Geometry Nodes: use virtual arrays in internal attribute api

A virtual array is a data structure that is similar to a normal array
in that its elements can be accessed by an index. However, a virtual
array does not have to be a contiguous array internally. Instead, its
elements can be layed out arbitrarily while element access happens
through a virtual function call. However, the virtual array data
structures are designed so that the virtual function call can be avoided
in cases where it could become a bottleneck.

Most commonly, a virtual array is backed by an actual array/span or
is a single value internally, that is the same for every index.
Besides those, there are many more specialized virtual arrays like the
ones that provides vertex positions based on the `MVert` struct or
vertex group weights.

Not all attributes used by geometry nodes are stored in simple contiguous
arrays. To provide uniform access to all kinds of attributes, the attribute
API has to provide virtual array functionality that hides the implementation
details of attributes.

Before this refactor, the attribute API provided its own virtual array
implementation as part of the `ReadAttribute` and `WriteAttribute` types.
That resulted in unnecessary code duplication with the virtual array system.
Even worse, it bound many algorithms used by geometry nodes to the specifics
of the attribute API, even though they could also use different data sources
(such as data from sockets, default values, later results of expressions, ...).

This refactor removes the `ReadAttribute` and `WriteAttribute` types and
replaces them with `GVArray` and `GVMutableArray` respectively. The `GV`
stands for "generic virtual". The "generic" means that the data type contained
in those virtual arrays is only known at run-time. There are the corresponding
statically typed types `VArray<T>` and `VMutableArray<T>` as well.

No regressions are expected from this refactor. It does come with one
improvement for users. The attribute API can convert the data type
on write now. This is especially useful when writing to builtin attributes
like `material_index` with e.g. the Attribute Math node (which usually
just writes to float attributes, while `material_index` is an integer attribute).

Differential Revision: https://developer.blender.org/D10994

1 files changed, 4 insertions, 4 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_mesh_primitive_uv_sphere.cc b/source/blender/nodes/geometry/nodes/node_geo_mesh_primitive_uv_sphere.cc
index fd95cdc81f7..cc93e71a5dd 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_mesh_primitive_uv_sphere.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_mesh_primitive_uv_sphere.cc
@@ -224,9 +224,9 @@ static void calculate_sphere_uvs(Mesh *mesh, const float segments, const float r
 {
   MeshComponent mesh_component;
   mesh_component.replace(mesh, GeometryOwnershipType::Editable);
-  OutputAttributePtr uv_attribute = mesh_component.attribute_try_get_for_output(
-      "uv_map", ATTR_DOMAIN_CORNER, CD_PROP_FLOAT2, nullptr);
-  MutableSpan<float2> uvs = uv_attribute->get_span_for_write_only<float2>();
+  OutputAttribute_Typed<float2> uv_attribute =
+      mesh_component.attribute_try_get_for_output_only<float2>("uv_map", ATTR_DOMAIN_CORNER);
+  MutableSpan<float2> uvs = uv_attribute.as_span();
 
   int loop_index = 0;
   const float dy = 1.0f / rings;
@@ -256,7 +256,7 @@ static void calculate_sphere_uvs(Mesh *mesh, const float segments, const float r
     uvs[loop_index++] = float2(segment / segments, 1.0f - dy);
   }
 
-  uv_attribute.apply_span_and_save();
+  uv_attribute.save();
 }
 
 static Mesh *create_uv_sphere_mesh(const float radius, const int segments, const int rings)