2010.02.08 23:05 "Re: [Tiff] FFT on two TIFF images", by Andy Cave

Hi Richard, Debora,

----- Original Message -----

From: "Richard Nolde" <richard.nolde@cybox.com> To: "Tiff Listserve" <tiff@lists.maptools.org>; <debora.gil@vodafone.com> Sent: Monday, February 08, 2010 9:39 PM Subject: [Tiff] FFT on two TIFF images

Fourier Transforms only work if the width of the sample is a power of two, which is pretty unlikely to be the case with faxes. You will need to pad each line out to a power of two with zeros before processing your data with an FFT.

Not true. Modern FFTs can compute FFTs on any sized data and incredibly fast too. The fastest FFTs around that can do that are FFTW (MIT) and MKL (Intel). FFTW is free for non-commercial use, otherwise it costs (but you get source). MKL is a lot cheaper if you want to buy it for commercial use, but you only get object (for one platform unless you purchase multiple licenses).

It has been a long time since I wrote an FFT, so I'm not too sure about this next comment. You may want to check if there is any value in doing an FFT on bilevel images. I think it is more common to do FFTs on grayscale images where the rate of transition from light to dark can vary over a wide area instead of being a simple on/off transition.

You can do a lot of anaylsis by doing FFTs on 1 bit images. Probably things you've not thought of doing though.

Regards,

Andy.