[Gimp-developer] jpeg quality factor.

[Robert L Krawitz]

   From: Sven Neumann <sven at gimp.org>
   Date: Sun, 08 Jul 2007 15:12:03 +0200

   On Sun, 2007-07-08 at 14:53 +0200, gg at catking.net wrote:

   > Does your reply indicate you take a "this feature not a bug"
   > approach here and you think is the best way gimp should deal with
   > this situation?

   Indeed. When you open a JPEG file, then you have a decoded
   image. The settings that were used to encode it are irrelevant
   since encoding it again as a JPEG file would not yield the same
   image anyway. Thus it is better to use the default values. Since we
   will very soon allow the user to change these defaults, this should
   be the best way we can handle this.

Think of the quality setting as an indication of expectations rather
than a specific outcome.  It may not be possible to get the exact same
outcome (and obviously -- at least to us -- there's no way to
retroactively "improve" the result), but the quality setting could be
treated as the user's expectation for the result.

It's certainly true that a couple of iterations of saving at a quality
setting of 85 (say) will yield a substantial degradation, and a couple
of iterations at 65 will yield even more degradation, but a couple of
iterations at a setting of 98 won't yield very much degradation at
all.

By this reasoning, if a user opens a file with a quality setting of
98, her expectation when saving the file is that the quality will
still be very high, while if the quality setting of the incoming file
is only 85, her expectations will be lower.  A single default setting
won't cover all cases.

If the choice really is that arbitrary (and you make a good argument
to that effect), why not simply use the quality setting of the
incoming file as the implied default?  I think it would at least align
better with user expectations, particularly for files shot at high
quality settings on digital cameras.

BTW, on the Canon S3, the Superfine, Fine, and Normal settings
correspond to 96, 90, and 68 respectively.  So anyone who shoots in
one of those two settings and then decides to do a quick edit will get
a rude surprise.

-- 
Robert Krawitz                                     <rlk at alum.mit.edu>

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