Alias-Free Shadow Maps

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Alias-Free Shadow Maps

• Timo Aila
• Samuli Laine
• Helsinki University of Technology

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Outline

• Brief intro to shadow mapping
• positive/negative features
• what is the fundamental problem?
• Our solution
• properties, performance
• Future work
• QA

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Shadow maps Williams78

• Widely used
• Simple
• General (anything that can be rasterized)
• Aliasing artifacts

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Aliasing resolution issues
Sen et al. 2003
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Aliasing incorrect self-shadowing
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Step 1. From light source

• Compute a shadow map(z-buffer)
• Dots are sampling points
• depth known only at these points

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Step 2. From camera

• Samples vs. pixels
• Project each visible sample to light source
• Determine shadow term using shadow map

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Shadow map sampling points
?
Points where the depth is computed/known
Points where the depth is queried
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Idea why not do it right?

• Project screen-space samples to image plane of
  light source
• Rasterize blocker geometry using them as sampling
  points
• Depth known at correct positions ?
• same result as shadow rays

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Rasterization

• Q How to rasterize using irregular sampling
  points?

blocker
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Rasterization depth test

• Test if sampling point covered
• we use edge functions Pineda88
• Depth test
• Lets make it practical!
• hierarchical processing of sampling points
• we use axis-aligned 2D BSP

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Properties

• Resolution issues disappear
• Bias term independent of scene
• Semi-transparent shadow receivers
• simply transform multiple samples per pixel
• Semi-transparent shadow casters
• can modulate the RGB color of shadows

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Performance (1/2)

• Scalability random triangles _at_ 1024x768
• transformation of samples 130ms
• BSP construction 400ms
• rasterization
• 1K 10K 100K 1M
• 145ms 381ms 1568ms 8102ms

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Performance (2/2)

• 2.9M semi-transparent shadow casting tris _at_
  1536x1088 12.6s/frame
• Plenty of room for optimizations

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Future work

• GPU implementation
• are HW modifications needed?
• many options, probably not BSP
• Soft shadows
• Smoothies Penumbra maps should work
• soft shadow volumes with shadow maps?
• new physically-based methods?
• Applications in ray tracing?

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Concurrent work
The Irregular Z-Buffer and its Application to
Shadow Mapping, Gregory S. Johnson, William R.
Mark, and Christopher A. Burns, The University of
Texas at Austin, Department of Computer Sciences.
Technical Report TR-04-09, April, 2004.

• Same core idea
• Different implementation
• Propose hardware modifications

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Thank you!

• Questions?
• Acknowledgements
• 3DR group at Helsinki University of Tech.
• Jukka Arvo, Ville Miettinen, Tim Weyrich
• The National Technology Agency of Finland,
  Bitboys, Hybrid Graphics, Nokia, Remedy
  Entertainment.