Sunday, September 30, 2012

Tools for curved surfaces

I have a love-hate relationship with NURBS. I love them for the same reason I like vector art: they scale to arbitrarily-high resolutions. I hate them because the NURBS tools often produce ugly artifacts, and it's not obvious how to fix them.

In two dimensions, curves are approximated using a large number of square pixels: zoom in too much, and you'll see the pixels. Vector-based images don't have this problem because they store the curves themselves, not the pixels.

Above: pixels.

In three dimensions, curved surfaces are approximated using a large number of flat faces: zoom in too much, and you'll see the triangles. NURBS-based geometry doesn't suffer from this problem because it stores the curves themselves, not the triangles.

Above: polygon edges.

This all sounds great in theory, but when I experimented with the Houdini tools in the NURBS category, I was repeatedly disappointed by unexpected artifacts in the generated curves. Sometimes the artifacts were tiny, but visible — and sometimes the generated curves were completely wrong. In both cases, there was no obvious way to fix the problem.

Left: subtle artifact.
Right: not so subtle.

As you can see below, eventually I did manage to produce some good-looking shapes. There was no magic artifact-fixing step; I sat down and learned how each tool worked, how they were calculating their results. This allowed my to understand why I saw the artifacts I saw, and guided me towards ways of using the tools which were less likely to generate artifacts. It's better to stick to profile-curves generated through the Carve tool, for instance.

17 down, 195 to go!
Yes, I know there are 20 icons,
but some of those have appeared before.

Oh, and one of those tools doesn't exist,
it's a digital asset I created myself.
Can you spot which one? 

I learned a lot during this series, but I still have many more tools to go through. Houdini has a lot of stuff!

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