Improvements to prefiltered environment maps (#1420)

As discussed with @jstone-lucasfilm, MaterialXView's pre-integrated IBL path (i.e. when FIS is turned off) is broken. This PR attempts to fix this issue.
AcademySoftwareFoundation · Dec 29, 2023 · 3580bb7 · 3580bb7
1 parent dddf104
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diff --git a/libraries/pbrlib/genglsl/lib/mx_environment_fis.glsl b/libraries/pbrlib/genglsl/lib/mx_environment_fis.glsl
@@ -1,15 +1,5 @@
 #include "mx_microfacet_specular.glsl"
 
-// https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch20.html
-// Section 20.4 Equation 13
-float mx_latlong_compute_lod(vec3 dir, float pdf, float maxMipLevel, int envSamples)
-{
-    const float MIP_LEVEL_OFFSET = 1.5;
-    float effectiveMaxMipLevel = maxMipLevel - MIP_LEVEL_OFFSET;
-    float distortion = sqrt(1.0 - mx_square(dir.y));
-    return max(effectiveMaxMipLevel - 0.5 * log2(float(envSamples) * pdf * distortion), 0.0);
-}
-
 vec3 mx_environment_radiance(vec3 N, vec3 V, vec3 X, vec2 alpha, int distribution, FresnelData fd)
 {
     // Generate tangent frame.

diff --git a/libraries/pbrlib/genglsl/lib/mx_environment_prefilter.glsl b/libraries/pbrlib/genglsl/lib/mx_environment_prefilter.glsl
@@ -1,12 +1,5 @@
 #include "mx_microfacet_specular.glsl"
 
-float mx_latlong_compute_lod(float alpha)
-{
-    // Select a mip level based on input alpha.
-    float lodBias = alpha < 0.25 ? sqrt(alpha) : 0.5*alpha + 0.375;
-    return lodBias * float($envRadianceMips);
-}
-
 vec3 mx_environment_radiance(vec3 N, vec3 V, vec3 X, vec2 alpha, int distribution, FresnelData fd)
 {
     N = mx_forward_facing_normal(N, V);
@@ -19,7 +12,7 @@ vec3 mx_environment_radiance(vec3 N, vec3 V, vec3 X, vec2 alpha, int distributio
     float G = mx_ggx_smith_G2(NdotV, NdotV, avgAlpha);
     vec3 FG = fd.refraction ? vec3(1.0) - (F * G) : F * G;
 
-    vec3 Li = mx_latlong_map_lookup(L, $envMatrix, mx_latlong_compute_lod(avgAlpha), $envRadiance);
+    vec3 Li = mx_latlong_map_lookup(L, $envMatrix, mx_latlong_alpha_to_lod(avgAlpha), $envRadiance);
     return Li * FG;
 }
 

