problem with convhulln degenerate

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teresa
teresa 2012-3-20
评论： Walter Roberson 2020-9-18
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

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回答（4 个）

Nshine
Nshine 2020-1-16
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 2012-3-20
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 个评论
Nshine
Nshine 2020-1-16
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 2020-1-28
编辑：ilPlus30 2020-1-28
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 个评论
Walter Roberson
Walter Roberson 2020-1-28
The last column of xyzw is all 0.
ilPlus30
ilPlus30 2020-1-28
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 2020-9-18
what id degeneracy of matrix
A=[-1 0 0 0
0 -1 0 0
0 0 -1 0
0 0 0 -1];
  1 个评论
Walter Roberson
Walter Roberson 2020-9-18
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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