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# How to make a skinned sweep that can be subdivided ?

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I can't find a way to take two curves, make a sweep, skin the result, and have an output that is topologically equivalent to a box, so that it subdivides nicely in all directions.

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well....look at things from the other side of the fence.....take a box have the divisions as 1x4x1

Subdiv that and hey....suddenly things are not so ideal anymore....

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What is the difference between using the Skin SOP after the Sweep, as compared to using the "Skin Output" option on the Sweep SOP?

I imagine you could do a PolyFill after skinning, then finding a way to subdivide that. Maybe with a PolyExtrude inset?

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Dive into the resample node , and change  'Treat Polygon as', to see difference  or ... edit your profile curve

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Here is how I solved it for simple (regular polygon looking) cross sections:

make the first and last point closer to the second and second from last, then set pscale to 0 for the first and last point.

then after the skin, fuse the points which are close to each other, and at this point you have the topology of a sphere. This works better if the "radius" of the cross section curve is about the same size of the segments of the sweep profile polygon, to keep the quadrilaterals about the same size. Otherwise, there will be abrupt changes in the curvature ( the same way you do angles in NURBS curves by doubling points)

For more complex curves (not convex), I haven't yet found a good solution, because I think this would require to compute the straight skeleton, which is not something which has a good implementation in Houdini. Polyexpand2D is the closest Houdini has to offer, but it seems hard to uses it as a straight skeleton operator. CGAL has it function

This works for polygons because the straight skeleton happens to be the center, which is the point where all the vertices converge when scaled to 0.

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