1. Stencil Routed A-Buffer
Kevin Myers and Louis Bavoil
NVIDIA

2. Our Cool Thing

3. What is it? A-Buffer Related Work 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]

4. Why do I need this? Often want more than nearest Alpha blending
Volume rendering
Collision detection
Refraction and caustics
Global illumination

5. 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

6. Things that don’t work Blending can’t just turn of z-buffering MRT
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]

7. A-Buffer “A list of fragments per-pixel” MSAA
Anything on the GPU that resembles this?
MSAA
“A list of samples per-pixel”
Samples store coverage

8. 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

9. Say What? MSAA samples == A-Buffer pixels??
MSAA sample patterns don’t help
Need all MSAA samples at pixel center

10. 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!!

11. 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

12. Stencil always increments
Stencil Routing
Stencil always increments
Stencil passes when 4

13. 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

14. 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

15. Stencil Routing and MSAA

16. A Stencil Test That Works
StencilFunc
D3D10_COMPARISON_EQUAL
StencilRef 2
More on this later
StencilPassOp and StencilFailOp
D3D10_STENCIL_OP_DECR_SAT

17. 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

18. Why start at 2? When all sub-samples are written When overflow occurs
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

19. Occlusion Query Test
Pixel did not
overflow
Pixel
overflowed

20. 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

21. Handling Overflow Occlusion query Good Bad Very fast
Allows for dynamic A-Buffer sizing
Bad
Requires some CPU intervention
Ideally A-Buffer size is fixed

22. Demo
Demo Time!

23. Secrets of the Dragon Single A-Buffer Post process sort 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

24. 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
29.5
110
3.7
1280x960
28.1
81.4
2.9
1600x1200
26.2
54.9
2.1
16 Layers
15.5
76.7
4.9
15.3
63.0
4.1
14.7
48.0
3.3
14.1
34.6
2.5
13.3
23.1
1.7

25. Limits…DOH! 254 layers of depth max Fragments at same depth MSAA
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

26. Summary Stencil Routed A-Buffer A-buffer can be dynamically resized
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

27. 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”

28. Thanks for all the fish
Claudio Silva, Steven Callahan, Joao Comba, Aaron Lefohn, Cass Everitt, Peach Myers

29. The last slide… ?