I implemented catmul clark based on this Wikipedia article. And it seems to work fine for meshes where the edge point count is equal to the face point count, but not for planes where the corners have 2 edge points and 1 face point.
The article states the following for this formula
$$ \frac{F + 2R + (n-3)P}{n} $$
(Note that from the perspective of a vertex P, the number of edges neighboring P is also the number of adjacent faces, hence n)
And that seems to be true for cube-like shapes, but not for planes.
Does this implementation not work for planes, or am I doing something wrong?