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Better roughness remapping for IBLs #1981

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Better roughness remapping for IBLs #1981

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282 changes: 282 additions & 0 deletions docs/math/IBL Roughness Remapping.nb
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12 changes: 8 additions & 4 deletions shaders/src/light_indirect.fs
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Expand Up @@ -82,11 +82,15 @@ vec3 diffuseIrradiance(const vec3 n) {
// IBL specular
//------------------------------------------------------------------------------

float perceptualRoughnessToLod(float perceptualRoughness) {
// See: https://s3.amazonaws.com/docs.knaldtech.com/knald/1.0.0/lys_power_drops.html
// (Pre-convolved Cube Maps vs Path Tracers)
// Below is a quadratic fit to the formula in the article above at NoV=1
return frameUniforms.iblMaxMipLevel.x * perceptualRoughness * (1.686 - 0.686 * perceptualRoughness);
}

vec3 prefilteredRadiance(const vec3 r, float perceptualRoughness) {
// lod = lod_count * sqrt(roughness), which is the mapping used by cmgen
// where roughness = perceptualRoughness^2
// using all the mip levels requires seamless cubemap sampling
float lod = frameUniforms.iblMaxMipLevel.x * perceptualRoughness;
float lod = perceptualRoughnessToLod(perceptualRoughness);
return decodeDataForIBL(textureLod(light_iblSpecular, r, lod));
}

Expand Down
18 changes: 12 additions & 6 deletions tools/cmgen/src/cmgen.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -890,6 +890,12 @@ void iblMipmapPrefilter(utils::JobSystem& js, const utils::Path& iname,
}
}

float lodToPerceptualRoughness(float lod) {
const float a = 1.686f;
const float b = -0.686f;
return saturate((std::sqrt(a * a + 4.0f * b * lod) - a) / (2.0f * b));
}

void iblRoughnessPrefilter(
utils::JobSystem& js, const utils::Path& iname, const std::vector<Cubemap>& levels,
bool prefilter, const utils::Path& dir) {
Expand Down Expand Up @@ -942,12 +948,12 @@ void iblRoughnessPrefilter(
const float lod = saturate(level / (numLevels - 1.0f));
// map the lod to a linear_roughness, here we're using ^2, but other mappings are possible.
// ==> lod = sqrt(linear_roughness)
const float linear_roughness = lod * lod;
const float perceptualRoughness = lodToPerceptualRoughness(lod);
const float roughness = perceptualRoughness * perceptualRoughness;
if (!g_quiet) {
std::cout << "Level " << level <<
std::setprecision(3)
<< ", roughness(lin) = " << linear_roughness
<< ", roughness = " << sqrt(linear_roughness)
std::cout << "Level " << level << std::setprecision(3)
<< ", roughness = " << roughness
<< ", roughness (perceptual) = " << perceptualRoughness
<< std::endl;
}
Image image;
Expand All @@ -957,7 +963,7 @@ void iblRoughnessPrefilter(
if (!g_quiet) {
updater.start();
}
CubemapIBL::roughnessFilter(js, dst, levels, linear_roughness, numSamples,
CubemapIBL::roughnessFilter(js, dst, levels, roughness, numSamples,
float3{ 1, 1, 1 }, prefilter,
[&updater, quiet = g_quiet](size_t index, float v) {
if (!quiet) {
Expand Down