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

source/blender/alembic/ABC_alembic.h

@@ -117,6 +117,12 @@ struct Mesh *ABC_read_mesh(struct CacheReader *reader,
                            const char **err_str,
                            int flags);
 
+bool ABC_mesh_topology_changed(struct CacheReader *reader,
+                               struct Object *ob,
+                               struct Mesh *existing_mesh,
+                               const float time,
+                               const char **err_str);
+
 void CacheReader_incref(struct CacheReader *reader);
 void CacheReader_free(struct CacheReader *reader);
 

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());
 

source/blender/alembic/intern/abc_mesh.h

@@ -110,6 +110,8 @@ class AbcMeshReader : public AbcObjectReader {
                          const Alembic::Abc::ISampleSelector &sample_sel,
                          int read_flag,
                          const char **err_str);
+  bool topology_changed(Mesh *existing_mesh,
+                        const Alembic::Abc::ISampleSelector &sample_sel) override;
 
  private:
   void readFaceSetsSample(Main *bmain,

source/blender/alembic/intern/abc_object.cc

@@ -246,6 +246,14 @@ struct Mesh *AbcObjectReader::read_mesh(struct Mesh *existing_mesh,
   return existing_mesh;
 }
 
+bool AbcObjectReader::topology_changed(Mesh * /*existing_mesh*/,
+                                       const Alembic::Abc::ISampleSelector & /*sample_sel*/)
+{
+  /* The default implementation of read_mesh() just returns the original mesh, so never changes the
+   * topology. */
+  return false;
+}
+
 void AbcObjectReader::setupObjectTransform(const float time)
 {
   bool is_constant = false;

source/blender/alembic/intern/abc_object.h

@@ -190,6 +190,8 @@ class AbcObjectReader {
                                  const Alembic::Abc::ISampleSelector &sample_sel,
                                  int read_flag,
                                  const char **err_str);
+  virtual bool topology_changed(Mesh *existing_mesh,
+                                const Alembic::Abc::ISampleSelector &sample_sel);
 
   /** Reads the object matrix and sets up an object transform if animated. */
   void setupObjectTransform(const float time);

source/blender/alembic/intern/alembic_capi.cc

@@ -937,12 +937,7 @@ void ABC_get_transform(CacheReader *reader, float r_mat[4][4], float time, float
 
 /* ************************************************************************** */
 
-Mesh *ABC_read_mesh(CacheReader *reader,
-                    Object *ob,
-                    Mesh *existing_mesh,
-                    const float time,
-                    const char **err_str,
-                    int read_flag)
+static AbcObjectReader *get_abc_reader(CacheReader *reader, Object *ob, const char **err_str)
 {
   AbcObjectReader *abc_reader = reinterpret_cast<AbcObjectReader *>(reader);
   IObject iobject = abc_reader->iobject();
@@ -958,12 +953,44 @@ Mesh *ABC_read_mesh(CacheReader *reader,
     return NULL;
   }
 
+  return abc_reader;
+}
+
+static ISampleSelector sample_selector_for_time(float time)
+{
   /* kFloorIndex is used to be compatible with non-interpolating
    * properties; they use the floor. */
-  ISampleSelector sample_sel(time, ISampleSelector::kFloorIndex);
+  return ISampleSelector(time, ISampleSelector::kFloorIndex);
+}
+
+Mesh *ABC_read_mesh(CacheReader *reader,
+                    Object *ob,
+                    Mesh *existing_mesh,
+                    const float time,
+                    const char **err_str,
+                    int read_flag)
+{
+  AbcObjectReader *abc_reader = get_abc_reader(reader, ob, err_str);
+  if (abc_reader == NULL) {
+    return NULL;
+  }
+
+  ISampleSelector sample_sel = sample_selector_for_time(time);
   return abc_reader->read_mesh(existing_mesh, sample_sel, read_flag, err_str);
 }
 
+bool ABC_mesh_topology_changed(
+    CacheReader *reader, Object *ob, Mesh *existing_mesh, const float time, const char **err_str)
+{
+  AbcObjectReader *abc_reader = get_abc_reader(reader, ob, err_str);
+  if (abc_reader == NULL) {
+    return NULL;
+  }
+
+  ISampleSelector sample_sel = sample_selector_for_time(time);
+  return abc_reader->topology_changed(existing_mesh, sample_sel);
+}
+
 /* ************************************************************************** */
 
 void CacheReader_free(CacheReader *reader)

source/blender/modifiers/intern/MOD_meshsequencecache.c

@@ -114,10 +114,20 @@ static Mesh *applyModifier(ModifierData *md, const ModifierEvalContext *ctx, Mes
     }
   }
 
+  /* If this invocation is for the ORCO mesh, and the mesh in Alembic hasn't changed topology, we
+   * must return the mesh as-is instead of deforming it. */
+  if (ctx->flag & MOD_APPLY_ORCO &&
+      !ABC_mesh_topology_changed(mcmd->reader, ctx->object, mesh, time, &err_str)) {
+    return mesh;
+  }
+
   if (me != NULL) {
     MVert *mvert = mesh->mvert;
     MEdge *medge = mesh->medge;
     MPoly *mpoly = mesh->mpoly;
+
+    /* TODO(sybren+bastien): possibly check relevant custom data layers (UV/color depending on
+     * flags) and duplicate those too. */
     if ((me->mvert == mvert) || (me->medge == medge) || (me->mpoly == mpoly)) {
       /* We need to duplicate data here, otherwise we'll modify org mesh, see T51701. */
       BKE_id_copy_ex(NULL,