Normals are ignored. The new points are build by averaging neighbouring polygon centers, edge centers, vertices... The different rules for subdivided corner points / edge-, poly-centers are simple, but because the process is recursive, it's difficult to accelerate.I've never modelled anything with catmull-clark surfaces -- is the tesselation shape dependent only on the vertex positions and normals, like phong tesselation?
actually, the artist have barely control over where bulging etc. happens
I've done a lot of modeling with catmull clark and also made my own editor because i was not happy with crease options from commercial apps.
For modeling organic shapes catmull clark is the best option. With proper creases it's also a very good alternative to nurbs for things like cars etc., while still easier to understand.
Cons are: You need to avoid triangles and use regular quad grids whenever possible. A good model will end up with mostly quads, some 5 sided and a few 6 sided polygons.
Subdividing a typical triangulated mesh makes no sense - you need to have the original quadbased model to get good results.
The first subdivision step is special, it does the most important work and ends up with a mesh containing quads only.
For a good HW-acceleration it gives sense to do it with its own algorithm, maybe on CPU.
For following steps it could give sense to switch to a more hardware friendly method, like bezier patches.
If anyone has experience with practical HW-acceleration i would like to hear something about it too...
Note that this can be a very good thing, because if you do the skinning with the low res control mesh, you get MUCH better final high res skinning! This also saves some work, as you don't need to skin the subdivided stuff.
Skinning is where difference to other tesselation methods shows up most noticeably. Because the corner vertices get smoothed too, not just the surface around them. Maybe it's hard for a programmer to get the point why they are so good compared th other methods - but with skinning the difference in visual quality is really huge. Trust me