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

2. Outline Brief intro to shadow mapping Our solution Future work Q&A
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 queried
Points where the depth is computed/known

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 1536x1088: 12.6s/frame
Plenty of room for optimizations

15. Future work GPU implementation Soft shadows
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
”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

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.