Screen Space Ambient Occlusion (SSAO) MVP

[JMS55]

Objective

• Add Screen space ambient occlusion (SSAO). SSAO approximates small-scale, local occlusion of indirect diffuse light between objects. SSAO does not apply to direct lighting, such as point or directional lights.
• This darkens creases, e.g. on staircases, and gives nice contact shadows where objects meet, giving entities a more "grounded" feel.
• Closes Screen Space Ambient Occlusion #3632.

Solution

• Implement the GTAO algorithm.
  • https://www.activision.com/cdn/research/Practical_Real_Time_Strategies_for_Accurate_Indirect_Occlusion_NEW%20VERSION_COLOR.pdf
  • https://blog.selfshadow.com/publications/s2016-shading-course/activision/s2016_pbs_activision_occlusion.pdf
• Source code heavily based on Intel's XeGTAO.
• Add an SSAO bevy example.

Algorithm Overview

• Run a depth and normal prepass
• Create downscaled mips of the depth texture (preprocess_depths pass)
• GTAO pass - for each pixel, take several random samples from the depth+normal buffers, reconstruct world position, raytrace in screen space to estimate occlusion. Rather then doing completely random samples on a hemisphere, you choose random slices of the hemisphere, and then can analytically compute the full occlusion of that slice. Also compute edges based on depth differences here.
• Spatial denoise pass - bilateral blur, using edge detection to not blur over edges. This is the final SSAO result.
• Main pass - if SSAO exists, sample the SSAO texture, and set occlusion to be the minimum of ssao/material occlusion. This then feeds into the rest of the PBR shader as normal.

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Future Improvements

• Maybe remove the low quality preset for now (too noisy)
• WebGPU fallback (see below)
• Faster depth->world position (see reverted code)
• Bent normals
• Try interleaved gradient noise or spatiotemporal blue noise
• Replace the spatial denoiser with a combined spatial+temporal denoiser
• Render at half resolution and use a bilateral upsample
• Better multibounce approximation (https://drive.google.com/file/d/1SyagcEVplIm2KkRD3WQYSO9O0Iyi1hfy/view)

Far-Future Performance Improvements

• F16 math (missing naga-wgsl support [meta] f16 support gfx-rs/wgpu#4384)
• Faster coordinate space conversion for normals
• Faster depth mipchain creation (https://github.com/GPUOpen-Effects/FidelityFX-SPD) (wgpu/naga does not currently support subgroup ops)
• Deinterleaved SSAO for better cache efficiency (https://developer.nvidia.com/sites/default/files/akamai/gameworks/samples/DeinterleavedTexturing.pdf)

Other Interesting Papers

• Visibility bitmask (https://link.springer.com/article/10.1007/s00371-022-02703-y, https://cdrinmatane.github.io/posts/cgspotlight-slides/)
• Screen space diffuse lighting (https://github.com/Patapom/GodComplex/blob/master/Tests/TestHBIL/2018%20Mayaux%20-%20Horizon-Based%20Indirect%20Lighting%20(HBIL).pdf)

Platform Support

• SSAO currently does not work on DirectX12 due to issues with wgpu and naga:
  • DX12 increase max_storage_buffers_per_shader_stage and max_storage_textures_per_shader_stage gfx-rs/wgpu#3798
  • Implement Pack/Unpack for HLSL gfx-rs/naga#2353
• SSAO currently does not work on WebGPU because r16float is not a valid storage texture format https://gpuweb.github.io/gpuweb/wgsl/#storage-texel-formats. We can fix this with a fallback to r32float.

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Changelog

• Added ScreenSpaceAmbientOcclusionSettings, ScreenSpaceAmbientOcclusionQualityLevel, and ScreenSpaceAmbientOcclusionBundle

[cart]

Fantastic work! I did a rough "port pass" over the shader code and things seem to line up well.

[cart]

Great work!

[cart]

I cancelled the merge because I just noticed that the ScreenSpaceAmbientOcclusionPlugin is added by default, which means we're spamming the console with this error by default:

ERROR bevy_pbr::ssao: SSAO is being used which requires Msaa::Off, but Msaa is currently set to Res(Sample4)

We'll need to resolve that first.