BKE_mesh: add polygon flipping tools.

Those new functions invert the winding of polygons, effectively inverting their normals.

A helper was also added to allow swapping two items in customdata layers.

Being able to invert normals outside of BMesh area is very important in several places,
like IO scripts or customnormals modifiers...

Reviewers: campbellbarton

Differential Revision: https://developer.blender.org/D1814

2 files changed, 29 insertions, 1 deletions

diff --git a/source/blender/makesrna/intern/rna_mesh.c b/source/blender/makesrna/intern/rna_mesh.c
index 686f8331ac4..1d734864833 100644
--- a/source/blender/makesrna/intern/rna_mesh.c
+++ b/source/blender/makesrna/intern/rna_mesh.c
@@ -418,6 +418,14 @@ static float rna_MeshPolygon_area_get(PointerRNA *ptr)
 	return BKE_mesh_calc_poly_area(mp, me->mloop + mp->loopstart, me->mvert);
 }
 
+static void rna_MeshPolygon_flip(ID *id, MPoly *mp)
+{
+	Mesh *me = (Mesh *)id;
+
+	BKE_mesh_polygon_flip(mp, me->mloop, &me->ldata);
+	BKE_mesh_tessface_clear(me);
+}
+
 static void rna_MeshTessFace_normal_get(PointerRNA *ptr, float *values)
 {
 	Mesh *me = rna_mesh(ptr);
@@ -2138,6 +2146,7 @@ static void rna_def_mpolygon(BlenderRNA *brna)
 {
 	StructRNA *srna;
 	PropertyRNA *prop;
+	FunctionRNA *func;
 
 	srna = RNA_def_struct(brna, "MeshPolygon", NULL);
 	RNA_def_struct_sdna(srna, "MPoly");
@@ -2216,6 +2225,11 @@ static void rna_def_mpolygon(BlenderRNA *brna)
 	RNA_def_property_clear_flag(prop, PROP_EDITABLE);
 	RNA_def_property_int_funcs(prop, "rna_MeshPolygon_index_get", NULL, NULL);
 	RNA_def_property_ui_text(prop, "Index", "Index of this polygon");
+
+	func = RNA_def_function(srna, "flip", "rna_MeshPolygon_flip");
+	RNA_def_function_flag(func, FUNC_USE_SELF_ID);
+	RNA_def_function_ui_description(func, "Invert winding of this polygon (flip its normal)");
+
 }
 
 /* mesh.loop_uvs */
diff --git a/source/blender/makesrna/intern/rna_mesh_api.c b/source/blender/makesrna/intern/rna_mesh_api.c
index 1459157112e..a3bc21b0170 100644
--- a/source/blender/makesrna/intern/rna_mesh_api.c
+++ b/source/blender/makesrna/intern/rna_mesh_api.c
@@ -200,6 +200,15 @@ static void rna_Mesh_transform(Mesh *mesh, float *mat, int shape_keys)
 	DAG_id_tag_update(&mesh->id, 0);
 }
 
+static void rna_Mesh_flip_normals(Mesh *mesh)
+{
+	BKE_mesh_polygons_flip(mesh->mpoly, mesh->mloop, &mesh->ldata, mesh->totpoly);
+	BKE_mesh_tessface_clear(mesh);
+	BKE_mesh_calc_normals(mesh);
+
+	DAG_id_tag_update(&mesh->id, 0);
+}
+
 #else
 
 void RNA_api_mesh(StructRNA *srna)
@@ -209,11 +218,16 @@ void RNA_api_mesh(StructRNA *srna)
 	const int normals_array_dim[] = {1, 3};
 
 	func = RNA_def_function(srna, "transform", "rna_Mesh_transform");
-	RNA_def_function_ui_description(func, "Transform mesh vertices by a matrix");
+	RNA_def_function_ui_description(func, "Transform mesh vertices by a matrix "
+	                                      "(Warning: inverts normals if matrix is negative)");
 	parm = RNA_def_float_matrix(func, "matrix", 4, 4, NULL, 0.0f, 0.0f, "", "Matrix", 0.0f, 0.0f);
 	RNA_def_property_flag(parm, PROP_REQUIRED);
 	RNA_def_boolean(func, "shape_keys", 0, "", "Transform Shape Keys");
 
+	func = RNA_def_function(srna, "flip_normals", "rna_Mesh_flip_normals");
+	RNA_def_function_ui_description(func, "Invert winding of all polygons "
+	                                      "(clears tessellation, does not handle custom normals)");
+
 	func = RNA_def_function(srna, "calc_normals", "BKE_mesh_calc_normals");
 	RNA_def_function_ui_description(func, "Calculate vertex normals");