add un-subdivude as an optional method for the decimate modifier, gives more even geometry & nicer results in some cases.

9 files changed, 414 insertions, 319 deletions

diff --git a/source/blender/bmesh/CMakeLists.txt b/source/blender/bmesh/CMakeLists.txt
index 3e0985471e1..a000cb59167 100644
--- a/source/blender/bmesh/CMakeLists.txt
+++ b/source/blender/bmesh/CMakeLists.txt
@@ -65,6 +65,7 @@ set(SRC
 	intern/bmesh_core.c
 	intern/bmesh_core.h
 	intern/bmesh_decimate_collapse.c
+	intern/bmesh_decimate_unsubdivide.c
 	intern/bmesh_decimate.h
 	intern/bmesh_inline.h
 	intern/bmesh_interp.c
diff --git a/source/blender/bmesh/intern/bmesh_decimate.h b/source/blender/bmesh/intern/bmesh_decimate.h
index 7fafb752e1e..18d86cfb1a5 100644
--- a/source/blender/bmesh/intern/bmesh_decimate.h
+++ b/source/blender/bmesh/intern/bmesh_decimate.h
@@ -29,4 +29,7 @@
 
 void BM_mesh_decimate_collapse(BMesh *bm, const float factor, float *vweights);
 
+void BM_mesh_decimate_unsubdivide_ex(BMesh *bm, const int iterations, const int tag_only);
+void BM_mesh_decimate_unsubdivide(BMesh *bm, const int iterations);
+
 #endif /* __BMESH_DECIMATE_H__ */
diff --git a/source/blender/bmesh/intern/bmesh_decimate_collapse.c b/source/blender/bmesh/intern/bmesh_decimate_collapse.c
index 75fe12a418a..ce10fe2fed1 100644
--- a/source/blender/bmesh/intern/bmesh_decimate_collapse.c
+++ b/source/blender/bmesh/intern/bmesh_decimate_collapse.c
@@ -20,10 +20,10 @@
  * ***** END GPL LICENSE BLOCK *****
  */
 
-/** \file blender/bmesh/intern/bmesh_decimate.c
+/** \file blender/bmesh/intern/bmesh_decimate_collapse.c
  *  \ingroup bmesh
  *
- * BMesh decimator.
+ * BMesh decimator that uses an edge collapse method.
  */
 