diff --git a/libraries/pbrlib/genglsl/lib/mx_microfacet_specular.glsl b/libraries/pbrlib/genglsl/lib/mx_microfacet_specular.glsl
@@ -593,3 +593,27 @@ vec3 mx_latlong_map_lookup(vec3 dir, mat4 transform, float lod, sampler2D envSam
     vec2 uv = mx_latlong_projection(envDir);
     return textureLod(envSampler, uv, lod).rgb;
 }
+
+// https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch20.html
+// Section 20.4 Equation 13
+float mx_latlong_compute_lod(vec3 dir, float pdf, float maxMipLevel, int envSamples)
+{
+    const float MIP_LEVEL_OFFSET = 1.5;
+    float effectiveMaxMipLevel = maxMipLevel - MIP_LEVEL_OFFSET;
+    float distortion = sqrt(1.0 - mx_square(dir.y));
+    return max(effectiveMaxMipLevel - 0.5 * log2(float(envSamples) * pdf * distortion), 0.0);
+}
+
+float mx_latlong_alpha_to_lod(float alpha)
+{
+    // Return the mip level associated with the given alpha in a prefiltered environment.
+    float lodBias = (alpha < 0.25) ? sqrt(alpha) : 0.5 * alpha + 0.375;
+    return lodBias * float($envRadianceMips - 1);
+}
+
+float mx_latlong_lod_to_alpha(float lod)
+{
+    // Return the alpha associated with the given mip level in a prefiltered environment.
+    float lodBias = lod / float($envRadianceMips - 1);
+    return (lodBias < 0.5) ? mx_square(lodBias) : 2.0 * (lodBias - 0.375);
+}
diff --git a/libraries/pbrlib/genglsl/lib/mx_prefilter_environment.glsl b/libraries/pbrlib/genglsl/lib/mx_prefilter_environment.glsl
@@ -0,0 +1,76 @@
+#include "mx_microfacet_specular.glsl"
+
+// Construct an orthonormal basis from a unit vector.
+// https://graphics.pixar.com/library/OrthonormalB/paper.pdf
+mat3 mx_orthonormal_basis(vec3 N)
+{
+    float sign = (N.z < 0.0) ? -1.0 : 1.0;
+    float a = -1.0 / (sign + N.z);
+    float b = N.x * N.y * a;
+    vec3 X = vec3(1.0 + sign * N.x * N.x * a, sign * b, -sign * N.x);
+    vec3 Y = vec3(b, sign + N.y * N.y * a, -N.y);
+    return mat3(X, Y, N);
+}
+
+// The inverse of mx_latlong_projection.
+vec3 mx_latlong_map_projection_inverse(vec2 uv)
+{
+    float latitude = (uv.y - 0.5) * M_PI;
+    float longitude = (uv.x - 0.5) * M_PI * 2.0;
+
+    float x = -cos(latitude) * sin(longitude);
+    float y = -sin(latitude);
+    float z = cos(latitude) * cos(longitude);
+
+    return vec3(x, y, z);
+}
+
+vec3 mx_prefilter_environment()
+{
+    vec2 uv = gl_FragCoord.xy * pow(2.0, $envPrefilterMip) / vec2(2048.0, 1024.0);
+    float alpha = mx_latlong_lod_to_alpha(float($envPrefilterMip));
+    if ($envPrefilterMip == 0)
+    {
+        return textureLod($envRadiance, uv, 0).rgb;
+    }
+
+    // Compute world normal and transform.
+    vec3 worldN = mx_latlong_map_projection_inverse(uv);
+    mat3 tangentToWorld = mx_orthonormal_basis(worldN);
+
+    // Local normal and view vectors are constant and aligned.
+    vec3 V = vec3(0.0, 0.0, 1.0);
+    float NdotV = 1.0;
+    float G1V = mx_ggx_smith_G1(NdotV, alpha);
+
+    // Integrate the LD term for the given environment and alpha.
+    vec3 radiance = vec3(0.0, 0.0, 0.0);
+    float weight = 0.0;
+    int envRadianceSamples = 1024;
+    for (int i = 0; i < envRadianceSamples; i++)
+    {
+        vec2 Xi = mx_spherical_fibonacci(i, envRadianceSamples);
+
+        // Compute the half vector and incoming light direction.
+        vec3 H = mx_ggx_importance_sample_VNDF(Xi, V, vec2(alpha));
+        vec3 L = -V + 2.0 * H.z * H;
+
+        // Compute dot products for this sample.
+        float NdotL = clamp(L.z, M_FLOAT_EPS, 1.0);
+
+        // Compute the geometric term.
+        float G = mx_ggx_smith_G2(NdotL, NdotV, alpha);
+
+        // Sample the environment light from the given direction.
+        vec3 Lw = tangentToWorld * L;
+        float pdf = mx_ggx_NDF(H, vec2(alpha)) * G1V / (4.0 * NdotV);
+        float lod = mx_latlong_compute_lod(Lw, pdf, float($envRadianceMips - 1), envRadianceSamples);
+        vec3 sampleColor = mx_latlong_map_lookup(Lw, $envMatrix, lod, $envRadiance);
+
+        // Add the radiance contribution of this sample.
+        radiance += G * sampleColor;
+        weight += G;
+    }
+
+    return radiance / weight;
+}
diff --git a/source/JsMaterialX/JsMaterialXGenShader/JsGenOptions.cpp b/source/JsMaterialX/JsMaterialXGenShader/JsGenOptions.cpp
@@ -42,5 +42,6 @@ EMSCRIPTEN_BINDINGS(GenOptions)
         .property("hwMaxActiveLightSources", &mx::GenOptions::hwMaxActiveLightSources)
         .property("hwNormalizeUdimTexCoords", &mx::GenOptions::hwNormalizeUdimTexCoords)
         .property("hwWriteAlbedoTable", &mx::GenOptions::hwWriteAlbedoTable)
+        .property("hwWriteEnvPrefilter", &mx::GenOptions::hwWriteEnvPrefilter)
         ;
 }
diff --git a/source/MaterialXGenGlsl/GlslShaderGenerator.cpp b/source/MaterialXGenGlsl/GlslShaderGenerator.cpp
@@ -553,7 +553,7 @@ void GlslShaderGenerator::emitPixelStage(const ShaderGraph& graph, GenContext& c
     bool lighting = requiresLighting(graph);
 
