Mesh: Move material indices to a generic attribute

This patch moves material indices from the mesh `MPoly` struct to a
generic integer attribute. The builtin material index was already
exposed in geometry nodes, but this makes it a "proper" attribute
accessible with Python and visible in the "Attributes" panel.

The goals of the refactor are code simplification and memory and
performance improvements, mainly because the attribute doesn't have
to be stored and processed if there are no materials. However, until
4.0, material indices will still be read and written in the old
format, meaning there may be a temporary increase in memory usage.

Further notes:
* Completely removing the `MPoly.mat_nr` after 4.0 may require
  changes to DNA or introducing a new `MPoly` type.
* Geometry nodes regression tests didn't look at material indices,
  so the change reveals a bug in the realize instances node that I fixed.
* Access to material indices from the RNA `MeshPolygon` type is slower
  with this patch. The `material_index` attribute can be used instead.
* Cycles is changed to read from the attribute instead.
* BMesh isn't changed in this patch. Theoretically it could be though,
  to save 2 bytes per face when less than two materials are used.
* Eventually we could use a 16 bit integer attribute type instead.

Ref T95967

Differential Revision: https://developer.blender.org/D15675

1 files changed, 88 insertions, 32 deletions

diff --git a/source/blender/geometry/intern/realize_instances.cc b/source/blender/geometry/intern/realize_instances.cc
index 87a610ac0a2..25ff705385c 100644
--- a/source/blender/geometry/intern/realize_instances.cc
+++ b/source/blender/geometry/intern/realize_instances.cc
@@ -9,6 +9,7 @@
 #include "DNA_object_types.h"
 #include "DNA_pointcloud_types.h"
 
+#include "BLI_devirtualize_parameters.hh"
 #include "BLI_noise.hh"
 #include "BLI_task.hh"
 
@@ -102,6 +103,7 @@ struct MeshRealizeInfo {
   Array<std::optional<GVArraySpan>> attributes;
   /** Vertex ids stored on the mesh. If there are no ids, this #Span is empty. */
   Span<int> stored_vertex_ids;
+  VArray<int> material_indices;
 };
 
 struct RealizeMeshTask {
@@ -182,6 +184,7 @@ struct AllMeshesInfo {
   /** Ordered materials on the output mesh. */
   VectorSet<Material *> materials;
   bool create_id_attribute = false;
+  bool create_material_index_attribute = false;
 };
 
 struct AllCurvesInfo {
@@ -787,7 +790,10 @@ static void execute_realize_pointcloud_tasks(const RealizeInstancesOptions &opti
  * \{ */
 
 static OrderedAttributes gather_generic_mesh_attributes_to_propagate(
-    const GeometrySet &in_geometry_set, const RealizeInstancesOptions &options, bool &r_create_id)
+    const GeometrySet &in_geometry_set,
+    const RealizeInstancesOptions &options,
+    bool &r_create_id,
+    bool &r_create_material_index)
 {
   Vector<GeometryComponentType> src_component_types;
   src_component_types.append(GEO_COMPONENT_TYPE_MESH);
@@ -800,10 +806,10 @@ static OrderedAttributes gather_generic_mesh_attributes_to_propagate(
       src_component_types, GEO_COMPONENT_TYPE_MESH, true, attributes_to_propagate);
   attributes_to_propagate.remove("position");
   attributes_to_propagate.remove("normal");
-  attributes_to_propagate.remove("material_index");
   attributes_to_propagate.remove("shade_smooth");
   attributes_to_propagate.remove("crease");
   r_create_id = attributes_to_propagate.pop_try("id").has_value();
+  r_create_material_index = attributes_to_propagate.pop_try("material_index").has_value();
   OrderedAttributes ordered_attributes;
   for (const auto item : attributes_to_propagate.items()) {
     ordered_attributes.ids.add_new(item.key);
@@ -833,13 +839,19 @@ static AllMeshesInfo preprocess_meshes(const GeometrySet &geometry_set,
 {
   AllMeshesInfo info;
   info.attributes = gather_generic_mesh_attributes_to_propagate(
-      geometry_set, options, info.create_id_attribute);
+      geometry_set, options, info.create_id_attribute, info.create_material_index_attribute);
 
   gather_meshes_to_realize(geometry_set, info.order);
   for (const Mesh *mesh : info.order) {
-    for (const int slot_index : IndexRange(mesh->totcol)) {
-      Material *material = mesh->mat[slot_index];
-      info.materials.add(material);
+    if (mesh->totcol == 0) {
+      /* Add an empty material slot for the default material. */
+      info.materials.add(nullptr);
+    }
+    else {
+      for (const int slot_index : IndexRange(mesh->totcol)) {
+        Material *material = mesh->mat[slot_index];
+        info.materials.add(material);
+      }
     }
   }
   info.realize_info.reinitialize(info.order.size());
@@ -849,11 +861,16 @@ static AllMeshesInfo preprocess_meshes(const GeometrySet &geometry_set,
     mesh_info.mesh = mesh;
 