 #include <stddef.h>
diff --git a/source/blender/bmesh/intern/bmesh_decimate_unsubdivide.c b/source/blender/bmesh/intern/bmesh_decimate_unsubdivide.c
new file mode 100644
index 00000000000..18bfa5159bf
--- /dev/null
+++ b/source/blender/bmesh/intern/bmesh_decimate_unsubdivide.c
@@ -0,0 +1,342 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * 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.
+ *
+ * Contributor(s): Campbell Barton
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ */
+
+/** \file blender/bmesh/intern/bmesh_decimate_unsubdivide.c
+ *  \ingroup bmesh
+ *
+ * BMesh decimator that uses a grid un-subdivide method.
+ */
+
+
+#include "MEM_guardedalloc.h"
+
+#include "BLI_math.h"
+
+#include "bmesh.h"
+
+#include "intern/bmesh_operators_private.h" /* own include */
+
+
+static int bm_vert_dissolve_fan_test(BMVert *v)
+{
+	/* check if we should walk over these verts */
+	BMIter iter;
+	BMEdge *e;
+
+	unsigned int tot_edge = 0;
+	unsigned int tot_edge_boundary = 0;
+	unsigned int tot_edge_manifold = 0;
+	unsigned int tot_edge_wire     = 0;
+
+	BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
+		if (BM_edge_is_boundary(e)) {
+			tot_edge_boundary++;
+		}
+		else if (BM_edge_is_manifold(e)) {
+			tot_edge_manifold++;
+		}
+		else if (BM_edge_is_wire(e)) {
+			tot_edge_wire++;
+		}
+		tot_edge++;
+	}
+
+	if ((tot_edge == 4) && (tot_edge_boundary == 0) && (tot_edge_manifold == 4)) {
+		return TRUE;
+	}
+	else if ((tot_edge == 3) && (tot_edge_boundary == 0) && (tot_edge_manifold == 3)) {
+		return TRUE;
+	}
+	else if ((tot_edge == 3) && (tot_edge_boundary == 2) && (tot_edge_manifold == 1)) {
+		return TRUE;
+	}
+	else if ((tot_edge == 2) && (tot_edge_wire == 2)) {
+		return TRUE;
+	}
+	return FALSE;
+}
+
+static int bm_vert_dissolve_fan(BMesh *bm, BMVert *v)
+{
+	/* collapse under 2 conditions.
+	 * - vert connects to 4 manifold edges (and 4 faces).
+	 * - vert connecrs to 1 manifold edge, 2 boundary edges (and 2 faces).
+	 *
+	 * This covers boundary verts of a quad grid and center verts.
+	 * note that surrounding faces dont have to be quads.
+	 */
+
+	BMIter iter;
+	BMEdge *e;
+
+	unsigned int tot_loop = 0;
+	unsigned int tot_edge = 0;
+	unsigned int tot_edge_boundary = 0;
+	unsigned int tot_edge_manifold = 0;
+	unsigned int tot_edge_wire     = 0;
+
+	BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
+		if (BM_edge_is_boundary(e)) {
+			tot_edge_boundary++;
+		}
+		else if (BM_edge_is_manifold(e)) {
+			tot_edge_manifold++;
+		}
+		else if (BM_edge_is_wire(e)) {
+			tot_edge_wire++;
+		}
+		tot_edge++;
+	}
+
+	if (tot_edge == 2) {
+		/* check for 2 wire verts only */
+		if (tot_edge_wire == 2) {
+			return (BM_vert_collapse_edge(bm, v->e, v, TRUE) != NULL);
+		}
+	}
+	else if (tot_edge == 4) {
+		/* check for 4 faces surrounding */
+		if (tot_edge_boundary == 0 && tot_edge_manifold == 4) {
+			/* good to go! */
+			tot_loop = 4;
+		}
+	}
+	else if (tot_edge == 3) {
+		/* check for 2 faces surrounding at a boundary */
+		if (tot_edge_boundary == 2 && tot_edge_manifold == 1) {
+			/* good to go! */
+			tot_loop = 2;
+		}
