Re: imtranslate

[Søren Hauberg]

lør, 29 12 2007 kl. 14:06 -0600, skrev Daniel Elliott:
> Hello,
> 
> I tried contacting the author, but the email address is no longer
> functional.  I have not done my homework before asking the question...
> 
> The imtranslate function takes the absolute value of the image as it
> is brought out of FFT-space.  Is this because of some property of FFT
> where it can potentially flip the sign of a pixel?
> 
> I am using this method to permute images that may have negative
> values.  Does anyone know why the absolute value is used here?
I haven't written (or even used) imtranslate so I don't know the
details. I assume that FFT sometimes returns complex values, so an abs()
is needed. Depending on your problem, I suggest you either
  1) Separate the signs from the data and transform each separately.
Something like:
    S = sign(im);
    im = abs(im);
    St = imtranslate(S, ...)
    imt = imtranslate(im, ...)
    T = St.*imt;
  2) Use the more general 'imperspectivewarp'. You can just apply an
affine transformation matrix like
    T = [eye(2), [Tx; Ty]];
where Tx and Ty are your translations.

Hope that helps,
Søren