T75631: New multires shrinkage problem

The old Subdivide button was behaving as if subdivision modifier was
applied on top of the multires. This was the source of shrinkage since
the behavior of the limit surface: limit surface of a sparse point
from another limit surface makes final result appear smaller.

The new behavior is based on propagating delta against base mesh's
limit surface to the top level. Effectively, this is as if we've
sculpted on old top level and then propagated to the new top level.

Differential Revision: https://developer.blender.org/D7505

3 files changed, 13 insertions, 3 deletions

diff --git a/source/blender/blenkernel/intern/multires_reshape.c b/source/blender/blenkernel/intern/multires_reshape.c
index 8674f5d2dbf..02cd5f094a6 100644
--- a/source/blender/blenkernel/intern/multires_reshape.c
+++ b/source/blender/blenkernel/intern/multires_reshape.c
@@ -202,7 +202,13 @@ void multiresModifier_subdivide_to_level(struct Object *object,
   multires_reshape_store_original_grids(&reshape_context);
   multires_reshape_ensure_grids(coarse_mesh, reshape_context.top.level);
   multires_reshape_assign_final_coords_from_orig_mdisps(&reshape_context);
-  multires_reshape_smooth_object_grids(&reshape_context);
+
+  /* Free original grids which makes it so smoothing with details thinks all the details were
+   * added against base mesh's limit surface. This is similar behavior to as if we've done all
+   * displacement in sculpt mode at the old top level and then propagated to the new top level. */
+  multires_reshape_free_original_grids(&reshape_context);
+
+  multires_reshape_smooth_object_grids_with_details(&reshape_context);
   multires_reshape_object_grids_to_tangent_displacement(&reshape_context);
   multires_reshape_context_free(&reshape_context);
 
diff --git a/source/blender/blenkernel/intern/multires_reshape.h b/source/blender/blenkernel/intern/multires_reshape.h
index 79d3c48869f..adfa2659661 100644
--- a/source/blender/blenkernel/intern/multires_reshape.h
+++ b/source/blender/blenkernel/intern/multires_reshape.h
@@ -166,6 +166,7 @@ bool multires_reshape_context_create_from_subdivide(MultiresReshapeContext *resh
                                                     struct MultiresModifierData *mmd,
                                                     int top_level);
 
+void multires_reshape_free_original_grids(MultiresReshapeContext *reshape_context);
 void multires_reshape_context_free(MultiresReshapeContext *reshape_context);
 
 /* ================================================================================================
diff --git a/source/blender/blenkernel/intern/multires_reshape_util.c b/source/blender/blenkernel/intern/multires_reshape_util.c
index 5b6e211cc9b..57d790d2c34 100644
--- a/source/blender/blenkernel/intern/multires_reshape_util.c
+++ b/source/blender/blenkernel/intern/multires_reshape_util.c
@@ -236,7 +236,7 @@ bool multires_reshape_context_create_from_subdivide(MultiresReshapeContext *resh
   return context_verify_or_free(reshape_context);
 }
 
-static void free_original_grids(MultiresReshapeContext *reshape_context)
+void multires_reshape_free_original_grids(MultiresReshapeContext *reshape_context)
 {
   MDisps *orig_mdisps = reshape_context->orig.mdisps;
   GridPaintMask *orig_grid_paint_masks = reshape_context->orig.grid_paint_masks;
@@ -259,6 +259,9 @@ static void free_original_grids(MultiresReshapeContext *reshape_context)
 
   MEM_SAFE_FREE(orig_mdisps);
   MEM_SAFE_FREE(orig_grid_paint_masks);
+
+  reshape_context->orig.mdisps = NULL;
+  reshape_context->orig.grid_paint_masks = NULL;
 }
 
 void multires_reshape_context_free(MultiresReshapeContext *reshape_context)
@@ -267,7 +270,7 @@ void multires_reshape_context_free(MultiresReshapeContext *reshape_context)
     BKE_subdiv_free(reshape_context->subdiv);
   }
 
-  free_original_grids(reshape_context);
+  multires_reshape_free_original_grids(reshape_context);
 
   MEM_freeN(reshape_context->face_start_grid_index);
   MEM_freeN(reshape_context->ptex_start_grid_index);