Geometry Nodes: Experimental rigid body integration.

This is an exploration of how geometry nodes might be coupled with
rigid bodies and iterative simulations in general. It's a very
rough-and-ready implementation, not meant as a final version, but rather
to prove the possiblity and to find challenging areas where redesign is
needed.

The core additions are:
- Geometry nodes to flag points and/or instances as rigid bodies.
- Depsgraph integration to ensure the necessary order of operations
    between modifiers and rigid body pre/post simulation updates.
- Simple cache feature to store arbitrary geometry and loop back into
    the next iteration.

3 files changed, 21 insertions, 2 deletions

diff --git a/source/blender/nodes/geometry/node_geometry_util.cc b/source/blender/nodes/geometry/node_geometry_util.cc
index 8f673d2264e..c67cfe77c04 100644
--- a/source/blender/nodes/geometry/node_geometry_util.cc
+++ b/source/blender/nodes/geometry/node_geometry_util.cc
@@ -45,7 +45,7 @@ bool geo_node_poll_default(bNodeType *UNUSED(ntype),
                            bNodeTree *ntree,
                            const char **r_disabled_hint)
 {
-  if (!STREQ(ntree->idname, "GeometryNodeTree")) {
+  if (!STR_ELEM(ntree->idname, "GeometryNodeTree", "SimulationNodeTree")) {
     *r_disabled_hint = TIP_("Not a geometry node tree");
     return false;
   }
diff --git a/source/blender/nodes/geometry/nodes/node_geo_join_geometry.cc b/source/blender/nodes/geometry/nodes/node_geo_join_geometry.cc
index 0c56b0e9804..e7aa209a099 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_join_geometry.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_join_geometry.cc
@@ -133,6 +133,18 @@ static void join_components(Span<const VolumeComponent *> src_components, Geomet
   UNUSED_VARS(src_components, dst_component);
 }
 
+static void join_components(Span<const SimulationComponent *> src_components, GeometrySet &result)
+{
+  SimulationComponent &dst_component = result.get_component_for_write<SimulationComponent>();
+
+  for (const SimulationComponent *src_component : src_components) {
+    for (const auto &shape : src_component->shapes()) {
+      dst_component.add_shape(shape);
+    }
+  }
+  join_attributes(to_base_components(src_components), dst_component, {"position"});
+}
+
 template<typename Component>
 static void join_component_type(Span<GeometrySet> src_geometry_sets, GeometrySet &result)
 {
@@ -156,7 +168,7 @@ static void join_component_type(Span<GeometrySet> src_geometry_sets, GeometrySet
   InstancesComponent &instances =
       instances_geometry_set.get_component_for_write<InstancesComponent>();
 
-  if constexpr (is_same_any_v<Component, InstancesComponent, VolumeComponent>) {
+  if constexpr (is_same_any_v<Component, InstancesComponent, SimulationComponent, VolumeComponent>) {
     join_components(components, result);
   }
   else {
@@ -185,6 +197,7 @@ static void node_geo_exec(GeoNodeExecParams params)
   join_component_type<InstancesComponent>(geometry_sets, geometry_set_result);
   join_component_type<VolumeComponent>(geometry_sets, geometry_set_result);
   join_component_type<CurveComponent>(geometry_sets, geometry_set_result);
+  join_component_type<SimulationComponent>(geometry_sets, geometry_set_result);
 
   params.set_output("Geometry", std::move(geometry_set_result));
 }
diff --git a/source/blender/nodes/geometry/nodes/node_geo_separate_components.cc b/source/blender/nodes/geometry/nodes/node_geo_separate_components.cc
index b29ebd0d4d4..124b977f63c 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_separate_components.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_separate_components.cc
@@ -12,6 +12,7 @@ static void node_declare(NodeDeclarationBuilder &b)
   b.add_output<decl::Geometry>(N_("Curve"));
   b.add_output<decl::Geometry>(N_("Volume"));
   b.add_output<decl::Geometry>(N_("Instances"));
+  b.add_output<decl::Geometry>(N_("Simulation"));
 }
 
 static void node_geo_exec(GeoNodeExecParams params)
@@ -23,6 +24,7 @@ static void node_geo_exec(GeoNodeExecParams params)
   GeometrySet volumes;
   GeometrySet curves;
   GeometrySet instances;
+  GeometrySet simulation;
 
   if (geometry_set.has<MeshComponent>()) {
     meshes.add(*geometry_set.get_component_for_read<MeshComponent>());
@@ -39,12 +41,16 @@ static void node_geo_exec(GeoNodeExecParams params)
   if (geometry_set.has<InstancesComponent>()) {
     instances.add(*geometry_set.get_component_for_read<InstancesComponent>());
   }
+  if (geometry_set.has<SimulationComponent>()) {
+    simulation.add(*geometry_set.get_component_for_read<SimulationComponent>());
+  }
 
   params.set_output("Mesh", meshes);
   params.set_output("Point Cloud", point_clouds);
   params.set_output("Curve", curves);
   params.set_output("Volume", volumes);
   params.set_output("Instances", instances);
+  params.set_output("Simulation", simulation);
 }
 
 }  // namespace blender::nodes::node_geo_separate_components_cc