Thank you Remi, actually i need to know which kind of features i may get by using FFT. Because i need to extract the features from Document images for identifying scripts. 

For example, you can use FFT-based convolution technique.

See R. C. Gonzalez, R. E. Woods. Digital image processing: Pearson prentice hall  // Upper Saddle River, NJ. —2008

Thanks to all.

It depends on your target you want to detect or recognize! 

For example, you can extract Fourier descriptors from a binary image that can be used as geometry features.

FFT is used for deriving the features in frequency domain.

Fourier Transform->number of changes in unit time.

if the time is very short FT will give infinite frequencies. So in case of an image one thing that can be known is, how the respective condition appears in the image.

(basically FFT is taken when signal is non-periodic).