Cleanup: Use const arguments for volume code

The problem was that you could getting write access to a grid from a
`const Volume *` without breaking const correctness. I encountered this
when working on support for volumes in the bounding box node. For
geometry nodes there is an important distinction between getting data
"for read" and "for write", with the former returning a `const` version
of the data.

Also, for volumes it was necessary to cast away const, since all of
the relevant functions in `volume.cc` didn't have const versions. This
patch adds `const` in these places, distinguising between "for read"
and "for write" versions of functions where necessary.

The downside is that loading and unloading in the global volume cache
needs const write-access to some member variables. I see that as an
inherent problem that comes up with caching that never has a beautiful
solution anyway.

Some of the const-ness could probably be propogated futher in EEVEE
code, but I'll leave that out, since there is another level of caching.

Differential Revision: https://developer.blender.org/D10916

1 files changed, 2 insertions, 2 deletions

diff --git a/intern/cycles/blender/blender_volume.cpp b/intern/cycles/blender/blender_volume.cpp
index 0ff4de846e1..772ab9f5c8a 100644
--- a/intern/cycles/blender/blender_volume.cpp
+++ b/intern/cycles/blender/blender_volume.cpp
@@ -26,7 +26,7 @@
 #ifdef WITH_OPENVDB
 #  include <openvdb/openvdb.h>
 openvdb::GridBase::ConstPtr BKE_volume_grid_openvdb_for_read(const struct Volume *volume,
-                                                             struct VolumeGrid *grid);
+                                                             const struct VolumeGrid *grid);
 #endif
 
 CCL_NAMESPACE_BEGIN
@@ -227,7 +227,7 @@ class BlenderVolumeLoader : public VDBImageLoader {
         const bool unload = !b_volume_grid.is_loaded();
 
         ::Volume *volume = (::Volume *)b_volume.ptr.data;
-        VolumeGrid *volume_grid = (VolumeGrid *)b_volume_grid.ptr.data;
+        const VolumeGrid *volume_grid = (VolumeGrid *)b_volume_grid.ptr.data;
         grid = BKE_volume_grid_openvdb_for_read(volume, volume_grid);
 
         if (unload) {