USD: port of UsdSkel import from the USD branch.

Also removed commented out code.

@@ -984,0 +1023,4 @@
+
+  if (!import_params_.import_skeletons || prim_.IsInstanceProxy()) {
+    /* Use the standard transform computation, since we are ignoring
+     * skinning data. Note that applying theUsdSkelBindingAPI to an

@@ -0,0 +47,4 @@
+namespace {
+
+/* Utility: create curve at the given array index. */
+FCurve *create_fcurve(int array_index, const char *rna_path)

@@ -0,0 +51,4 @@
+{
+  FCurve *fcu = BKE_fcurve_create();
+  fcu->flag = (FCURVE_VISIBLE | FCURVE_SELECTED);
+  fcu->rna_path = BLI_strdupn(rna_path, strlen(rna_path));

@@ -0,0 +59,4 @@
+/* Utility: create curve at the given array index and
+ * adds it as a channel to a group. */
+FCurve *create_chan_fcurve(
+    bAction *act, bActionGroup *grp, int array_index, const char *rna_path, int totvert)

@@ -0,0 +78,4 @@
+  bez.f1 = bez.f2 = bez.f3 = SELECT;
+  bez.h1 = bez.h2 = HD_AUTO;
+  insert_bezt_fcurve(fcu, &bez, INSERTKEY_NOFLAGS);
+  BKE_fcurve_handles_recalc(fcu);

@@ -0,0 +238,4 @@
+
+  /* Set the curve samples. */
+  for (double frame : samples) {
+

@@ -0,0 +253,4 @@
+    }
+
+    for (int i = 0; i < joint_local_xforms.size(); ++i) {
+

@@ -0,0 +406,4 @@
+  std::set<pxr::TfToken> shapekey_names;
+
+  for (int i = 0; i < targets.size(); ++i) {
+

@@ -0,0 +559,4 @@
+  std::vector<FCurve *> curves;
+
+  for (auto blendshape_name : blendshapes) {
+

@@ -0,0 +685,4 @@
+    return;
+  }
+
+  /* Check if any bone natrices have negative determinants,

@@ -0,0 +689,4 @@
+   * indicating negative scales, possibly due to mirroring
+   * operations.  Such matrices can't be propery converted
+   * to Blender's axis/roll bone representation (see
+   * https://developer.blender.org/T82930).  If we detect

@@ -0,0 +228,4 @@
+
+  pxr::VtMatrix4dArray joint_local_bind_xforms(bind_xforms.size());
+  for (int i = 0; i < bind_xforms.size(); ++i) {
+    int parent_id = skel_topology.GetParent(i);

@@ -0,0 +269,4 @@
+      }
+
+      float re = qrot.GetReal();
+      pxr::GfVec3f im = qrot.GetImaginary();

@@ -0,0 +272,4 @@
+      pxr::GfVec3f im = qrot.GetImaginary();
+
+      for (int j = 0; j < 3; ++j) {
+        int k = 3 * i + j;

@@ -0,0 +283,4 @@
+      }
+
+      for (int j = 0; j < 4; ++j) {
+        int k = 4 * i + j;

@@ -0,0 +299,4 @@
+      }
+
+      for (int j = 0; j < 3; ++j) {
+        int k = 3 * i + j;

@@ -0,0 +560,4 @@
+    return;
+  }
+
+  size_t num_samples = times.size();

@@ -0,0 +669,4 @@
+  }
+
+  /* Sanity check: we should have created a bone for each joint. */
+  size_t num_joints = skel_topology.GetNumJoints();

@@ -0,0 +703,4 @@
+   * https://projects.blender.org/blender/blender/issues/82930).
+   * If we detect such matrices, we will flag an error and won't
+   * try to import the animation, since the rotations would
+   * be incorrect in such cases.  Unfortunately, the Pixar

@@ -0,0 +755,4 @@
+  std::vector<std::vector<int>> child_bones(num_joints);
+
+  for (size_t i = 0; i < num_joints; ++i) {
+    int parent_idx = skel_topology.GetParent(i);

@@ -0,0 +802,4 @@
+    pxr::GfVec3f parent_head(parent->head);
+    pxr::GfVec3f parent_tail(parent->tail);
+
+    float new_len = (avg_child_head - parent_head).GetLength();

@@ -0,0 +805,4 @@
+    float new_len = (avg_child_head - parent_head).GetLength();
+
+    /* Be sure not to scale by zero. */
+    if (new_len > .00001) {

@@ -0,0 +815,4 @@
+  /* Scale terminal bones by the average length scale. */
+  avg_len_scale /= num_joints;
+
+  if (avg_len_scale > .00001) {

@@ -0,0 +1011,4 @@
+    for (int j = 0; j < joint_weights_elem_size; ++j) {
+      int k = offset + j;
+      float w = joint_weights[k];
+      if (w < .00001) {

@@ -0,0 +44,4 @@
+                        bool import_anim = true);
+
+/**
+ * Import the given USD sekeleton as an armature object. Optionally, if the

@@ -0,0 +59,4 @@
+                     const pxr::UsdSkelSkeleton &skel,
+                     bool import_anim = true);
+/**
+ * Import skinning data from a source USD prim as deform groups and an armature

@@ -45,6 +45,11 @@ class USDStageReader {
 
   void collect_readers(struct Main *bmain);
 
+  /* Complete setting up the armature modifiers that

@@ -171,0 +155,4 @@
+                                         pxr::GfMatrix4d *r_xform,
+                                         bool *r_is_constant) const
+{
+  if (!r_xform) {

@@ -53,0 +59,4 @@
+   * \param r_is_constant: Return value flag, set to false if the transform is
+   *                       time varying
+   */
+  virtual bool get_local_usd_xform(const float time,

@@ -97,0 +110,4 @@
+   * Override transform computation to account for the binding
+   * transformation for skinned meshes.
+   */
+  std::optional<XformResult> get_local_usd_xform(const float time) const override;

@@ -321,0 +350,4 @@
+    if (!reader->object()) {
+      continue;
+    }
+    if (const USDMeshReader *mesh_reader = dynamic_cast<const USDMeshReader *>(reader)) {

@@ -171,0 +169,4 @@
+    return std::nullopt;
+  }
+
+  return XformResult(pxr::GfMatrix4f(xform), is_constant);

@@ -12,6 +12,8 @@
 
 namespace blender::io::usd {
 
+using XformResult = std::tuple<pxr::GfMatrix4f, bool>;

@@ -53,0 +58,4 @@
+   *
+   * \param time: Time code for evaluating the transform.
+   *
+   * \return: Optional tuple with the following elements: