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Soft Shadow Volumes for Ray TracingSoft 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
We presentWe 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
Soft Shadows – A Quick RecapSoft Shadows – A Quick Recap
• Area lights give soft shadows
Point light Area light
Hard shadow Soft shadow
Classic solutionClassic solution
• Multiple shadow rays
pixelpixel
Lots of shadow rays per pixel
Our solution - overviewOur solution - overview
• Replace the shadow rays– With soft shadow volume computations
– Plus one reference shadow ray
pixel
Lots of shadow rays per pixel
pixel
One shadow ray + soft shadow volume computations
Classic approach Our approach
What’s a Soft Shadow Volume?What’s a 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
Light source
Soft Shadow Volume
Shadow caster
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
Our solution - overviewOur solution - overview
Finding potential silhouettesFinding potential silhouettes
Arbitrary shadow casterArbitrary shadow caster
Arbitrary Arbitrary shadow shadow receiving receiving point point pp
Fast method for determining silhouette edges that overlap light source from p
HOW?HOW?
Bottom face of hemicube
hemicube
Each cell contains list of edges
Hemicube ConstructionHemicube Construction
Rasterize soft shadow volumes into a hemicube for each light source
Each cell contains list of edges
pp
This cell contains potential silhouette edges from pp
Bottom face of hemicube
Rasterize soft shadow volumes into a hemicube for each light source
Each cell contains list of edges
Wedge marked to all cells it even partially overlaps → no artifacts
Hemicube ConstructionHemicube Construction
Bottom face of hemicube
Each cell contains list of edges
Wedge marked to all cells it even partially overlaps → no artifacts
Hemicube ConstructionHemicube Construction
Lots of edges does not need to be considered
Hemicube ConstructionHemicube 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
pp11
pp22
Hemicube ConstructionHemicube Construction
Wedge Creation Criterions Wedge Creation Criterions
1. Wedges are created for all edges that are silhouettes from any point on the light source
2. The wedge includes all positions from which the edge projects onto (occludes) the light source.
• Hemicube returns a conservative set of edges
– All the necessary edges but also many more
• We only want edges that are
1. Silhouettes from pp
2. Project onto the light source.
• Tested with Point-inside-wedge
• Now we have the silhouette edges from p
During RenderingDuring Rendering
pp
pp
Visibility ReconstructionVisibility 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
Depth ComplexityDepth Complexity
• Depth complexity of si = number of surfaces
between p and si
1
11
1 0
0
2
Light source as seen from p Depth complexity function
0
• 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
From Silhouette Edges to Relative Depth ComplexityFrom Silhouette Edges to Relative Depth Complexity
1
11
1 00
2
Integration: ExampleIntegration: Example
• Left-to-right integration of a triangular silhouette
Light source as seen from p Depth complexity function
+1
-1
-10 1
1
0
1
Integration: Sampling PointsIntegration: Sampling Points
• We don’t 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 inside
update region and update theirdepth complexity counters
+1
• There’s 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
Integration: RulesIntegration: Rules
We don’t have this edge!
• Solution: apply special rules to edges that cross the left side of the light source
• This accountsfor potentiallymissing edges
Integration: RulesIntegration: Rules
• 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
Integration: OptimizationIntegration: Optimization
• 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 lowest
relative depth complexity
• If blocked, all si are blocked
• Otherwise, all si with lowest rel.
depth complexity are visible
From Relative Depth Complexity to VisibilityFrom Relative Depth Complexity to Visibility
Experimental ResultsExperimental 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
Columns: 580 triangles, adaptive AA, 960x540
L = 256 L = 1024
Speedup factor 103 242
Formula: 60K triangles, adaptive AA, 960x540
L = 200 L = 800
Speedup factor 30 65
Sponza: 109K triangles, adaptive AA, 960x540
L = 150 L = 600
Speedup factor 11 33
Robots: 1.3M triangles, adaptive AA, 960x540
L = 200 L = 800
Speedup factor 21 58
Ring: 374K triangles, adaptive AA, 960x540
L = 150 L = 600
Speedup factor 13 32
ConclusionsConclusions
• 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)
Future WorkFuture Work
• GPU implementation?
• Improvement of the acceleration structure
– To pump up the edge acceptance ratio (currently < 10%)
ThanksThanks
• 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
Semi-transparent shadow castersSemi-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
Visible Blocked
Blocked???
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