Geometry Nodes: new geometry attribute API

Currently, there are two attribute API. The first, defined in `BKE_attribute.h` is
accessible from RNA and C code. The second is implemented with `GeometryComponent`
and is only accessible in C++ code. The second is widely used, but only being
accessible through the `GeometrySet` API makes it awkward to use, and even impossible
for types that don't correspond directly to a geometry component like `CurvesGeometry`.

This patch adds a new attribute API, designed to replace the `GeometryComponent`
attribute API now, and to eventually replace or be the basis of the other one.

The basic idea is that there is an `AttributeAccessor` class that allows code to
interact with a set of attributes owned by some geometry. The accessor itself has
no ownership. `AttributeAccessor` is a simple type that can be passed around by
value. That makes it easy to return it from functions and to store it in containers.

For const-correctness, there is also a `MutableAttributeAccessor` that allows
changing individual and can add or remove attributes.

Currently, `AttributeAccessor` is composed of two pointers. The first is a pointer
to the owner of the attribute data. The second is a pointer to a struct with
function pointers, that is similar to a virtual function table. The functions
know how to access attributes on the owner.

The actual attribute access for geometries is still implemented with the `AttributeProvider`
pattern, which makes it easy to support different sources of attributes on a
geometry and simplifies dealing with built-in attributes.

There are different ways to get an attribute accessor for a geometry:
* `GeometryComponent.attributes()`
* `CurvesGeometry.attributes()`
* `bke::mesh_attributes(const Mesh &)`
* `bke::pointcloud_attributes(const PointCloud &)`

All of these also have a `_for_write` variant that returns a `MutabelAttributeAccessor`.

Differential Revision: https://developer.blender.org/D15280

4 files changed, 29 insertions, 47 deletions

diff --git a/source/blender/editors/sculpt_paint/curves_sculpt_add.cc b/source/blender/editors/sculpt_paint/curves_sculpt_add.cc
index 401550b3b3a..26145a386f5 100644
--- a/source/blender/editors/sculpt_paint/curves_sculpt_add.cc
+++ b/source/blender/editors/sculpt_paint/curves_sculpt_add.cc
@@ -176,12 +176,9 @@ struct AddOperationExecutor {
     /* Find UV map. */
     VArraySpan<float2> surface_uv_map;
     if (curves_id_->surface_uv_map != nullptr) {
-      MeshComponent surface_component;
-      surface_component.replace(surface_, GeometryOwnershipType::ReadOnly);
-      surface_uv_map = surface_component
-                           .attribute_try_get_for_read(curves_id_->surface_uv_map,
-                                                       ATTR_DOMAIN_CORNER)
-                           .typed<float2>();
+      const bke::AttributeAccessor surface_attributes = bke::mesh_attributes(*surface_);
+      surface_uv_map = surface_attributes.lookup<float2>(curves_id_->surface_uv_map,
+                                                         ATTR_DOMAIN_CORNER);
     }
 
     /* Find normals. */
diff --git a/source/blender/editors/sculpt_paint/curves_sculpt_density.cc b/source/blender/editors/sculpt_paint/curves_sculpt_density.cc
index 826b0611e81..e211a568705 100644
--- a/source/blender/editors/sculpt_paint/curves_sculpt_density.cc
+++ b/source/blender/editors/sculpt_paint/curves_sculpt_density.cc
@@ -64,7 +64,6 @@ struct DensityAddOperationExecutor {
   Mesh *surface_ = nullptr;
   Span<MLoopTri> surface_looptris_;
   Span<float3> corner_normals_su_;
-  VArraySpan<float2> surface_uv_map_;
 
   const CurvesSculpt *curves_sculpt_ = nullptr;
   const Brush *brush_ = nullptr;
@@ -228,12 +227,9 @@ struct DensityAddOperationExecutor {
     /* Find UV map. */
     VArraySpan<float2> surface_uv_map;
     if (curves_id_->surface_uv_map != nullptr) {
-      MeshComponent surface_component;
-      surface_component.replace(surface_, GeometryOwnershipType::ReadOnly);
-      surface_uv_map = surface_component
-                           .attribute_try_get_for_read(curves_id_->surface_uv_map,
-                                                       ATTR_DOMAIN_CORNER)
-                           .typed<float2>();
+      bke::AttributeAccessor surface_attributes = bke::mesh_attributes(*surface_);
+      surface_uv_map = surface_attributes.lookup<float2>(curves_id_->surface_uv_map,
+                                                         ATTR_DOMAIN_CORNER);
     }
 
