>> The difference in size is only very slight, about 1%, and it is not due
>> to excessive data being stripped off. The image content is actually
>> slightly different. I compared the two images via
>>
>> convert original.jpg extracted.jpg -compose difference -composite
>> comparison.png
>>
>> and then counted the colours.
>
> A difference in color count does indicate a difference in image content
> (although the same color count would not prove identical image
> content).

You misunderstood me:
I counted the colours in comparison.png, which is 1 if the two images 
are identical. (This is the result I get when I combine the JPEGs with 
Acrobat and then extract them with pdfimages.)

> Not only is there no benefit to the re-encoding, there are actually two
> problems that result: it takes more processor power, and every
> re-encoding loses a bit of quality.

Well, that's why I'm asking all this ;-)

Greetings from Münster, germany
Wolfgang Hugemann
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