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

1 files changed, 5 insertions, 2 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_points_to_volume.cc b/source/blender/nodes/geometry/nodes/node_geo_points_to_volume.cc
index 42cee4c0efe..28a01a5cbce 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_points_to_volume.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_points_to_volume.cc
@@ -163,12 +163,15 @@ static void gather_point_data_from_component(GeoNodeExecParams &params,
                                              Vector<float3> &r_positions,
                                              Vector<float> &r_radii)
 {
-  VArray<float3> positions = component.attribute_get_for_read<float3>(
+  if (component.is_empty()) {
+    return;
+  }
+  VArray<float3> positions = component.attributes()->lookup_or_default<float3>(
       "position", ATTR_DOMAIN_POINT, {0, 0, 0});
 
   Field<float> radius_field = params.get_input<Field<float>>("Radius");
   GeometryComponentFieldContext field_context{component, ATTR_DOMAIN_POINT};
-  const int domain_num = component.attribute_domain_num(ATTR_DOMAIN_POINT);
+  const int domain_num = component.attribute_domain_size(ATTR_DOMAIN_POINT);
 
   r_positions.resize(r_positions.size() + domain_num);
   positions.materialize(r_positions.as_mutable_span().take_back(domain_num));