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[Octave-bug-tracker] [bug #50494] griddata loses precision in the linear


From: El Colmo
Subject: [Octave-bug-tracker] [bug #50494] griddata loses precision in the linear interpolation of 3 column vectors (using Delaunay triangles)
Date: Wed, 8 Mar 2017 11:48:26 -0500 (EST)
User-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.75 Safari/537.36

URL:
  <http://savannah.gnu.org/bugs/?50494>

                 Summary: griddata loses precision in the linear interpolation
of 3 column vectors (using Delaunay triangles)
                 Project: GNU Octave
            Submitted by: elcolmo
            Submitted on: Wed 08 Mar 2017 04:48:24 PM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Inaccurate Result
                  Status: None
             Assigned to: None
         Originator Name: Colmo
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 4.2.1
        Operating System: Any

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Details:

The griddata function loses precision when calculating the linear
interpolation of 3 column vectors (using Delaunay triangles), since only the x
and y parameters are passed to "delaunay" (ie, using the default QHull
options). This results in triangle elimination in "delaunay".  When the
generated (reduced) set of triangles is searched by "tsearch", the resultant
interpolated value is less precise than expected.

The fix is trivial - to change the parameters passed to "delaunay", so that
the default QHull options are no longer used, resulting in *no* triangle
elimination by "delaunay", resulting in greatly increased precision of the
interpolated values.

The fix is trivial. In Change the following line (in
"/usr/share/octave/4.0.3/m/geometry/griddata.m", in my deployment) from:

tri = delaunay (x, y);

to either..

tri = delaunay (x, y, {"Qt", "Qc", "Qz"});   #remove default option Qbb, in
order to slightly increase precision (by increasing the triangle count)

or...

tri = delaunay (x, y, {"Qt", "Qbb", "Qc"});   #remove default option Qz, in
order to greatly increase precision (by increasing the triangle count)


Note: this is also an issue in the latest available version of Octave (4.2.1).




    _______________________________________________________

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