     /* Find normals. */
diff --git a/source/blender/editors/sculpt_paint/curves_sculpt_slide.cc b/source/blender/editors/sculpt_paint/curves_sculpt_slide.cc
index dedc880988d..aabe6fd93e4 100644
--- a/source/blender/editors/sculpt_paint/curves_sculpt_slide.cc
+++ b/source/blender/editors/sculpt_paint/curves_sculpt_slide.cc
@@ -148,12 +148,9 @@ struct SlideOperationExecutor {
                          BKE_mesh_runtime_looptri_len(surface_)};
 
     if (curves_id_->surface_uv_map != nullptr) {
-      MeshComponent surface_component;
-      surface_component.replace(surface_, GeometryOwnershipType::ReadOnly);
-      surface_uv_map_ = surface_component
-                            .attribute_try_get_for_read(curves_id_->surface_uv_map,
-                                                        ATTR_DOMAIN_CORNER)
-                            .typed<float2>();
+      const bke::AttributeAccessor surface_attributes = bke::mesh_attributes(*surface_);
+      surface_uv_map_ = surface_attributes.lookup<float2>(curves_id_->surface_uv_map,
+                                                          ATTR_DOMAIN_CORNER);
     }
 
     if (stroke_extension.is_first) {
diff --git a/source/blender/editors/sculpt_paint/paint_vertex_color_ops.cc b/source/blender/editors/sculpt_paint/paint_vertex_color_ops.cc
index 50f8854d90f..8b726c7b942 100644
--- a/source/blender/editors/sculpt_paint/paint_vertex_color_ops.cc
+++ b/source/blender/editors/sculpt_paint/paint_vertex_color_ops.cc
@@ -92,11 +92,9 @@ static bool vertex_paint_from_weight(Object *ob)
     return false;
   }
 
-  MeshComponent component;
-  component.replace(me, GeometryOwnershipType::Editable);
+  bke::MutableAttributeAccessor attributes = bke::mesh_attributes_for_write(*me);
 
-  bke::WriteAttributeLookup color_attribute = component.attribute_try_get_for_write(
-      active_color_layer->name);
+  bke::GAttributeWriter color_attribute = attributes.lookup_for_write(active_color_layer->name);
   if (!color_attribute) {
     BLI_assert_unreachable();
     return false;
@@ -104,7 +102,7 @@ static bool vertex_paint_from_weight(Object *ob)
 
   /* Retrieve the vertex group with the domain and type of the existing color
    * attribute, in order to let the attribute API handle both conversions. */
-  const GVArray vertex_group = component.attribute_get_for_read(
+  const GVArray vertex_group = attributes.lookup(
       deform_group->name,
       ATTR_DOMAIN_POINT,
       bke::cpp_type_to_custom_data_type(color_attribute.varray.type()));
@@ -113,14 +111,11 @@ static bool vertex_paint_from_weight(Object *ob)
     return false;
   }
 
-  GVArraySpan interpolated{component.attribute_try_adapt_domain(
-      vertex_group, ATTR_DOMAIN_POINT, color_attribute.domain)};
+  GVArraySpan interpolated{
+      attributes.adapt_domain(vertex_group, ATTR_DOMAIN_POINT, color_attribute.domain)};
 
   color_attribute.varray.set_all(interpolated.data());
-
-  if (color_attribute.tag_modified_fn) {
-    color_attribute.tag_modified_fn();
-  }
+  color_attribute.finish();
   tag_object_after_update(ob);
 
   return true;
@@ -167,29 +162,28 @@ static IndexMask get_selected_indices(const Mesh &mesh,
   Span<MVert> verts(mesh.mvert, mesh.totvert);
   Span<MPoly> faces(mesh.mpoly, mesh.totpoly);
 
