un-subdivide bmesh operator, useful for making lower polygon versions of models, can give nicer results then edge collapsing which tends to give a lot of sharp triangles.

works on edges and faces, has iteration option to further reduce the poly count.

access from the edge menu, under subdivide.

example: http://www.graphicall.org/ftp/ideasman42/bmesh_unsubdivide.png

release/scripts/startup/bl_ui/space_view3d.py

@@ -1821,6 +1821,7 @@ class VIEW3D_MT_edit_mesh_edges(Menu):
 
         layout.operator("mesh.edge_face_add")
         layout.operator("mesh.subdivide")
+        layout.operator("mesh.unsubdivide")
 
         layout.separator()
 

source/blender/bmesh/CMakeLists.txt

@@ -56,6 +56,7 @@ set(SRC
 	operators/bmo_subdivide.c
 	operators/bmo_subdivide.h
 	operators/bmo_triangulate.c
+	operators/bmo_unsubdivide.c
 	operators/bmo_utils.c
 	operators/bmo_wireframe.c
 

source/blender/bmesh/intern/bmesh_core.c

@@ -1143,6 +1143,8 @@ static BMFace *bm_face_create__sfme(BMesh *bm, BMFace *UNUSED(example))
 /**
  * \brief Split Face Make Edge (SFME)
  *
+ * \warning this is a low level function, most likely you want to use #BM_face_split()
+ *
  * Takes as input two vertices in a single face. An edge is created which divides the original face
  * into two distinct regions. One of the regions is assigned to the original face and it is closed off.
  * The second region has a new face assigned to it.

source/blender/bmesh/intern/bmesh_iterators.h

@@ -111,7 +111,7 @@ typedef struct BMIter {
 		long        l;
 		float       f;
 	} filter;
-	int count;
+	int count;  /* note, only some iterators set this, don't rely on it */
 	char itype;
 } BMIter;
 

source/blender/bmesh/intern/bmesh_opdefines.c

@@ -698,6 +698,15 @@ static BMOpDefine bmo_triangulate_def = {
 	BMO_OP_FLAG_UNTAN_MULTIRES
 };
 
+static BMOpDefine bmo_unsubdivide_def = {
+	"unsubdivide",
+	{{BMO_OP_SLOT_ELEMENT_BUF, "verts"}, /* input vertices */
+	 {BMO_OP_SLOT_INT, "iterations"},
+	 {0} /* null-terminating sentinel */},
+	bmo_unsubdivide_exec,
+	BMO_OP_FLAG_UNTAN_MULTIRES
+};
+
 static BMOpDefine bmo_subdivide_edges_def = {
 	"subdivide_edges",
 	{{BMO_OP_SLOT_ELEMENT_BUF, "edges"},
@@ -1274,6 +1283,7 @@ BMOpDefine *opdefines[] = {
 	&bmo_translate_def,
 	&bmo_triangle_fill_def,
 	&bmo_triangulate_def,
+	&bmo_unsubdivide_def,
 	&bmo_weld_verts_def,
 	&bmo_wireframe_def,
 

source/blender/bmesh/intern/bmesh_operators_private.h

@@ -101,6 +101,7 @@ void bmo_transform_exec(BMesh *bm, BMOperator *op);
 void bmo_translate_exec(BMesh *bm, BMOperator *op);
 void bmo_triangle_fill_exec(BMesh *bm, BMOperator *op);
 void bmo_triangulate_exec(BMesh *bm, BMOperator *op);
+void bmo_unsubdivide_exec(BMesh *bm, BMOperator *op);
 void bmo_weld_verts_exec(BMesh *bm, BMOperator *op);
 void bmo_wireframe_exec(BMesh *bm, BMOperator *op);
 

source/blender/bmesh/operators/bmo_unsubdivide.c

@@ -0,0 +1,348 @@
+/*
+ * ***** 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/operators/bmo_unsubdivide.c
+ *  \ingroup bmesh
+ */
+
+#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 */
+// #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;
+
+	/* tag verts */
+	BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
+		BM_elem_flag_disable(v, BM_ELEM_TAG);
+	}
+	for (v_index = 0; v_index < vinput->len; v_index++) {
+		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! */
+			}
+		}
+		/* dont 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
+
+}
+

source/blender/editors/mesh/editmesh_select.c

@@ -2165,11 +2165,19 @@ static void walker_deselect_nth(BMEditMesh *em, int nth, int offset, BMHeader *h
 	         BMW_FLAG_NOP, /* don't use BMW_FLAG_TEST_HIDDEN here since we want to desel all */
 	         BMW_NIL_LAY);
 