     // Define directional albedo approach
-    if (lighting || context.getOptions().hwWriteAlbedoTable)
+    if (lighting || context.getOptions().hwWriteAlbedoTable || context.getOptions().hwWriteEnvPrefilter)
     {
         emitLine("#define DIRECTIONAL_ALBEDO_METHOD " + std::to_string(int(context.getOptions().hwDirectionalAlbedoMethod)), stage, false);
         emitLineBreak(stage);
@@ -591,6 +591,13 @@ void GlslShaderGenerator::emitPixelStage(const ShaderGraph& graph, GenContext& c
         emitLineBreak(stage);
     }
 
+    // Emit environment prefiltering code
+    if (context.getOptions().hwWriteEnvPrefilter)
+    {
+        emitLibraryInclude("pbrlib/genglsl/lib/mx_prefilter_environment.glsl", context, stage);
+        emitLineBreak(stage);
+    }
+
     // Set the include file to use for uv transformations,
     // depending on the vertical flip flag.
     if (context.getOptions().fileTextureVerticalFlip)
@@ -636,6 +643,10 @@ void GlslShaderGenerator::emitPixelStage(const ShaderGraph& graph, GenContext& c
     {
         emitLine(outputSocket->getVariable() + " = vec4(mx_generate_dir_albedo_table(), 1.0)", stage);
     }
+    else if (context.getOptions().hwWriteEnvPrefilter)
+    {
+        emitLine(outputSocket->getVariable() + " = vec4(mx_prefilter_environment(), 1.0)", stage);
+    }
     else
     {
         // Add all function calls.

diff --git a/source/MaterialXGenMsl/MslShaderGenerator.cpp b/source/MaterialXGenMsl/MslShaderGenerator.cpp
@@ -1059,6 +1059,13 @@ void MslShaderGenerator::emitPixelStage(const ShaderGraph& graph, GenContext& co
             emitLineBreak(stage);
         }
 
+        // Emit environment prefiltering code
+        if (context.getOptions().hwWriteEnvPrefilter)
+        {
+            emitLibraryInclude("pbrlib/genglsl/lib/mx_prefilter_environment.glsl", context, stage);
+            emitLineBreak(stage);
+        }
+
         // Set the include file to use for uv transformations,
         // depending on the vertical flip flag.
         if (context.getOptions().fileTextureVerticalFlip)
@@ -1104,6 +1111,10 @@ void MslShaderGenerator::emitPixelStage(const ShaderGraph& graph, GenContext& co
         {
             emitLine(outputSocket->getVariable() + " = float4(mx_generate_dir_albedo_table(), 1.0)", stage);
         }
+        else if (context.getOptions().hwWriteEnvPrefilter)
+        {
+            emitLine(outputSocket->getVariable() + " = float4(mx_prefilter_environment(), 1.0)", stage);
+        }
         else
         {
             // Add all function calls.

diff --git a/source/MaterialXGenShader/GenOptions.h b/source/MaterialXGenShader/GenOptions.h
@@ -90,6 +90,7 @@ class MX_GENSHADER_API GenOptions
         hwMaxActiveLightSources(3),
         hwNormalizeUdimTexCoords(false),
         hwWriteAlbedoTable(false),
+        hwWriteEnvPrefilter(false),
         hwImplicitBitangents(true),
         emitColorTransforms(true)
     {
@@ -174,6 +175,10 @@ class MX_GENSHADER_API GenOptions
     /// Defaults to false.
     bool hwWriteAlbedoTable;
 
+    /// Enables the generation of a prefiltered environment map.
+    /// Defaults to false.
+    bool hwWriteEnvPrefilter;
+
     /// Calculate fallback bitangents from existing normals and tangents
     /// inside the bitangent node.
     bool hwImplicitBitangents;

