Re: JPEG compression only retains magnitude information? How about phase information?

In message <[EMAIL PROTECTED]>, walala <[EMAIL PROTECTED]> 
writes
> I guess phase information is not useful at all, so JPEG just ignore them...

It doesn't ignore them.
The phases are identically zero by construction in the DCT.

Hint:   2cos(x)  =  exp(ix) + exp(-ix)

> But is there any application that phase information can be useful?

Phases are essential in the classical DFT. Where if memory serves
64 reals in image space transform to 4 reals and 30 complex numbers.

It is convenient to think about the Fourier transform of an image as 
working between real and complex number representations.
> And if we want to get the phase information,

There is no phase information to be had in the DCT.

64 reals in and 64 reals out through an orthogonal transform.

Regards,
--
Martin Brown