Fix T50920: Adds missing edges on return of bisect operator

source/blender/bmesh/intern/bmesh_opdefines.c

@@ -1284,7 +1284,7 @@ static BMOpDefine bmo_bisect_plane_def = {
 	 {"clear_inner",   BMO_OP_SLOT_BOOL},    /* when enabled. remove all geometry on the negative side of the plane */
 	 {{'\0'}},
 	},
-	{{"geom_cut.out", BMO_OP_SLOT_ELEMENT_BUF, {BM_VERT | BM_EDGE}},  /* output new geometry from the cut */
+	{{"geom_cut.out", BMO_OP_SLOT_ELEMENT_BUF, {BM_VERT | BM_EDGE}},  /* output geometry aligned with the plane (new and existing) */
 	 {"geom.out",     BMO_OP_SLOT_ELEMENT_BUF, {BM_VERT | BM_EDGE | BM_FACE}},  /* input and output geometry (result of cut)  */
 	 {{'\0'}}},
 	bmo_bisect_plane_exec,

source/blender/bmesh/operators/bmo_bisect_plane.c

@@ -38,7 +38,8 @@
 #include "intern/bmesh_operators_private.h" /* own include */
 
 #define ELE_NEW 1
-#define ELE_INPUT 2
+#define ELE_CUT 2
+#define ELE_INPUT 4
 
 void bmo_bisect_plane_exec(BMesh *bm, BMOperator *op)
 {
@@ -69,7 +70,7 @@ void bmo_bisect_plane_exec(BMesh *bm, BMOperator *op)
 
 
 	BM_mesh_bisect_plane(bm, plane, use_snap_center, true,
-	                     ELE_NEW, dist);
+	                     ELE_CUT, ELE_NEW, dist);
 
 
 	if (clear_outer || clear_inner) {
@@ -108,5 +109,5 @@ void bmo_bisect_plane_exec(BMesh *bm, BMOperator *op)
 	}
 
 	BMO_slot_buffer_from_enabled_flag(bm, op, op->slots_out, "geom.out", BM_ALL_NOLOOP, ELE_NEW | ELE_INPUT);
-	BMO_slot_buffer_from_enabled_flag(bm, op, op->slots_out, "geom_cut.out", BM_VERT | BM_EDGE, ELE_NEW);
+	BMO_slot_buffer_from_enabled_flag(bm, op, op->slots_out, "geom_cut.out", BM_VERT | BM_EDGE, ELE_CUT);
 }

source/blender/bmesh/tools/bmesh_bisect_plane.c

@@ -110,7 +110,7 @@ static int bm_vert_sortval_cb(const void *v_a_v, const void *v_b_v)
 }
 
 
-static void bm_face_bisect_verts(BMesh *bm, BMFace *f, const float plane[4], const short oflag_center)
+static void bm_face_bisect_verts(BMesh *bm, BMFace *f, const float plane[4], const short oflag_center, const short oflag_new)
 {
 	/* unlikely more than 2 verts are needed */
 	const unsigned int f_len_orig = (unsigned int)f->len;
@@ -154,10 +154,11 @@ static void bm_face_bisect_verts(BMesh *bm, BMFace *f, const float plane[4], con
 			/* common case, just cut the face once */
 			BM_face_split(bm, f, l_a, l_b, &l_new, NULL, true);
 			if (l_new) {
-				if (oflag_center) {
+				if (oflag_center | oflag_new) {
-					BMO_edge_flag_enable(bm, l_new->e, oflag_center);
+					BMO_edge_flag_enable(bm, l_new->e, oflag_center | oflag_new);
-					BMO_face_flag_enable(bm, l_new->f, oflag_center);
+				}
-					BMO_face_flag_enable(bm, f,        oflag_center);
+				if (oflag_new) {
+					BMO_face_flag_enable(bm, l_new->f, oflag_new);
 				}
 			}
 		}
@@ -269,10 +270,11 @@ static void bm_face_bisect_verts(BMesh *bm, BMFace *f, const float plane[4], con
 						f_tmp = BM_face_split(bm, face_split_arr[j], l_a, l_b, &l_new, NULL, true);
 
 						if (l_new) {
-							if (oflag_center) {
+							if (oflag_center | oflag_new) {
-								BMO_edge_flag_enable(bm, l_new->e,          oflag_center);
+								BMO_edge_flag_enable(bm, l_new->e, oflag_center | oflag_new);
-								BMO_face_flag_enable(bm, l_new->f,          oflag_center);
+							}
-								BMO_face_flag_enable(bm, face_split_arr[j], oflag_center);
+							if (oflag_new) {
+								BMO_face_flag_enable(bm, l_new->f, oflag_new);
 							}
 						}
 
@@ -307,7 +309,7 @@ finally:
 void BM_mesh_bisect_plane(
         BMesh *bm, const float plane[4],
         const bool use_snap_center, const bool use_tag,
-        const short oflag_center, const float eps)
+        const short oflag_center, const short oflag_new, const float eps)
 {
 	unsigned int einput_len;
 	unsigned int i;
@@ -390,7 +392,7 @@ void BM_mesh_bisect_plane(
 		const float dist[2] = {BM_VERT_DIST(e->v1), BM_VERT_DIST(e->v2)};
 
 		if (side[0] && side[1] && (side[0] != side[1])) {
-			const float e_fac = fabsf(dist[0]) / fabsf(dist[0] - dist[1]);
+			const float e_fac = dist[0] / (dist[0] - dist[1]);
 			BMVert *v_new;
 
 			if (e->l) {
@@ -404,10 +406,17 @@ void BM_mesh_bisect_plane(
 				} while ((l_iter = l_iter->radial_next) != l_first);
 			}
 
-			v_new = BM_edge_split(bm, e, e->v1, NULL, e_fac);
+			{
+				BMEdge *e_new;
+				v_new = BM_edge_split(bm, e, e->v1, &e_new, e_fac);
+				if (oflag_new) {
+					BMO_edge_flag_enable(bm, e_new, oflag_new);
+				}
+			}
+
 			vert_is_center_enable(v_new);
-			if (oflag_center) {
+			if (oflag_new | oflag_center) {
-				BMO_vert_flag_enable(bm, v_new, oflag_center);
+				BMO_vert_flag_enable(bm, v_new, oflag_new | oflag_center);
 			}
 
 			BM_VERT_DIR(v_new) = 0;
@@ -448,7 +457,7 @@ void BM_mesh_bisect_plane(
 	MEM_freeN(edges_arr);
 
 	while ((f = BLI_LINKSTACK_POP(face_stack))) {
-		bm_face_bisect_verts(bm, f, plane, oflag_center);
+		bm_face_bisect_verts(bm, f, plane, oflag_center, oflag_new);
 	}
 
 	/* now we have all faces to split in the stack */

source/blender/bmesh/tools/bmesh_bisect_plane.h

@@ -30,6 +30,6 @@
 void BM_mesh_bisect_plane(
         BMesh *bm, const float plane[4],
         const bool use_snap_center, const bool use_tag,
-        const short oflag_center, const float eps);
+        const short oflag_center, const short oflag_new, const float eps);
 
 #endif /* __BMESH_BISECT_PLANE_H__ */