Updated nurbsCurve static exporter to export usd curve spec

plugin/al/translators/NurbsCurve.cpp

@@ -223,6 +223,11 @@ void NurbsCurve::writeEdits(
     MProfilingScope profilerScope(
         _nurbsCurveProfilerCategory, MProfiler::kColorE_L3, "Write edits");
 
+    const int curveOrder = fnCurve.degree() + 1;
+    if (!TF_VERIFY(curveOrder <= fnCurve.numCVs())) {
+        return;
+    }
+
     uint32_t diff_curves = AL::usdmaya::utils::kAllNurbsCurveComponents;
     bool     performDiff = !writeAll;
     if (performDiff) {

plugin/al/translators/tests/testTranslators.py

@@ -5,6 +5,7 @@
 
 import maya.cmds as mc
 import maya.mel as mel
+from maya.api import OpenMaya
 
 from pxr import Tf, Usd, UsdGeom, Gf
 import translatortestutils
@@ -773,7 +774,44 @@ def testTranslatedPrimDeactiveCrashIssue(self):
         # When a prim is within a translated hierarchy, deactivating it's parent should not crash Maya or clear the children of proxyShapeTransform.
         self._performDisablePrimTest('./testTranslatedPrimDisableCrashIssue.usda')
 
+    def testUSDSpecNurbsCurveExportImport(self):
+        """Testing exporting maya nurbsCurves in USD's curve spec, and importing USD curve spec in Maya"""
+        tempFile = tempfile.NamedTemporaryFile(suffix=".usda", prefix="test_USDNurbsCurveExportImport_", delete=False)
+        mc.CreateNURBSCircle()
+        spans =  mc.getAttr('nurbsCircleShape1.spans')
+        degree = mc.getAttr('nurbsCircleShape1.degree')
+        mayaKnotCount = spans + 2 * degree - 1
+        usdnotCount = spans + 2 * degree + 1
+
+        # Export Maya curve to USD.
+        mc.select('nurbsCircle1', r=True)
+        mc.file(tempFile.name, exportSelected=True, force=True, type="AL usdmaya export", 
+            options="Dynamic_Attributes=1;Duplicate_Instances=1;Merge_Transforms=1;Merge_Offset_Parent_Matrix=0;Animation=0;Use_Timeline_Range=0;Frame_Min=0;Frame_Max=1;Sub_Samples=1;Filter_Sample=0;Export_At_Which_Time=0;Export_In_World_Space=0;Activate_all_Plugin_Translators=1;Active_Translator_List=;Inactive_Translator_List=;Nurbs_Curves=1;Meshes=1;Mesh_Face_Connects=1;Mesh_Points=1;Mesh_Extents=1;Mesh_Normals=1;Mesh_Vertex_Creases=1;Mesh_Edge_Creases=1;Mesh_UVs=1;Mesh_UV_Only=0;Mesh_Points_as_PRef=0;Mesh_Colours=1;Default_RGB=0.18;Default_Alpha=1;Custom_Colour_Threshold=1;Colour_Threshold_Value=1e-05;Mesh_Holes=1;Write_Normals_as_Primvars=1;Reverse_Opposite_Normals=1;Subdivision_scheme=0;Compaction_Level=3;"
+        )
+
+        # Check whether exported curve's knot cout matches USD's curve spec.
+        # Reference: https://graphics.pixar.com/usd/release/api/class_usd_geom_nurbs_curves.html#details
+        stage = Usd.Stage.Open(tempFile.name)
+        prim = stage.GetPrimAtPath('/nurbsCircle1')
+        knotValues = prim.GetAttribute('knots').Get()
+        self.assertEqual(len(knotValues), usdnotCount)
+
+        # Import exported USD curve back to Maya.
+        mc.file(f=True, new=True)
+        mc.file(tempFile.name, i=True, force=True, type="AL usdmaya import",  ignoreVersion=True, ra=True, 
+            options="Parent_Path=;Prim_Path=;Import_Animations=1;Import_Dynamic_Attributes=1;Load_None=0;Read_Default_Values=1;Activate_all_Plugin_Translators=1;Active_Translator_List=;Inactive_Translator_List=;Import_Curves=1;Import_Meshes=1;", 
+            pr=True, importFrameRate=True, importTimeRange='override'
+        )
+
+        # Check whether exported curve's knot cout matches USD's curve spec.
+        mc.select('nurbsCircle1Shape', r=True)
+        sel = OpenMaya.MGlobal.getActiveSelectionList()
+        curveFn = OpenMaya.MFnNurbsCurve(sel.getDependNode(0))
+        self.assertEqual(list(curveFn.knots()), list(knotValues[1:-1]))
 
