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[Bugfix] Use single-bounce reflectance for IBL diffuse energy conservation - #1329
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…ation computeIBLContribution attenuated diffuse by the multi-bounce-compensated reflectance from environmentReflectance, which conflates two different things: the energy specular's own approximation needs restored via later bounces, and the share a surface diverts away from diffuse in the first place. Factor the single-bounce term out into singleBounceReflectance and use that for the diffuse term instead, keeping the compensated value for specular only. Fixes #1296
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Sync fork develop with upstream: per-eye shading, LOD hidden parts, IBL diffuse (untoldengine#1285, untoldengine#1324, untoldengine#1329)
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Summary
computeIBLContributionattenuated the diffuse term by the full multi-bounce-compensated reflectance fromenvironmentReflectance, instead of the single-bounce reflectance. The multi-bounce compensation exists to restore energy lost by specular's own approximation, not to measure how much a surface diverts away from diffuse scattering.singleBounceReflectancehelper (ShadersUtils.h/.metal), and switched the diffuse-energy-conservation term incomputeIBLParts(LightShader.metal) to use it, leaving the fully compensated value for the specular term only..metallibkernels for all platforms viabuildkernels.sh.Test plan
EnvironmentReflectionShadingTests(10 tests) pass, including the existingtestAWhiteSurfaceGivesBackAllTheLightWhateverItsFinishandtestARoughColoredMetalStaysWithinItsColor.testARoughHighF0SurfaceDiffuseGivesUpOnlyTheSingleBounceShare, targeting a roughness-1.0, F0≈0.5 material where the single-bounce and multi-bounce-compensated reflectance clearly diverge.Closes #1296