Hi @song.liu ,
Sorry, my answer was not very clear: by “streched” I refer to the shape of the triangle.
You are right, subdividing the triangles will not improve their shapes but it can help because in your input mesh, you don’t have any node “internal” to the orange and green surfaces. So the surface model relies entirely on the normals computed at the interfaces of the blue+(orange&green) surfaces and yellow+(orange&green) surfaces. Thus, adding other nodes can help to have a better computation of normals.
I am not sure that it will work though (). If it doesn’t work, another solution can be to provide the normals by yourself (if you know them)…