+		else if (tot_edge_boundary == 0 && tot_edge_manifold == 3) {
+			/* good to go! */
+			tot_loop = 3;
+		}
+	}
+
+	if (tot_loop) {
+		BMLoop *f_loop[4];
+		unsigned int i;
+
+		/* ensure there are exactly tot_loop loops */
+		BLI_assert(BM_iter_at_index(bm, BM_LOOPS_OF_VERT, v, tot_loop) == NULL);
+		BM_iter_as_array(bm, BM_LOOPS_OF_VERT, v, (void **)f_loop, tot_loop);
+
+		for (i = 0; i < tot_loop; i++) {
+			BMLoop *l = f_loop[i];
+			if (l->f->len > 3) {
+				BLI_assert(l->prev->v != l->next->v);
+				BM_face_split(bm, l->f, l->prev->v, l->next->v, NULL, NULL, TRUE);
+			}
+		}
+
+		return BM_vert_dissolve(bm, v);
+	}
+
+	return FALSE;
+}
+
+enum {
+	VERT_INDEX_DO_COLLAPSE  = -1,
+	VERT_INDEX_INIT         =  0,
+	VERT_INDEX_IGNORE       =  1
+};
+
+// #define USE_WALKER  /* gives uneven results, disable for now */
+
+/* - BMVert.flag & BM_ELEM_TAG:  shows we touched this vert
+ * - BMVert.index == -1:         shows we will remove this vert
+ */
+
+/**
+ * \param tag_only so we can call this from an operator */
+void BM_mesh_decimate_unsubdivide_ex(BMesh *bm, const int iterations, const int tag_only)
+{
+#ifdef USE_WALKER
+#  define ELE_VERT_TAG 1
+#else
+	BMVert **vert_seek_a = MEM_mallocN(sizeof(BMVert *) * bm->totvert, __func__);
+	BMVert **vert_seek_b = MEM_mallocN(sizeof(BMVert *) * bm->totvert, __func__);
+	unsigned vert_seek_a_tot = 0;
+	unsigned vert_seek_b_tot = 0;
+#endif
+
+	BMVert *v;
+	BMIter iter;
+
+	const unsigned int offset = 0;
+	const unsigned int nth = 2;
+
+	int iter_step;
+
+	/* if tag_only is set, we assyme the caller knows what verts to tag
+	 * needed for the operator */
+	if (tag_only == FALSE) {
+		BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
+			BM_elem_flag_enable(v, BM_ELEM_TAG);
+		}
+	}
+
+	for (iter_step = 0; iter_step < iterations; iter_step++) {
+		int iter_done;
+
+		BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
+			if (BM_elem_flag_test(v, BM_ELEM_TAG) && bm_vert_dissolve_fan_test(v)) {
+#ifdef USE_WALKER
+				BMO_elem_flag_enable(bm, v, ELE_VERT_TAG);
+#endif
+				BM_elem_index_set(v, VERT_INDEX_INIT);  /* set_dirty! */
+			}
+			else {
+				BM_elem_index_set(v, VERT_INDEX_IGNORE);  /* set_dirty! */
+			}
+		}
+		/* done with selecting tagged verts */
+
+
+		/* main loop, keep tagging until we can't tag any more islands */
+		while (TRUE) {
+#ifdef USE_WALKER
+			BMWalker walker;
+#else
+			unsigned int depth = 1;
+			unsigned int i;
+#endif
+			BMVert *v_first = NULL;
+			BMVert *v;
+
+			/* we could avoid iterating from the start each time */
+			BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
+				if (v->e && (BM_elem_index_get(v) == VERT_INDEX_INIT)) {
+#ifdef USE_WALKER
+					if (BMO_elem_flag_test(bm, v, ELE_VERT_TAG))
+#endif
+					{
+						/* check again incase the topology changed */
+						if (bm_vert_dissolve_fan_test(v)) {
+							v_first = v;
+						}
+						break;
+					}
+				}
+			}
+			if (v_first == NULL) {
+				break;
+			}
+
+#ifdef USE_WALKER
+			/* Walk over selected elements starting at active */
+			BMW_init(&walker, bm, BMW_CONNECTED_VERTEX,
