Ray Tracing Depth Maps Using Precomputed Edge Tables

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Ray Tracing Depth Maps Using Precomputed Edge
Tables

• Kevin Egan
• Rhythm Hues Studios

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Overview

• Shadowing
• Ray tracing depth maps
• Our new technique
• Analysis and future work

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Shadowing

• Shadows provide realism and a sense of depth
• We will focus on opaque objects casting shadows
  from an area light
• Ray traced shadows
• Depth maps

area light
occluder
umbra
penumbra
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Area Lights and Depth Maps

• Create and sample many depth maps
• Percentage closer filtering with an expanded
  filter region
• Incorrect self-shadowing
• Ray tracing depth maps

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Ray Tracing Depth Maps

• Agrawala et al. introduced techniques for ray
  tracing through depth maps (SIGGRAPH 2000)
• Correctly cast rays from shading point to area
  light
• Hierarchical depth map
• Caching shadow rays
• Trace one ray at a time

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Side View
depth map projection point
area light
shadow ray
pixel frusta
shading point
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Side View
depth map projection point
area light
shadow ray
pixel frusta
shading point
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Side View
depth map projection point
area light
shadow ray
pixel frusta
shading point
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Overhead view
shadow ray
pixel frusta
shading point
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Overhead view
shadow ray
pixel frusta
shading point
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Overhead view
shadow ray
pixel frusta
shading point
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Ray Tracing Depth Maps

• In Agrawala implementation tracing many rays
  leads to repeated depth map lookups

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Overhead view
shadow rays
pixel frusta
shading point
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Our Work

• Same basic idea as Agrawala et al.
• Correctly cast rays from shading point to area
  light
• New datastructure for tracing many shadow ray
  segments in parallel

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Precomputed Edge Tables

• Pick a shading position to precompute
• For each pixel frustum edge compute intersection
  with all shadow rays
• For each intersection store bitmask recording
  which rays intersect below intersection point
• We call this set of bitmasks an edge table

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Side View
depth map projection point
area light
shadow ray
pixel frusta
occluder
shading position
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Side View
depth map projection point
area light
shadow ray
bitmasks
pixel frusta
shading position
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Precomputed Edge Tables

• We can efficiently find all rays occluded by a
  single depth map pixel

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Overhead view
shadow rays
pixel frusta
shading point
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Precomputed Edge Tables

• For each pixel in filter region
• Lookup depth from depth map
• Find nearest bitmasks for all edges
• XOR bitmasks for incoming edges with outgoing
• edges
• Mark all occluded rays
• Record percentage of occluded rays

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Precomputed Edge Tables

• Edge table masks store all relevant edge tables
• Efficient and accurate computation for one
  shading position
• Assume the light source is planar and
  perpendicular to the axis of projection
• One edge table mask can be used for many pixels

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Precomputed Edge Tables

• Generate masks for some number of positions

precomputed mask positions
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Precomputed Edge Tables

• Masks generated for one pixel can be reused for
  all pixels

replicated mask positions
precomputed mask positions
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Precomputed Edge Tables

• For new shading point move the nearest masks to
  the new shading position and linearly blend the
  results
• Moving a mask from its precomputed position
  effectively shrinks or shifts the light

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Precomputed Edge Tables
new shading position
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Results
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Results
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Drawbacks

• Rendering time and memory consumption are
  dependent on
• Filter size (penumbra width)
• Depth map resolution
• Density of edge table masks
• Undersampling filter region does not give good
  results

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Benefits

• Improvement to Agrawala implementation
• Especially when shadow ray caching is ineffective
• Precomputation for geometry and shadow rays
• More robust than percentage closer filtering
• Faster than ray traced shadows

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Future Work

• Mixing ray tracing and precomputed depth maps
• Accuracy along silhouette edges
• Efficiency for other areas
• Multi-resolution depth map
• GPU implementation

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Thanks!

• Ivan Neulander
• Rhythm Hues Studios