How to determine and extract surface points of 3D object?

Question

I have a 3D object which is not hollow, so there are many 3D points. How would you determine which of these points of such an object (especially with a very curvaceous surface) are on the surface? I understand how to extract them, but I need either a function somewhat like libraryUNK.surfacePoint ... Which I don't know.

Or better an understanding of what is considered to be a surface point, which I don't know either and couldn't (yet) develop (for myself) any proper definition.

I know I can do triangulation to get the surface. But I don't get what to do next, as I will be left now with a set of triangles, some of which are on the surface, some of which are not... but again I have no definition how to consider what's on surface and what is not...

Answer 1

It seems to me that you want to compute the convex hull of your 3D points cloud.

It's not an easy problem, but there's plently of solutions (and algorithms) to do that. One of the efficients one is called "convex hull". There's a ConvexHull function in scipy.spatial.

Here is the details with an example (2D, but it works in any dimension)

http://scipy.github.io/devdocs/generated/scipy.spatial.ConvexHull.html

This function use the QHull library

http://www.qhull.org/

There's plently of ressources on the QHull page. There's also a Wikipedia page (Again, this is not the only method to compute convex hulls, you may want to try others):

https://en.wikipedia.org/wiki/Quickhull

Edit: after re-reading the question, it seems your volume may not be convex. Unfortunately, if it isn't, there's no way to tell whether a point is inside the volume or in the surface without additional informations on the volume or on the points cloud.