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Alias-Free Shadow Maps

Alias-Free Shadow Maps. Timo Aila Samuli Laine Helsinki University of Technology. Outline. Brief intro to shadow mapping positive/negative features what is the fundamental problem? Our solution properties, performance Future work Q&A. Shadow maps [Williams78]. Widely used Simple

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Alias-Free Shadow Maps

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  1. Alias-Free Shadow Maps Timo Aila Samuli Laine Helsinki University of Technology

  2. Outline • Brief intro to shadow mapping • positive/negative features • what is the fundamental problem? • Our solution • properties, performance • Future work • Q&A

  3. Shadow maps [Williams78] • Widely used • Simple • General (anything that can be rasterized) • Aliasing artifacts

  4. Aliasing: resolution issues [Sen et al. 2003]

  5. Aliasing: incorrect self-shadowing

  6. Step 1. From light source: • Compute a shadow map(z-buffer) • Dots are sampling points • depth known only at these points

  7. Step 2. From camera: • Samples vs. pixels • Project each visible sample to light source • Determine shadow term using shadow map

  8. Shadow map sampling points  Points where the depth is computed/known Points where the depth is queried

  9. 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

  10. Rasterization • Q: How to rasterize using irregular sampling points? blocker

  11. Rasterization + depth test • Test if sampling point covered • we use edge functions [Pineda88] • Depth test Let’s make it practical! • hierarchical processing of sampling points • we use axis-aligned 2D BSP

  12. 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

  13. Performance (1/2) • Scalability: random triangles @ 1024x768: • transformation of samples: ~130ms • BSP construction: ~400ms • rasterization: 1K 10K 100K 1M 145ms 381ms 1568ms 8102ms

  14. Performance (2/2) • 2.9M semi-transparent shadow casting tris @ 1536x1088: 12.6s/frame • Plenty of room for optimizations

  15. 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?

  16. Concurrent work • Same core idea • Different implementation • Propose hardware modifications ”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.

  17. 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.

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