye view resolution
• ? mip-mapping capability of graphics H/W

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Adaptive Shadow Map

• Removing nodes
• A particular nodes memory need to be reclaimed
• Use LRU on all nodes that were not used in last
  frame
• If there are no such nodes, remove existing node
  only if new node has more benefit that existing
  one

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Implementation

• Mip-mapping
• To determine when resolution mismatches occur
• Graphics H/W tells interpolated mip-map level
• Combining ID and depth comparison
• First, ID test
• If it fails, do the conventional depth test

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Optimization

• ASM queries using depth culling
• Store most recently queried leaf node in cache
• SSE/SSE2
• Frustum culling for each cell in top level of
  hierarchy

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Conclusion

• Adaptive shadow map
• Adaptive subdivision to address aliasing
• View-driven
• Progressive
• Confined to a use-specifiable memory limit
• Compatible with conventional shadow map