maintain UV connectivity across export and re-import round trips

lib/mayaUsd/fileio/utils/meshReadUtils.cpp

@@ -190,7 +190,6 @@ bool assignUVSetPrimvarToMesh(const UsdGeomPrimvar& primvar, MFnMesh& meshFn, bo
 {
     const TfToken& primvarName = primvar.GetPrimvarName();
 
-    // Get the raw data before applying any indexing.
     VtVec2fArray uvValues;
     if (!primvar.Get(&uvValues) || uvValues.empty()) {
         TF_WARN(
@@ -200,53 +199,7 @@ bool assignUVSetPrimvarToMesh(const UsdGeomPrimvar& primvar, MFnMesh& meshFn, bo
         return false;
     }
 
-    // This is the number of UV values assuming the primvar is NOT indexed.
-    VtIntArray assignmentIndices;
-    if (primvar.GetIndices(&assignmentIndices)) {
-        // The primvar IS indexed, so the indices array is what determines the
-        // number of UV values.
-        int unauthoredValuesIndex = primvar.GetUnauthoredValuesIndex();
-
-        // Replace any index equal to unauthoredValuesIndex with -1.
-        if (unauthoredValuesIndex != -1) {
-            for (int& index : assignmentIndices) {
-                if (index == unauthoredValuesIndex) {
-                    index = -1;
-                }
-            }
-        }
-
-        // Furthermore, if unauthoredValuesIndex is valid for uvValues, then
-        // remove it from uvValues and shift the indices (we don't want to
-        // import the unauthored value into Maya, where it has no meaning).
-        if (unauthoredValuesIndex >= 0
-            && static_cast<size_t>(unauthoredValuesIndex) < uvValues.size()) {
-            // This moves [unauthoredValuesIndex + 1, end) to
-            // [unauthoredValuesIndex, end - 1), erasing the
-            // unauthoredValuesIndex.
-            std::move(
-                uvValues.begin() + unauthoredValuesIndex + 1,
-                uvValues.end(),
-                uvValues.begin() + unauthoredValuesIndex);
-            uvValues.pop_back();
-
-            for (int& index : assignmentIndices) {
-                if (index > unauthoredValuesIndex) {
-                    index = index - 1;
-                }
-            }
-        }
-    }
-
-    // Go through the UV data and add the U and V values to separate
-    // MFloatArrays.
-    MFloatArray uCoords;
-    MFloatArray vCoords;
-    for (const GfVec2f& v : uvValues) {
-        uCoords.append(v[0]);
-        vCoords.append(v[1]);
-    }
-
+    // Determine the name to use for the Maya UV set.
     MStatus status { MS::kSuccess };
     MString uvSetName(primvarName.GetText());
     bool    createUVSet = true;
@@ -260,7 +213,7 @@ bool assignUVSetPrimvarToMesh(const UsdGeomPrimvar& primvar, MFnMesh& meshFn, bo
         // If map1 still exists, we rename and re-use it:
         MStringArray uvSetNames;
         meshFn.getUVSetNames(uvSetNames);
-        if (uvSetNames[0] == UsdMayaMeshPrimvarTokens->DefaultMayaTexcoordName.GetText()) {
+        if (uvSetNames[0u] == UsdMayaMeshPrimvarTokens->DefaultMayaTexcoordName.GetText()) {
             meshFn.renameUVSet(
                 UsdMayaMeshPrimvarTokens->DefaultMayaTexcoordName.GetText(), uvSetName);
             createUVSet = false;
@@ -287,8 +240,23 @@ bool assignUVSetPrimvarToMesh(const UsdGeomPrimvar& primvar, MFnMesh& meshFn, bo
     MString currentSet = meshFn.currentUVSetName();
     meshFn.setCurrentUVSetName(currentSet);
 
-    // Create UVs on the mesh from the values we collected out of the primvar.
-    // We'll assign mesh components to these values below.
+    // Set the UVs on the mesh from the values we collected out of the primvar.
+    // We'll check whether there is an unauthored value in the primvar and skip
+    // it if so to ensure that we don't import it into Maya where it has no
+    // meaning.
+    const int unauthoredValuesIndex = primvar.GetUnauthoredValuesIndex();
+
+    MFloatArray uCoords;
+    MFloatArray vCoords;
+
+    for (size_t uvId = 0u; uvId < uvValues.size(); ++uvId) {
+        if (unauthoredValuesIndex < 0 || uvId != static_cast<size_t>(unauthoredValuesIndex)) {
+            const GfVec2f& v = uvValues[uvId];
+            uCoords.append(v[0u]);
+            vCoords.append(v[1u]);
+        }
+    }
+
     status = meshFn.setUVs(uCoords, vCoords, &uvSetName);
     if (status != MS::kSuccess) {
         TF_WARN(
@@ -298,6 +266,23 @@ bool assignUVSetPrimvarToMesh(const UsdGeomPrimvar& primvar, MFnMesh& meshFn, bo
         return false;
     }
 
+    VtIntArray assignmentIndices;
+    if (primvar.GetIndices(&assignmentIndices)) {
+        if (unauthoredValuesIndex >= 0) {
+            // Since the unauthored value was removed above, we need to fix up
+            // the assignment indices to replace any index equal to the
+            // unauthored value index with -1, and decrement any index that was
+            // after the unauthored value index by 1.
+            for (int& index : assignmentIndices) {
+                if (index == unauthoredValuesIndex) {
+                    index = -1;
+                } else if (index > unauthoredValuesIndex) {
+                    index -= 1;
+                }
+            }
+        }
+    }
+
     const TfToken& interpolation = primvar.GetInterpolation();
 
