Difference between convhull() and convhulln()?

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david dang
david dang 2015-7-11
评论： david dang 2015-7-12
In MATLAB's function libraries, there are 2 functions that achieve descriptively the same purpose: 1) convhull() 2) convhulln()
I notice that these 2, when applied to the same set of input, gives me slightly different answers for volume (negligible error, but I am confused why there is a difference since I thought the algorithm gives an EXACT volume). And for some sets of vertices, convhulln() outputs an error while convhull() works.
Can someone explain to me the difference between the two?

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回答（3 个）

Walter Roberson
Walter Roberson 2015-7-11
convhulln can operate in higher dimensional spaces.
Negligible error would be due to floating point roundoff, probably.
Image Analyst
Image Analyst 2015-7-11
convhull() operates on 2D data - points in an infinitely thin plane. How are you computing the volume from that? It won't have a volume. Or a volume of zero.
convhulln operates on higher dimensional data. In 3D it would be like putting a tight balloon around your points. It would return the points that are "pointy" in your balloon envelope. How do you compute the volume from that?
John D'Errico
John D'Errico 2015-7-12
Welcome to the wonderful, wacky world of floating point arithmetic.
There is no such thing as an "exact" computation when done in floating point arithmetic by two distinct code sequences. That you get subtly (though negligibly) different results when two different computations are applied is no surprise at all. Many such mathematically, theoretically identical operations, when implemented in floating point arithmetic, will fail to yield numerically identical results.

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