>From: William Sommerwerck <williams@xxxxxxxxxx>
...
>It is not necessarily true that repeated saves in a lossy format cause
>continued loss of quality. Once the data have been reduced, the image is
>now in the "preferred" data format for the lossy compression, and little
>or no further degradation should occur. (A similar line of reasoning
>applies to AC-3 and other forms of lossy audio compression.)
>
>Does anyone out there have more-definitive information on this?
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?
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.
I'm still very interested and hopeful about newer compression schemes,
especially wavelet and fractal schemes. As I mentioned in another post,
Genuine Fractals claims lossless compression at a slight size penalty, and
you can buy it today. I looked at the URL posted for the wavelet encoder,
but they don't seem to have a real product -- it looks like they're looking
to license the technology.
BTW: Vivitar crashed and burned trying to do wavelets. I had their ViviCam
3000, which did wavelet compression, but it died. They replaced it, and the
replacement died. They replaced that with a ViviCam 3100, saying the 3000
was no longer available. The 3000 uses JPEG compression, NOT wavelet.
: Jan Steinman <mailto:jans@xxxxxxxxxxx>
: 19280 Rydman Court, West Linn, OR 97068-1331 USA
: +1.503.635.3229
< 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 >
|