Fix T56408: Hair children recalc on every frame on Alembic mesh

This fixes the glitching hairs described in T56408, T63534, and possibly
also T63534.

The fix consists of returning the original mesh (i.e. as visible in edit
mode) when constructing the ORCO mesh. This allows a static set of
coordinates to be used when computing the child hair positions.

The original mesh is only returned when it has the same topology (at
least same number of vertices, loops, and polys. It's up the author of
the Alembic file to ensure stable geometry when it's desired to be
compatible with Blender's hair system.

Reviewers: mont29, brecht

Differential Revision: https://developer.blender.org/D5492

1 files changed, 26 insertions, 4 deletions

diff --git a/source/blender/alembic/intern/abc_mesh.cc b/source/blender/alembic/intern/abc_mesh.cc
index 6647ca83bd6..be2793e38f7 100644
--- a/source/blender/alembic/intern/abc_mesh.cc
+++ b/source/blender/alembic/intern/abc_mesh.cc
@@ -1131,6 +1131,31 @@ bool AbcMeshReader::accepts_object_type(
   return true;
 }
 
+bool AbcMeshReader::topology_changed(Mesh *existing_mesh, const ISampleSelector &sample_sel)
+{
+  IPolyMeshSchema::Sample sample;
+  try {
+    sample = m_schema.getValue(sample_sel);
+  }
+  catch (Alembic::Util::Exception &ex) {
+    printf("Alembic: error reading mesh sample for '%s/%s' at time %f: %s\n",
+           m_iobject.getFullName().c_str(),
+           m_schema.getName().c_str(),
+           sample_sel.getRequestedTime(),
+           ex.what());
+    // A similar error in read_mesh() would just return existing_mesh.
+    return false;
+  }
+
+  const P3fArraySamplePtr &positions = sample.getPositions();
+  const Alembic::Abc::Int32ArraySamplePtr &face_indices = sample.getFaceIndices();
+  const Alembic::Abc::Int32ArraySamplePtr &face_counts = sample.getFaceCounts();
+
+  return positions->size() != existing_mesh->totvert ||
+         face_counts->size() != existing_mesh->totpoly ||
+         face_indices->size() != existing_mesh->totloop;
+}
+
 Mesh *AbcMeshReader::read_mesh(Mesh *existing_mesh,
                                const ISampleSelector &sample_sel,
                                int read_flag,
@@ -1162,10 +1187,7 @@ Mesh *AbcMeshReader::read_mesh(Mesh *existing_mesh,
   ImportSettings settings;
   settings.read_flag |= read_flag;
 
-  bool topology_changed = positions->size() != existing_mesh->totvert ||
-                          face_counts->size() != existing_mesh->totpoly ||
-                          face_indices->size() != existing_mesh->totloop;
-  if (topology_changed) {
+  if (topology_changed(existing_mesh, sample_sel)) {
     new_mesh = BKE_mesh_new_nomain_from_template(
         existing_mesh, positions->size(), 0, 0, face_indices->size(), face_counts->size());