source/blender/blenkernel/intern/cachefile.c

@@ -188,7 +188,8 @@ void BKE_cachefile_reader_open(CacheFile *cache_file,
     case CACHEFILE_TYPE_ALEMBIC:
 #  ifdef WITH_ALEMBIC
       /* Open Alembic cache reader. */
-      *reader = CacheReader_open_alembic_object(cache_file->handle, *reader, object, object_path);
+      *reader = CacheReader_open_alembic_object(
+          cache_file->handle, *reader, object, object_path, cache_file->is_sequence);
 #  endif
       break;
     case CACHEFILE_TYPE_USD:

source/blender/io/alembic/ABC_alembic.h

@@ -141,7 +141,8 @@ void ABC_CacheReader_free(struct CacheReader *reader);
 struct CacheReader *CacheReader_open_alembic_object(struct CacheArchiveHandle *handle,
                                                     struct CacheReader *reader,
                                                     struct Object *object,
-                                                    const char *object_path);
+                                                    const char *object_path,
+                                                    bool is_sequence);
 
 #ifdef __cplusplus
 }

source/blender/io/alembic/intern/abc_reader_mesh.cc

@@ -465,6 +465,39 @@ static void read_velocity(const V3fArraySamplePtr &velocities,
   }
 }
 
+static bool samples_have_same_topology(const IPolyMeshSchema::Sample &sample,
+                                       const IPolyMeshSchema::Sample &ceil_sample)
+{
+  const P3fArraySamplePtr &positions = sample.getPositions();
+  const Alembic::Abc::Int32ArraySamplePtr &face_indices = sample.getFaceIndices();
+  const Alembic::Abc::Int32ArraySamplePtr &face_counts = sample.getFaceCounts();
+
+  const P3fArraySamplePtr &ceil_positions = ceil_sample.getPositions();
+  const Alembic::Abc::Int32ArraySamplePtr &ceil_face_indices = ceil_sample.getFaceIndices();
+  const Alembic::Abc::Int32ArraySamplePtr &ceil_face_counts = ceil_sample.getFaceCounts();
+
+  /* It the counters are different, we can be sure the topology is different. */
+  const bool different_counters = positions->size() != ceil_positions->size() ||
+                                  face_counts->size() != ceil_face_counts->size() ||
+                                  face_indices->size() != ceil_face_indices->size();
+  if (different_counters) {
+    return false;
+  }
+
+  /* Otherwise, we need to check the connectivity as files from e.g. videogrammetry may have the
+   * same face count, but different connections between faces. */
+
+  if (memcmp(face_counts->get(), ceil_face_counts->get(), face_counts->size() * sizeof(int))) {
+    return false;
+  }
+
+  if (memcmp(face_indices->get(), ceil_face_indices->get(), face_indices->size() * sizeof(int))) {
+    return false;
+  }
+
+  return true;
+}
+
 static void read_mesh_sample(const std::string &iobject_full_name,
                              ImportSettings *settings,
                              const IPolyMeshSchema &schema,
@@ -483,7 +516,10 @@ static void read_mesh_sample(const std::string &iobject_full_name,
   if (config.weight != 0.0f) {
     Alembic::AbcGeom::IPolyMeshSchema::Sample ceil_sample;
     schema.get(ceil_sample, Alembic::Abc::ISampleSelector(config.ceil_index));
-    abc_mesh_data.ceil_positions = ceil_sample.getPositions();
+    if (samples_have_same_topology(sample, ceil_sample)) {
+      /* Only set interpolation data if the samples are compatible. */
+      abc_mesh_data.ceil_positions = ceil_sample.getPositions();
+    }
   }
 
