Olympus-OM
[Top] [All Lists]

Re: [OM] OT: JPEG lossiness

Subject: Re: [OM] OT: JPEG lossiness
From: Christopher Biggs <chris@xxxxxxxxxxxxxx>
Date: 15 Jan 1999 10:57:27 +1000
Jan Steinman <jans@xxxxxxxxxxx> moved upon the face of the 'Net and spake 
thusly:

> A simple empirical test suggests otherwise for JPEG: just look at the file
> size continue to go down as you compress it over and over. If it is not
> getting progressively less quality where did the bits go? Why weren't they
> removed the first time, if they don't contribute to the photo quality?
> 

JPEG will continue to degrade over multiple iterations.

> As I understand it, JPEG works by taking areas of similar color and
> replacing them with a single color. It will continue to do so, regardless
> of whether it has already been done. In common use, the reduction in
> quality tends to be asymptotic -- it approaches a limit -- but it still
> reduces quality each time it is used.

JPEG works by 

        1.  Convert to YUV colourspace
        2.  decimate the colour resolution to half the luminance
                resolution
        3.  consider the image in 8x8 blocks
        4.  perform "Discrete Cosine Transform" on each block and
                encode the coefficients of the DCT output
                using a configurable amount of precision (this
                is the variable compression).

For the non-mathematical, a DCT has the effect of losing fine detail
but keeping the general "trend" of colour change across each block.

For signal-theory types this is analogous to doing noise reduction by
performing a Fourier transform, clipping the output and then inverting 
the transform.

HTH
        Chris.

ObOM: Got the test roll back for my "new" OM2n.  Spot on.  Wonderful
        chromes!   OTOH, my new $10 Screwmount 135/3.5 vignettes badly
        at f/22 using an optical M42->OM adaptor.

--
 chris@xxxxxxxxxxxxxx, when he visits the Real World, is Christopher J. Biggs
 Stallion Technologies, Australia.   Ph. +61-7-3270-4266 Fax. +61-7-3270-4245
 I dig PGP, MIME and Rush. Send mail with "Subject: sendpgpkey" for my pubkey
 ------------------ Power grows out of the barrel of a GNU ------------------


< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >


<Prev in Thread] Current Thread [Next in Thread>
Sponsored by Tako
Impressum | Datenschutz