A Hierarchical Shadow Volume Algorithm

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A Hierarchical Shadow Volume Algorithm

• Timo Aila1,2
• Tomas Akenine-Möller3

1Helsinki University of Technology 2Hybrid
Graphics 3Lund University
2
Outline

• Brief intro to shadow volumes
• fillrate problem, existing solutions
• Our solution
• idea
• implementation
• Results
• QA

3
Shadow volumes Crow77

• Shadow volumes define closed volumes of space
  that are in shadow

infinitesimallight source
shadow caster light cap
dark cap
extrudedside quads
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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
R
2D illustration
P2
P3
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Using graphics hardware

• R at 8 behind pixel (z-fail) BilodeauSongy,
  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

-

view frustum
shadow volume
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Amount of pixel processing
Adapted from Chan and Durand 2004
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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

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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 ChanDurand 2004
• use SVs only at shadow boundaries
• boundary pixels determined using shadow map
• artifacts due to limited shadow map resolution

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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

camera
2D illustration
shadow volume
visible pixels
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Outline

• Brief intro to shadow volumes
• fillrate problem, existing solutions
• Our solution
• idea
• implementation
• Results
• QA

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Reference image
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Shadow volume algorithm executed once per 8x8
pixel tile
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Green tiles may contain shadow boundary - other
tiles were correct
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Low-res (gray) per-pixel computed boundaries
(dark)
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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

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Outline

• Brief intro to shadow volumes
• fillrate problem, existing solutions
• Our solution
• idea
• implementation
• Results
• QA

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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

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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

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Implementation Stage 1
SV triangles
Low-res shadows
Boundary?
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

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Implementation Stage 2
Boundary?
SV triangles
Low-resolution rasterizer
No
Copy to2-level stencil
boundary tile?
Yes
Per-pixel rasterizer
Stencil ops
Update 2-level stencil
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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 SVs Stage 1

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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

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Outline

• Brief intro to shadow volumes
• fillrate problem, existing solutions
• Our solution
• idea
• implementation
• Results
• QA

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Results Simple scene (1280x1024)
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Results Knights (1280x1024)
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Results Powerplant (1280x1024)
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Summary

• Hierarchical rendering method for shadow volumes
• 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

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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 (2004-2005)