   if ((settings->read_flag & MOD_MESHSEQ_READ_UV) != 0) {
@@ -668,9 +704,47 @@ bool AbcMeshReader::topology_changed(const Mesh *existing_mesh, const ISampleSel
   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;
+  /* It the counters are different, we can be sure the topology is different. */
+  const bool different_counters = positions->size() != existing_mesh->totvert ||
+                                  face_counts->size() != existing_mesh->totpoly ||
+                                  face_indices->size() != existing_mesh->totloop;
+  if (different_counters) {
+    return true;
+  }
+
+  /* Check first if we indeed have multiple samples, unless we read a file sequence in which case
+   * we need to do a full topology comparison. */
+  if (!m_is_reading_a_file_sequence && (m_schema.getFaceIndicesProperty().getNumSamples() == 1 &&
+                                        m_schema.getFaceCountsProperty().getNumSamples() == 1))
+  {
+    return false;
+  }
+
+  /* Otherwise, we need to check the connectivity as files from e.g. videogrammetry may have the
+   * same face count, but different connections between faces. */
+  uint abc_index = 0;
+
+  const int *mesh_corners = existing_mesh->corner_verts().data();
+  const int *mesh_poly_offsets = existing_mesh->poly_offsets().data();
+
+  for (int i = 0; i < face_counts->size(); i++) {
+    if (mesh_poly_offsets[i] != abc_index) {
+      return true;
+    }
+
+    const int abc_face_size = (*face_counts)[i];
+    /* NOTE: Alembic data is stored in the reverse order. */
+    uint rev_loop_index = abc_index + (abc_face_size - 1);
+    for (int f = 0; f < abc_face_size; f++, abc_index++, rev_loop_index--) {
+      const int mesh_vert = mesh_corners[rev_loop_index];
+      const int abc_vert = (*face_indices)[abc_index];
+      if (mesh_vert != abc_vert) {
+        return true;
+      }
+    }
+  }
+
+  return false;
 }
 
 Mesh *AbcMeshReader::read_mesh(Mesh *existing_mesh,

source/blender/io/alembic/intern/abc_reader_object.cc

@@ -33,6 +33,7 @@ AbcObjectReader::AbcObjectReader(const IObject &object, ImportSettings &settings
     : m_object(nullptr),
       m_iobject(object),
       m_settings(&settings),
+      m_is_reading_a_file_sequence(settings.is_sequence),
       m_min_time(std::numeric_limits<chrono_t>::max()),
       m_max_time(std::numeric_limits<chrono_t>::min()),
       m_refcount(0),

source/blender/io/alembic/intern/abc_reader_object.h

@@ -77,7 +77,11 @@ class AbcObjectReader {
   Object *m_object;
   Alembic::Abc::IObject m_iobject;
 
+  /* XXX - TODO(kevindietrich) : this references stack memory... */
   ImportSettings *m_settings;
+  /* This is initialised from the ImportSettings above on construction. It will need to be removed
+   * once we fix the stack memory reference situation.  */
+  bool m_is_reading_a_file_sequence = false;
 
   chrono_t m_min_time;
   chrono_t m_max_time;

source/blender/io/alembic/intern/alembic_capi.cc

@@ -822,6 +822,7 @@ bool ABC_mesh_topology_changed(CacheReader *reader,
     return false;
   }
 
+  std::cerr << __func__ << '\n';
   ISampleSelector sample_sel = sample_selector_for_time(time);
   return abc_reader->topology_changed(existing_mesh, sample_sel);
 }
@@ -847,7 +848,8 @@ void ABC_CacheReader_incref(CacheReader *reader)
 CacheReader *CacheReader_open_alembic_object(CacheArchiveHandle *handle,
                                              CacheReader *reader,
                                              Object *object,
-                                             const char *object_path)
+                                             const char *object_path,
+                                             const bool is_sequence)
 {
   if (object_path[0] == '\0') {
     return reader;
@@ -867,6 +869,7 @@ CacheReader *CacheReader_open_alembic_object(CacheArchiveHandle *handle,
   }
 
   ImportSettings settings;
+  settings.is_sequence = is_sequence;
   AbcObjectReader *abc_reader = create_reader(iobject, settings);
   if (abc_reader == nullptr) {
     /* This object is not supported */