-  MeshComponent component;
-  component.replace(&const_cast<Mesh &>(mesh), GeometryOwnershipType::ReadOnly);
+  bke::AttributeAccessor attributes = bke::mesh_attributes(mesh);
 
   if (mesh.editflag & ME_EDIT_PAINT_FACE_SEL) {
-    const VArray<bool> selection = component.attribute_try_adapt_domain(
+    const VArray<bool> selection = attributes.adapt_domain(
         VArray<bool>::ForFunc(faces.size(),
                               [&](const int i) { return faces[i].flag & ME_FACE_SEL; }),
         ATTR_DOMAIN_FACE,
         domain);
 
     return index_mask_ops::find_indices_from_virtual_array(
-        IndexMask(component.attribute_domain_num(domain)), selection, 4096, indices);
+        IndexMask(attributes.domain_size(domain)), selection, 4096, indices);
   }
   if (mesh.editflag & ME_EDIT_PAINT_VERT_SEL) {
-    const VArray<bool> selection = component.attribute_try_adapt_domain(
+    const VArray<bool> selection = attributes.adapt_domain(
         VArray<bool>::ForFunc(verts.size(), [&](const int i) { return verts[i].flag & SELECT; }),
         ATTR_DOMAIN_POINT,
         domain);
 
     return index_mask_ops::find_indices_from_virtual_array(
-        IndexMask(component.attribute_domain_num(domain)), selection, 4096, indices);
+        IndexMask(attributes.domain_size(domain)), selection, 4096, indices);
   }
-  return IndexMask(component.attribute_domain_num(domain));
+  return IndexMask(attributes.domain_size(domain));
 }
 
 static void face_corner_color_equalize_vertices(Mesh &mesh, const IndexMask selection)
@@ -202,17 +196,15 @@ static void face_corner_color_equalize_vertices(Mesh &mesh, const IndexMask sele
     return;
   }
 
-  MeshComponent component;
-  component.replace(&mesh, GeometryOwnershipType::Editable);
+  bke::AttributeAccessor attributes = bke::mesh_attributes(mesh);
 
-  if (component.attribute_get_meta_data(active_color_layer->name)->domain == ATTR_DOMAIN_POINT) {
+  if (attributes.lookup_meta_data(active_color_layer->name)->domain == ATTR_DOMAIN_POINT) {
     return;
   }
 
-  GVArray color_attribute_point = component.attribute_try_get_for_read(active_color_layer->name,
-                                                                       ATTR_DOMAIN_POINT);
+  GVArray color_attribute_point = attributes.lookup(active_color_layer->name, ATTR_DOMAIN_POINT);
 
-  GVArray color_attribute_corner = component.attribute_try_adapt_domain(
+  GVArray color_attribute_corner = attributes.adapt_domain(
       color_attribute_point, ATTR_DOMAIN_POINT, ATTR_DOMAIN_CORNER);
 
   color_attribute_corner.materialize(selection, active_color_layer->data);
@@ -278,11 +270,9 @@ static bool transform_active_color(Mesh &mesh, const TransformFn &transform_fn)
     return false;
   }
 
-  MeshComponent component;
-  component.replace(&mesh, GeometryOwnershipType::Editable);
+  bke::MutableAttributeAccessor attributes = bke::mesh_attributes_for_write(mesh);
 
-  bke::WriteAttributeLookup color_attribute = component.attribute_try_get_for_write(
-      active_color_layer->name);
+  bke::GAttributeWriter color_attribute = attributes.lookup_for_write(active_color_layer->name);
   if (!color_attribute) {
     BLI_assert_unreachable();
     return false;
@@ -310,6 +300,8 @@ static bool transform_active_color(Mesh &mesh, const TransformFn &transform_fn)
     });
   });
 
+  color_attribute.finish();
+
   DEG_id_tag_update(&mesh.id, 0);
 
   return true;