Interp2 cubic. Diff between Matlab and Octave

[Ben Abbott]

On May 10, 2009, at 5:55 PM, Andres Sepulveda wrote:

> Hi,
>
> We want to port a Matlab code to Octave but where it uses the interp2
> function with the 'cubic'  flag we get the following diference. Any
> idea why this happens?
>
> Andres Sepulveda
> University of Concepcion, CHILE
>
> clear;
> close all;
> clc;
>
> if exist('OCTAVE_VERSION')
> 	sufix = 'octave';
> else
> 	sufix = 'matlab';
> end
>
> % original data --> {x,y,data}
> [x,y] = meshgrid(1:11,1:3);
> data = [0.41 0.40 0.73 0.77 0.48 0.11 0.74 0.06 0.57 0.83 0.46; ...
>        0.98 0.32 0.54 0.38 0.19 0.30 0.33 0.03 0.68 0.07 0.44; ...
>        0.69 0.98 0.16 0.95 0.67 0.25 0.35 0.50 0.40 0.46 0.63];
>
> % interpolated data --> {lon,lat,rr_}
> lon = linspace(x(1),x(end),25);
> lat = linspace(y(1),y(end),25);
> rr1 = interp2(x,y,data,lon,lat,'linear'); % linear
> rr2 = interp2(x,y,data,lon,lat,'cubic');  % cubic
>
> % plot result
> figure;
> plot(rr1,'linewidth',2);
> axis([1 25 0 1]);
> title(['linear interpolation ',sufix]);
> print(['linear_',sufix,'.eps'],'-depsc');
>
> figure;
> plot(rr2,'linewidth',2);
> axis([1 25 0 1]);
> title(['cubic interpolation ',sufix]);
> print(['cubic_',sufix,'.eps'],'-depsc');

Using Matlab, both results look quite similar.

It appears to me that the cubic interpolation is not working correctly.

Jaroslav, can you confirm?

Ben