Geometry Nodes: Handle multiple grids in the volume to mesh node

In future use cases, a volume can contain many grids that represent the
density information. In this case, it's better if the volume to mesh node
creates a mesh based on all of the grids in the volume.

This is also a benefit to share-ability, since one doesn't have to
specify the grid name in the node. Instead, in the future we can have
a way to split particular grids into separate volumes, if only one
grid should be considered.

The code changes are relatively simple:
 - Move the old volume to mesh node to the legacy folder.
 - Run the volume to mesh node on all instance geometry, like elsewhere.
 - Make the blenkernel's volume to mesh API a bit more specific.

Differential Revision: https://developer.blender.org/D12997

1 files changed, 32 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_volume_to_mesh.hh b/source/blender/blenkernel/BKE_volume_to_mesh.hh
index 1f6e89636c4..9532da8c23c 100644
--- a/source/blender/blenkernel/BKE_volume_to_mesh.hh
+++ b/source/blender/blenkernel/BKE_volume_to_mesh.hh
@@ -14,6 +14,8 @@
  * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  */
 
+#include "BLI_span.hh"
+
 #include "DNA_modifier_types.h"
 
 #ifdef WITH_OPENVDB
@@ -33,10 +35,40 @@ struct VolumeToMeshResolution {
 };
 
 #ifdef WITH_OPENVDB
+
+/**
+ * The result of converting a volume grid to mesh data, in the format used by the OpenVDB API.
+ */
+struct OpenVDBMeshData {
+  std::vector<openvdb::Vec3s> verts;
+  std::vector<openvdb::Vec3I> tris;
+  std::vector<openvdb::Vec4I> quads;
+  bool is_empty() const
+  {
+    return verts.empty();
+  }
+};
+
 struct Mesh *volume_to_mesh(const openvdb::GridBase &grid,
                             const VolumeToMeshResolution &resolution,
                             const float threshold,
                             const float adaptivity);
+
+struct OpenVDBMeshData volume_to_mesh_data(const openvdb::GridBase &grid,
+                                           const VolumeToMeshResolution &resolution,
+                                           const float threshold,
+                                           const float adaptivity);
+
+void fill_mesh_from_openvdb_data(const Span<openvdb::Vec3s> vdb_verts,
+                                 const Span<openvdb::Vec3I> vdb_tris,
+                                 const Span<openvdb::Vec4I> vdb_quads,
+                                 const int vert_offset,
+                                 const int poly_offset,
+                                 const int loop_offset,
+                                 MutableSpan<MVert> verts,
+                                 MutableSpan<MPoly> polys,
+                                 MutableSpan<MLoop> loops);
+
 #endif
 
 }  // namespace blender::bke