diff --git a/source/MaterialXGenShader/HwShaderGenerator.cpp b/source/MaterialXGenShader/HwShaderGenerator.cpp
@@ -59,6 +59,7 @@ const string T_ENV_RADIANCE                   = "$envRadiance";
 const string T_ENV_RADIANCE_MIPS              = "$envRadianceMips";
 const string T_ENV_RADIANCE_SAMPLES           = "$envRadianceSamples";
 const string T_ENV_IRRADIANCE                 = "$envIrradiance";
+const string T_ENV_PREFILTER_MIP              = "$envPrefilterMip";
 const string T_REFRACTION_TWO_SIDED           = "$refractionTwoSided";
 const string T_ALBEDO_TABLE                   = "$albedoTable";
 const string T_ALBEDO_TABLE_SIZE              = "$albedoTableSize";
@@ -113,6 +114,7 @@ const string ENV_RADIANCE                     = "u_envRadiance";
 const string ENV_RADIANCE_MIPS                = "u_envRadianceMips";
 const string ENV_RADIANCE_SAMPLES             = "u_envRadianceSamples";
 const string ENV_IRRADIANCE                   = "u_envIrradiance";
+const string ENV_PREFILTER_MIP                = "u_envPrefilterMip";
 const string REFRACTION_TWO_SIDED             = "u_refractionTwoSided";
 const string ALBEDO_TABLE                     = "u_albedoTable";
 const string ALBEDO_TABLE_SIZE                = "u_albedoTableSize";
@@ -222,6 +224,7 @@ HwShaderGenerator::HwShaderGenerator(SyntaxPtr syntax) :
     _tokenSubstitutions[HW::T_AMB_OCC_GAIN] = HW::AMB_OCC_GAIN;
     _tokenSubstitutions[HW::T_VERTEX_DATA_INSTANCE] = HW::VERTEX_DATA_INSTANCE;
     _tokenSubstitutions[HW::T_LIGHT_DATA_INSTANCE] = HW::LIGHT_DATA_INSTANCE;
+    _tokenSubstitutions[HW::T_ENV_PREFILTER_MIP] = HW::ENV_PREFILTER_MIP;
 
     // Setup closure contexts for defining closure functions
     //
@@ -359,6 +362,16 @@ ShaderPtr HwShaderGenerator::createShader(const string& name, ElementPtr element
         psPrivateUniforms->add(Type::INTEGER, HW::T_ALBEDO_TABLE_SIZE, Value::createValue<int>(64));
     }
 
+    // Add uniforms for environment prefiltering.
+    if (context.getOptions().hwWriteEnvPrefilter)
+    {
+        psPrivateUniforms->add(Type::FILENAME, HW::T_ENV_RADIANCE);
+        psPrivateUniforms->add(Type::INTEGER, HW::T_ENV_PREFILTER_MIP, Value::createValue<int>(1));
+        const Matrix44 yRotationPI = Matrix44::createScale(Vector3(-1, 1, -1));
+        psPrivateUniforms->add(Type::MATRIX44, HW::T_ENV_MATRIX, Value::createValue(yRotationPI));
+        psPrivateUniforms->add(Type::INTEGER, HW::T_ENV_RADIANCE_MIPS, Value::createValue<int>(1));
+    }
+
     // Create uniforms for the published graph interface
     for (ShaderGraphInputSocket* inputSocket : graph->getInputSockets())
     {

diff --git a/source/MaterialXGenShader/HwShaderGenerator.h b/source/MaterialXGenShader/HwShaderGenerator.h
@@ -126,6 +126,7 @@ extern MX_GENSHADER_API const string T_ENV_RADIANCE;
 extern MX_GENSHADER_API const string T_ENV_RADIANCE_MIPS;
 extern MX_GENSHADER_API const string T_ENV_RADIANCE_SAMPLES;
 extern MX_GENSHADER_API const string T_ENV_IRRADIANCE;
+extern MX_GENSHADER_API const string T_ENV_PREFILTER_MIP;
 extern MX_GENSHADER_API const string T_REFRACTION_TWO_SIDED;
 extern MX_GENSHADER_API const string T_ALBEDO_TABLE;
 extern MX_GENSHADER_API const string T_ALBEDO_TABLE_SIZE;
@@ -182,6 +183,7 @@ extern MX_GENSHADER_API const string ENV_RADIANCE;
 extern MX_GENSHADER_API const string ENV_RADIANCE_MIPS;
 extern MX_GENSHADER_API const string ENV_RADIANCE_SAMPLES;
 extern MX_GENSHADER_API const string ENV_IRRADIANCE;
+extern MX_GENSHADER_API const string ENV_PREFILTER_MIP;
 extern MX_GENSHADER_API const string REFRACTION_TWO_SIDED;
 extern MX_GENSHADER_API const string ALBEDO_TABLE;
 extern MX_GENSHADER_API const string ALBEDO_TABLE_SIZE;

diff --git a/source/MaterialXRender/ImageHandler.cpp b/source/MaterialXRender/ImageHandler.cpp
@@ -165,7 +165,7 @@ void ImageHandler::unbindImages()
     }
 }
 
