Diameter measurement

[Daniel Coelho]

Hello forum!

Can anyone give me some tips on how to measure the internal and external diameters of this 3D object that I've created?

I thought about cropping the external and internal surfaces, but I couldn't figure out how to do so, since there's no "circular" option on the cropping tool.

I also tried using the "cross section" tool, and projeting the slice(s) points on their best fit plane. But after doing that, I don't know how to proceed to get the avarege diameter out of it.

I would appreciate any help!

[WargodHernandez]

One way to get diameter would be to use the RANSAC plugin to search for Cylinders, with the right settings I think you should be able to find 2 cylinders, 1 for the inner surface and 1 for the outer.

If you want to post a cloud with just that small section I'd be happy to try it out

[daniel]

Yes I was going to suggest the same.

You can also roughly segment the inner and outer surfaces (with the scissors tool) before fitting the cylinders (so as to get more chances of find the right cylinder right away).

[Daniel Coelho]

First of all, thank you very much for both answers!

So I took the advice and did the following steps:

- Used the "Cross section" tool to get rid of the top part of my model;
- Used the "Segment" tool to separate both inner and outer surfaces from each other (in order to apply the RANSAC plugin, individually in each one);
- Applied the "RANSAC" plugin individually, using different "min support points per primitive" since one of the surfaces has twice as much points as the other one.

However, even doing so, the cylinder shape generated was not satisfying, due to the fact that it is not oriented on the same axis as my model...

So, is it possible to delimit the cylinder generated using the RANSAC to be paralel with the "z" axis? I think that would be enough to solve the problem.

[daniel]

Ah, that's strange, it's maybe due to the ripples on the cloud surface?

Now that I think about it, another option would be to create a cylinder primitive (via the 'File > Primitive factory', with roughly the right radius). Pre-position this cylinder so that it's roughly aligned with the cloud, and then use the ICP registration tool between the cloud and the cylinder. Make sure the 'Adjust scale' option is checked.

This will eventually find the best matching, and especially with the right scaling. Divide the cylinder radius by this scale, and you'll get a quite good estimation of the cloud 'radius'.

Well, that's definitely a hack, I would have expected the Ransac SD plugin to work better...