Add hair nodes to MDL backend (#2059)

This PR introduces support for hair nodes to the MDL backend. Nodes to add: - `<chiang_hair_bsdf>` - `<deon_hair_absorption_from_melanin>` - `<chiang_hair_absorption_from_color>` - `<chiang_hair_roughness>`
AcademySoftwareFoundation · Oct 13, 2024 · 0ba2ad0 · 0ba2ad0
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diff --git a/libraries/pbrlib/genmdl/pbrlib_genmdl_impl.mtlx b/libraries/pbrlib/genmdl/pbrlib_genmdl_impl.mtlx
@@ -25,6 +25,9 @@
   <!-- <sheen_bsdf> -->
   <implementation name="IM_sheen_bsdf_genmdl" nodedef="ND_sheen_bsdf" sourcecode="materialx::pbrlib_{{MDL_VERSION_SUFFIX}}::mx_sheen_bsdf(mxp_weight:{{weight}}, mxp_color:{{color}}, mxp_roughness:{{roughness}}, mxp_normal:{{normal}}, mxp_base:{{base}})" target="genmdl" />
 
+  <!-- <chiang_hair_bsdf> -->
+  <implementation name="IM_chiang_hair_bsdf_genmdl" nodedef="ND_chiang_hair_bsdf" sourcecode="materialx::pbrlib_{{MDL_VERSION_SUFFIX}}::mx_chiang_hairbsdf(mxp_tint_R:{{tint_R}}, mxp_tint_TT:{{tint_TT}}, mxp_tint_TRT:{{tint_TRT}}, mxp_ior:{{ior}}, mxp_roughness_R:{{roughness_R}}, mxp_roughness_TT:{{roughness_TT}}, mxp_roughness_TRT:{{roughness_TRT}}, mxp_cuticle_angle:{{cuticle_angle}}, mxp_absorption_coefficient:{{absorption_coefficient}}, mxp_curve_direction:{{curve_direction}})" target="genmdl" />
+
   <!-- <uniform_edf> -->
   <implementation name="IM_uniform_edf_genmdl" nodedef="ND_uniform_edf" sourcecode="materialx::pbrlib_{{MDL_VERSION_SUFFIX}}::mx_uniform_edf(mxp_color:{{color}})" target="genmdl" />
 
@@ -90,4 +93,13 @@
   <!-- <blackbody> -->
   <implementation name="IM_blackbody_genmdl" nodedef="ND_blackbody" sourcecode="materialx::pbrlib_{{MDL_VERSION_SUFFIX}}::mx_blackbody(mxp_temperature:{{temperature}})" target="genmdl" />
 
+  <!-- <deon_hair_absorption_from_melanin> -->
+  <implementation name="IM_deon_hair_absorption_from_melanin_genmdl" nodedef="ND_deon_hair_absorption_from_melanin" sourcecode="materialx::pbrlib_{{MDL_VERSION_SUFFIX}}::mx_deon_hair_absorption_from_melanin(mxp_melanin_concentration:{{melanin_concentration}}, mxp_melanin_redness:{{melanin_redness}}, mxp_eumelanin_color:{{eumelanin_color}}, mxp_pheomelanin_color:{{pheomelanin_color}})" function="mx_deon_hair_absorption_from_melanin" target="genmdl" />
+
+  <!-- <chiang_hair_absorption_from_color> -->
+  <implementation name="IM_chiang_hair_absorption_from_color_genmdl" nodedef="ND_chiang_hair_absorption_from_color" sourcecode="materialx::pbrlib_{{MDL_VERSION_SUFFIX}}::mx_chiang_hair_absorption_from_color(mxp_color:{{color}}, mxp_azimuthal_roughness:{{azimuthal_roughness}})" function="mx_chiang_hair_absorption_from_color" target="genmdl" />
+
+  <!-- <chiang_hair_roughness> -->
+  <implementation name="IM_chiang_hair_roughness_genmdl" nodedef="ND_chiang_hair_roughness" file="mx_hair_bsdf.glsl" sourcecode="materialx::pbrlib_{{MDL_VERSION_SUFFIX}}::mx_chiang_hair_roughness(mxp_longitudinal:{{longitudinal}}, mxp_azimuthal:{{azimuthal}}, mxp_scale_TT:{{scale_TT}}, mxp_scale_TRT:{{scale_TRT}})" target="genmdl" />
+
 </materialx>
diff --git a/source/MaterialXGenMdl/mdl/materialx/pbrlib_1_6.mdl b/source/MaterialXGenMdl/mdl/materialx/pbrlib_1_6.mdl
@@ -428,6 +428,35 @@ export material mx_thin_film_bsdf(
     volume: mxp_base.volume
 );
 