+					 ELE_VERT_TAG, BMW_MASK_NOP, BMW_MASK_NOP,
+					 BMW_FLAG_NOP, /* don't use BMW_FLAG_TEST_HIDDEN here since we want to desel all */
+					 BMW_NIL_LAY);
+
+			BLI_assert(walker.order == BMW_BREADTH_FIRST);
+			for (v = BMW_begin(&walker, v_first); v != NULL; v = BMW_step(&walker)) {
+				/* Deselect elements that aren't at "nth" depth from active */
+				if (BM_elem_index_get(v) == VERT_INDEX_INIT) {
+					if ((offset + BMW_current_depth(&walker)) % nth) {
+						/* tag for removal */
+						BM_elem_index_set(v, VERT_INDEX_DO_COLLAPSE);  /* set_dirty! */
+					}
+					else {
+						/* works better to allow these verts to be checked again */
+						//BM_elem_index_set(v, VERT_INDEX_IGNORE);  /* set_dirty! */
+					}
+				}
+			}
+			BMW_end(&walker);
+#else
+
+			BM_elem_index_set(v_first, (offset + depth) % nth ? VERT_INDEX_IGNORE : VERT_INDEX_DO_COLLAPSE);  /* set_dirty! */
+
+			vert_seek_b_tot = 0;
+			vert_seek_b[vert_seek_b_tot++] = v_first;
+
+			while (TRUE) {
+				BMEdge *e;
+
+				if ((offset + depth) % nth) {
+					vert_seek_a_tot = 0;
+					for (i = 0; i < vert_seek_b_tot; i++) {
+						v = vert_seek_b[i];
+						BLI_assert(BM_elem_index_get(v) == VERT_INDEX_IGNORE);
+						BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
+							BMVert *v_other = BM_edge_other_vert(e, v);
+							if (BM_elem_index_get(v_other) == VERT_INDEX_INIT) {
+								BM_elem_index_set(v_other, VERT_INDEX_DO_COLLAPSE);  /* set_dirty! */
+								vert_seek_a[vert_seek_a_tot++] = v_other;
+							}
+						}
+					}
+					if (vert_seek_a_tot == 0) {
+						break;
+					}
+				}
+				else {
+					vert_seek_b_tot = 0;
+					for (i = 0; i < vert_seek_a_tot; i++) {
+						v = vert_seek_a[i];
+						BLI_assert(BM_elem_index_get(v) == VERT_INDEX_DO_COLLAPSE);
+						BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
+							BMVert *v_other = BM_edge_other_vert(e, v);
+							if (BM_elem_index_get(v_other) == VERT_INDEX_INIT) {
+								BM_elem_index_set(v_other, VERT_INDEX_IGNORE);  /* set_dirty! */
+								vert_seek_b[vert_seek_b_tot++] = v_other;
+							}
+						}
+					}
+					if (vert_seek_b_tot == 0) {
+						break;
+					}
+				}
+
+				depth++;
+			}
+#endif  /* USE_WALKER */
+
+		}
+
+		/* now we tagged all verts -1 for removal, lets loop over and rebuild faces */
+		iter_done = FALSE;
+		BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
+			if (BM_elem_index_get(v) == VERT_INDEX_DO_COLLAPSE) {
+				iter_done |= bm_vert_dissolve_fan(bm, v);
+			}
+		}
+
+		if (iter_done == FALSE) {
+			break;
+		}
+	}
+
+	bm->elem_index_dirty |= BM_VERT;
+
+#ifndef USE_WALKER
+	MEM_freeN(vert_seek_a);
+	MEM_freeN(vert_seek_b);
+#endif
+}
+
+void BM_mesh_decimate_unsubdivide(BMesh *bm, const int iterations)
+{
+	BM_mesh_decimate_unsubdivide_ex(bm, iterations, FALSE);
+}
diff --git a/source/blender/bmesh/operators/bmo_unsubdivide.c b/source/blender/bmesh/operators/bmo_unsubdivide.c
index 64b7151aee5..75f886856dc 100644
--- a/source/blender/bmesh/operators/bmo_unsubdivide.c
+++ b/source/blender/bmesh/operators/bmo_unsubdivide.c
@@ -24,171 +24,23 @@
  *  \ingroup bmesh
  */
 
