1. A Hierarchical Shadow Volume Algorithm
Timo Aila1,2
Tomas Akenine-Möller3
1Helsinki University of Technology Hybrid Graphics Lund University

2. Outline Brief intro to shadow volumes Our solution Results Q&A
fillrate problem, existing solutions
Our solution
idea
implementation
Results
Q&A

3. Shadow volumes [Crow77]
Shadow volumes define closed volumes of space that are in shadow
infinitesimal light source
shadow caster = light cap
dark cap
extruded side quads

4. Is point inside shadow volume?
Pick reference point R outside shadow volume
any such point is OK
Span line from R to point to be classified
Compute sum of enter (+1) and exit (-1) events P1
shadow volume
2D illustration: R P2 P3

5. Using graphics hardware
R at ∞ behind pixel (z-fail) [Bilodeau&Songy, Carmack]
infinity always outside SVs – robust
must not clip to far plane of view frustum
sum hidden events to stencil buffer, sign from backface culling
visible samples (or pixels)
2D illustration: - +
camera R + - +
shadow volume
view frustum

6. Amount of pixel processing
Adapted from [Chan and Durand 2004]

7. Fillrate problem
50+ fps without shadows on ATI Radeon 9800XT at 1280x1024, 1 sample/pixel
1 fps when shadow volumes rasterized
2.2 billion pixels per frame

8. Existing solutions (1/2)
CC shadow volumes [Lloyd et al. 2004]
draw SVs only where receivers exist
good when lots of empty space
Hybrid shadow maps and volumes [Chan&Durand 2004]
use SVs only at shadow boundaries
boundary pixels determined using shadow map
artifacts due to limited shadow map resolution

9. Existing solutions (2/2)
Depth bounds [Nvidia 2003]
application supplies min & max depth values separately for each shadow volume
rasterize shadow volume only when visible geometry between [min,max]
optimal bounds hard to compute
min
max
camera
2D illustration:
shadow volume
visible pixels

10. Outline Brief intro to shadow volumes Our solution Results Q&A
fillrate problem, existing solutions
Our solution
idea
implementation
Results
Q&A

11. Reference image

12. Shadow volume algorithm executed once per 8x8 pixel tile

13. Green tiles may contain shadow boundary - other tiles were correct

14. Low-res (gray) + per-pixel computed boundaries (dark)

15. How to detect shadow boundaries?
Two facts about shadow volumes
always closed
SV triangles mark potential shadow boundaries
If 3D volume in scene not intersected by shadow volume triangles
fully lit or fully in shadow
single sample classifies entire volume

16. Outline Brief intro to shadow volumes Our solution Results Q&A
fillrate problem, existing solutions
Our solution
idea
implementation
Results
Q&A

17. Detecting boundary tiles
Bound tile with axis-aligned bounding box
8x8 pixel region
Zmin, Zmax
Triangle vs. AA Box intersection test
low-resolution rasterization
Zmin and Zmax tests 8
8 pixels
Zmax
Zmin

18. Fast update of non-boundary tiles
Copy low-res shadows to stencil buffer
writing 64 per-pixel values would be slow
Two-level stencil buffer saves the day
maintain [Smin, Smax] per tile
always test the higher level first
often no need to validate per-pixel values
stencil values of non-boundary tiles are constant

19. Implementation – Stage 1
SV triangles
Boundary?
Low-res shadows
Low-resolution rasterizer
Per-tile operations
Buffers built separately for each shadow volume
Classifications ready when entire SV processed
application marks begin/end of shadow volumes

20. Implementation – Stage 2
Boundary?
Low-res shadows
SV triangles
Low-resolution rasterizer
boundary
tile? No
Copy to 2-level stencil
Yes
Per-pixel rasterizer
Stencil ops
Update 2-level stencil

21. Alternative implementations
Two pass
Pass 1 = Stage 1
Pass 2 = Stage 2
How to keep pixel units busy during Stage 1?
maybe assign per-tile operations to pixel shaders?
Single pass
Separate stages using delay stream [Aila et al. 2003]
Stage 2 of current SV executes simultaneously with next SV’s Stage 1

22. Hardware resources Two-level stencil buffer Per-tile operations
Optionally
delay stream *
duplicate low-res rasterizer & Zmin/Zmax units *
cache for per shadow volume buffers
multiple buffers for pipelined operation
allocate from external memory
* If not already there for occlusion culling purposes

23. Outline Brief intro to shadow volumes Our solution Results Q&A
fillrate problem, existing solutions
Our solution
idea
implementation
Results
Q&A

24. Results – Simple scene (1280x1024)
Depth bounds
Hierarchical
Improvement
Ratio in #pixels
1.1
12.7
11.5
Ratio in bandwidth
1.03
17.6
17.2

25. Results – Knights (1280x1024) Ratio in #pixels 2.6 7.4 2.8
Depth bounds
Hierarchical
Improvement
Ratio in #pixels
2.6
7.4
2.8
Ratio in bandwidth
2.4
5.6

26. Results – Powerplant (1280x1024)
Depth bounds
Hierarchical
Improvement
Ratio in #pixels
2.4
22.9
9.5
Ratio in bandwidth
2.3
16.0
6.9

27. Summary Hierarchical rendering method for shadow volumes Future work
significant fillrate savings compared to other hardware methods
also works for soft shadow volumes
Future work
would it make sense to extend programmability to per-tile operations?
how many pipeline bubbles are created?
requires chip-level simulations

28. Thank you! Questions? Acknowledgements
Ville Miettinen, Jacob Ström, Eric Haines, Ulf Assarsson, Lauri Savioja, Jonas Svensson, Ulf Borgenstam, Karl Schultz, 3DR group at Helsinki University of Technology
The National Technology Agency of Finland, Hybrid Graphics, Bitboys, Nokia and Remedy Entertainment
ATI for granting fellowship to Timo ( )