Fix "Origin to Center of Mass" not working well with ngons

Now we do simple triangulation and calculate signed area of triangles to
account for concave polygons.

This only works correct for planar polygons but gives better results
overall.

1 files changed, 32 insertions, 3 deletions

diff --git a/source/blender/blenkernel/intern/mesh.c b/source/blender/blenkernel/intern/mesh.c
index 21641e77b0b..c9c86e6739f 100644
--- a/source/blender/blenkernel/intern/mesh.c
+++ b/source/blender/blenkernel/intern/mesh.c
@@ -2999,6 +2999,36 @@ float BKE_mesh_calc_poly_area(MPoly *mpoly, MLoop *loopstart,
 	}
 }
 
+/* note, results won't be correct if polygon is non-planar */
+static float mesh_calc_poly_planar_area_centroid(MPoly *mpoly, MLoop *loopstart, MVert *mvarray, float cent[3])
+{
+	int i;
+	float tri_area;
+	float total_area = 0.0f;
+	float v1[3], v2[3], v3[3], normal[3], tri_cent[3];
+
+	BKE_mesh_calc_poly_normal(mpoly, loopstart, mvarray, normal);
+	copy_v3_v3(v1, mvarray[loopstart[0].v].co);
+	copy_v3_v3(v2, mvarray[loopstart[1].v].co);
+	zero_v3(cent);
+
+	for (i = 2; i < mpoly->totloop; i++) {
+		copy_v3_v3(v3, mvarray[loopstart[i].v].co);
+
+		tri_area = area_tri_signed_v3(v1, v2, v3, normal);
+		total_area += tri_area;
+
+		cent_tri_v3(tri_cent, v1, v2, v3);
+		madd_v3_v3fl(cent, tri_cent, tri_area);
+
+		copy_v3_v3(v2, v3);
+	}
+
+	mul_v3_fl(cent, 1.0f / total_area);
+
+	return total_area;
+}
+
 /**
  * This function takes the difference between 2 vertex-coord-arrays
  * (\a vert_cos_src, \a vert_cos_dst),
@@ -3292,9 +3322,8 @@ int BKE_mesh_center_centroid(Mesh *me, float cent[3])
 	
 	/* calculate a weighted average of polygon centroids */
 	for (mpoly = me->mpoly; i--; mpoly++) {
-		BKE_mesh_calc_poly_center(mpoly, me->mloop + mpoly->loopstart, me->mvert, poly_cent);
-		poly_area = BKE_mesh_calc_poly_area(mpoly, me->mloop + mpoly->loopstart, me->mvert, NULL);
-		
+		poly_area = mesh_calc_poly_planar_area_centroid(mpoly, me->mloop + mpoly->loopstart, me->mvert, poly_cent);
+
 		madd_v3_v3fl(cent, poly_cent, poly_area);
 		total_area += poly_area;
 	}