[HdSt] Added functions to interpolate patch coord w/o GS

Added methods to compute interpolated PatchCoords for triangles
and quads when there is no geometry shader. These are similar to
the method recently added for triangulated quads.

This allows some additional cleanup of interstage plumbing.

(Internal change: 2200684)

pxr/imaging/hdSt/meshShaderKey.cpp

@@ -114,6 +114,8 @@ TF_DEFINE_PRIVATE_TOKENS(
     ((mainTriQuadGS,           "Mesh.Geometry.TriQuad"))
     ((mainQuadGS,              "Mesh.Geometry.Quad"))
     ((mainPatchCoordFS,        "Mesh.Fragment.PatchCoord"))
+    ((mainPatchCoordTriangleFS,"Mesh.Fragment.PatchCoord.Triangle"))
+    ((mainPatchCoordQuadFS,    "Mesh.Fragment.PatchCoord.Quad"))
     ((mainPatchCoordTriQuadFS, "Mesh.Fragment.PatchCoord.TriQuad"))
     ((mainFS,                  "Mesh.Fragment"))
 
@@ -462,7 +464,11 @@ HdSt_MeshShaderKey::HdSt_MeshShaderKey(
                                       _tokens->pointIdFallbackFS;
     FS[fsIndex++] = hasTopologicalVisibility? _tokens->topVisFS :
                                               _tokens->topVisFallbackFS;
-    if (isPrimTypeTriQuads) {
+    if (isPrimTypeTris && !gsStageEnabled) {
+        FS[fsIndex++] = _tokens->mainPatchCoordTriangleFS;
+    } else if (isPrimTypeQuads && !gsStageEnabled) {
+        FS[fsIndex++] = _tokens->mainPatchCoordQuadFS;
+    } else if (isPrimTypeTriQuads) {
         FS[fsIndex++] = _tokens->mainPatchCoordTriQuadFS;
     } else {
         FS[fsIndex++] = _tokens->mainPatchCoordFS;

pxr/imaging/hdSt/shaders/mesh.glslfx

@@ -45,8 +45,6 @@ out VertexData
     vec3 Neye;
 } outData;
 
-out vec4 gsPatchCoord;
-
 // Fwd declare methods defined in pointId.glslfx, that are used below.
 int GetPointId();
 float GetPointRasterSize(int);
@@ -62,12 +60,6 @@ void main(void)
 
     outData.Neye = GetNormal(vec3(0), 0); // normalized
 
-    // Fill out redundantly, since at this point we don't know
-    // the following stages include tessellation or geometry shader.
-    // we rely on that compiler (linker) optimizes out these redundant
-    // assignment and attribute plumbing.
-    gsPatchCoord = vec4(0);
-
     int pointId = GetPointId();
     gl_PointSize = GetPointRasterSize(pointId);
     ProcessPointId(pointId);
@@ -316,7 +308,6 @@ out VertexData
 
 out vec4 gsPatchCoord;
 
-
 vec4 GetPatchCoord(int index)
 {
     return tesPatchCoord[index];
@@ -710,6 +701,39 @@ vec4 GetInterpolatedPatchCoord()
     return gsPatchCoord;
 }
 
+--- --------------------------------------------------------------------------
+-- glsl Mesh.Fragment.PatchCoord.Triangle
+
+vec3 GetBarycentricCoord(); // codeGen
+
+vec2 GetPatchCoordLocalST()
+{
+    vec3 barycentric = GetBarycentricCoord();
+    return barycentric.yz;
+}
+
+vec4 GetInterpolatedPatchCoord()
+{
+    return OsdInterpolatePatchCoordTriangle(
+                        GetPatchCoordLocalST(), GetPatchParam());
+}
+
+--- --------------------------------------------------------------------------
+-- glsl Mesh.Fragment.PatchCoord.Quad
+
+vec3 GetBarycentricCoord(); // codeGen
+
+vec2 GetPatchCoordLocalST()
+{
+    vec3 barycentric = GetBarycentricCoord();
+    return barycentric.yz;
+}
+
+vec4 GetInterpolatedPatchCoord()
+{
+    return OsdInterpolatePatchCoord(GetPatchCoordLocalST(), GetPatchParam());
+}
+
 --- --------------------------------------------------------------------------
 -- glsl Mesh.Fragment.PatchCoord.TriQuad