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, 11 insertions, 9 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_attribute_domain_size.cc b/source/blender/nodes/geometry/nodes/node_geo_attribute_domain_size.cc
index 8ab0eb678e7..7e4904a7a6a 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_attribute_domain_size.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_attribute_domain_size.cc
@@ -72,11 +72,11 @@ static void node_geo_exec(GeoNodeExecParams params)
     case GEO_COMPONENT_TYPE_MESH: {
       if (geometry_set.has_mesh()) {
         const MeshComponent *component = geometry_set.get_component_for_read<MeshComponent>();
-        params.set_output("Point Count", component->attribute_domain_num(ATTR_DOMAIN_POINT));
-        params.set_output("Edge Count", component->attribute_domain_num(ATTR_DOMAIN_EDGE));
-        params.set_output("Face Count", component->attribute_domain_num(ATTR_DOMAIN_FACE));
-        params.set_output("Face Corner Count",
-                          component->attribute_domain_num(ATTR_DOMAIN_CORNER));
+        const AttributeAccessor attributes = *component->attributes();
+        params.set_output("Point Count", attributes.domain_size(ATTR_DOMAIN_POINT));
+        params.set_output("Edge Count", attributes.domain_size(ATTR_DOMAIN_EDGE));
+        params.set_output("Face Count", attributes.domain_size(ATTR_DOMAIN_FACE));
+        params.set_output("Face Corner Count", attributes.domain_size(ATTR_DOMAIN_CORNER));
       }
       else {
         params.set_default_remaining_outputs();
@@ -86,8 +86,9 @@ static void node_geo_exec(GeoNodeExecParams params)
     case GEO_COMPONENT_TYPE_CURVE: {
       if (geometry_set.has_curves()) {
         const CurveComponent *component = geometry_set.get_component_for_read<CurveComponent>();
-        params.set_output("Point Count", component->attribute_domain_num(ATTR_DOMAIN_POINT));
-        params.set_output("Spline Count", component->attribute_domain_num(ATTR_DOMAIN_CURVE));
+        const AttributeAccessor attributes = *component->attributes();
+        params.set_output("Point Count", attributes.domain_size(ATTR_DOMAIN_POINT));
+        params.set_output("Spline Count", attributes.domain_size(ATTR_DOMAIN_CURVE));
       }
       else {
         params.set_default_remaining_outputs();
@@ -98,7 +99,7 @@ static void node_geo_exec(GeoNodeExecParams params)
       if (geometry_set.has_pointcloud()) {
         const PointCloudComponent *component =
             geometry_set.get_component_for_read<PointCloudComponent>();
-        params.set_output("Point Count", component->attribute_domain_num(ATTR_DOMAIN_POINT));
+        params.set_output("Point Count", component->attributes()->domain_size(ATTR_DOMAIN_POINT));
       }
       else {
         params.set_default_remaining_outputs();
@@ -109,7 +110,8 @@ static void node_geo_exec(GeoNodeExecParams params)
       if (geometry_set.has_instances()) {
         const InstancesComponent *component =
             geometry_set.get_component_for_read<InstancesComponent>();
-        params.set_output("Instance Count", component->attribute_domain_num(ATTR_DOMAIN_INSTANCE));
+        params.set_output("Instance Count",
+                          component->attributes()->domain_size(ATTR_DOMAIN_INSTANCE));
       }
       else {
         params.set_default_remaining_outputs();