Subdivision Surfaces

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Overview

Subdivision surfaces are a type of smooth surface defined by a polyhedron - the smooth surface follows the "outline" of the polygons in an intuitive and useful way that has become very popular with modelers.

There are various K-3D tools to create SDS:

  • MakeSDS is a modifier that flags a polyhedron as a subdivision surface (SDS), thus you can think of it as a conversion from one geometric datatype (polyhedron) to another (SDS).
  • SDSCrease can be used to assign variable-sharpness creases to the edges of an SDS.
  • SDSCorner does the same thing for individual points.
  • SDSHole tags individual polygons as "holes" that don't show up in the SDS, even though they contribute to its shape.
  • CatmullClark modifier is a polygon subdivison filter - it applies the SDS algorithms to a collection of polygons, producing a different set of smaller, smoother output polygons.

The distinction between K-3D SDS preview and CatmullClark output isn't obvious in the viewport because our SDS preview has limited fidelity and both use the same algorithm. But if you render your scene, you'll see that an SDS always renders as a perfectly smooth shape, while the CatmullClark filter is simply producing additional flat polygons.

Same thing for MakeCreases - it only affects geometry that has had creases applied with SDSCrease.

Creases

Bart Janssens wrote the SDS tools using the most common scheme: Catmull-Clark. It's a well known subdivision algorithm and you won't see any difference when rendering your scene with different renderers.

However, the SDSCrease modifier will show dissimilar shapes depending on the renderer. The method used in K-3D differs (not dramatically) from the ones implemented in Aqsis and 3Delight. It can be used with any renderer supporting catmull-clark without creases, provided users apply the MakeCreases modifier before rendering, which adds the crease geometry to the mesh. After that, the creases will appear exactly as in the preview after application of the regular catmull-clark scheme by the renderer. This method is independent of the renderer, previews well even at low SDS levels (one could consider implementing the Aqsis method as another option, but the current preview should be close enough for most purposes).

Aqsis uses its own method to calculate creases, which differs from the way 3delight does it because of a Pixar patent.

External links

Wikipedia's Subdivision Surface article

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