Stencil Routed A-Buffer Kevin Myers and Louis Bavoil NVIDIA

Our Cool Thing

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]

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 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]

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

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 always increments Stencil Routing 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

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

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

Occlusion Query Test Pixel did not overflow Pixel overflowed

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 Bad 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 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

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

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

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

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… ? kmyers@nvidia.com lbavoil@nvidia.com