     /* Create material index mapping. */
-    mesh_info.material_index_map.reinitialize(mesh->totcol);
-    for (const int old_slot_index : IndexRange(mesh->totcol)) {
-      Material *material = mesh->mat[old_slot_index];
-      const int new_slot_index = info.materials.index_of(material);
-      mesh_info.material_index_map[old_slot_index] = new_slot_index;
+    mesh_info.material_index_map.reinitialize(std::max<int>(mesh->totcol, 1));
+    if (mesh->totcol == 0) {
+      mesh_info.material_index_map.first() = info.materials.index_of(nullptr);
+    }
+    else {
+      for (const int old_slot_index : IndexRange(mesh->totcol)) {
+        Material *material = mesh->mat[old_slot_index];
+        const int new_slot_index = info.materials.index_of(material);
+        mesh_info.material_index_map[old_slot_index] = new_slot_index;
+      }
     }
 
     /* Access attributes. */
@@ -874,6 +891,8 @@ static AllMeshesInfo preprocess_meshes(const GeometrySet &geometry_set,
         mesh_info.stored_vertex_ids = ids_attribute.varray.get_internal_span().typed<int>();
       }
     }
+    mesh_info.material_indices = attributes.lookup_or_default<int>(
+        "material_index", ATTR_DOMAIN_FACE, 0);
   }
   return info;
 }
@@ -883,7 +902,8 @@ static void execute_realize_mesh_task(const RealizeInstancesOptions &options,
                                       const OrderedAttributes &ordered_attributes,
                                       Mesh &dst_mesh,
                                       MutableSpan<GSpanAttributeWriter> dst_attribute_writers,
-                                      MutableSpan<int> all_dst_vertex_ids)
+                                      MutableSpan<int> all_dst_vertex_ids,
+                                      MutableSpan<int> all_dst_material_indices)
 {
   const MeshRealizeInfo &mesh_info = *task.mesh_info;
   const Mesh &mesh = *mesh_info.mesh;
@@ -893,14 +913,19 @@ static void execute_realize_mesh_task(const RealizeInstancesOptions &options,
   const Span<MLoop> src_loops{mesh.mloop, mesh.totloop};
   const Span<MPoly> src_polys{mesh.mpoly, mesh.totpoly};
 
-  MutableSpan<MVert> dst_verts{dst_mesh.mvert + task.start_indices.vertex, mesh.totvert};
-  MutableSpan<MEdge> dst_edges{dst_mesh.medge + task.start_indices.edge, mesh.totedge};
-  MutableSpan<MLoop> dst_loops{dst_mesh.mloop + task.start_indices.loop, mesh.totloop};
-  MutableSpan<MPoly> dst_polys{dst_mesh.mpoly + task.start_indices.poly, mesh.totpoly};
+  const IndexRange dst_vert_range(task.start_indices.vertex, src_verts.size());
+  const IndexRange dst_edge_range(task.start_indices.edge, src_edges.size());
+  const IndexRange dst_poly_range(task.start_indices.poly, src_polys.size());
+  const IndexRange dst_loop_range(task.start_indices.loop, src_loops.size());
+
+  MutableSpan dst_verts = MutableSpan(dst_mesh.mvert, dst_mesh.totvert).slice(dst_vert_range);
+  MutableSpan dst_edges = MutableSpan(dst_mesh.medge, dst_mesh.totedge).slice(dst_edge_range);
+  MutableSpan dst_polys = MutableSpan(dst_mesh.mpoly, dst_mesh.totpoly).slice(dst_poly_range);
+  MutableSpan dst_loops = MutableSpan(dst_mesh.mloop, dst_mesh.totloop).slice(dst_loop_range);
 
   const Span<int> material_index_map = mesh_info.material_index_map;
 
