Re: [PLUG] which image transfer format?

[Jeff Weisberg <jaw+plug@tcp4me.com>]

| > So, my answer is that I don't know.  8^)  A JPEG at 100% quality must be
| >  better than it is at 75%, but I don't know if it's lossless.
|
| Len can probably tell you for certain, but it's my understanding that
| saving as JPEG loses information, whether the setting is 100% or not.
| Every save in the JPEG format loses some data, because the JPEG algorithm
| by definition loses some data.


what the "quality" setting actually does varies from program to
program, but, in most software, it will cause the quantization
table to be all "1"s--resulting in 0 loss in the quantization
phase. there will still (most likely) be loss due to round-off
errors in various intermediate steps.

there are draft standards though for truly lossless jpegs
(LOCO aka JPEG-LS[1])

in practice, your digicam or scanner's sensor introduces
far more visible error[2] than a jpeg at a high quality setting,
and the jpeg is far better than good enough for most purposes.


	--jeff


[1] http://www.hpl.hp.com/loco/HPL-98-193.pdf
    and http://www.jpeg.org/public/fcd14495p.pdf

[2] the real world is not 24 bit color

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