     // Build an array of value assignments for each face vertex in the mesh.

lib/mayaUsd/fileio/utils/meshWriteUtils.cpp

@@ -30,6 +30,7 @@
 #include <pxr/base/tf/staticTokens.h>
 #include <pxr/base/tf/token.h>
 #include <pxr/base/vt/array.h>
+#include <pxr/pxr.h>
 #include <pxr/usd/usdGeom/mesh.h>
 #include <pxr/usd/usdGeom/pointBased.h>
 #include <pxr/usd/usdGeom/primvar.h>
@@ -846,39 +847,51 @@ bool UsdMayaMeshWriteUtils::getMeshUVSetData(
     TfToken*       interpolation,
     VtIntArray*    assignmentIndices)
 {
-    MStatus status { MS::kSuccess };
-
-    // Sanity check first to make sure this UV set even has assigned values
-    // before we attempt to do anything with the data.
+    // Check first to make sure this UV set even has assigned values before we
+    // attempt to do anything with the data. We cannot directly use this data
+    // otherwise though since we need a uvId for every face vertex, and the
+    // returned uvIds MIntArray may be shorter than that if there are unmapped
+    // faces.
     MIntArray uvCounts, uvIds;
-    status = mesh.getAssignedUVs(uvCounts, uvIds, &uvSetName);
-    if (status != MS::kSuccess) {
-        return false;
-    }
-    if (uvCounts.length() == 0 || uvIds.length() == 0) {
+    MStatus   status = mesh.getAssignedUVs(uvCounts, uvIds, &uvSetName);
+    CHECK_MSTATUS_AND_RETURN(status, false);
+
+    if (uvCounts.length() == 0u || uvIds.length() == 0u) {
         return false;
     }
 
-    // using itFV.getUV() does not always give us the right answer, so
-    // instead, we have to use itFV.getUVIndex() and use that to index into the
-    // UV set.
+    // Transfer the UV values directly to USD, in the same order as they are in
+    // the Maya mesh.
     MFloatArray uArray;
     MFloatArray vArray;
-    mesh.getUVs(uArray, vArray, &uvSetName);
+    status = mesh.getUVs(uArray, vArray, &uvSetName);
+    CHECK_MSTATUS_AND_RETURN(status, false);
+
     if (uArray.length() != vArray.length()) {
         return false;
     }
 
-    // We'll populate the assignment indices for every face vertex, but we'll
-    // only push values into the data if the face vertex has a value. All face
-    // vertices are initially unassigned/unauthored.
-    const unsigned int numFaceVertices = mesh.numFaceVertices(&status);
     uvArray->clear();
-    assignmentIndices->assign((size_t)numFaceVertices, -1);
+    uvArray->reserve(static_cast<size_t>(uArray.length()));
+    for (unsigned int uvId = 0u; uvId < uArray.length(); ++uvId) {
+#if PXR_VERSION >= 2011
+        uvArray->emplace_back(uArray[uvId], vArray[uvId]);
+#else
+        GfVec2f value(uArray[uvId], vArray[uvId]);
+        uvArray->push_back(value);
+#endif
+    }
+
+    // Now iterate through all the face vertices and fill in the faceVarying
+    // assignmentIndices array, again in the same order as in the Maya mesh.
+    const unsigned int numFaceVertices = mesh.numFaceVertices(&status);
+    CHECK_MSTATUS_AND_RETURN(status, false);
+
+    assignmentIndices->assign(static_cast<size_t>(numFaceVertices), -1);
     *interpolation = UsdGeomTokens->faceVarying;
 
     MItMeshFaceVertex itFV(mesh.object());
-    unsigned int      fvi = 0;
+    unsigned int      fvi = 0u;
     for (itFV.reset(); !itFV.isDone(); itFV.next(), ++fvi) {
         if (!itFV.hasUVs(uvSetName)) {
             // No UVs for this faceVertex, so leave it unassigned.
@@ -887,17 +900,16 @@ bool UsdMayaMeshWriteUtils::getMeshUVSetData(
 
         int uvIndex;
         itFV.getUVIndex(uvIndex, &uvSetName);
-        if (uvIndex < 0 || static_cast<size_t>(uvIndex) >= uArray.length()) {
+        if (uvIndex < 0 || static_cast<unsigned int>(uvIndex) >= uArray.length()) {
             return false;
         }
 
-        GfVec2f value(uArray[uvIndex], vArray[uvIndex]);
-        uvArray->push_back(value);
-        (*assignmentIndices)[fvi] = uvArray->size() - 1;
+        (*assignmentIndices)[fvi] = uvIndex;
     }
 
-    UsdMayaUtil::MergeEquivalentIndexedValues(uvArray, assignmentIndices);
-    UsdMayaUtil::CompressFaceVaryingPrimvarIndices(mesh, interpolation, assignmentIndices);
+    // We do not merge indexed values or compress indices here in an effort to
+    // maintain the same UV shells and connectivity across export/import
+    // round-trips.
 
     return true;
 }