Re: why cell arrays are used in 'interp2'

[Ben Abbott]

On Dec 24, 2012, at 10:25 PM, Sergei Steshenko wrote:

> Hello,
> 
> looking into 'octave-3.6.2/share/octave/3.6.2/m/general/interp2.m' file I see:
> 
> "
>     265       ## construct the cubic hermite base functions in x, y
>     266
>     267       ## formulas:
>     268       ## b{1,1} =    ( 2*t.^3 - 3*t.^2     + 1);
>     269       ## b{2,1} = h.*(   t.^3 - 2*t.^2 + t    );
>     270       ## b{1,2} =    (-2*t.^3 + 3*t.^2        );
>     271       ## b{2,2} = h.*(   t.^3 -   t.^2        );
>     272
>     273       ## optimized equivalents of the above:
>     274       t1 = tx.^2;
>     275       t2 = tx.*t1 - t1;
>     276       xb{2,2} = hx.*t2;
>     277       t1 = t2 - t1;
>     278       xb{2,1} = hx.*(t1 + tx);
>     279       t2 += t1;
>     280       xb{1,2} = -t2;
>     281       xb{1,1} = t2 + 1;
>     282
>     283       t1 = ty.^2;
>     284       t2 = ty.*t1 - t1;
>     285       yb{2,2} = hy.*t2;
>     286       t1 = t2 - t1;
>     287       yb{2,1} = hy.*(t1 + ty);
>     288       t2 += t1;
>     289       yb{1,2} = -t2;
>     290       yb{1,1} = t2 + 1;
>     291
>     292       ZI = zeros (size (XI));
>     293       for i = 1:2
>     294         for j = 1:2
>     295           zidx = sub2ind (size (Z), yidx+(j-1), xidx+(i-1));
>     296           ZI += xb{1,i} .* yb{1,j} .*   Z(zidx);
>     297           ZI += xb{2,i} .* yb{1,j} .*  DX(zidx);
>     298           ZI += xb{1,i} .* yb{2,j} .*  DY(zidx);
>     299           ZI += xb{2,i} .* yb{2,j} .* DXY(zidx);
>     300         endfor
>     301       endfor
> 
> ".
> 
> It looks to me only numeric data is used as array elements, so why cell 
> arrays and not regular 2d matrices ?
> 
> Speed ? Space ? Something else I couldn't think of ?
> 
> 
> Thanks,
>   Sergei.

The cell-arrays contian 2D matrices whose sizes are [numel(yi), numel(xi)].

Ben