Python API: add volume.grids.save(filepath) to save grids and metadata to disk

Particularly useful now that we can dynamically generate volume using
modifiers. Not exposed in the user interface currently, but it can be used
by add-ons that need to export an entire scene including volumes.

source/blender/blenkernel/BKE_volume.h

@@ -30,6 +30,7 @@ struct BoundBox;
 struct Depsgraph;
 struct Main;
 struct Object;
+struct ReportList;
 struct Scene;
 struct Volume;
 struct VolumeGridVector;
@@ -143,6 +144,12 @@ struct VolumeGrid *BKE_volume_grid_add(struct Volume *volume,
                                        VolumeGridType type);
 void BKE_volume_grid_remove(struct Volume *volume, struct VolumeGrid *grid);
 
+/* File Save */
+bool BKE_volume_save(struct Volume *volume,
+                     struct Main *bmain,
+                     struct ReportList *reports,
+                     const char *filename);
+
 #ifdef __cplusplus
 }
 #endif

source/blender/blenkernel/intern/volume.cc

@@ -44,6 +44,7 @@
 #include "BKE_modifier.h"
 #include "BKE_object.h"
 #include "BKE_packedFile.h"
+#include "BKE_report.h"
 #include "BKE_scene.h"
 #include "BKE_volume.h"
 
@@ -401,7 +402,7 @@ struct VolumeGrid {
  * the actual grids are always saved in a VDB file. */
 
 struct VolumeGridVector : public std::list<VolumeGrid> {
-  VolumeGridVector()
+  VolumeGridVector() : metadata(new openvdb::MetaMap())
   {
     filepath[0] = '\0';
   }
@@ -803,6 +804,40 @@ void BKE_volume_unload(Volume *volume)
 #endif
 }
 
+/* File Save */
+
+bool BKE_volume_save(Volume *volume, Main *bmain, ReportList *reports, const char *filepath)
+{
+#ifdef WITH_OPENVDB
+  if (!BKE_volume_load(volume, bmain)) {
+    BKE_reportf(reports, RPT_ERROR, "Could not load volume for writing");
+    return false;
+  }
+
+  VolumeGridVector &grids = *volume->runtime.grids;
+  openvdb::GridCPtrVec vdb_grids;
+
+  for (VolumeGrid &grid : grids) {
+    vdb_grids.push_back(BKE_volume_grid_openvdb_for_read(volume, &grid));
+  }
+
+  try {
+    openvdb::io::File file(filepath);
+    file.write(vdb_grids, *grids.metadata);
+    file.close();
+  }
+  catch (const openvdb::IoError &e) {
+    BKE_reportf(reports, RPT_ERROR, "Could not write volume: %s", e.what());
+    return false;
+  }
+
+  return true;
+#else
+  UNUSED_VARS(volume, bmain, reports, filepath);
+  return false;
+#endif
+}
+
 BoundBox *BKE_volume_boundbox_get(Object *ob)
 {
   BLI_assert(ob->type == OB_VOLUME);

source/blender/makesrna/intern/rna_volume.c

@@ -343,6 +343,14 @@ static void rna_def_volume_grids(BlenderRNA *brna, PropertyRNA *cprop)
 
   func = RNA_def_function(srna, "unload", "BKE_volume_unload");
   RNA_def_function_ui_description(func, "Unload all grid and voxel data from memory");
+
+  func = RNA_def_function(srna, "save", "BKE_volume_save");
+  RNA_def_function_ui_description(func, "Save grids and metadata to file");
+  RNA_def_function_flag(func, FUNC_USE_MAIN | FUNC_USE_REPORTS);
+  parm = RNA_def_string_file_path(func, "filepath", NULL, 0, "", "File path to save to");
+  RNA_def_parameter_flags(parm, 0, PARM_REQUIRED);
+  parm = RNA_def_boolean(func, "success", 0, "", "True if grid list was successfully loaded");
+  RNA_def_function_return(func, parm);
 }
 
 static void rna_def_volume_display(BlenderRNA *brna)