Trimming Surface to Edge

If I crate a patch then do Show Control Points, I see there are points outside the visible edge of the surface, which I assume explains why, when I do smooth and have Fixed Boundaries, the edge moves.

Is it possible to rebuild the surface to get rid of these extra points?

Hi Jim - the only case where you can ‘get rid’ of them is when the trims are exactly on isocurves (not necessarily the displayed ones) - then you can ShrinkTrimmedSrf. If the trims are not on isocurves then shrinking may help, sort of, but there will always be points outside the trims.


Is it possible at all to SMOOTH a patch while preserving the edge?

Can you post an example, including all the inputs to the Patch?


Problem51.3dm (523.0 KB)

I’d say there is no good way to ‘smooth’ the result here. But patch seems like a third rate way to build this shape, to me. See

for some other ideas.


My problem is that I am not designing but rather recreating from data. The data is horizontal and vertical but the hull edge goes diagonally, giving problems for doing, say, networksrf.

Sure, but that does not mean you need to make the thing as one surface - you could for instance make it up from multiple surfaces made in different ways, depending on the input. Also, there is no reason you absolutely must use all of the curves and no other curves - the key is to come to within some tolerance of the data, right, not necessarily to use only the data as inputs ? So where needed you can make your own curves or use some subset of the curves. Like, for instance the bulk of it can be made with a NetworkSrf, using maybe every fourth curve or so of the vertical curves. Then build the harder surfaces separately.


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I would definitely have to build it from different parts.

The problem I have now is fairing (smoothing). I would built the final version using fewer points. The problem I have found is that if do the fairing with fewer points, the shape gets seriously distorted. In fact, I have found that the supplemental data points the original designers add are critical to getting the right shape. Part of the problem is you don’t know which points you can get rid of at the start.

If I were designing a ship from scratch, it would be no problem. But when you are doing a model of an existing ship that people are going to take a micrometer to and compare, you have less flexibility.

If I could do every other or every forth frame and waterline and be able to manipulate them at the same time, I could get away with that. But apparently, I am forced to do frames and waterlines separately (I cannot correct a waterline and have the corresponding frame be updated automatically). So I have to fair with more curves than I would really like to.

I did a beautiful fairing of the stern using a subset of the curves. Going back and forth between the frames and waterlines, I got both sets really fair. Then I built the surface and there were huge bumps in the frame gaps.

This might be the answer to your problem.