An Improved PhysicallyBased Soft Shadow Volume Algorithm

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An Improved Physically-BasedSoft Shadow Volume
Algorithm

• Jaakko Lehtinen,Samuli Laine, and Timo Aila
• SIGGRAPH 2005
• Date 8/09/2006
• Presented by Edward

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Outline

• Introduction
• Soft Shadow in Ray Tracing
• Soft Shadow Volume for Ray Tracing
• Hemi-cube footprint
• Reconstruction of visibility function
• An Improved Physically-Based Soft Shadow Volume
  Algorithm
• Wedge BSP Tree
• Silhouette regions
• Result
• Discussion

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Introduction

• A fast algorithm for rendering physically-based
  soft shadows in ray tracing-based renderers.
• This paper speedup the algorithm of previous
  paper.
• Samuli Laine, Timo Aila, Ulf Assarsson, Jaakko
  Lehtinen and Tomas Akenine-Möller, "Soft Shadow
  Volumes for Ray Tracing", ACM SIGGRAPH 2005.

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Soft Shadow in Ray Tracing

• Compute light source visibility by sampling an
  area light source using shadow rays

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Soft Shadow Volume for Ray Tracing

• Support shadow ray casting and compute light
  source visibility in ray tracing based rendering
  by soft shadow volume

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Soft Shadow Volume for Ray Tracing

• For a given point P, we need
• Identify the edges which will cut the light
  source from P
• Compute how many sample fall in the shadow region

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Soft Shadow Volume for Ray Tracing

• Method can separate into preprocessing pass and
  run-time pass
• preprocessing pass
• Find potential silhouette edges for area light
  source
• run-time pass
• For each given point P find actual silhouette
  edges
• Compute how many sample fall in the shadow region

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preprocessing pass-potential silhouette edges

• For each edge define four subspaces by two
  connected triangles.
• If the light source lies entirely inside the -
  or - subspace, the edge is a potential
  silhouette edge.

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preprocessing pass -Hemicube footprint

• In order to quickly find out actual silhouette
  edges in run-time
• Store wedge footprint in hemi-cube

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preprocessing pass -Hemicube footprint

• Use multi-resolution hemicube to reduce memory
  storage

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run-time pass -find actual silhouette edges

• For each given point P in scene
• Step 1 Query wedge list from hemicube
• Step 2 Test if point P lie inside wedge or not.

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run-time pass- visibility function

• Store relative changes of the depth complexity
  function.
• if and only if sample has zero depth complexity,
  it is visible.

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run-time pass- visibility function

• Integration rules for updating the depth
  complexity counters of the light samples edge by
  edge.

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An Improved Physically-Based Soft Shadow Volume
Algorithm

• In this paper they propose some different data
  structure to speedup the algorithm.
• Use BSP tree to replace hemicube structure.
• Modify the construction of wedge by silhouette
  regions.

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An Improved Physically-Based Soft Shadow Volume
Algorithm

• Problem of hemi-cube structure
• In some case, wedge will cover too much
  unnecessary point by using hemi-cube.

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wedge BSP tree

• Instead of hemi-cube structure, they use wedge
  BSP tree.

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wedge BSP tree

• In order to reduce memory storage they use lazy
  construction of the wedge BSP Tree.
• Intermix the construction of the tree with the
  wedge queries, and only subdivide the tree in
  regions that actually receive queries.

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lazy construction of the wedge BSP Tree

• preprocessing pass
• Initialize a tree root node which cover whole
  scene.

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lazy construction of the wedge BSP Tree

• run-time pass for each given point
• Traverse BSP Tree
• If node is leaf node try to split it.

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lazy construction of the wedge BSP Tree

• Stop condition of spilt leaf node
• Grid is small enough
• Number of wedge in node is small enough

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Silhouette regions

• Beside using BSP tree, they modify the according
  to the silhouette region.

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Result
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Result
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Result
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Result
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Result
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Result

• The tests were performed on a PC with a 2.8GHz
  Pentium 4 processor and 2GB of memory.

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Result
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Discussion

• This paper only speedup the algorithm of previous
  paper.
• Use BSP tree data structure to replace hemicube.
• Modify wedge by silhouette region.
• Major contribution
• Lazy construction of wedge BSP tree.

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• The End