bmesh inset: another small improvement - use the shared edge vector rather then the cross product between 2 faces if the faces infact share an edge - works best for non planer faces.

also added utility function - BM_loop_other_vert_loop

source/blender/bmesh/intern/bmesh_queries.c

@@ -151,6 +151,56 @@ BMLoop *BM_face_other_vert_loop(BMFace *f, BMVert *v_prev, BMVert *v)
 	}
 }
 
+/**
+ * \brief Other Loop in Face Sharing a Vert
+ *
+ * Finds the other loop that shares \a v with \a e loop in \a f.
+ *
+ *     +----------+ <-- return the face loop of this vertex.
+ *     |          |
+ *     |          |
+ *     |          |
+ *     +----------+ <-- This vertex defines the direction.
+ *           l    v
+ *           ^ <------- This loop defines both the face to search
+ *                      and the edge, in combination with 'v'
+ *                      The faces loop direction is ignored.
+ */
+
+BMLoop *BM_loop_other_vert_loop(BMLoop *l, BMVert *v)
+{
+#if 0 /* works but slow */
+	return BM_face_other_vert_loop(l->f, BM_edge_other_vert(l->e, v), v);
+#else
+	BMEdge *e = l->e;
+	BMVert *v_prev = BM_edge_other_vert(e, v);
+	if (l->v == v) {
+		if (l->prev->v == v_prev) {
+			return l->next;
+		}
+		else {
+			BLI_assert(l->next->v == v_prev);
+
+			return l->prev;
+		}
+	}
+	else {
+		BLI_assert(l->v == v_prev);
+
+		if (l->prev->v == v) {
+			return l->prev->prev;
+		}
+		else {
+			BLI_assert(l->next->v == v);
+			return l->next->next;
+		}
+	}
+
+
+
+#endif
+}
+
 /**
  * Returns TRUE if the vertex is used in a given face.
  */

source/blender/bmesh/intern/bmesh_queries.h

@@ -43,6 +43,7 @@ int     BM_edge_loop_pair(BMEdge *e, BMLoop **r_la, BMLoop **r_lb);
 BMVert *BM_edge_other_vert(BMEdge *e, BMVert *v);
 BMLoop *BM_face_other_edge_loop(BMFace *f, BMEdge *e, BMVert *v);
 BMLoop *BM_face_other_vert_loop(BMFace *f, BMVert *v_prev, BMVert *v);
+BMLoop *BM_loop_other_vert_loop(BMLoop *l, BMVert *v);
 
 int     BM_vert_edge_count_nonwire(BMVert *v);
 int     BM_vert_edge_count(BMVert *v);

source/blender/bmesh/operators/bmo_inset.c

@@ -274,7 +274,7 @@ void bmo_inset_exec(BMesh *bm, BMOperator *op)
 							 * if both edges use the same face OR both faces have the same normal,
 							 * ...then we can calculate an edge that fits nicely between the 2 edge normals.
 							 *
-							 * Otherwise use the corner defined by these 2 edge-face normals,
+							 * Otherwise use the shared edge OR the corner defined by these 2 face normals,
 							 * when both edges faces are adjacent this works best but even when this vertex
 							 * fans out faces it should work ok.
 							 */
@@ -293,12 +293,27 @@ void bmo_inset_exec(BMesh *bm, BMOperator *op)
 								normalize_v3(tvec);
 							}
 							else {
-								float tno[3];
-								cross_v3_v3v3(tno, f_a->no, f_b->no);
-								if (dot_v3v3(tvec, tno) < 0.0f) {
-									negate_v3(tno);
+								/* these lookups are very quick */
+								BMLoop *l_other_a = BM_loop_other_vert_loop(e_info_a->l, v_split);
+								BMLoop *l_other_b = BM_loop_other_vert_loop(e_info_b->l, v_split);
+
+								if (l_other_a->v == l_other_b->v) {
+									/* both edges faces are adjacent, but we don't need to know the shared edge
+									 * having both verts is enough. */
+									sub_v3_v3v3(tvec, l_other_a->v->co, v_split->co);
 								}
-								copy_v3_v3(tvec, tno);
+								else {
+									/* faces don't touch,
+									 * just get cross product of their normals, its *good enough*
+									 */
+									float tno[3];
+									cross_v3_v3v3(tno, f_a->no, f_b->no);
+									if (dot_v3v3(tvec, tno) < 0.0f) {
+										negate_v3(tno);
+									}
+									copy_v3_v3(tvec, tno);
+								}
+
 								normalize_v3(tvec);
 							}