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Re: image-dired does not rotate the original image


From: Mathias Dahl
Subject: Re: image-dired does not rotate the original image
Date: Tue, 24 Jul 2007 08:52:59 +0200
User-agent: Gnus/5.11 (Gnus v5.11) Emacs/22.1 (windows-nt)

Thien-Thi Nguyen <address@hidden> writes:

>    As I mentioned earlier, jpegtran does lossless rotation, something
>    `convert' cannot do (or could not the last time I checked).
>
> does this limitation also exist w/ mogrify(1) (or alternatively gm(1)
> subcommand mogrify)?

I don't know. What I do know is that `convert' makes a lossy rotation
of JPEGs. I found this out myself while creating Tumme; while in the
thumbnail buffer, rotate a thumbnail over and over again. Because of
the small size you soon begin to see changes.

>  cp foo.jpg foo.orig.jpg
>  gm mogrify -rotate -90 foo.jpg
>  gm mogrify -rotate  90 foo.jpg
>  cmp foo.jpg foo.orig.jpg
>
> however, doing the above (even w/ "-quality 100" for the mogrify commands)
> always results in cmp(1) noting a difference, even though visually i could
> not tell.

If you do that in a loop, say ten or more times, maybe you will start
to see a difference (save the original and compare it visually).

>  i'm not a graphics format expert; could you suggest a better
> way to check for loss?

How about rotating the image in Gimp, for comparison?

Maybe there should be an option in `image-dired' to allow lossy
rotation, so that people who accept that do not need to have
`jpegtran'?

/Mathias




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