I think it would be a very useful feature to be able to open a JPEG in
the GIMP, do some editing that touches just a small minority of the
pixels like correcting red-eyes, and save just the edited parts,
keeping the rest of the JPEG unchanged. (Not talking about individual
pixels, of course, but those 8x8 (?) pixel blocks.)

I have no idea what would be a good way to implement this, or how the
user interface could look. Probably one would need to keep a bitmap of
dirty pixel blocks (or whetever they are called), and when saving, if
only a reasonably small minority of the pixel blocks are dirty, the
plug-in could offer the user to keep the untouched pixel blocks intact
and save just the changed ones.

There is a patch to jpegtran that enables it do to lossless
cropping. The same patch also enable you to "insert" a smaller jpeg in
a larger one without loss. See http://www.reporter.de/digifoto/jpegcrop/ .

Another possibly useful JPEG feature would be to be able to use
variable quality when saving an image as a JPEG. For instance the
selection mask could be used to indicate the relative importance of
various parts of an image, and the plug-in would use higher quality
the more important a pixel block is. Hmm, is this doable in the JPEG
format at all? Of course, one could achieve something similar by
simply blurring the unimportant parts before saving.


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