-#include "MEM_guardedalloc.h"
-
 #include "BLI_math.h"
+#include "BLI_utildefines.h"
 
 #include "bmesh.h"
 
 #include "intern/bmesh_operators_private.h" /* own include */
 
-
-static int bm_vert_dissolve_fan_test(BMVert *v)
-{
-	/* check if we should walk over these verts */
-	BMIter iter;
-	BMEdge *e;
-
-	unsigned int tot_edge = 0;
-	unsigned int tot_edge_boundary = 0;
-	unsigned int tot_edge_manifold = 0;
-	unsigned int tot_edge_wire     = 0;
-
-	BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
-		if (BM_edge_is_boundary(e)) {
-			tot_edge_boundary++;
-		}
-		else if (BM_edge_is_manifold(e)) {
-			tot_edge_manifold++;
-		}
-		else if (BM_edge_is_wire(e)) {
-			tot_edge_wire++;
-		}
-		tot_edge++;
-	}
-
-	if ((tot_edge == 4) && (tot_edge_boundary == 0) && (tot_edge_manifold == 4)) {
-		return TRUE;
-	}
-	else if ((tot_edge == 3) && (tot_edge_boundary == 0) && (tot_edge_manifold == 3)) {
-		return TRUE;
-	}
-	else if ((tot_edge == 3) && (tot_edge_boundary == 2) && (tot_edge_manifold == 1)) {
-		return TRUE;
-	}
-	else if ((tot_edge == 2) && (tot_edge_wire == 2)) {
-		return TRUE;
-	}
-	return FALSE;
-}
-
-static int bm_vert_dissolve_fan(BMesh *bm, BMVert *v)
-{
-	/* collapse under 2 conditions.
-	 * - vert connects to 4 manifold edges (and 4 faces).
-	 * - vert connecrs to 1 manifold edge, 2 boundary edges (and 2 faces).
-	 *
-	 * This covers boundary verts of a quad grid and center verts.
-	 * note that surrounding faces dont have to be quads.
-	 */
-
-	BMIter iter;
-	BMEdge *e;
-
-	unsigned int tot_loop = 0;
-	unsigned int tot_edge = 0;
-	unsigned int tot_edge_boundary = 0;
-	unsigned int tot_edge_manifold = 0;
-	unsigned int tot_edge_wire     = 0;
-
-	BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
-		if (BM_edge_is_boundary(e)) {
-			tot_edge_boundary++;
-		}
-		else if (BM_edge_is_manifold(e)) {
-			tot_edge_manifold++;
-		}
-		else if (BM_edge_is_wire(e)) {
-			tot_edge_wire++;
-		}
-		tot_edge++;
-	}
-
-	if (tot_edge == 2) {
-		/* check for 2 wire verts only */
-		if (tot_edge_wire == 2) {
-			return (BM_vert_collapse_edge(bm, v->e, v, TRUE) != NULL);
-		}
-	}
-	else if (tot_edge == 4) {
-		/* check for 4 faces surrounding */
-		if (tot_edge_boundary == 0 && tot_edge_manifold == 4) {
-			/* good to go! */
-			tot_loop = 4;
-		}
-	}
-	else if (tot_edge == 3) {
-		/* check for 2 faces surrounding at a boundary */
-		if (tot_edge_boundary == 2 && tot_edge_manifold == 1) {
-			/* good to go! */
-			tot_loop = 2;
-		}
-		else if (tot_edge_boundary == 0 && tot_edge_manifold == 3) {
-			/* good to go! */
-			tot_loop = 3;
-		}
-	}
-
-	if (tot_loop) {
-		BMLoop *f_loop[4];
-		unsigned int i;
-
-		/* ensure there are exactly tot_loop loops */
-		BLI_assert(BM_iter_at_index(bm, BM_LOOPS_OF_VERT, v, tot_loop) == NULL);
-		BM_iter_as_array(bm, BM_LOOPS_OF_VERT, v, (void **)f_loop, tot_loop);
-
-		for (i = 0; i < tot_loop; i++) {
-			BMLoop *l = f_loop[i];
-			if (l->f->len > 3) {
-				BLI_assert(l->prev->v != l->next->v);
-				BM_face_split(bm, l->f, l->prev->v, l->next->v, NULL, NULL, TRUE);
-			}
-		}
-
-		return BM_vert_dissolve(bm, v);
-	}
-
-	return FALSE;
-}
-
-enum {
-	VERT_INDEX_DO_COLLAPSE  = -1,
-	VERT_INDEX_INIT         =  0,
-	VERT_INDEX_IGNORE       =  1
-};
-
-// #define USE_WALKER  /* gives uneven results, disable for now */
-// #define USE_ALL_VERTS
-
 /* - BMVert.flag & BM_ELEM_TAG:  shows we touched this vert
  * - BMVert.index == -1:         shows we will remove this vert
  */
 void bmo_unsubdivide_exec(BMesh *bm, BMOperator *op)
 {
-#ifdef USE_WALKER
-#  define ELE_VERT_TAG 1
-#else
-	BMVert **vert_seek_a = MEM_mallocN(sizeof(BMVert *) * bm->totvert, __func__);
-	BMVert **vert_seek_b = MEM_mallocN(sizeof(BMVert *) * bm->totvert, __func__);
-	unsigned vert_seek_a_tot = 0;
-	unsigned vert_seek_b_tot = 0;
-#endif
-
 	BMVert *v;
 	BMIter iter;
 
