Philip Rhoades wrote:
> What still doesn't make sense is that if the original file is JPG and
> one simply opens it and then saves it as another JPG file with 100%
> quality - you are saying that introduced artifacts are adding about 150%
> to the file size? (681 KB to 1.618 MB) How could the compression
> algorithms be so different as to cause this sort of result?  - At worst
> I would have expected maybe a 10% increase in size . .

well firstly, 1.6MB are not that bad in comparison to 9MB of raw RGB data, 
(just try saving to uncompressed BMP as Greg suggested).

Btw, 100% quality for JPG gives very little visible advantage over the
default 90% setting.

The relationship between quality value, file size and perceived image quality
is very delicate. Any assertion has to be made with a lot of weasel words.
So yes, compression artifacts have a tendency to hinder compression,
resulting in larger file sizes after re-compression to comparable quality.
A similar effect is caused by noise. So to get optimal JPG files, it's
best to use a RAW->XCF->JPG workflow where the JPG is created only once.

Just have a look at the advanced settings in the JPG save dialog to get
a first impression of what machinery is at work here. There also was a
very long thread on gimp.developer on that very quality setting...

How does image quality compare if you adjust the quality slider
such that the resulting file size is about 680KB?


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