[Octave-bug-tracker] [bug #51624] griddata example fails

[Dan Sebald]

Follow-up Comment #8, bug #51624 (project octave):

I'm still a bit curious as to why the following example, which only adds the
four points at the corners, results in the attached reconstructed surface:


rand ("state", 1);
x = 2*rand (1000,1) - 1;
y = 2*rand (size (x)) - 1;
x = [x; 1; 1; -1; -1];
y = [y; 1; -1; -1; 1];
z = sin (2*(x.^2+y.^2));
[xx,yy] = meshgrid (linspace (-1,1,32));
zz = griddata (x, y, z, xx, yy);
mesh (xx, yy, zz);


I can imagine that the algorithm somehow fills in those side surfaces by
assuming continuity between those corner points and all the points along the
arches at x=1, at y=1, etc.  But I'm wondering if that is something it should
be doing under the circumstances.  When there are none of the four corners,
things look reasonable.  Implicit in that statement of "reasonable" is that
the reconstructed surface has a certain degree of smoothness, i.e., that it
can't make drastic, high-order transitions between sample points.  Experiment
with the above data with:


zz = griddata (x, y, z, xx, yy);
mesh (xx, yy, zz);

zz = griddata (x, y, z, xx, yy, "nearest");
mesh (xx, yy, zz);

zz = griddata (x, y, z, xx, yy, "cubic");
mesh (xx, yy, zz);


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