-  threading::parallel_for(IndexRange(mesh.totvert), 1024, [&](const IndexRange vert_range) {
+  threading::parallel_for(src_verts.index_range(), 1024, [&](const IndexRange vert_range) {
     for (const int i : vert_range) {
       const MVert &src_vert = src_verts[i];
       MVert &dst_vert = dst_verts[i];
@@ -908,7 +933,7 @@ static void execute_realize_mesh_task(const RealizeInstancesOptions &options,
       copy_v3_v3(dst_vert.co, task.transform * float3(src_vert.co));
     }
   });
-  threading::parallel_for(IndexRange(mesh.totedge), 1024, [&](const IndexRange edge_range) {
+  threading::parallel_for(src_edges.index_range(), 1024, [&](const IndexRange edge_range) {
     for (const int i : edge_range) {
       const MEdge &src_edge = src_edges[i];
       MEdge &dst_edge = dst_edges[i];
@@ -917,7 +942,7 @@ static void execute_realize_mesh_task(const RealizeInstancesOptions &options,
       dst_edge.v2 += task.start_indices.vertex;
     }
   });
-  threading::parallel_for(IndexRange(mesh.totloop), 1024, [&](const IndexRange loop_range) {
+  threading::parallel_for(src_loops.index_range(), 1024, [&](const IndexRange loop_range) {
     for (const int i : loop_range) {
       const MLoop &src_loop = src_loops[i];
       MLoop &dst_loop = dst_loops[i];
@@ -926,21 +951,38 @@ static void execute_realize_mesh_task(const RealizeInstancesOptions &options,
       dst_loop.e += task.start_indices.edge;
     }
   });
-  threading::parallel_for(IndexRange(mesh.totpoly), 1024, [&](const IndexRange poly_range) {
+  threading::parallel_for(src_polys.index_range(), 1024, [&](const IndexRange poly_range) {
     for (const int i : poly_range) {
       const MPoly &src_poly = src_polys[i];
       MPoly &dst_poly = dst_polys[i];
       dst_poly = src_poly;
       dst_poly.loopstart += task.start_indices.loop;
-      if (src_poly.mat_nr >= 0 && src_poly.mat_nr < mesh.totcol) {
-        dst_poly.mat_nr = material_index_map[src_poly.mat_nr];
+    }
+  });
+  if (!all_dst_material_indices.is_empty()) {
+    MutableSpan<int> dst_material_indices = all_dst_material_indices.slice(dst_poly_range);
+    if (mesh.totcol == 0) {
+      /* The material index map contains the index of the null material in the result. */
+      dst_material_indices.fill(material_index_map.first());
+    }
+    else {
+      if (mesh_info.material_indices.is_single()) {
+        const int src_index = mesh_info.material_indices.get_internal_single();
+        const bool valid = IndexRange(mesh.totcol).contains(src_index);
+        dst_material_indices.fill(valid ? material_index_map[src_index] : 0);
       }
       else {
-        /* The material index was invalid before. */
-        dst_poly.mat_nr = 0;
+        VArraySpan<int> indices_span(mesh_info.material_indices);
+        threading::parallel_for(src_polys.index_range(), 1024, [&](const IndexRange poly_range) {
+          for (const int i : poly_range) {
+            const int src_index = indices_span[i];
+            const bool valid = IndexRange(mesh.totcol).contains(src_index);
+            dst_material_indices[i] = valid ? material_index_map[src_index] : 0;
+          }
+        });
       }
     }
-  });
+  }
 
   if (!all_dst_vertex_ids.is_empty()) {
     create_result_ids(options,
@@ -956,13 +998,13 @@ static void execute_realize_mesh_task(const RealizeInstancesOptions &options,
       [&](const eAttrDomain domain) {
         switch (domain) {
           case ATTR_DOMAIN_POINT:
-            return IndexRange(task.start_indices.vertex, mesh.totvert);
+            return dst_vert_range;
           case ATTR_DOMAIN_EDGE:
-            return IndexRange(task.start_indices.edge, mesh.totedge);
-          case ATTR_DOMAIN_CORNER:
-            return IndexRange(task.start_indices.loop, mesh.totloop);
+            return dst_edge_range;
           case ATTR_DOMAIN_FACE:
-            return IndexRange(task.start_indices.poly, mesh.totpoly);
+            return dst_poly_range;
+          case ATTR_DOMAIN_CORNER:
+            return dst_loop_range;
           default:
             BLI_assert_unreachable();
             return IndexRange();
@@ -1013,6 +1055,12 @@ static void execute_realize_mesh_tasks(const RealizeInstancesOptions &options,
   if (all_meshes_info.create_id_attribute) {
     vertex_ids = dst_attributes.lookup_or_add_for_write_only_span<int>("id", ATTR_DOMAIN_POINT);
   }
+  /* Prepare material indices. */
+  SpanAttributeWriter<int> material_indices;
+  if (all_meshes_info.create_material_index_attribute) {
+    material_indices = dst_attributes.lookup_or_add_for_write_only_span<int>("material_index",
+                                                                             ATTR_DOMAIN_FACE);
+  }
 
   /* Prepare generic output attributes. */
   Vector<GSpanAttributeWriter> dst_attribute_writers;
@@ -1028,8 +1076,13 @@ static void execute_realize_mesh_tasks(const RealizeInstancesOptions &options,
   threading::parallel_for(tasks.index_range(), 100, [&](const IndexRange task_range) {
     for (const int task_index : task_range) {
       const RealizeMeshTask &task = tasks[task_index];
-      execute_realize_mesh_task(
-          options, task, ordered_attributes, *dst_mesh, dst_attribute_writers, vertex_ids.span);
+      execute_realize_mesh_task(options,
+                                task,
+                                ordered_attributes,
+                                *dst_mesh,
+                                dst_attribute_writers,
+                                vertex_ids.span,
+                                material_indices.span);
     }
   });
 
@@ -1040,6 +1093,9 @@ static void execute_realize_mesh_tasks(const RealizeInstancesOptions &options,
   if (vertex_ids) {
     vertex_ids.finish();
   }
+  if (material_indices) {
+    material_indices.finish();
+  }
 }
 
 /** \} */