GPU Pro 6

42 I Geometry Manipulation

(a) (b)

Figure 3.8. Edge overlap update for a single patch: (a) The direction of the overlap

updates originates from the blue patch. (b) The two adjacent patches (red and yellow)

receive their resulting overlap data by gathering the information from the blue patch.

Catmull-Clark subdivision surfaces two patch types are possible:

• Regular patches: All vertices of a patch have exactly four incoming edges.

• Irregular patches: At least one vertex of the patch has a valence different

from four.

In the regular case, the corner values of a patch can simply be copied to the

diagonal patch’s boundary corner (see Figure 3.9(a)). In