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Fix OpenPBR thin-walled subsurface color scaling #3066

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Merged
jstone-lucasfilm merged 5 commits into from
Sep 18, 2026
+29 −33
Merged

Fix OpenPBR thin-walled subsurface color scaling #3066

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9baaf05
Fix OpenPBR thin-walled subsurface color scaling
LouisDeconinck Sep 10, 2026
47b8e60
test: Address GCC warning in OpenPBR regression test
LouisDeconinck Sep 10, 2026
59ca080
Express thin-walled subsurface as a single mix of the two lobes
LouisDeconinck Sep 18, 2026
623faad
Merge branch 'main' into fix/openpbr-thin-walled-subsurface-color
jstone-lucasfilm Sep 18, 2026
c5aac9b
Minor adjustment of subsurface note in functional definition
jstone-lucasfilm Sep 18, 2026
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14 changes: 7 additions & 7 deletions documents/Specification/MaterialX.PBRSpec.md
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Original file line number Diff line number Diff line change
Expand Up @@ -2005,15 +2005,15 @@ surfaceshader open_pbr_surface(
// Coat facing reflectance (F0), reused by the coat-darkening and emission terms below.
float coat_F0 = ior_to_f0(coat_ior);

// Thin-walled subsurface: a translucent reflection/transmission pair scaled by the subsurface
// color, with 'subsurface_scatter_anisotropy' shifting weight from reflection toward
// transmission, blended in equal measure.
// Thin-walled subsurface: diffuse reflection and transmission lobes that share the subsurface
// color as their albedo, with 'subsurface_scatter_anisotropy' shifting weight from reflection
// toward transmission. The two weights sum to one, so the total albedo is the subsurface color.
color3 ss_color = max(subsurface_color, 0.0);
BSDF ss_reflection = oren_nayar_diffuse_bsdf(color = ss_color, roughness = base_diffuse_roughness,
normal = geometry_normal) * (subsurface_color * (1.0 - subsurface_scatter_anisotropy));
BSDF ss_transmission = translucent_bsdf(color = ss_color, normal = geometry_normal)
* (subsurface_color * (1.0 + subsurface_scatter_anisotropy));
BSDF subsurface_thin_walled = mix(ss_reflection, ss_transmission, 0.5);
normal = geometry_normal);
BSDF ss_transmission = translucent_bsdf(color = ss_color, normal = geometry_normal);
BSDF subsurface_thin_walled = mix(ss_transmission, ss_reflection,
0.5 * (1.0 + subsurface_scatter_anisotropy));

// Closed (non-thin-walled) subsurface: volumetric scattering over the scaled radius.
BSDF subsurface_volume = subsurface_bsdf(color = ss_color,
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36 changes: 10 additions & 26 deletions libraries/bxdf/open_pbr_surface.mtlx
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Expand Up @@ -142,38 +142,22 @@
<input name="roughness" type="float" interfacename="base_diffuse_roughness" />
<input name="normal" type="vector3" interfacename="geometry_normal" />
</oren_nayar_diffuse_bsdf>
<subtract name="one_minus_subsurface_scatter_anisotropy" type="float">
<input name="in1" type="float" value="1.0" />
<input name="in2" type="float" interfacename="subsurface_scatter_anisotropy" />
</subtract>
<multiply name="subsurface_thin_walled_brdf_factor" type="color3">
<input name="in1" type="color3" interfacename="subsurface_color" />
<input name="in2" type="float" nodename="one_minus_subsurface_scatter_anisotropy" />
</multiply>
<multiply name="subsurface_thin_walled_reflection" type="BSDF">
<input name="in1" type="BSDF" nodename="subsurface_thin_walled_reflection_bsdf" />
<input name="in2" type="color3" nodename="subsurface_thin_walled_brdf_factor" />
</multiply>
<translucent_bsdf name="subsurface_thin_walled_transmission_bsdf" type="BSDF">
<input name="color" type="color3" nodename="subsurface_color_nonnegative" />
<input name="normal" type="vector3" interfacename="geometry_normal" />
</translucent_bsdf>
<add name="one_plus_subsurface_scatter_anisotropy" type="float">
<input name="in1" type="float" value="1.0" />
<input name="in2" type="float" interfacename="subsurface_scatter_anisotropy" />
</add>
<multiply name="subsurface_thin_walled_btdf_factor" type="color3">
<input name="in1" type="color3" interfacename="subsurface_color" />
<input name="in2" type="float" nodename="one_plus_subsurface_scatter_anisotropy" />
</multiply>
<multiply name="subsurface_thin_walled_transmission" type="BSDF">
<input name="in1" type="BSDF" nodename="subsurface_thin_walled_transmission_bsdf" />
<input name="in2" type="color3" nodename="subsurface_thin_walled_btdf_factor" />
<multiply name="subsurface_scatter_anisotropy_half" type="float">
<input name="in1" type="float" interfacename="subsurface_scatter_anisotropy" />
<input name="in2" type="float" value="0.5" />
</multiply>
<add name="subsurface_thin_walled_mix" type="float">
<input name="in1" type="float" value="0.5" />
<input name="in2" type="float" nodename="subsurface_scatter_anisotropy_half" />
</add>
<mix name="subsurface_thin_walled" type="BSDF">
<input name="fg" type="BSDF" nodename="subsurface_thin_walled_reflection" />
<input name="bg" type="BSDF" nodename="subsurface_thin_walled_transmission" />
<input name="mix" type="float" value="0.5" />
<input name="fg" type="BSDF" nodename="subsurface_thin_walled_transmission_bsdf" />
<input name="bg" type="BSDF" nodename="subsurface_thin_walled_reflection_bsdf" />
<input name="mix" type="float" nodename="subsurface_thin_walled_mix" />
</mix>

<!-- Subsurface (non-thin-walled) -->
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12 changes: 12 additions & 0 deletions resources/Materials/TestSuite/pbrlib/surfaceshader/open_pbr_thin_walled_subsurface.mtlx
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@@ -0,0 +1,12 @@
<?xml version="1.0"?>
<materialx version="1.39" colorspace="lin_rec709">
<surfacematerial name="M_open_pbr_thin_walled_subsurface" type="material">
<input name="surfaceshader" type="surfaceshader" nodename="SR_open_pbr_thin_walled_subsurface" />
</surfacematerial>
<open_pbr_surface name="SR_open_pbr_thin_walled_subsurface" type="surfaceshader">
<input name="geometry_thin_walled" type="boolean" value="true" />
<input name="subsurface_weight" type="float" value="1.0" />
<input name="subsurface_color" type="color3" value="0.816, 0.140, 0.124" />
<input name="subsurface_scatter_anisotropy" type="float" value="0.5" />
</open_pbr_surface>
</materialx>
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