Remove explicit colorspace transformations

Not needed anymore since USD v23.08

src/guc/main.c

@@ -29,7 +29,6 @@ void print_usage()
   fprintf(stderr, "Options:\n");
   fprintf(stderr, "--emit-mtlx                              Emit MaterialX materials in addition to UsdPreviewSurfaces\n");
   fprintf(stderr, "--mtlx-as-usdshade                       Convert and inline MaterialX materials with UsdMtlx\n");
-  fprintf(stderr, "--explicit-colorspace-transforms         Explicitly transform colorspaces using MaterialX nodes\n");
   fprintf(stderr, "--hdstorm-compat                         Apply compatibility tweaks for the USD hdStorm renderer\n");
   fprintf(stderr, "--default-material-variant <number>      Index of the material variant that is selected by default\n");
 }
@@ -48,7 +47,6 @@ int main(int argc, const char* argv[])
   struct guc_options options;
   options.emit_mtlx = false;
   options.mtlx_as_usdshade = false;
-  options.explicit_colorspace_transforms = false;
   options.hdstorm_compat = false;
   options.default_material_variant = 0;
 
@@ -70,11 +68,6 @@ int main(int argc, const char* argv[])
         options.mtlx_as_usdshade = true;
         continue;
       }
-      else if (!strcmp(arg, "explicit-colorspace-transforms"))
-      {
-        options.explicit_colorspace_transforms = true;
-        continue;
-      }
       else if (!strcmp(arg, "hdstorm-compat"))
       {
         options.hdstorm_compat = true;

src/libguc/include/guc.h

@@ -37,19 +37,7 @@ struct guc_options
   // versions.
   bool mtlx_as_usdshade;
 
-  // MaterialX's 'colorspace' functionality may not be fully supported by an
-  // application. We work around this by implementing colorspace transformations using
-  // native MaterialX math nodes. MaterialX image nodes are assumed to return raw,
-  // untransformed values, since the default document colorspace is 'linear'.
-  bool explicit_colorspace_transforms;
-
-  // HdMtlx and therefore Storm do not seem to properly support MaterialX colorspaces.
-  // https://github.com/PixarAnimationStudios/USD/issues/1523
-  // https://github.com/PixarAnimationStudios/USD/issues/1632
-  // To work around this issue, we force-enable explicit colorspace transformations and
-  // undo colorspace transformations that exist because of USD's sRGB detection logic:
-  // https://github.com/PixarAnimationStudios/USD/blob/857ffda41f4f1553fe1019ac7c7b4f08c233a7bb/pxr/imaging/plugin/hioOiio/oiioImage.cpp#L470-L471
-  // Additionally, we make hdStorm recognize alpha materials as translucent.
+  // Workaround to make hdStorm recognize alpha materials as translucent.
   bool hdstorm_compat;
 
   // If the asset supports the KHR_materials_variants extension, select the material

src/libguc/src/converter.cpp

@@ -236,7 +236,7 @@ namespace guc
     , m_stage(stage)
     , m_params(params)
     , m_mtlxDoc(mx::createDocument())
-    , m_mtlxConverter(m_mtlxDoc, m_imgMetadata, params.explicitColorspaceTransforms, params.hdStormCompat)
+    , m_mtlxConverter(m_mtlxDoc, m_imgMetadata, params.hdStormCompat)
     , m_usdPreviewSurfaceConverter(m_stage, m_imgMetadata)
   {
   }

src/libguc/src/converter.h

@@ -46,7 +46,6 @@ namespace guc
       bool genRelativePaths;
       bool emitMtlx;
       bool mtlxAsUsdShade;
-      bool explicitColorspaceTransforms;
       bool hdStormCompat;
       int defaultMaterialVariant;
     };

src/libguc/src/fileFormat.cpp

@@ -134,7 +134,6 @@ bool UsdGlTFFileFormat::Read(SdfLayer* layer,
   params.genRelativePaths = false;
   params.emitMtlx = data->emitMtlx;
   params.mtlxAsUsdShade = true;
-  params.explicitColorspaceTransforms = false;
   params.hdStormCompat = false;
   params.defaultMaterialVariant = 0;
 

src/libguc/src/guc.cpp

@@ -52,7 +52,6 @@ bool convertToUsd(fs::path src_dir,
   params.genRelativePaths = true;
   params.emitMtlx = options->emit_mtlx;
   params.mtlxAsUsdShade = options->mtlx_as_usdshade;
-  params.explicitColorspaceTransforms = options->explicit_colorspace_transforms;
   params.hdStormCompat = options->hdstorm_compat;
   params.defaultMaterialVariant = options->default_material_variant;
 

src/libguc/src/image.cpp

@@ -179,7 +179,7 @@ namespace guc
     return false;
   }
 
-  bool readImageMetadata(const char* path, int& channelCount, bool& isSrgbInUSD)
+  bool readImageMetadata(const char* path, int& channelCount)
   {
 #ifdef GUC_USE_OIIO
     using namespace OIIO;
@@ -189,9 +189,6 @@ namespace guc
     {
       const ImageSpec& spec = image->spec();
       channelCount = spec.nchannels;
-      // Detection logic of HioOIIO_Image::IsColorSpaceSRGB for _sourceColorSpace auto (default value)
-      // https://github.com/PixarAnimationStudios/USD/blob/857ffda41f4f1553fe1019ac7c7b4f08c233a7bb/pxr/imaging/plugin/hioOiio/oiioImage.cpp
-      isSrgbInUSD = (channelCount == 3 || channelCount == 4) && spec.format == TypeDesc::UINT8;
       image->close();
       return true;
     }
@@ -200,8 +197,6 @@ namespace guc
     int ok = stbi_info(path, &width, &height, &channelCount);
     if (ok)
     {
-      bool isHdr = bool(stbi_is_hdr(path));
-      isSrgbInUSD = (channelCount == 3 || channelCount == 4) && !isHdr; // UINT8 by default in stb_image
       return true;
     }
 #endif
@@ -313,7 +308,7 @@ namespace guc
     metadata.refPath = dstRefPath;
 
     // Read the metadata required for MaterialX shading network creation
-    if (!readImageMetadata(dstFilePath.c_str(), metadata.channelCount, metadata.isSrgbInUSD))
+    if (!readImageMetadata(dstFilePath.c_str(), metadata.channelCount))
     {
       TF_RUNTIME_ERROR("unable to read metadata of image %s", dstFilePath.c_str());
       return std::nullopt;

src/libguc/src/image.h

@@ -30,13 +30,7 @@ namespace guc
   {
     std::string filePath;
     std::string refPath;
-    // Needed to determine the type of MaterialX <image> nodes
-    int channelCount;
-    // USD makes an incorrect assumption that we have to work around by undoing an incorrect sRGB-to-linear transform in our MaterialX network gen:
-    // https://github.com/PixarAnimationStudios/USD/blob/857ffda41f4f1553fe1019ac7c7b4f08c233a7bb/pxr/imaging/plugin/hioOiio/oiioImage.cpp#L470-L471
-    // The stb_image sRGB detection logic is slightly different, but since guc requires OIIO, we don't have to care about it.
-    // Our UsdPreviewSurface generator is fine too, since there we set explicit sourceColorSpace inputs.
-    bool isSrgbInUSD;
+    int channelCount; // Needed to determine the type of MaterialX <image> nodes
   };
 
   using ImageMetadataMap = std::unordered_map<const cgltf_image*, ImageMetadata>;