Fix: Mantaflow OpenVDB cache hollow volume artifacts

If an OpenVDB grid has an 8x8x8 block with the same value, it is stored as
a tile with a single value. Now iterate over the bounding box and copy the
value to all voxels for this case.

Fix #91174: Hollow emission artifacts
Fix #124064: Hollow density artifacts

Pull Request: https://projects.blender.org/blender/blender/pulls/139231

extern/mantaflow/README.blender

@@ -5,3 +5,4 @@ Copyright: Copyright 2011 Tobias Pfaff, Nils Thuerey
 Upstream version: 0.13
 Local modifications:
 * ./patches/local_namespace.diff to support loading MANTA variables into an isolated __main__ name-space.
+* ./patches/import-openvdb-empty-cubes-fix.patch to fix OpenVDB import visual glitches, by copying full 8x8x8 nodes.

extern/mantaflow/patches/import-openvdb-empty-cubes-fix.patch

@@ -0,0 +1,46 @@
+commit 725df6eb883dadf2225938baecf97c005da5b549
+Author: Bartosz Kosiorek <gang65@poczta.onet.pl>
+Date:   Wed May 21 22:44:01 2025 +0200
+
+    Physics: Fix OpenVDB import visual glitches, by copying full 8x8x8 nodes
+    
+    If grid has 8x8x8 block with the same value, it is stored as Tile with single value.
+    We need to iterate over the bounding box and copy such value to all voxels in 8x8x8 node.
+    
+    Fixes: #91174 #124064
+
+diff --git a/extern/mantaflow/preprocessed/fileio/iovdb.cpp b/extern/mantaflow/preprocessed/fileio/iovdb.cpp
+index 4b7463782da..85ab3781e5a 100644
+--- a/extern/mantaflow/preprocessed/fileio/iovdb.cpp
++++ b/extern/mantaflow/preprocessed/fileio/iovdb.cpp
+@@ -54,14 +54,28 @@ template<class GridType, class T> void importVDB(typename GridType::Ptr from, Gr
+   using ValueT = typename GridType::ValueType;
+ 
+   // Check if current grid is to be read as a sparse grid, active voxels (only) will be copied
++  
+   if (to->saveSparse()) {
+     to->clear();  // Ensure that destination grid is empty before writing
+     for (typename GridType::ValueOnCIter iter = from->cbeginValueOn(); iter.test(); ++iter) {
+       ValueT vdbValue = *iter;
+-      openvdb::Coord coord = iter.getCoord();
+       T toMantaValue;
+       convertFrom(vdbValue, &toMantaValue);
+-      to->set(coord.x(), coord.y(), coord.z(), toMantaValue);
++      // #91174 #124064 - Check if iteration is Voxel or Tile
++      if (iter.isVoxelValue()) {
++        openvdb::Coord coord = iter.getCoord();
++        to->set(coord.x(), coord.y(), coord.z(), toMantaValue);
++      }
++      else {
++        openvdb::CoordBBox bbox;
++        iter.getBoundingBox(bbox);
++        // If grid has 8x8x8 block with the same value, it is stored as Tile with single value.
++        // We need to iterate over the bounding box and copy such value to all voxels in 8x8x8 node.
++        for (openvdb::CoordBBox::Iterator<true> ijk(bbox); ijk; ++ijk) {
++          openvdb::Coord coord = *ijk;
++          to->set(coord.x(), coord.y(), coord.z(), toMantaValue);
++        }
++      }
+     }
+   }
+   // When importing all grid cells, using a grid accessor is usually faster than a value iterator

extern/mantaflow/preprocessed/fileio/iovdb.cpp

@@ -54,14 +54,28 @@ template<class GridType, class T> void importVDB(typename GridType::Ptr from, Gr
   using ValueT = typename GridType::ValueType;
 
   // Check if current grid is to be read as a sparse grid, active voxels (only) will be copied
+  
   if (to->saveSparse()) {
     to->clear();  // Ensure that destination grid is empty before writing
     for (typename GridType::ValueOnCIter iter = from->cbeginValueOn(); iter.test(); ++iter) {
       ValueT vdbValue = *iter;
-      openvdb::Coord coord = iter.getCoord();
       T toMantaValue;
       convertFrom(vdbValue, &toMantaValue);
-      to->set(coord.x(), coord.y(), coord.z(), toMantaValue);
+      // #91174 #124064 - Check if iteration is Voxel or Tile
+      if (iter.isVoxelValue()) {
+        openvdb::Coord coord = iter.getCoord();
+        to->set(coord.x(), coord.y(), coord.z(), toMantaValue);
+      }
+      else {
+        openvdb::CoordBBox bbox;
+        iter.getBoundingBox(bbox);
+        // If grid has 8x8x8 block with the same value, it is stored as Tile with single value.
+        // We need to iterate over the bounding box and copy such value to all voxels in 8x8x8 node.
+        for (openvdb::CoordBBox::Iterator<true> ijk(bbox); ijk; ++ijk) {
+          openvdb::Coord coord = *ijk;
+          to->set(coord.x(), coord.y(), coord.z(), toMantaValue);
+        }
+      }
     }
   }
   // When importing all grid cells, using a grid accessor is usually faster than a value iterator