Hi Philip,

On 15 Jan 10 18:27 Philip Rhoades <p...@pricom.com.au> said:
> - when the JPG is uncompressed by GIMP into RAM, there is no loss of
> information (?)

No further loss, but the restored image is subject to those averages 
created when the image was originally compressed.
> - when GIMP then saves the same image as a new JPG at 100% quality 
> (I would have thought that this meant not losing any more 
> information),

You shouldn't take 100% too literally. Think of it more as "best 
quality", but the best that the JPG algorithm achieves is not "no 

> that the second JPG would be compressed/created in much the same way
> as the first and therefore would be about the same size . .

Remember that it is working on data that is already corrupted so it 
further corrupts it, again averaging the larger areas of similar 
colour. If you didn't want it to compress it a bit (even at 100% 
quality) you wouldn't be using selecting a JPG format when saving it 
would you? You'd choose a lossless format instead.
> of course I have no control over the file format that the camera 
> uses

Most cameras do have a range of compression options available within 
their menu system. It won't have the variability of the GIMP but 
probably will have a "Normal" setting plus a high and low option.

> and cropping a camera image and actually getting a result that is 
> 2.5 times the size of the original is a bit annoying . .

But now you realise how much the data is compressed at even "normal" 
levels, you realise how good the algorithm was that the JPEG came up 
with! :-)

Greg Chapman
Helping new users of KompoZer and The GIMP
Gimp-user mailing list

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