[Octave-bug-tracker] [bug #46946] isosurface throws errors where Matlab does not

[Markus Mützel]

Follow-up Comment #7, bug #46946 (project octave):

No problem if it takes a little longer. Thus, the patch can "mature" a
little:
I realized that "unique" does not really do the job for recognizing shared
vertices. This is because very close doubles are treated as different values:

a = [2/3 0 0; 2/3 1 0; 1-1/3 0 0];
unique(a, "rows")

Result:

ans =
   0.66667   0.00000   0.00000
   0.66667   1.00000   0.00000
   0.66667   0.00000   0.00000

This is because

>> a(1,:)-a(3,:)
ans =
  -1.1102e-016  0.0000e+000  0.0000e+000

>> eps(2/3)
ans =   1.1102e-016


At first I thought this was a bug. But Matlab behaves exactly the same. And it
is true that 2/3 != 1-1/3 in double precision.

Therefore, I changed the logic for detection of shared vertices by calculating
the mutual distances between all vertices and comparing to "eps"
(isosurface_4.patch). There is probably a much better way than my current
implementation since it performs e.g. more than twice the calculations really
needed. But I cannot think of one without using loops which would perform even
worse in Octave.
So feel free to give advise.


(file #36360, file #36361)
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Additional Item Attachment:

File name: isosurface_4.patch             Size:19 KB
File name: isosurface.m                   Size:18 KB


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