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Re: Discuss Digest, Vol 38, Issue 22

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jim_klessig

Re: Discuss Digest, Vol 38, Issue 22

"Actually it is possible, but probably not inside OpenSCAD. Many years ago I

was in the land surveying business and I wrote a routine to take randomly
spaced 3D points"

I would disagree. Your routine would be making some (probably un stated) assumptions about the shape. Such as that the points do not form a "cave", and probably what points represent a boundary.

For example how would your routine deal with the example I gave, of 8 points representing the corners of a cube.

There are multiple "correct" representations of that set of points.

I would suspect that your routine would assume those points are either a cube shaped bump in a flat plane, or a cubical hole in an otherwise flat plane. Both of which are making perfectly reasonable assumptions for making it topographical map.

But those assumptions would be/could be incorrect for just that set of 8 points, "floating in space" with no other information.

Essentially there are too many degrees of freedom for the problem to be solvable as he posed it.

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jim_klessig

Re: Discuss Digest, Vol 38, Issue 22

Also, for whatever reason, i can never access any of the pictures people attach here, so no, i can not see it.

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doug.moen

Re: Discuss Digest, Vol 38, Issue 22

In reply to this post by jim_klessig

This idea has been discussed before. The CGAL "Alpha Shapes" module is one way to implement this. Note that we already use CGAL for boolean operations like union.

https://doc.cgal.org/latest/Alpha_shapes_3/index.html

On 23 January 2018 at 13:09, jim.klessig <[hidden email]> wrote:

"Actually it is possible, but probably not inside OpenSCAD. Many years ago I
was in the land surveying business and I wrote a routine to take randomly
spaced 3D points"

I would disagree. Your routine would be making some (probably un stated) assumptions about the shape. Such as that the points do not form a "cave", and probably what points represent a boundary.

For example how would your routine deal with the example I gave, of 8 points representing the corners of a cube.
There are multiple "correct" representations of that set of points.

I would suspect that your routine would assume those points are either a cube shaped bump in a flat plane, or a cubical hole in an otherwise flat plane. Both of which are making perfectly reasonable assumptions for making it topographical map.
But those assumptions would be/could be incorrect for just that set of 8 points, "floating in space" with no other information.
Essentially there are too many degrees of freedom for the problem to be solvable as he posed it.

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cbernhardt

Re: Discuss Digest, Vol 38, Issue 22

In reply to this post by jim_klessig

jim_klessig wrote

> I would disagree. Your routine would be making some (probably un stated)
> assumptions about the shape. Such as that the points do not form a
> "cave", and probably what points represent a boundary.
> For example how would your routine deal with the example I gave, of 8
> points representing the corners of a cube.There are multiple "correct"
> representations of that set of points.
> I would suspect that your routine would assume those points are either a
> cube shaped bump in a flat plane, or a cubical hole in an otherwise flat
> plane. Both of which are making perfectly reasonable assumptions for
> making it topographical map.But those assumptions would be/could be
> incorrect for just that set of 8 points, "floating in space" with no other
> information.Essentially there are too many degrees of freedom for the
> problem to be solvable as he posed it.

You are correct. My routine eliminates any duplicate XY points, so it would
not tessellate an exact cube or an overhang correctly; however, for purposes
of my headlight bezel, or 99% of our topo maps, it works fine.
Charles

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NateTG

Re: Discuss Digest, Vol 38, Issue 22

Sure, you can, for example, do a Delauney triangulation in the XY plane to
generate a mesh, and then do a quadratic or bezier fit. That's technically
possible using OpenSCAD functions, but I would be looking at existing mesh
manipulation stuff like blender or meshlab.

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