HYDRA-1326 : Various fixes for the custom GLSLFX shader (#224)

* HYDRA-1326 : Various fixes for the custom GLSLFX shader

* Add comments

lib/flowViewport/usdPlugins/shadersDiscoveryPlugin/README

@@ -3,3 +3,34 @@ The shaders defined in shadersDef.usda will be loaded by usd/hydra and available
 
 You can add the plugInfo.json file path in the environment variable PXR_PLUGINPATH_NAME or use another .json file to load it
 so USD/Hydra loads it at startup.
+
+To use the FVP_CustomBasicLightingShader GLSLFX shader into a usd file, just add this definition to your file :
+
+def Scope "mtl"
+{
+    def Material "CustomGLSL"
+    {
+        token outputs:surface.connect = </mtl/CustomGLSL/CustomGLSLShader1.outputs:surface>
+
+        def Shader "CustomGLSLShader1"
+        {
+            uniform token info:id = "FVP_CustomBasicLightingShader"
+            token outputs:surface
+        }
+    }
+}
+
+You can also assign the material binding to a prim directly by adding  :
+
+def Mesh "Mesh1" (
+    prepend apiSchemas = ["MaterialBindingAPI"]
+)
+{
+    uniform bool doubleSided = 1
+    float3[] extent = [(-1.0000002, -1, -1.0000005), (1, 1, 1.0000001)]
+    int[] faceVertexCounts = ...
+    int[] faceVertexIndices = ...
+    rel material:binding = </mtl/CustomGLSL>
+    point3f[] points = ...
+    ...
+}

lib/flowViewport/usdPlugins/shadersDiscoveryPlugin/shaders/customBasicLightingShader.glslfx

@@ -76,11 +76,15 @@ surfaceShader(vec4 Peye, vec3 Neye, vec4 color, vec4 patchCoord)
     //vec2 st = HdGet_st().xy;//This doesn't work unless you add a texture node in the material network
 
     //Get the vertex colors from the prim vars of the quad primitive
-    vec3 vertexColors = HdGet_displayColor().rgb;
+	#ifdef HD_HAS_displayColor
+		vec3 vertexColors = HdGet_displayColor().rgb;
+	#else
+		vec3 vertexColors = vec3(1);
+	#endif
 
     vec3 ambientColor = vec3(0);//Hardcoded
     //Get the value of the custom color float3 parameter from the material, it is named "FVP_CustomColor"
-    vec3 diffuseColor = HdGet_FVP_CustomColor() *0.5 +  vertexColors*0.5;//Blend the custom color with the vertex colors
+    vec3 diffuseColor = HdGet_FVP_CustomColor() * vertexColors;//Blend the custom color with the vertex colors
     vec3 specularColor = vec3(1);//Hardcoded
 
     vec3 n = Neye;

lib/mayaHydra/flowViewportAPIExamples/customShadersNode/mhCustomShadersNode.cpp

@@ -25,6 +25,37 @@
 // 
 //  createNode("MhCustomShaders")
 //
+// To use the custom shader into a usd file, please add to your file :
+/*
+def Scope "mtl"
+{
+    def Material "CustomGLSL"
+    {
+        token outputs:surface.connect = </mtl/CustomGLSL/CustomGLSLShader1.outputs:surface>
+
+        def Shader "CustomGLSLShader1"
+        {
+            uniform token info:id = "FVP_CustomBasicLightingShader"
+            token outputs:surface
+        }
+    }
+}
+
+You can also assign the material binding to a prim directly by adding  :
+
+def Mesh "Mesh1" (
+    prepend apiSchemas = ["MaterialBindingAPI"]
+)
+{
+    uniform bool doubleSided = 1
+    float3[] extent = [(-1.0000002, -1, -1.0000005), (1, 1, 1.0000001)]
+    int[] faceVertexCounts = ...
+    int[] faceVertexIndices = ...
+    rel material:binding = </mtl/CustomGLSL>
+    point3f[] points = ...
+    ...
+}
+*/
 ////////////////////////////////////////////////////////////////////////
 
 //maya headers
@@ -608,6 +639,11 @@ void MhCustomShaders::addedToModelCb()
     //Data producer scene index interface is used to add the retained scene index to all viewports with all render delegates
     auto& dataProducerSceneIndexInterface = Fvp::DataProducerSceneIndexInterface::get();
     dataProducerSceneIndexInterface.addDataProducerSceneIndex(_retainedSceneIndex, noPrefix, (void*)&obj, FvpViewportAPITokens->allViewports,FvpViewportAPITokens->allRenderers);
+
+    //Update color
+    MPlug colorPlug(obj, mColor);
+    const double3& color = colorPlug.asMDataHandle().asDouble3();
+    UpdateColorInShader(color);
 }
 
 void MhCustomShaders::removedFromModelCb()