3D Data: Blobs

What's it all about?

The object at right was created by "rounding" or "filleting" one edge of a rectanguar extrusion, but that's not important. We can understand the shape as a set of planar regions (the polygons) plus one curved region at the rounded edge. The flat bits at top, sides, and bottom are understood by knowing the edges (the polygon boundaries) and the nature of planes (intersect in straight lines, flat shaded, etc). The curved bit can be understood in terms of the straight edges, the arcs at the end, and the notion of uniform cross-section.

Many complexly-curved objects, including most mesh surfaces, break down into regions that behave according to particular mathematics. These are often called patches. Patches are generally one-sided surfaces, not volumes. The processes of joining or cutting patches, analagous to boolean operations, are called stitching and trimming.

There are lots of different kinds of patches in the modeling world. The ideal patch would be able to represent everything from a flat polygon to a cylindrical tube, to a "saddle" shape like a hyperbolic parabaloid. The process of calculating the intersection of two patches would be well-defined, and the result would produce the same kind of patch to an arbitrary level of detail.

Non-uniform rational b-splines (NURBS) are one type of patch.

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