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, 2 insertions, 0 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_triangulate.cc b/source/blender/nodes/geometry/nodes/node_geo_triangulate.cc
index 32fa32a9f13..0c754ad3b37 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_triangulate.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_triangulate.cc
@@ -65,6 +65,8 @@ static void geo_node_triangulate_exec(GeoNodeExecParams params)
   GeometryNodeTriangulateNGons ngon_method = static_cast<GeometryNodeTriangulateNGons>(
       params.node().custom2);
 
+  geometry_set = geometry_set_realize_instances(geometry_set);
+
   /* #triangulate_mesh might modify the input mesh currently. */
   Mesh *mesh_in = geometry_set.get_mesh_for_write();
   if (mesh_in != nullptr) {