Why is scatteredInterpolant slower than TriScatteredInterp ?

I've written a code that uses TriScatteredInterp, but I read in Matlab's documentation that this will not be supported in future release and that I should instead use scatteredInterpolant. So I did, and found to be twice slower for a 512 by 512 matrix. The only difference in my code was just using:

    F = scatteredInterpolant(double(x'),double(y'),double(z'),'natural');

instead of

    F = TriScatteredInterp(double(x'),double(y'),double(z'),'natural');

Why is that and what can I do to make it faster?

[Matt J]

Your calls to double() and ctranspose() could have affected the speed comparison.

[Adi Natan]

I doubt it. I use the exact same input for TriScatteredInterp.

[Matt J]

Even if the construction time does matter to you, I cannot reproduce the slow behavior. I even see slightly faster behavior in scatteredInterpolant for a 512^2 data,

    x = rand(512)*4-2;
    y = rand(512)*4-2;
    z = x.*exp(-x.^2-y.^2);

    tic;
    obj1 = TriScatteredInterp(x(:),y(:),z(:));
    toc
    %Elapsed time is 1.475620 seconds.

    tic
    obj2 = scatteredInterpolant(x(:),y(:),z(:));
    toc
    %Elapsed time is 1.303282 seconds.

Answer 1

[Matt J]

You're not supposed to care about the time to construct F. That's only a one-time set-up computation. You're supposed to care about the speed of the actual interpolation operations, which presumably you would be doing repeatedly and on many more points.

In my tests below, I do find scatteredInterpolant to be a bit slower, but certainly not by a factor of 2, and the difference varies with the data sizes. There might be robustness improvements that explain the slower interpolation speed of the new version (better error checking, for example...)

    x = rand(100,1)*4-2;
    y = rand(100,1)*4-2;
    z = x.*exp(-x.^2-y.^2);

    obj1 = TriScatteredInterp(x,y,z);

    obj2 = scatteredInterpolant(x,y,z);

    ti = linspace(-2,2,2e3);
    [qx,qy] = meshgrid(ti,ti);

    tic;
     qz1 = obj1(qx,qy);
    toc;
    %Elapsed time is 4.324792 seconds.

    tic;
     qz2 = obj2(qx,qy);
    toc
    %Elapsed time is 4.990118 seconds.