FPGA Central

I guess phase information is not useful at all, so JPEG just ignore them...

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

And if we want to get the phase information,

1) How to get at the encoder side along with DCT coefficients magnitude
information? and then it can be transmitted to the decoder side...

2) How to get at the decoder side by inferencing from DCT coefficient
magnitude information, if they are not obtained at the encoder side and
transmitted to the decoder side...

Thanks a lot,

-Walalla

walala wrote:

> I guess phase information is not useful at all, so JPEG just ignore them...
>
> But is there any application that phase information can be useful?
>
> And if we want to get the phase information,

Holograms record phase information, all other photographic methods
record only intensity.

-- glen

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

Hi,

> I guess phase information is not useful at all, so JPEG just ignore them...

Not quite. A transform I would "ignore" information in case some information
is definitely lost. Lost in the sense that for a transform "I" there is
no inverse transformation "I^{-1}". Since DCT is completely reversible, not
a single bit of information is lost, and thus no kind of "phase" is lost
either. Information gets lots only because a) you quantize and b) DCT is
implemented with limited precision. If a system had infinite computing
accuracy, DCT would be reversible.

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

Phase information of what? The relative phases of the light signal captured
as an image? That information is already lost at the time you take a photo,
unless it's a hologram.

> 1) How to get at the encoder side along with DCT coefficients magnitude
> information? and then it can be transmitted to the decoder side...

Make a hologram, digitialize it, then transmit this information. Classical
image compression algorithms might be unsuitable here because they might be
not precise enough/might be lossy for the wrong kind of the hologram.
(Kinna like: Image compression with gzip is possible, though not smart).

> 2) How to get at the decoder side by inferencing from DCT coefficient
> magnitude information, if they are not obtained at the encoder side and
> transmitted to the decoder side...

Again, there is no definite answer because there is no definite
question. (-; Like in "The Hitchhiker's Guide Thru Galaxy", you first need
to find the proper question before getting the answer 42. (-;

So long,
Thomas

Thomas Richter <[email protected]> wrote in message news:<[email protected]>...
> Hi,
>
> > I guess phase information is not useful at all, so JPEG just ignore them...
>
> Not quite. A transform I would "ignore" information in case some information
> is definitely lost. Lost in the sense that for a transform "I" there is
> no inverse transformation "I^{-1}". Since DCT is completely reversible, not
> a single bit of information is lost, and thus no kind of "phase" is lost
> either. Information gets lots only because a) you quantize and b) DCT is
^^^^^^
Hence the two categories of compression method: lotsy and lossy ;-)

Jon C.