-	const unsigned int offset = 0;
-	const unsigned int nth = 2;
-
 	const int iterations = maxi(1, BMO_slot_int_get(op, "iterations"));
-	int iter_step;
 
-#ifdef USE_ALL_VERTS
-	(void)op;
-	BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
-		BM_elem_flag_enable(v, BM_ELEM_TAG);
-	}
-#else  /* USE_ALL_VERTS */
 	BMOpSlot *vinput = BMO_slot_get(op, "verts");
 	BMVert **vinput_arr = (BMVert **)vinput->data.p;
 	int v_index;
@@ -201,148 +53,7 @@ void bmo_unsubdivide_exec(BMesh *bm, BMOperator *op)
 		v = vinput_arr[v_index];
 		BM_elem_flag_enable(v, BM_ELEM_TAG);
 	}
-#endif  /* USE_ALL_VERTS */
-
-
-	for (iter_step = 0; iter_step < iterations; iter_step++) {
-		int iter_done;
-
-		BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
-			if (BM_elem_flag_test(v, BM_ELEM_TAG) && bm_vert_dissolve_fan_test(v)) {
-#ifdef USE_WALKER
-				BMO_elem_flag_enable(bm, v, ELE_VERT_TAG);
-#endif
-				BM_elem_index_set(v, VERT_INDEX_INIT);  /* set_dirty! */
-			}
-			else {
-				BM_elem_index_set(v, VERT_INDEX_IGNORE);  /* set_dirty! */
-			}
-		}
-		/* done with selecting tagged verts */
-
-
-		/* main loop, keep tagging until we can't tag any more islands */
-		while (TRUE) {
-#ifdef USE_WALKER
-			BMWalker walker;
-#else
-			unsigned int depth = 1;
-			unsigned int i;
-#endif
-			BMVert *v_first = NULL;
-			BMVert *v;
-
-			/* we could avoid iterating from the start each time */
-			BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
-				if (v->e && (BM_elem_index_get(v) == VERT_INDEX_INIT)) {
-#ifdef USE_WALKER
-					if (BMO_elem_flag_test(bm, v, ELE_VERT_TAG))
-#endif
-					{
-						/* check again incase the topology changed */
-						if (bm_vert_dissolve_fan_test(v)) {
-							v_first = v;
-						}
-						break;
-					}
-				}
-			}
-			if (v_first == NULL) {
-				break;
-			}
-
-#ifdef USE_WALKER
-			/* Walk over selected elements starting at active */
-			BMW_init(&walker, bm, BMW_CONNECTED_VERTEX,
-					 ELE_VERT_TAG, BMW_MASK_NOP, BMW_MASK_NOP,
-					 BMW_FLAG_NOP, /* don't use BMW_FLAG_TEST_HIDDEN here since we want to desel all */
-					 BMW_NIL_LAY);
-
-			BLI_assert(walker.order == BMW_BREADTH_FIRST);
-			for (v = BMW_begin(&walker, v_first); v != NULL; v = BMW_step(&walker)) {
-				/* Deselect elements that aren't at "nth" depth from active */
-				if (BM_elem_index_get(v) == VERT_INDEX_INIT) {
-					if ((offset + BMW_current_depth(&walker)) % nth) {
-						/* tag for removal */
-						BM_elem_index_set(v, VERT_INDEX_DO_COLLAPSE);  /* set_dirty! */
-					}
-					else {
-						/* works better to allow these verts to be checked again */
-						//BM_elem_index_set(v, VERT_INDEX_IGNORE);  /* set_dirty! */
-					}
-				}
-			}
-			BMW_end(&walker);
-#else
-
-			BM_elem_index_set(v_first, (offset + depth) % nth ? VERT_INDEX_IGNORE : VERT_INDEX_DO_COLLAPSE);  /* set_dirty! */
-
-			vert_seek_b_tot = 0;
-			vert_seek_b[vert_seek_b_tot++] = v_first;
-
-			while (TRUE) {
-				BMEdge *e;
-
-				if ((offset + depth) % nth) {
-					vert_seek_a_tot = 0;
-					for (i = 0; i < vert_seek_b_tot; i++) {
-						v = vert_seek_b[i];
-						BLI_assert(BM_elem_index_get(v) == VERT_INDEX_IGNORE);
-						BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
-							BMVert *v_other = BM_edge_other_vert(e, v);
-							if (BM_elem_index_get(v_other) == VERT_INDEX_INIT) {
-								BM_elem_index_set(v_other, VERT_INDEX_DO_COLLAPSE);  /* set_dirty! */
-								vert_seek_a[vert_seek_a_tot++] = v_other;
-							}
-						}
-					}
-					if (vert_seek_a_tot == 0) {
-						break;
-					}
-				}
-				else {
-					vert_seek_b_tot = 0;
-					for (i = 0; i < vert_seek_a_tot; i++) {
-						v = vert_seek_a[i];
-						BLI_assert(BM_elem_index_get(v) == VERT_INDEX_DO_COLLAPSE);
-						BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
-							BMVert *v_other = BM_edge_other_vert(e, v);
-							if (BM_elem_index_get(v_other) == VERT_INDEX_INIT) {
-								BM_elem_index_set(v_other, VERT_INDEX_IGNORE);  /* set_dirty! */
-								vert_seek_b[vert_seek_b_tot++] = v_other;
-							}
-						}
-					}
-					if (vert_seek_b_tot == 0) {
-						break;
-					}
-				}
-
-				depth++;
-			}
-#endif  /* USE_WALKER */
-
-		}
-
-		/* now we tagged all verts -1 for removal, lets loop over and rebuild faces */
-		iter_done = FALSE;
-		BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
-			if (BM_elem_index_get(v) == VERT_INDEX_DO_COLLAPSE) {
-				iter_done |= bm_vert_dissolve_fan(bm, v);
-			}
-		}
-
-		if (iter_done == FALSE) {
-			break;
-		}
-	}
-
-	bm->elem_index_dirty |= BM_VERT;
-
-#ifndef USE_WALKER
-	MEM_freeN(vert_seek_a);
-	MEM_freeN(vert_seek_b);
-#endif
 
+	/* do all the real work here */
+	BM_mesh_decimate_unsubdivide_ex(bm, iterations, TRUE);
 }
-
diff --git a/source/blender/makesdna/DNA_modifier_types.h b/source/blender/makesdna/DNA_modifier_types.h
index 05c1871ae41..f53287beb9d 100644
--- a/source/blender/makesdna/DNA_modifier_types.h
+++ b/source/blender/makesdna/DNA_modifier_types.h
@@ -361,15 +361,23 @@ typedef struct UVProjectModifierData {
 typedef struct DecimateModifierData {
 	ModifierData modifier;
 
