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Fix interpretation of subsurface weight in MDL #2536

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Merged
jstone-lucasfilm merged 5 commits into from
Sep 2, 2025
Merged

Fix interpretation of subsurface weight in MDL #2536

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ef9f6d2
The subsurface weight should not be applied to the expected color but…
krohmerNV Aug 27, 2025
45cd70a
increase the epsilon for subsurface scattering
krohmerNV Sep 2, 2025
32f53aa
add all pbr nodes to the renderer tests
krohmerNV Sep 2, 2025
0dd223e
increase recursion depth
krohmerNV Sep 2, 2025
53dae8f
Restore render test paths for now
jstone-lucasfilm Sep 2, 2025
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17 changes: 11 additions & 6 deletions source/MaterialXGenMdl/mdl/materialx/pbrlib_1_6.mdl
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Original file line number Diff line number Diff line change
Expand Up @@ -344,15 +344,16 @@ export material mx_subsurface_bsdf(
color mxp_color = color(0.18),
color mxp_radius = color(1.0),
float mxp_anisotropy = 0.0,
float3 mxp_normal = state::normal() [[ anno::unused() ]]
float3 mxp_normal = state::normal()
) [[
anno::usage( "materialx:bsdf")
]]
= let {
// https://blog.selfshadow.com/publications/s2017-shading-course/imageworks/s2017_pbs_imageworks_slides_v2.pdf
// Phase function eccentricity 'g' has been omitted here since we pass that directly
// to anisotropic_vdf(directional_bias: g).
color C = math::saturate(mxp_weight) * mxp_color;
float w = math::saturate(mxp_weight);
color C = mxp_color;
color albedo = color(4.09712) +
(4.20863f * C) -
math::sqrt(
Expand All @@ -364,7 +365,7 @@ export material mx_subsurface_bsdf(
color white = color(1.0, 1.0, 1.0);
color alpha = (white - albedo_sq) / (white - mxp_anisotropy * albedo_sq);

color radius_inv = white / math::max(color(0.001), mxp_radius);
color radius_inv = white / math::max(0.00000001, mxp_radius);

color sigma_s = alpha * radius_inv;
color sigma_a = radius_inv - sigma_s;
Expand All @@ -374,12 +375,16 @@ export material mx_subsurface_bsdf(
scattering: df::anisotropic_vdf (
directional_bias: mxp_anisotropy
),
scattering_coefficient: sigma_s,
absorption_coefficient: sigma_a
scattering_coefficient: w * sigma_s,
absorption_coefficient: w * sigma_a
),

surface: material_surface(
scattering: df::diffuse_transmission_bsdf(tint: white)
scattering: df::weighted_layer(
weight: w,
layer: df::diffuse_transmission_bsdf(tint: white),
normal: mxp_normal
)
)
);

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2 changes: 1 addition & 1 deletion source/MaterialXTest/MaterialXGenMdl/GenMdl.cpp
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Expand Up @@ -315,7 +315,7 @@ void MdlShaderGeneratorTester::compileSource(const std::vector<mx::FilePath>& so
if (renderArgs.empty())
{
// Assume MDL example DXR is being used and set reasonable arguments automatically
renderCommand += " --nogui --res 512 512 --iterations 1024 --max_path_length 3 --noaux --no_firefly_clamp";
renderCommand += " --nogui --res 512 512 --iterations 1024 --max_path_length 4 --noaux --no_firefly_clamp";
renderCommand += " --background 0.073239 0.073239 0.083535";
}
else
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