Geometry Nodes: Make instances real on-demand

This commit makes the geometry output of the collection info usable.
The output is the geometry of a collection instance, but this commit
adds a utility to convert the instances to real geometry, used in the
background whenever it is needed, like copy on write.

The recursive nature of the "realize instances" code is essential,
because collection instances in the `InstancesComponent`, might have no
geometry sets of their own containing even more collection instances,
which might then contain object instances, etc.

Another consideration is that currently, every single instance contains
a reference to its data. This is inefficient since most of the time
there are many locations and only a few sets of unique data. So this
commit adds a `GeometryInstanceGroup` to support this future optimization.
The API for instances returns a vector of `GeometryInstanceGroup`.
This may be less efficient when there are many instances, but it makes
more complicated operations like point distribution that need to iterate
over input geometry multiple times much simpler.

Any code that needs to change data, like most of the attribute nodes,
can simply call `geometry_set_realize_instances(geometry_set)`,
which will move any geometry in the `InstancesComponent` to new "real"
geometry components.

Many nodes can support read-only access to instances in order to avoid
making them real, this will be addressed where needed in the near future.

Instances from the existing "dupli" system are not supported yet.

Differential Revision: https://developer.blender.org/D10327

1 files changed, 17 insertions, 0 deletions

diff --git a/source/blender/nodes/geometry/node_geometry_util.hh b/source/blender/nodes/geometry/node_geometry_util.hh
index b7b2afeefcb..7ca44d82a38 100644
--- a/source/blender/nodes/geometry/node_geometry_util.hh
+++ b/source/blender/nodes/geometry/node_geometry_util.hh
@@ -49,4 +49,21 @@ CustomDataType attribute_data_type_highest_complexity(Span<CustomDataType>);
 Array<uint32_t> get_geometry_element_ids_as_uints(const GeometryComponent &component,
                                                   const AttributeDomain domain);
 
+GeometrySet geometry_set_realize_instances(const GeometrySet &geometry_set);
+
+struct AttributeInfo {
+  CustomDataType data_type;
+  AttributeDomain domain;
+};
+
+/**
+ * Add information about all the attributes on every component of the type. The resulting info
+ * will contain the highest complexity data type and the highest priority domain among every
+ * attribute with the given name on all of the input components.
+ */
+void gather_attribute_info(Map<std::string, AttributeInfo> &attributes,
+                           const GeometryComponentType component_type,
+                           Span<GeometryInstanceGroup> set_groups,
+                           const Set<std::string> &ignored_attributes);
+
 }  // namespace blender::nodes