-bool ImageHandler::createRenderResources(ImagePtr, bool)
+bool ImageHandler::createRenderResources(ImagePtr, bool, bool)
 {
     return false;
 }

diff --git a/source/MaterialXRender/ImageHandler.h b/source/MaterialXRender/ImageHandler.h
@@ -230,7 +230,7 @@ class MX_RENDER_API ImageHandler
     }
 
     /// Create rendering resources for the given image.
-    virtual bool createRenderResources(ImagePtr image, bool generateMipMaps);
+    virtual bool createRenderResources(ImagePtr image, bool generateMipMaps, bool useAsRenderTarget = false);
 
     /// Release rendering resources for the given image, or for all cached images
     /// if no image pointer is specified.

diff --git a/source/MaterialXRender/LightHandler.h b/source/MaterialXRender/LightHandler.h
@@ -37,6 +37,7 @@ class MX_RENDER_API LightHandler
         _lightTransform(Matrix44::IDENTITY),
         _directLighting(true),
         _indirectLighting(true),
+        _usePrefilteredMap(false),
         _envSampleCount(DEFAULT_ENV_SAMPLE_COUNT),
         _refractionTwoSided(false)
     {
@@ -101,6 +102,30 @@ class MX_RENDER_API LightHandler
         return _envRadianceMap;
     }
 
+    /// Set the environment radiance map for the prefiltered environment lighting model.
+    void setEnvPrefilteredMap(ImagePtr map)
+    {
+        _envPrefilteredMap = map;
+    }
+
+    /// Return the environment radiance map for the prefiltered environment lighting model.
+    ImagePtr getEnvPrefilteredMap() const
+    {
+        return _envPrefilteredMap;
+    }
+
+    /// Set whether to use the prefiltered environment lighting model.
+    void setUsePrefilteredMap(bool val)
+    {
+        _usePrefilteredMap = val;
+    }
+
+    /// Return whether to use the prefiltered environment lighting model.
+    bool getUsePrefilteredMap()
+    {
+        return _usePrefilteredMap;
+    }
+
     /// Set the environment irradiance map
     void setEnvIrradianceMap(ImagePtr map)
     {
@@ -216,8 +241,10 @@ class MX_RENDER_API LightHandler
     Matrix44 _lightTransform;
     bool _directLighting;
     bool _indirectLighting;
+    bool _usePrefilteredMap;
 
     ImagePtr _envRadianceMap;
+    ImagePtr _envPrefilteredMap;
     ImagePtr _envIrradianceMap;
     int _envSampleCount;
 

diff --git a/source/MaterialXRender/Util.cpp b/source/MaterialXRender/Util.cpp
@@ -76,6 +76,26 @@ ShaderPtr createAlbedoTableShader(GenContext& context,
     return shader;
 }
 
+ShaderPtr createEnvPrefilterShader(GenContext& context,
+                                        DocumentPtr stdLib,
+                                        const string& shaderName)
+{
+    // Construct a dummy nodegraph.
+    DocumentPtr doc = createDocument();
+    doc->importLibrary(stdLib);
+    NodeGraphPtr nodeGraph = doc->addNodeGraph();
+    NodePtr constant = nodeGraph->addNode("constant");
+    OutputPtr output = nodeGraph->addOutput();
+    output->setConnectedNode(constant);
+
+    // Generate the shader
+    GenContext tableContext = context;
+    tableContext.getOptions().hwWriteEnvPrefilter = true;
+    ShaderPtr shader = createShader(shaderName, tableContext, output);
+
+    return shader;
+}
+
 ShaderPtr createBlurShader(GenContext& context,
                            DocumentPtr stdLib,
                            const string& shaderName,

diff --git a/source/MaterialXRender/Util.h b/source/MaterialXRender/Util.h
@@ -47,6 +47,11 @@ MX_RENDER_API ShaderPtr createAlbedoTableShader(GenContext& context,
                                                 DocumentPtr stdLib,
                                                 const string& shaderName);
 
+/// Create a shader that generates a prefiltered environment map.
+MX_RENDER_API ShaderPtr createEnvPrefilterShader(GenContext& context,
+                                                 DocumentPtr stdLib,
+                                                 const string& shaderName);
+
 /// Create a blur shader, using the given standard libraries for code generation.
 MX_RENDER_API ShaderPtr createBlurShader(GenContext& context,
                                          DocumentPtr stdLib,