On Fri, Jan 15, 2010 at 12:27 PM, Philip Rhoades <p...@pricom.com.au> wrote:
> - there was a loss of information when the first JPG was saved in the
> digital camera memory from the CCD


> - when the JPG is uncompressed by GIMP into RAM, there is no loss of
> information (?)

Since JPG is not lossless, there is always a loss of information. Or
more specifically the same JPG can be interpreted differently by
different software, so opening it in GIMP might look different than
another program perhaps. Once an image is saved as JPG there's no way
to get the original image back from that JPG file.

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

100% quality does not mean no loss of info, just means that it's as
close to the original as JPG is capable of getting. It's still not
going to be identical to the original.

> Good to know that this happens anyhow . . of course I have no control
> over the file format that the camera uses and cropping a camera image
> and actually getting a result that is 2.5 times the size of the original
> is a bit annoying . .

That's why the expensive/professeional cameras output in raw format,
so the photographer can have total control. :) FWIW, if you have a
Canon you very well might be able to install CHDK and get raw images,
that's what I've done with my SD550 and SD1000.

Depending on your purpose for the final JPG file, there are other ways
to make the file smaller. Saving as progressive vs baseline usually
makes a small difference in size. You can use the program jpegoptim to
optimize (losslessly) and reduce filesize, and you can strip out
EXIF/thumbnail/etc header info to make the file smaller without
affecting the actual image data either (assuming you're not using the
EXIF for rotation/etc in whatever program you're displaying the image

For best quality/size trade-off when you save your JPG in GIMP I would
use the "show preview in window" (something like that) option which
will show the resulting compressed JPEG in a window. Then you can
adjust the quality and other settings and see how it looks. I usually
adjust it as low as I can until the image appears to degrade, then I
bump it up a notch and choose that. For me, around 75% seems to be a
pretty good combination of small file size and good quality.

There are also JPEG tools that allow you to losslessly crop/rotate etc
your pictures. Check out jpegtran from jpegclub.org or one of these
programs which supposedly include the same functionality:
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