+	/* use tag to avoid touching the same verts twice */
+	BM_ITER_MESH (ele, &iter, bm, itertype) {
+		BM_elem_flag_disable(ele, BM_ELEM_TAG);
+	}
+
 	BLI_assert(walker.order == BMW_BREADTH_FIRST);
 	for (ele = BMW_begin(&walker, h_act); ele != NULL; ele = BMW_step(&walker)) {
-		/* Deselect elements that aren't at "nth" depth from active */
-		if ((offset + BMW_current_depth(&walker)) % nth) {
-			BM_elem_select_set(bm, ele, FALSE);
+		if (!BM_elem_flag_test(ele, BM_ELEM_TAG)) {
+			/* Deselect elements that aren't at "nth" depth from active */
+			if ((offset + BMW_current_depth(&walker)) % nth) {
+				BM_elem_select_set(bm, ele, FALSE);
+			}
+			BM_elem_flag_enable(ele, BM_ELEM_TAG);
 		}
 	}
 	BMW_end(&walker);

source/blender/editors/mesh/editmesh_tools.c

@@ -156,6 +156,51 @@ void MESH_OT_subdivide(wmOperatorType *ot)
 }
 
 
+static int edbm_unsubdivide_exec(bContext *C, wmOperator *op)
+{
+	Object *obedit = CTX_data_edit_object(C);
+	BMEditMesh *em = BMEdit_FromObject(obedit);
+	BMOperator bmop;
+
+	int iterations = RNA_int_get(op->ptr, "iterations");
+
+	EDBM_op_init(em, &bmop, op,
+	             "unsubdivide verts=%hv iterations=%i", BM_ELEM_SELECT, iterations);
+
+	BMO_op_exec(em->bm, &bmop);
+
+	if (!EDBM_op_finish(em, &bmop, op, TRUE)) {
+		return 0;
+	}
+
+	if ((em->selectmode & SCE_SELECT_VERTEX) == 0) {
+		EDBM_selectmode_flush_ex(em, SCE_SELECT_VERTEX);  /* need to flush vert->face first */
+	}
+	EDBM_selectmode_flush(em);
+
+	EDBM_update_generic(C, em, TRUE);
+
+	return OPERATOR_FINISHED;
+}
+
+void MESH_OT_unsubdivide(wmOperatorType *ot)
+{
+	/* identifiers */
+	ot->name = "Un-Subdivide";
+	ot->description = "UnSubdivide selected edges & faces";
+	ot->idname = "MESH_OT_unsubdivide";
+
+	/* api callbacks */
+	ot->exec = edbm_unsubdivide_exec;
+	ot->poll = ED_operator_editmesh;
+
+	/* flags */
+	ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
+
+	/* props */
+	RNA_def_int(ot->srna, "iterations", 2, 1, INT_MAX, "Iterations", "Number of times to unsubdivide", 1, 100);
+}
+
 void EMBM_project_snap_verts(bContext *C, ARegion *ar, BMEditMesh *em)
 {
 	Object *obedit = em->ob;

source/blender/editors/mesh/mesh_intern.h

@@ -144,6 +144,7 @@ extern struct EnumPropertyItem *corner_type_items;
 
 void MESH_OT_merge(struct wmOperatorType *ot);
 void MESH_OT_subdivide(struct wmOperatorType *ot);
+void MESH_OT_unsubdivide(struct wmOperatorType *ot);
 void MESH_OT_remove_doubles(struct wmOperatorType *ot);
 void MESH_OT_spin(struct wmOperatorType *ot);
 void MESH_OT_screw(struct wmOperatorType *ot);

source/blender/editors/mesh/mesh_ops.c

@@ -74,6 +74,7 @@ void ED_operatortypes_mesh(void)
 	WM_operatortype_append(MESH_OT_normals_make_consistent);
 	WM_operatortype_append(MESH_OT_merge);
 	WM_operatortype_append(MESH_OT_subdivide);
+	WM_operatortype_append(MESH_OT_unsubdivide);
 	WM_operatortype_append(MESH_OT_faces_select_linked_flat);
 	WM_operatortype_append(MESH_OT_edges_select_sharp);
 	WM_operatortype_append(MESH_OT_primitive_plane_add);