Rule

The mathematical method for calculating the subdivisions.

• Catmull-Clark produces rounded shapes. The generated polygons are all quadrilateral. The higher the Subdivision Depth, the more this method approximates a bicubic standard B-spline surface. At regular vertices (exactly four edges), the surface has C2 (curvature) continuity; in other areas, the surface has C1 (tangential or parametric) continuity. The geometry produced is compatible with the method described in “Recursively generated B-spline surfaces on arbitrary topological surfaces” by E. Catmull and J. Clark (Computer-Aided Design 10(6):350-355, November 1978).

• XSI-Doo-Sabin produces shapes with the same silhouettes as traditional Doo-Sabin but with different tesselation. One advantage over traditional Doo-Sabin is that XSI-Doo-Sabin correctly propagates clusters and cluster properties (including discontinuities) such as texture UVs, vertex colors, and weight maps. Another advantage is that XSI-Doo-Sabin handles creases and hard edges better.

Like traditional Doo-Sabin, XSI-Doo-Sabin produces shapes that follow the original mesh more closely than Catmull-Clark. If the original mesh has N polynodes, then each level L of subdivision has (4^L)N quad polygons. The higher the subdivision level, the more this method approximates a biquadratic uniform B-spline surface. At regular vertices, the surface has C1 (tangential or parametric) continuity; in other areas, the surface has G1 continuity (i.e., the tangents have the same direction but not necessarily the same length).

• Linear subdivides the mesh without smoothing it. This is useful when you want an object to deform well but do not want to change its basic shape.

Use Loop for Triangles

Subdivides triangles into smaller triangles, giving better results than quad-based methods. The Loop method avoids bulges and other artifacts when subdividing triangles. Catmull-Clark or linear is still used for non-triangles, and there is a smooth transition at boundaries between Catmull-Clark and Loop. Not available with XSI-Doo-Sabin.