Fix T60244: Bisect in mirror modifier doesn't work properly.

That was indeed not working properly, not at all. Except for
the basic case, but as soon as you used another object to define the
mirror plane, it would be utterly broken, in several different ways!

1 files changed, 22 insertions, 25 deletions

diff --git a/source/blender/modifiers/intern/MOD_mirror.c b/source/blender/modifiers/intern/MOD_mirror.c
index ae3f28ab764..297169a17f8 100644
--- a/source/blender/modifiers/intern/MOD_mirror.c
+++ b/source/blender/modifiers/intern/MOD_mirror.c
@@ -84,18 +84,17 @@ static void updateDepsgraph(ModifierData *md, const ModifierUpdateDepsgraphConte
 
 static Mesh *doBiscetOnMirrorPlane(
         MirrorModifierData *mmd,
-        Object *ob,
         const Mesh *mesh,
-        Object *mirror_ob,
         int axis,
-        float mirrormat[4][4])
+        float plane_co[3],
+        float plane_no[3])
 {
 	bool do_bisect_flip_axis = (
 	        (axis == 0 && mmd->flag & MOD_MIR_BISECT_FLIP_AXIS_X) ||
 	        (axis == 1 && mmd->flag & MOD_MIR_BISECT_FLIP_AXIS_Y) ||
 	        (axis == 2 && mmd->flag & MOD_MIR_BISECT_FLIP_AXIS_Z));
 
-	const float bisect_distance = 0.001;
+	const float bisect_distance = 0.001f;
 
 	Mesh *result;
 	BMesh *bm;
@@ -110,21 +109,14 @@ static Mesh *doBiscetOnMirrorPlane(
 			.cd_mask_extra = CD_MASK_ORIGINDEX,
 		});
 
-	/* prepare data for bisecting */
+	/* Define bisecting plane (aka mirror plane). */
 	float plane[4];
-	float plane_co[3] = {0, 0, 0};
-	float plane_no[3];
-	copy_v3_v3(plane_no, mirrormat[axis]);
-
-	if (mirror_ob != NULL) {
-		float tmp[4][4];
-		invert_m4_m4(tmp, ob->obmat);
-		mul_m4_m4m4(tmp, tmp, mirror_ob->obmat);
-
-		copy_v3_v3(plane_no, tmp[axis]);
-		copy_v3_v3(plane_co, tmp[3]);
+	if (!do_bisect_flip_axis) {
+		/* That reversed condition is a tad weird, but for some reason that's how you keep
+		 * the part of the mesh which is on the non-mirrored side when flip option is disabled,
+		 * think that that is the expected behavior. */
+		negate_v3(plane_no);
 	}
-
 	plane_from_point_normal_v3(plane, plane_co, plane_no);
 
 	BM_mesh_bisect_plane(bm, plane, false, false, 0, 0, bisect_distance);
@@ -134,10 +126,6 @@ static Mesh *doBiscetOnMirrorPlane(
 	copy_v3_v3(plane_offset, plane);
 	plane_offset[3] = plane[3] - bisect_distance;
 
-	if (do_bisect_flip_axis) {
-		negate_v3(plane_offset);
-	}
-
 	/* Delete verts across the mirror plane. */
 	BM_ITER_MESH_MUTABLE(v, v_next, &viter, bm, BM_VERTS_OF_MESH) {
 		if (plane_point_side_v3(plane_offset, v->co) > 0.0f) {
@@ -173,6 +161,7 @@ static Mesh *doMirrorOnAxis(
 	MLoop *ml;
 	MPoly *mp;
 	float mtx[4][4];
+	float plane_co[3], plane_no[3];
 	int i;
 	int a, totshape;
 	int *vtargetmap = NULL, *vtmap_a = NULL, *vtmap_b = NULL;
@@ -197,14 +186,22 @@ static Mesh *doMirrorOnAxis(
 		/* combine matrices to get a single matrix that translates coordinates into
 		 * mirror-object-relative space, does the mirror, and translates back to
 		 * origin-relative space */
-		mul_m4_m4m4(mtx, mtx, tmp);
-		mul_m4_m4m4(mtx, itmp, mtx);
-	}
+		mul_m4_series(mtx, itmp, mtx, tmp);
 
+		if (do_bisect) {
+			copy_v3_v3(plane_co, itmp[3]);
+			copy_v3_v3(plane_no, itmp[axis]);
+		}
+	}
+	else if(do_bisect) {
+		copy_v3_v3(plane_co, mtx[3]);
+		/* Need to negate here, since that axis is inverted (for mirror transform). */
+		negate_v3_v3(plane_no, mtx[axis]);
+	}
 
 	Mesh *mesh_bisect = NULL;
 	if (do_bisect) {
-		mesh_bisect = doBiscetOnMirrorPlane(mmd, ob, mesh, mirror_ob, axis, mtx);
+		mesh_bisect = doBiscetOnMirrorPlane(mmd, mesh, axis, plane_co, plane_no);
 		mesh = mesh_bisect;
 	}