From: "James Turner" <james_turner_81@hotmail.co.uk>
Path: news.mathworks.com!newsfeed-00.mathworks.com!webcrossing
Newsgroups: comp.soft-sys.matlab
Subject: Re: Dummies guide to DCT
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Date: Fri, 17 Nov 2006 04:40:02 -0500
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Ken Davis wrote:
>
>
> "James Turner" <james_turner_81@hotmail.co.uk> wrote in
message
> news:ef462d4.2@webcrossing.raydaftYaTP...
>> sturlamolden wrote:
>>>
>>>
>>>
>>> James Turner wrote:
>>>
>>>> I was wondering if someone could explain to me the
Discrete
>>> Cosine
>>>> Transform, in as simple as possible English.
>>>
>>> In the DCT the signal is decomposed into a sum of cosines,
as
>>> opposed
>>> to the Discrete Fourier transform (DFT) where the signal is
>>> decomposed
>>> into a sum of sines and cosines.
>>>
>>>
>>
>>
>> Thanks for the respone guys, but I think I need even more
> reading. I
>> am lost at "the signal is decomposed into a sum of cosines". I
> don't
>> understand what a sum of cosines is! Is there not a way of
> explaining
>> DCT assuming that a person ONLY understands what a digital
image
> is
>> (i.e. a collection of pixels described using bits)? I'm talking
>> simple so a kid could understand it kind of stuff!
>
> I fear you may be in over your head. You really need a little
> linear algebra
> and maybe a little signal/image processing to understand this
> stuff.
>
> That said, the short answer is that you decompose your image into a
> weighted
> sum of "primitive" images. These primitive images are just
> sinusoidal waves
> of different frequencies in both the x and y directions. When you
> do a DCT
> you are calculating what the values of the weights are. You can
> transmit the
> complete set of weights (or a selected subset of them) and
> reconstruct the
> original image by applying the weights to the appropriate primitive
> images
> and adding them all together.
>
> When you encode information into an image by perturbing the
> weights, the
> effect of the perturbation may be very difficult to see in the
> reconstructed
> image.
>
>
>
  

Thank you for your helpful response Ken. Your explanation definitly
helps, and you are right, I need to do more reading. I can't expect
to understand the DCT straight out without understanding the basics
:)

Regards,

James