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diff --git a/source/blender/blenkernel/intern/multires_unsubdivide.h b/source/blender/blenkernel/intern/multires_unsubdivide.h
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+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2020 Blender Foundation.
+ * All rights reserved.
+ */
+
+/** \file
+ * \ingroup bke
+ */
+
+#ifndef __BKE_INTERN_MULTIRES_UNSUBDIVIDE_H__
+#define __BKE_INTERN_MULTIRES_UNSUBDIVIDE_H__
+
+#include "BLI_sys_types.h"
+
+struct BMesh;
+struct Depsgraph;
+struct Mesh;
+struct MultiresModifierData;
+struct Object;
+
+typedef struct MultiresUnsubdivideGrid {
+  /* For sanity checks. */
+  int grid_index;
+  int grid_size;
+
+  /** Grid coordinates in object space. */
+  float (*grid_co)[3];
+
+} MultiresUnsubdivideGrid;
+
+typedef struct MultiresUnsubdivideContext {
+  /* Input Mesh to un-subdivide. */
+  struct Mesh *original_mesh;
+  struct MDisps *original_mdisp;
+
+  /** Number of subdivision in the grids of the input mesh. */
+  int num_original_levels;
+
+  /** Level 0 base mesh after applying the maximum amount of unsubdivisions. */
+  struct Mesh *base_mesh;
+
+  /** Limit on how many levels down the unsubdivide operation should create, if possible. */
+  int max_new_levels;
+
+  /** New levels that were created after unsubdividing. */
+  int num_new_levels;
+
+  /**
+   * Number of subdivisions that should be applied to the base mesh.
+   * (num_new_levels + num_original_levels).
+   */
+  int num_total_levels;
+
+  /** Data for the new grids, indexed by base mesh loop index. */
+  int num_grids;
+  struct MultiresUnsubdivideGrid *base_mesh_grids;
+
+  /* Private data. */
+  struct BMesh *bm_original_mesh;
+  int *loop_to_face_map;
+  int *base_to_orig_vmap;
+} MultiresUnsubdivideContext;
+
+/* --------------------------------------------------------------------
+ * Construct/destruct reshape context.
+ */
+
+void multires_unsubdivide_context_init(MultiresUnsubdivideContext *context,
+                                       struct Mesh *original_mesh,
+                                       struct MultiresModifierData *mmd);
+void multires_unsubdivide_context_free(MultiresUnsubdivideContext *context);
+
+/* --------------------------------------------------------------------
+ * Rebuild Lower Subdivisions.
+ */
+
+/* Rebuilds all subdivision to the level 0 base mesh. */
+bool multires_unsubdivide_to_basemesh(MultiresUnsubdivideContext *context);
+
+#endif /* __BKE_INTERN_MULTIRES_UNSUBDIVIDE_H__ */