Fix #32398: Mirror modifier with "Merge" enabled producing pairs of faces

sharing the same set of vertices.

Modified the CDDM_merge_verts function (currently only used by the Mirror
modifier) to skip faces using all merged vertices.

1 files changed, 29 insertions, 8 deletions

diff --git a/source/blender/blenkernel/intern/cdderivedmesh.c b/source/blender/blenkernel/intern/cdderivedmesh.c
index a6b303f3556..c0e6bd54f53 100644
--- a/source/blender/blenkernel/intern/cdderivedmesh.c
+++ b/source/blender/blenkernel/intern/cdderivedmesh.c
@@ -2254,6 +2254,11 @@ void CDDM_calc_normals_tessface(DerivedMesh *dm)
  * this is a really horribly written function.  ger. - joeedh
  *
  * note, CDDM_recalc_tessellation has to run on the returned DM if you want to access tessfaces.
+ *
+ * Note: This function is currently only used by the Mirror modifier, so it
+ *       skips any faces that have all vertices merged (to avoid creating pairs
+ *       of faces sharing the same set of vertices). If used elsewhere, it may
+ *       be necessary to make this functionality optional.
  */
 DerivedMesh *CDDM_merge_verts(DerivedMesh *dm, const int *vtargetmap)
 {
@@ -2297,14 +2302,11 @@ DerivedMesh *CDDM_merge_verts(DerivedMesh *dm, const int *vtargetmap)
 			newv[i] = newv[vtargetmap[i]];
 		}
 	}
-	
-	/* find-replace merged vertices with target vertices */
-	ml = cddm->mloop;
-	for (i = 0; i < totloop; i++, ml++) {
-		if (vtargetmap[ml->v] != -1) {
-			ml->v = vtargetmap[ml->v];
-		}
-	}
+
+	/* Don't remap vertices in cddm->mloop, because we need to know the original
+	   indices in order to skip faces with all vertices merged.
+	   The "update loop indices..." section further down remaps vertices in mloop.
+	*/
 
 	/* now go through and fix edges and faces */
 	med = cddm->medge;
@@ -2338,6 +2340,25 @@ DerivedMesh *CDDM_merge_verts(DerivedMesh *dm, const int *vtargetmap)
 		
 		ml = cddm->mloop + mp->loopstart;
 
+		/* skip faces with all vertices merged */
+		{
+			int all_vertices_merged = TRUE;
+
+			for (j = 0; j < mp->totloop; j++, ml++) {
+				if (vtargetmap[ml->v] == -1) {
+					all_vertices_merged = FALSE;
+					break;
+				}
+			}
+
+			if (UNLIKELY(all_vertices_merged))
+			{
+				continue;
+			}
+		}
+
+		ml = cddm->mloop + mp->loopstart;
+
 		c = 0;
 		for (j = 0; j < mp->totloop; j++, ml++) {
 			med = cddm->medge + ml->e;