+        os.remove(tempFile.name)
+
+
 tests = unittest.TestLoader().loadTestsFromTestCase(TestTranslator)
 result = unittest.TextTestRunner(verbosity=2).run(tests)
 

plugin/al/usdmayautils/AL/usdmaya/utils/NurbsCurveUtils.cpp

@@ -109,9 +109,30 @@ void copyKnots(const MFnNurbsCurve& fnCurve, const UsdAttribute& knotsAttr, UsdT
 {
     MDoubleArray knots;
     fnCurve.getKnots(knots);
-    const uint32_t  count = knots.length();
-    VtArray<double> dataKnots(count);
-    memcpy((double*)dataKnots.cdata(), (const double*)&knots[0], sizeof(double) * count);
+    const uint32_t count = knots.length();
+
+    // USD's NurbsCurve representation requires 2 more knots
+    // Reference:
+    // https://graphics.pixar.com/usd/release/api/class_usd_geom_nurbs_curves.html#details
+    VtArray<double> dataKnots(count + 2);
+    for (uint32_t i = 0; i < count; ++i) {
+        dataKnots[i + 1] = knots[i];
+    }
+
+    auto curveForm = fnCurve.form();
+    if (curveForm == MFnNurbsCurve::kClosed || curveForm == MFnNurbsCurve::kPeriodic) {
+        // From USD spec: knot[0] = knot[1] - (knots[-2] - knots[-3]);
+        dataKnots[0]
+            = dataKnots[1] - (dataKnots[dataKnots.size() - 2] - dataKnots[dataKnots.size() - 3]);
+        // From USD spec: knot[-1] = knot[-2] + (knot[2] - knots[1]);
+        dataKnots[dataKnots.size() - 1]
+            = dataKnots[dataKnots.size() - 2] + (dataKnots[2] - dataKnots[1]);
+    } else {
+        // From USD spec: knot[0] = knot[1];
+        dataKnots[0] = dataKnots[1];
+        // From USD spec: knot[-1] = knot[-2];
+        dataKnots[dataKnots.size() - 1] = dataKnots[dataKnots.size() - 2];
+    }
     knotsAttr.Set(dataKnots, time);
 }
 
@@ -247,19 +268,41 @@ bool createMayaCurves(
 
     size_t currentPointIndex = 0;
     size_t currentKnotIndex = 0;
+
     for (size_t i = 0, ncurves = dataCurveVertexCounts.size(); i < ncurves; ++i) {
         const int32_t numPoints = dataCurveVertexCounts[i];
         controlVertices.setLength(numPoints);
 
-        const int32_t numKnots = numPoints + dataOrder[i] - 2;
-        knotSequences.setLength(numKnots);
-
-        const float*  pstart = (const float*)&dataPoints[currentPointIndex];
-        const double* kstart = &dataKnots[currentKnotIndex];
-        memcpy(&knotSequences[0], kstart, sizeof(double) * numKnots);
+        // Maya's curve spec requires (numberOfPoints + degree - 1) knots
+        uint32_t numKnots = numPoints + dataOrder[i] - 2;
+
+        // For backward compatibility, we check whether the knot count is in Maya's spec.
+        // If yes, we use the orignal knot values unchanged otherwise we need to convert to
+        // Maya's knot sepc from USD's knot spec
+        if (numKnots == dataKnots.size()) {
+            knotSequences.setLength(numKnots);
+            const double* kstart = &dataKnots[currentKnotIndex];
+            memcpy(&knotSequences[0], kstart, sizeof(double) * numKnots);
+            currentKnotIndex += numKnots;
+        } else {
+            // Converting knot values to Maya's spec. Maya requires 2 less knots than USD does.
+            // See
+            // https://graphics.pixar.com/usd/release/api/class_usd_geom_nurbs_curves.html#details
+            // and Maya MFnNurbsCurve API's Detailed Description session for more info.
+            numKnots = dataKnots.size() - 2;
+            knotSequences.setLength(numKnots);
+
+            // From Maya MFnNurbsCurve API's doc:
+            // To convert from one of these external representations into the Maya representation,
+            // simply omit the first and last knots from the external representation when creating
+            // the Maya representation
+            for (uint32_t j = 0; j < numKnots; ++j) {
+                knotSequences[j] = dataKnots[j + 1];
+            }
+        }
 
+        const float* pstart = (const float*)&dataPoints[currentPointIndex];
         currentPointIndex += numPoints;
-        currentKnotIndex += numKnots;
 
         AL::usdmaya::utils::convert3DFloatArrayTo4DDoubleArray(
             pstart, (double*)&controlVertices[0], numPoints);