(octave.info)Interpolation on Scattered Data

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30.4 Interpolation on Scattered Data
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An important use of the Delaunay tessellation is that it can be used to
interpolate from scattered data to an arbitrary set of points.  To do
this the N-simplex of the known set of points is calculated with
‘delaunay’ or ‘delaunayn’.  Then the simplices in to which the desired
points are found are identified.  Finally the vertices of the simplices
are used to interpolate to the desired points.  The functions that
perform this interpolation are ‘griddata’, ‘griddata3’ and ‘griddatan’.

 -- : ZI = griddata (X, Y, Z, XI, YI)
 -- : ZI = griddata (X, Y, Z, XI, YI, METHOD)
 -- : [XI, YI, ZI] = griddata (...)

     Generate a regular mesh from irregular data using interpolation.

     The function is defined by ‘Z = f (X, Y)’.  Inputs ‘X, Y, Z’ are
     vectors of the same length or ‘X, Y’ are vectors and ‘Z’ is matrix.

     The interpolation points are all ‘(XI, YI)’.  If XI, YI are vectors
     then they are made into a 2-D mesh.

     The interpolation method can be "nearest", "cubic" or "linear".  If
     method is omitted it defaults to "linear".

     See also: Note: griddata3, *note griddatan:
     XREFgriddatan, Note: delaunay.

 -- : VI = griddata3 (X, Y, Z, V, XI, YI, ZI)
 -- : VI = griddata3 (X, Y, Z, V, XI, YI, ZI, METHOD)
 -- : VI = griddata3 (X, Y, Z, V, XI, YI, ZI, METHOD, OPTIONS)

     Generate a regular mesh from irregular data using interpolation.

     The function is defined by ‘V = f (X, Y, Z)’.  The interpolation
     points are specified by XI, YI, ZI.

     The interpolation method can be "nearest" or "linear".  If method
     is omitted it defaults to "linear".

     The optional argument OPTIONS is passed directly to Qhull when
     computing the Delaunay triangulation used for interpolation.  See
     ‘delaunayn’ for information on the defaults and how to pass
     different values.

     See also: Note: griddata, *note griddatan:
     XREFgriddatan, Note: delaunayn.

 -- : YI = griddatan (X, Y, XI)
 -- : YI = griddatan (X, Y, XI, METHOD)
 -- : YI = griddatan (X, Y, XI, METHOD, OPTIONS)

     Generate a regular mesh from irregular data using interpolation.

     The function is defined by ‘Y = f (X)’.  The interpolation points
     are all XI.

     The interpolation method can be "nearest" or "linear".  If method
     is omitted it defaults to "linear".

     The optional argument OPTIONS is passed directly to Qhull when
     computing the Delaunay triangulation used for interpolation.  See
     ‘delaunayn’ for information on the defaults and how to pass
     different values.

     See also: Note: griddata, *note griddata3:
     XREFgriddata3, Note: delaunayn.

   An example of the use of the ‘griddata’ function is

     rand ("state", 1);
     x = 2*rand (1000,1) - 1;
     y = 2*rand (size (x)) - 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);

that interpolates from a random scattering of points, to a uniform grid.