-	float percent;
-	int faceCount;  /* runtime only */
+	float percent;  /* (mode == MOD_DECIM_MODE_COLLAPSE) */
+	int   iter;     /* (mode == MOD_DECIM_MODE_UNSUBDIV) */
 
 	char defgrp_name[64];	/* MAX_VGROUP_NAME */
-	int flag, pad;
+	short flag, mode;
+
+	/* runtime only */
+	int face_count;
 } DecimateModifierData;
 
 enum {
-	MOD_DECIM_INVERT_VGROUP	= (1 << 0)
+	MOD_DECIM_FLAG_INVERT_VGROUP = (1 << 0)
+};
+
+enum {
+	MOD_DECIM_MODE_COLLAPSE,
+	MOD_DECIM_MODE_UNSUBDIV
 };
 
 /* Smooth modifier flags */
diff --git a/source/blender/makesdna/intern/makesdna.c b/source/blender/makesdna/intern/makesdna.c
index 77d2b00dfc2..bef5e5cc161 100644
--- a/source/blender/makesdna/intern/makesdna.c
+++ b/source/blender/makesdna/intern/makesdna.c
@@ -1195,7 +1195,7 @@ int main(int argc, char **argv)
 }
 
 /* handy but fails on struct bounds which makesdna doesnt care about
- * unless structs are nested */
+ * with quite the same strictness as GCC does */
 #if 0
 /* include files for automatic dependencies */
 
diff --git a/source/blender/makesrna/intern/rna_modifier.c b/source/blender/makesrna/intern/rna_modifier.c
index 67ab1469318..36bdb6cc896 100644
--- a/source/blender/makesrna/intern/rna_modifier.c
+++ b/source/blender/makesrna/intern/rna_modifier.c
@@ -1110,6 +1110,12 @@ static void rna_def_modifier_mirror(BlenderRNA *brna)
 
 static void rna_def_modifier_decimate(BlenderRNA *brna)
 {
+	static EnumPropertyItem modifier_decim_mode_items[] = {
+		{MOD_DECIM_MODE_COLLAPSE, "COLLAPSE", 0, "Collapse", "Use edge collapsing"},
+		{MOD_DECIM_MODE_UNSUBDIV, "UNSUBDIV", 0, "Un-Subdivide", "Use un-subdivide face reduction"},
+		{0, NULL, 0, NULL, NULL}
+	};
+
 	StructRNA *srna;
 	PropertyRNA *prop;
 
@@ -1118,6 +1124,12 @@ static void rna_def_modifier_decimate(BlenderRNA *brna)
 	RNA_def_struct_sdna(srna, "DecimateModifierData");
 	RNA_def_struct_ui_icon(srna, ICON_MOD_DECIM);
 
+	prop = RNA_def_property(srna, "decimate_type", PROP_ENUM, PROP_NONE);
+	RNA_def_property_enum_sdna(prop, NULL, "mode");
+	RNA_def_property_enum_items(prop, modifier_decim_mode_items);
+	RNA_def_property_ui_text(prop, "Mode", "");
+	RNA_def_property_update(prop, 0, "rna_Modifier_update");
+
 	prop = RNA_def_property(srna, "ratio", PROP_FLOAT, PROP_NONE);
 	RNA_def_property_float_sdna(prop, NULL, "percent");
 	RNA_def_property_range(prop, 0, 1);
@@ -1125,6 +1137,13 @@ static void rna_def_modifier_decimate(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Ratio", "Ratio of triangles to reduce to");
 	RNA_def_property_update(prop, 0, "rna_Modifier_update");
 
+	prop = RNA_def_property(srna, "iterations", PROP_INT, PROP_UNSIGNED);
+	RNA_def_property_int_sdna(prop, NULL, "iter");
+	RNA_def_property_range(prop, 0, SHRT_MAX);
+	RNA_def_property_ui_range(prop, 1, 100, 1, 0);
+	RNA_def_property_ui_text(prop, "Iterations", "Number of times to unsubdivide");
+	RNA_def_property_update(prop, 0, "rna_Modifier_update");
+
 	prop = RNA_def_property(srna, "vertex_group", PROP_STRING, PROP_NONE);
 	RNA_def_property_string_sdna(prop, NULL, "defgrp_name");
 	RNA_def_property_ui_text(prop, "Vertex Group", "Vertex group name");
@@ -1132,12 +1151,11 @@ static void rna_def_modifier_decimate(BlenderRNA *brna)
 	RNA_def_property_update(prop, 0, "rna_Modifier_update");
 
