problem with convhulln degenerate

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teresa
teresa on 20 Mar 2012
Commented: Walter Roberson on 18 Sep 2020
Hi my name is noé alvarado I want to find de convex hull of the next point:
xyzw = [1 -1 -1.375 0;-0.5 -0.8 -0.2830 0;-1.3 0.7 0.217 0;1.5 1.3 1.74 0;0.6 0 0 0]
xyzw =
1.0000 -1.0000 -1.3750 0
-0.5000 -0.8000 -0.2830 0
-1.3000 0.7000 0.2170 0
1.5000 1.3000 1.7400 0
0.6000 0 0 0
tess = convhulln(xyzw)
however i have the error:
Error using ==> qhullmx The data is degenerate in at least one dimension - ND set of points lying in (N+1)D space.
Could yoyu help me?
thanks

Answers (4)

Nshine
Nshine on 16 Jan 2020
xyzw = [1 -1 -1.375 0;-0.5 -0.8 -0.2830 0;-1.3 0.7 0.217 0;1.5 1.3 1.74 0;0.6 0 0 0]
convhulln(xyzw)
The error occurs because the last column is all zeros. If you delete the last column and try again:
xyzw(:,4) = [];
convhulln(xyzw)
you get the answer
ans =
1 4 3
4 2 3
2 1 3
1 2 4
ie: facet 1 is made by the triangle connecting vertex 1, 4 and 3, etc.

Image Analyst
Image Analyst on 20 Mar 2012
I'm not sure I understand. You have a 2D matrix. Why not use the 2D version of convex hull, convhull()?
And what is "the next point"? What were the prior points for that matter?
You can't have the convex hull of just a point - you need at least 3 points to give a meaningfull convex hull.
  1 Comment
Nshine
Nshine on 16 Jan 2020
This is an old thread, but the the argument for convhulln, X is an m-by-n array (ie: 2D array) representing m points in n-D space, as can be read in the documentation:
help convhulln

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ilPlus30
ilPlus30 on 28 Jan 2020
Edited: ilPlus30 on 28 Jan 2020
Hi everybody,
even keeping the last column , you could try this way:
xyzw = [1 -1 -1.375 0;-0.5 -0.8 -0.2830 0;-1.3 0.7 0.217 0;1.5 1.3 1.74 0;0.6 0 0 0]
convhulln (xyzw, {'QJ'})
  2 Comments
ilPlus30
ilPlus30 on 28 Jan 2020
Sorry you're right, however in that way the last column could go on, for example it could be important not to delete it.

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Shannu
Shannu on 18 Sep 2020
what id degeneracy of matrix
A=[-1 0 0 0
0 -1 0 0
0 0 -1 0
0 0 0 -1];
  1 Comment
Walter Roberson
Walter Roberson on 18 Sep 2020
You have 4 points in a 4 dimensional space. In order to generate a convex hull, you need at least 5 points for a 4 dimensional space.

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