Mesh: Store skin modifier data as generic attributes

Replace the internal storage of skin radius and the "root" and "loose"
flags with generic attributes with names "skin_modifier_{radius/root/
loose}". File format backward and forward compatibility is maintained,
as is API compatibility. Overall this means the data should be
propagated in more places and supported in Geometry Nodes, node tools,
etc. One possible behavior change is that the default value is not
baked into the type anymore, but the existing operators used to create
the data maintain the old default.

Pull Request: https://projects.blender.org/blender/blender/pulls/162049

tests/python/bl_geometry_attributes.py

@@ -174,6 +174,64 @@ class TestMeshAttributeConvert(MeshObjectTest):
         self.assertTrue(self.mesh.uv_layers.active.name == "UVA")
 
 
+class TestMeshSkinVertices(MeshObjectTest):
+    def test_no_skin_data_by_default(self):
+        self.assertEqual(len(self.mesh.skin_vertices), 0)
+
+    def test_add_and_clear_skin_data(self):
+        bpy.ops.mesh.customdata_skin_add()
+        self.assertEqual(len(self.mesh.skin_vertices), 1)
+        skin_layer = self.mesh.skin_vertices[0]
+        self.assertEqual(skin_layer.name, "skin_modifier_radius")
+        self.assertEqual(len(skin_layer.data), len(self.mesh.vertices))
+
+        bpy.ops.mesh.customdata_skin_clear()
+        self.assertEqual(len(self.mesh.skin_vertices), 0)
+
+    def test_skin_vertex_radius(self):
+        bpy.ops.mesh.customdata_skin_add()
+        skin_layer = self.mesh.skin_vertices[0]
+
+        # Newly added skin data defaults to a radius of 0.25 on both axes.
+        vert = skin_layer.data[0]
+        self.assertAlmostEqual(vert.radius[0], 0.25)
+        self.assertAlmostEqual(vert.radius[1], 0.25)
+
+        vert.radius = (0.5, 0.75)
+        self.assertAlmostEqual(skin_layer.data[0].radius[0], 0.5)
+        self.assertAlmostEqual(skin_layer.data[0].radius[1], 0.75)
+
+    def test_skin_vert_root_and_loose(self):
+        bpy.ops.mesh.customdata_skin_add()
+        skin_layer = self.mesh.skin_vertices[0]
+
+        # An arbitrary vertex (the first) is marked as root by default, the rest are not,
+        # and no vertex is loose by default.
+        self.assertTrue(skin_layer.data[0].use_root)
+        self.assertFalse(skin_layer.data[1].use_root)
+        self.assertFalse(skin_layer.data[0].use_loose)
+
+        skin_layer.data[1].use_root = True
+        self.assertTrue(skin_layer.data[1].use_root)
+        skin_layer.data[1].use_root = False
+        self.assertFalse(skin_layer.data[1].use_root)
+
+        skin_layer.data[2].use_loose = True
+        self.assertTrue(skin_layer.data[2].use_loose)
+        skin_layer.data[2].use_loose = False
+        self.assertFalse(skin_layer.data[2].use_loose)
+
+    def test_skin_vertices_ignores_unrelated_float2_point_attribute(self):
+        # skin_vertices is identified by name, not just by domain and type, so an unrelated
+        # float2 point attribute should not be treated as skin data.
+        self.mesh.attributes.new("my_custom_data", 'FLOAT2', 'POINT')
+        self.assertEqual(len(self.mesh.skin_vertices), 0)
+
+        bpy.ops.mesh.customdata_skin_add()
+        self.assertEqual(len(self.mesh.skin_vertices), 1)
+        self.assertEqual(self.mesh.skin_vertices[0].name, "skin_modifier_radius")
+
+
 if __name__ == '__main__':
     import sys
     sys.argv = [__file__] + (sys.argv[sys.argv.index("--") + 1:] if "--" in sys.argv else [])