 	prop = RNA_def_property(srna, "invert_vertex_group", PROP_BOOLEAN, PROP_NONE);
-	RNA_def_property_boolean_sdna(prop, NULL, "flag", MOD_DECIM_INVERT_VGROUP);
+	RNA_def_property_boolean_sdna(prop, NULL, "flag", MOD_DECIM_FLAG_INVERT_VGROUP);
 	RNA_def_property_ui_text(prop, "Invert", "Invert vertex group influence");
 	RNA_def_property_update(prop, 0, "rna_Modifier_update");
 
 	prop = RNA_def_property(srna, "face_count", PROP_INT, PROP_NONE);
-	RNA_def_property_int_sdna(prop, NULL, "faceCount");
 	RNA_def_property_clear_flag(prop, PROP_EDITABLE);
 	RNA_def_property_ui_text(prop, "Face Count", "The current number of faces in the decimated mesh");
 }
diff --git a/source/blender/modifiers/intern/MOD_decimate.c b/source/blender/modifiers/intern/MOD_decimate.c
index 1cb6853447a..6eb2f8d6c79 100644
--- a/source/blender/modifiers/intern/MOD_decimate.c
+++ b/source/blender/modifiers/intern/MOD_decimate.c
@@ -72,8 +72,10 @@ static void copyData(ModifierData *md, ModifierData *target)
 	DecimateModifierData *tdmd = (DecimateModifierData *) target;
 
 	tdmd->percent = dmd->percent;
+	tdmd->iter = dmd->iter;
 	BLI_strncpy(tdmd->defgrp_name, dmd->defgrp_name, sizeof(tdmd->defgrp_name));
 	tdmd->flag = dmd->flag;
+	tdmd->mode = dmd->mode;
 }
 
 static CustomDataMask requiredDataMask(Object *UNUSED(ob), ModifierData *md)
@@ -110,42 +112,52 @@ static DerivedMesh *applyModifier(ModifierData *md, Object *ob,
 		return dm;
 	}
 
-	if (dmd->defgrp_name[0]) {
-		MDeformVert *dvert;
-		int defgrp_index;
+	if (dmd->mode == MOD_DECIM_MODE_COLLAPSE) {
+		if (dmd->defgrp_name[0]) {
+			MDeformVert *dvert;
+			int defgrp_index;
 
-		modifier_get_vgroup(ob, dm, dmd->defgrp_name, &dvert, &defgrp_index);
+			modifier_get_vgroup(ob, dm, dmd->defgrp_name, &dvert, &defgrp_index);
 
-		if (dvert) {
-			const unsigned int vert_tot = dm->getNumVerts(dm);
-			unsigned int i;
+			if (dvert) {
+				const unsigned int vert_tot = dm->getNumVerts(dm);
+				unsigned int i;
 
-			vweights = MEM_mallocN(vert_tot * sizeof(float), __func__);
+				vweights = MEM_mallocN(vert_tot * sizeof(float), __func__);
 
-			if (dmd->flag & MOD_DECIM_INVERT_VGROUP) {
-				for (i = 0; i < vert_tot; i++) {
-					vweights[i] = 1.0f - defvert_find_weight(&dvert[i], defgrp_index);
+				if (dmd->flag & MOD_DECIM_FLAG_INVERT_VGROUP) {
+					for (i = 0; i < vert_tot; i++) {
+						vweights[i] = 1.0f - defvert_find_weight(&dvert[i], defgrp_index);
+					}
 				}
-			}
-			else {
-				for (i = 0; i < vert_tot; i++) {
-					vweights[i] = defvert_find_weight(&dvert[i], defgrp_index);
+				else {
+					for (i = 0; i < vert_tot; i++) {
+						vweights[i] = defvert_find_weight(&dvert[i], defgrp_index);
+					}
 				}
 			}
 		}
 	}
 
-
 	em = DM_to_editbmesh(dm, NULL, FALSE);
 	bm = em->bm;
 
-	BM_mesh_decimate_collapse(bm, dmd->percent, vweights);
+	switch (dmd->mode) {
+		case MOD_DECIM_MODE_COLLAPSE:
+			BM_mesh_decimate_collapse(bm, dmd->percent, vweights);
+			break;
+		case MOD_DECIM_MODE_UNSUBDIV:
+			BM_mesh_decimate_unsubdivide(bm, dmd->iter);
+			break;
+	}
+
 
 	if (vweights) {
 		MEM_freeN(vweights);
 	}
 
-	dmd->faceCount = bm->totface;
+	/* update for display only */
+	dmd->face_count = bm->totface;
 
 	BLI_assert(em->looptris == NULL);
 	result = CDDM_from_BMEditMesh(em, NULL, TRUE, FALSE);