Transcript Stencil Routed A-Buffer

Stencil Routed
A-Buffer
Kevin Myers and Louis Bavoil
NVIDIA
Our Cool Thing
What is it?
• A-Buffer
– Simply a list of fragments per-pixel
• “The A-buffer, an antialiased hidden surface method”
[Carpenter 84]
• Related Work
– Depth Peeling [Mammen 89] [Everitt 01]
– k-Buffer [Bavoil et al. 07]
Why do I need this?
• Often want more than nearest
– Alpha blending
– Volume rendering
– Collision detection
– Refraction and caustics
– Global illumination
Why is it hard?
• GPU’s optimized to capture nearest layer
– Z buffering and early z test
– Fine for most real-time lighting models
– Wasteful if not rendering front to back
Things that don’t work
• Blending can’t just turn of z-buffering
– Most operations non-commutative
• MRT
– Can’t direct output
• Reading what you’re writing
– Hazardous
• “Multi-Layer Depth Peeling via Fragment Sort” [Liu et al. 06]
• k-Buffer [Bavoil et al. 07]
A-Buffer
• “A list of fragments per-pixel”
– Anything on the GPU that resembles this?
• MSAA
– “A list of samples per-pixel”
– Samples store coverage
MSAA in review
• Multisampled Antialiasing
– Fragments are rasterized at a higher res
• 8xMSAA == 8 x aliased resolution
– Pixel shader is run once per-pixel
– Frame buffer storage is at sample resolution
Say What?
• MSAA samples == A-Buffer pixels??
• MSAA sample patterns don’t help
• Need all MSAA samples at pixel center
Line up your Sub-samples
• Turn off multisampling
– Still render to an MSAA buffer
– Pixel shader output bloats to all sub-samples
– BOOL D3D10_RASTERIZER_DESC::MultisampleEnable
• Now writing 8 samples per pixel
– All have the same value!!
Bloating Your Pixel
• Applause?
• Meets the definition
– “List of fragments per-pixel”
• Not exactly what we want
– Each item contains same value
– Next fragment will clobber the entire list
– Need to update one entry in the list
• Once and only once
Stencil Routing
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Stencil always increments
Stencil passes when 4
Stencil Routing
• First introduced by Purcell et al 2003
– Did not work for general rasterization
• Tile aligned points
– Fat point is spread across four pixels
• Four pixels get same value
• Stencil allows one pixel to update
Stencil Routing and MSAA
• Stencil always operates at sample res
– Regardless of MultisampleEnable state
– DX10 Spec
• Use sub-samples to route
– Allows any pixel shader output to be routed
• Arbitrary primitives
Stencil Routing and MSAA
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A Stencil Test That Works
• StencilFunc
– D3D10_COMPARISON_EQUAL
• StencilRef
–2
• More on this later
• StencilPassOp and StencilFailOp
– D3D10_STENCIL_OP_DECR_SAT
Initializing Stencil
• Clear stencil buffer to pass value ( 2 )
– Initializes sample 0 to 2
• Use SampleMask to selectively update
– Stencil set to replace with refrence value
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Why start at 2?
• When all sub-samples are written
– Most stencil values will be 0
• Except the last one written
– Last sample written stencil == 1
• When overflow occurs
– All stencil values will be 0
Occlusion Query Test
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Pixel
overflowed
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Handling Overflow
• Set sample mask to last sample updated
• Draw full screen quad
– Issue an occlusion query
– Set stencil to pass if stencil == 0
• Check occlusion query
– Sample pass count == overflow count
Handling Overflow
• Occlusion query
– Good
• Very fast
• Allows for dynamic A-Buffer sizing
– Bad
• Requires some CPU intervention
– Ideally A-Buffer size is fixed
Demo
Demo Time!
Secrets of the Dragon
• Single A-Buffer
– RG32F
• R is packed color
• G is depth
– Saves on texture loads
• Post process sort
– 8 fragment per-pixel bitonic sort
• Additional fragments, insertion sort
8800 GTX Performance
Alpha Blended Stanford Dragon
8 Layers
Depth Peeling
8xABuffer
ABuffer Speedup
640x480
30.9
164
5.3
800x600
30.4
139
4.6
1024x768
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3.7
1280x960
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81.4
2.9
1600x1200
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2.1
Depth Peeling
8xABuffer
ABuffer Speedup
640x480
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76.7
4.9
800x600
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63.0
4.1
1024x768
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48.0
3.3
1280x960
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2.5
1600x1200
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23.1
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16 Layers
Limits…DOH!
• 254 layers of depth max
– 8-bit stencil ( 255 – 1 for overflow bit )
– If you do this call us cause that’s crazy
• Fragments at same depth
– Must be handled in post-process
• MSAA
Summary
• Stencil Routed A-Buffer
– Ideally suited for complex geometries
• Much faster than depth peeling
• A-buffer can be dynamically resized
– Use an occlusion query
– Best to pre-determine size
Future Work
• Render target arrays
– Each target has its own stencil buffer
– Target replaces sub-sample
• Or augments sub-sample
– #arrays * MSAA level in one “CPU pass”
• With dx10 saturates 254 layers
– Use instancing for additional “GPU passes”
Thanks for all the fish
• Claudio Silva, Steven Callahan, Joao Comba,
Aaron Lefohn, Cass Everitt, Peach Myers
The last slide…
• ?
– [email protected]
– [email protected]