GPv3: Convex Hull node

We create one convex hull per layer, following what we do for instances.

This is covered as part of the design in #113602, and as such not
commented on the code itself.

Part of #113602.

Pull Request: https://projects.blender.org/blender/blender/pulls/113788

source/blender/nodes/geometry/nodes/node_geo_convex_hull.cc

@@ -7,6 +7,8 @@
 #include "DNA_pointcloud_types.h"
 
 #include "BKE_curves.hh"
+#include "BKE_grease_pencil.hh"
+#include "BKE_instances.hh"
 #include "BKE_material.h"
 #include "BKE_mesh.hh"
 
@@ -207,6 +209,53 @@ static Mesh *compute_hull(const GeometrySet &geometry_set)
   return hull_from_bullet(geometry_set.get_mesh(), positions);
 }
 
+static void convex_hull_grease_pencil(GeometrySet &geometry_set)
+{
+  using namespace blender::bke::greasepencil;
+
+  const GreasePencil &grease_pencil = *geometry_set.get_grease_pencil();
+  Array<Mesh *> mesh_by_layer(grease_pencil.layers().size(), nullptr);
+
+  for (const int layer_index : grease_pencil.layers().index_range()) {
+    const Drawing *drawing = get_eval_grease_pencil_layer_drawing(grease_pencil, layer_index);
+    if (drawing == nullptr) {
+      continue;
+    }
+    const bke::CurvesGeometry &curves = drawing->strokes();
+    const Span<float3> positions_span = curves.evaluated_positions();
+    if (positions_span.is_empty()) {
+      continue;
+    }
+    mesh_by_layer[layer_index] = hull_from_bullet(nullptr, positions_span);
+  }
+
+  if (mesh_by_layer.is_empty()) {
+    return;
+  }
+
+  InstancesComponent &instances_component =
+      geometry_set.get_component_for_write<InstancesComponent>();
+  bke::Instances *instances = instances_component.get_for_write();
+  if (instances == nullptr) {
+    instances = new bke::Instances();
+    instances_component.replace(instances);
+  }
+  for (Mesh *mesh : mesh_by_layer) {
+    if (!mesh) {
+      /* Add an empty reference so the number of layers and instances match.
+       * This makes it easy to reconstruct the layers afterwards and keep their attributes.
+       * Although in this particular case we don't propagate the attributes. */
+      const int handle = instances->add_reference(bke::InstanceReference());
+      instances->add_instance(handle, float4x4::identity());
+      continue;
+    }
+    GeometrySet temp_set = GeometrySet::from_mesh(mesh);
+    const int handle = instances->add_reference(bke::InstanceReference{temp_set});
+    instances->add_instance(handle, float4x4::identity());
+  }
+  geometry_set.replace_grease_pencil(nullptr);
+}
+
 #endif /* WITH_BULLET */
 
 static void node_geo_exec(GeoNodeExecParams params)
@@ -221,6 +270,9 @@ static void node_geo_exec(GeoNodeExecParams params)
       geometry::debug_randomize_mesh_order(mesh);
     }
     geometry_set.replace_mesh(mesh);
+    if (geometry_set.has_grease_pencil()) {
+      convex_hull_grease_pencil(geometry_set);
+    }
     geometry_set.keep_only_during_modify({GeometryComponent::Type::Mesh});
   });