Soft Shadow Volumes for Ray Tracing

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Soft Shadow Volumes for Ray Tracing
Timo Aila Helsinki University of
Technology, Hybrid Graphics Ltd.
Samuli Laine Helsinki University of
Technology, Hybrid Graphics Ltd.
Ulf Assarsson ARTIS, GRAVIR /IMAG
INRIA, Chalmers University of Technology
Jaakko Lehtinen Helsinki University of
Technology, Remedy Entertainment Ltd.
Tomas Akenine-Möller Lund University
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We present

• An algorithm for computing Soft Shadows
• For planar area light sources
• Exactly the same shadows as using shadow rays
• Faster - up to 2 orders of magnitude
• Can compute very high resolution soft shadows
  with low extra cost
• E.g. allows for 1000 shadow samples per pixel
• Requires computation of silhouettes
• Trivial for polygon-based scenes

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Soft Shadows A Quick Recap

• Area lights give soft shadows

Point light
Area light
Hard shadow
Soft shadow
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Classic solution

• Multiple shadow rays

Lots of shadow rays per pixel
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Our solution - overview

• Replace the shadow rays
• With soft shadow volume computations
• Plus one reference shadow ray

One shadow ray soft shadow volume computations
Lots of shadow rays per pixel
pixel
pixel
Classic approach Our approach
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Whats a Soft Shadow Volume?
Light source
Shadow caster
Soft Shadow Volume

• Soft Shadow Volume
• A. Volume from which an edge projects onto the
  light source
• B. Region of penumbra caused by an edge

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Our solution - overview

• Two parts
• from any receiving point p, we need to find
  silhouette edges affecting the visibility
• A method for computing the visibility from
  silhouette information

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Finding potential silhouettes

• Fast method for determining silhouette edges that
  overlap light source from p

Arbitrary shadow caster
Arbitrary shadow receiving point p
HOW?
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Hemicube Construction

• Rasterize soft shadow volumes into a hemicube for
  each light source

Each cell contains list of edges
Each cell contains list of edges
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Hemicube Construction

• Rasterize soft shadow volumes into a hemicube for
  each light source

Wedge marked to all cells it even partially
overlaps ? no artifacts
Each cell contains list of edges
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Hemicube Construction
Wedge marked to all cells it even partially
overlaps ? no artifacts
Each cell contains list of edges
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Hemicube Construction

• Lots of edges does not need to be considered

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Hemicube Construction

• Lots of edges does not need to be considered

Because these edges cannot be silhouettes
projected onto the light source from any point p
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Wedge Creation Criterions

• Wedges are created for all edges that are
  silhouettes from any point on the light source
• The wedge includes all positions from which the
  edge projects onto (occludes) the light source.

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During Rendering

• Hemicube returns a conservative set of edges
• All the necessary edges but also many more
• We only want edges that are
• Silhouettes from p
• Project onto the light source.
• Tested with Point-inside-wedge
• Now we have the silhouette edges from p

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Visibility Reconstruction

• Which light samples si are visible from point p?
• Brute force cast a shadow ray for each si
• Our recipe
• 1. Find silhouette edges between p and light
  source
• 2. Project them onto light source ? reduces to
  2D
• 3. Compute relative depth complexity for every
  si
• 4. Solve visibility with a single shadow ray
• 5. Profit

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Depth Complexity

• Depth complexity of si number of surfaces
  between p and si

?
0
Light source as seen from p
Depth complexity function
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From Silhouette Edges to Relative Depth Complexity

• Projected silhouette edges define the first
  derivative of the depth complexity function
• Hence, relative depth complexity can be solved by
  integrating the silhouette edges over the light
  source
• Integration islinear ? can beperformed oneedge
  at a time

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Integration Example

• Left-to-right integration of a triangular
  silhouette

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?
0
Light source as seen from p
Depth complexity function
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Integration Sampling Points

• We dont need the value of the depth complexity
  function except at the sampling points si
• Sufficient to maintain a depth complexity counter
  for each si
• Integration find si that are insideupdate
  region and update theirdepth complexity counters

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Integration Rules

• Theres a caveat - we only have the edges that
  overlap the light source
• Loops are not necessarily closed, since parts
  outside the light source may be missing

We dont have this edge!
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Integration Rules

• Solution apply special rules to edges that cross
  the left side of the light source
• This accountsfor potentiallymissing edges

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Integration Optimization

• Place the light samples into uniformly sized
  buckets
• During integration, only process affected buckets
• Most edges span only one bucket
• We use approx. sqrt(N)buckets for N light
  samples
• We also process 4 samplesin parallel using SSE
  vectorinstructions

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From Relative Depth Complexity to Visibility

• We are not done yet, since the constant of
  integration is not known ? cannot solve
  visibility
• Solution cast a shadow rayray to one si with
  lowestrelative depth complexity
• If blocked, all si are blocked
• Otherwise, all si with lowest rel.depth
  complexity are visible

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Experimental Results

• Comparisons done using a commercial ray tracer
• Turtle by Illuminate Labs
• Enabled heuristic shadow ray casting stop if
  first 50 of rays agree
• Our method was implemented into this ray tracer
• We use the same ray caster for casting the
  reference ray

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Columns 580 triangles, adaptive AA, 960x540
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Formula 60K triangles, adaptive AA, 960x540
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Sponza 109K triangles, adaptive AA, 960x540
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Robots 1.3M triangles, adaptive AA, 960x540
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Ring 374K triangles, adaptive AA, 960x540
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Conclusions

• Fast shadow algorithm in wide range of scenes
• Easy to plug into an existing ray tracer
• Scalability considerations
• Number of light samples excellent ( sqrt M)
• Number of triangles good (silhouettes
  N(something 1))
• Output resolution not so good (linear)
• Spatial size of the light source not so good (
  linear)

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Future Work

• GPU implementation?
• Improvement of the acceleration structure
• To pump up the edge acceptance ratio (currently lt
  10)

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Thanks

• Questions

Thanks for the cash Bitboys, Hybrid Graphics,
Remedy Entertainment, Nokia, National Technology
Agency of Finland, ATI, Illuminate Labs, French
Ministry of Research, Chalmers, Hans Werthén
foundation, Carl Tryggers foundation, Ernhold
Lundströms foundation, Swedish Foundation for
Strategic Research
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Semi-transparent shadow casters

• Detect the light samples that are occluded only
  by semi-transparent occluders
• Determine the color for these light samples by
  casting shadow rays

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