Re:

[Francesco Potorti`]

Your code does not look like it should be slow.  Just some small
possible improvements (all untested):

>    ycbcr(:,:,1) = y;
>    ycbcr(:,:,2) = cb;
>    ycbcr(:,:,3) = cr;

This could be slightly faster:
  ycbcr = cat(3,y,cb,cr);
Or even:
  ycbcr = reshape([y(:);cb(:);cr(:)],[size(y) 3]);

or reverse the order of assignments, like Søren suggested.

>yn(find(yn < 0)) = 0;

This can be rewrote more simply:
  yn(yn < 0) = 0;

and the others as well.

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