+export material mx_chiang_hair_bsdf(
+    // TODO: MDL's chiang_hair BSDF has no support tinting each lobes
+    color mxp_tint_R = color(1.0),
+    color mxp_tint_TT = color(1.0),
+    color mxp_tint_TRT = color(1.0),
+    float mxp_ior = 1.55,
+    float2 mxp_roughness_R = float2(0.1, 0.1),
+    float2 mxp_roughness_TT = float2(0.05, 0.05),
+    float2 mxp_roughness_TRT = float2(0.2, 0.2),
+    float mxp_cuticle_angle = 0.5,
+    float3 mxp_absorption_coefficient = float3(0.0),
+    // TODO: MDL's chiang_hair BSDF has no support user tangent vector
+    float3 mxp_curve_direction = state::texture_tangent_u(0)
+) [[
+    anno::usage( "materialx:bsdf")
+]]
+= material(
+    hair: df::chiang_hair_bsdf(
+        diffuse_reflection_weight: 0.0,
+        diffuse_reflection_tint: color(0.0),
+        roughness_R: mxp_roughness_R,
+        roughness_TT: mxp_roughness_TT,
+        roughness_TRT: mxp_roughness_TRT,
+        cuticle_angle: mxp_cuticle_angle,
+        absorption_coefficient: mxp_absorption_coefficient,
+        ior: mxp_ior
+    )
+);
+
 // EDF Nodes
 
 export material mx_uniform_edf(
@@ -980,3 +1009,64 @@ export mx_artistic_ior__result mx_artistic_ior(
     color k = math::sqrt(k2);
     return mx_artistic_ior__result(n,k);
 }
+
+export float3 mx_dion_hair_absorption_from_melanin(
+    float mxp_melanin_concentration = 0.25,
+    float mxp_melanin_redness = 0.5,
+    color mxp_eumelanin_color = color(0.657704, 0.498077, 0.254107),
+    color mxp_pheomelanin_color = color(0.829444, 0.67032, 0.349938)
+) {
+    float melanin = -math::log(math::max(1.0 - mxp_melanin_concentration, 0.0001));
+    float eumelanin = melanin * (1.0 = mxp_melanin_redness);
+    float pheomelanin = melanin * mxp_melanin_redness;
+    return math::max(
+        eumelanin * -math::log(mxp_eumelanin_color) + pheomelanin * -math::log(mxp_pheomelanin_color),
+        float3(0.0)
+    );
+}
+
+export float3 mx_chiang_hair_absorption_from_color(
+    color mxp_color = color(1.0, 1.0, 1.0),
+    float mxp_azimuthal_roughness = 0.2
+) {
+    float r2 = mxp_azimuthal_roughness * mxp_azimuthal_roughness;
+    float r4 = r2 * r2;
+    float r_fac =
+        5.969 - 
+    (0.215 * mxp_azimuthal_roughness) + 
+    (2.532 * r2) -
+    (10.73 * r2 * mxp_azimuthal_roughness) + 
+    (5.574 * r4) +
+    (0.245 * r4 * mxp_azimuthal_roughness);
+    float3 sigma = math::log(math::min(math::max(mxp_color, 0.001), float3(1.0))) / r_fac;
+    return (sigma * sigma);
+}
+
+// Supportive struct type for the three outputs of mx_chiang_hair_roughness
+export struct mx_chiang_hair_roughness__result {
+    float2 mxp_roughness_R;
+    float2 mxp_roughness_TT;
+    float2 mxp_roughness_TRT;
+};
+
+export mx_chiang_hair_roughness__result mx_chiang_hair_roughness(
+    float mxp_longitudinal = 0.1,
+    float mxp_azimuthal = 0.2,
+    float mxp_scale_TT = 0.5,
+    float mxp_scale_TRT = 2.0
+) {
+    float lr = math::clamp(mxp_longitudinal, 0.001, 1.0);
+    float ar = math::clamp(mxp_azimuthal, 0.001, 1.0);
+
+    // longitudinal variance
+    float v = 0.726 * lr + 0.812 * lr * lr + 3.7 * math::pow(lr, 20);
+    v = v * v;
+
+    float s = 0.265 * ar + 1.194 * ar * ar + 5.372 * math::pow(ar, 22);
+
+    float2 roughness_R = float2(v, s);
+    float2 roughness_TT = float2(v * mxp_scale_TT * mxp_scale_TT, s);
+    float2 roughness_TRT = float2(v * mxp_scale_TRT * mxp_scale_TRT, s);
+
+    return mx_chiang_hair_roughness__result(roughness_R, roughness_TT, roughness_TRT);
+}
diff --git a/source/MaterialXTest/MaterialXGenMdl/GenMdl.cpp b/source/MaterialXTest/MaterialXGenMdl/GenMdl.cpp
@@ -93,12 +93,8 @@ TEST_CASE("GenShader: MDL Implementation Check", "[genmdl]")
     generatorSkipNodeTypes.insert("light");
 
     mx::StringSet generatorSkipNodeDefs;
-    generatorSkipNodeDefs.insert("ND_chiang_hair_roughness");
-    generatorSkipNodeDefs.insert("ND_chiang_hair_absorption_from_color");
-    generatorSkipNodeDefs.insert("ND_deon_hair_absorption_from_melanin");
-    generatorSkipNodeDefs.insert("ND_chiang_hair_bsdf");
 
-    GenShaderUtil::checkImplementations(context, generatorSkipNodeTypes, generatorSkipNodeDefs, 35);
+    GenShaderUtil::checkImplementations(context, generatorSkipNodeTypes, generatorSkipNodeDefs, 31);
 }
 
 

diff --git a/source/MaterialXTest/MaterialXGenMdl/GenMdl.h b/source/MaterialXTest/MaterialXGenMdl/GenMdl.h
@@ -50,9 +50,6 @@ class MdlShaderGeneratorTester : public GenShaderUtil::ShaderGeneratorTester
             _skipFiles.insert("heighttonormal_in_nodegraph.mtlx");
         }
 
-        _skipFiles.insert("hair_bsdf.mtlx");
-        _skipFiles.insert("hair_surfaceshader.mtlx");
-
         ShaderGeneratorTester::addSkipFiles();
     }