severity 355199 normal
quit
On Sat, Mar 04, 2006 at 12:55:11AM +0100, Marco Candrian wrote:
> Package: libjpeg62
> Version: 6b-11
> Severity: critical
> Justification: causes serious data loss
>
> jpegtran -rotate 180 source.jpg > target.jpg
>
> damages borders, see: http://www.calmar.ws/jpegtrans.png
Hello Marco,
Your image size seems to be 1000x669 which are not multiple of 16 so it
seems you triggered a documented jpegtran feature, see man jpegtran:
The transpose transformation has no restrictions regarding image dimen-
sions. The other transformations operate rather oddly if the image
dimensions are not a multiple of the iMCU size (usually 8 or 16 pix-
els), because they can only transform complete blocks of DCT coeffi-
cient data in the desired way.
jpegtran's default behavior when transforming an odd-size image is
designed to preserve exact reversibility and mathematical consistency
of the transformation set. As stated, transpose is able to flip the
entire image area. Horizontal mirroring leaves any partial iMCU column
at the right edge untouched, but is able to flip all rows of the image.
Similarly, vertical mirroring leaves any partial iMCU row at the bottom
edge untouched, but is able to flip all columns. The other transforms
can be built up as sequences of transpose and flip operations; for con-
sistency, their actions on edge pixels are defined to be the same as
the end result of the corresponding transpose-and-flip sequence.
Theses restrictions on lossless transformations are inherent to the JPEG
image format, not to jpegtran.
If you do not like the current behaviour, there is 3 ways around it:
1) use -trim, that will cut the image borders that cannot be rotated.
2) use -perfect, that will cause jpegtran to fail with an error
when the image size does not allow for a perfect rotation.
3) use a lossy rotation when jpegtran -perfect fails, like in
(jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg| pnmflip -r90 | cjpeg)
>PS: it's critical and causes serious data loss on application, where
>the original picture gets overwritten with the rotated one - I hope
>that counts)
This is not correct. jpegtran does not cause any data loss because you
can rotate back the image and the exact same image you started with. By
contrast other JPEG rotations software will lose data due to the lossy
compression, but I doubt you will consider that a serious data loss.
Just try to do
jpegtran -rotate 180 target.jpg > target2.jpg
and compare source.jpg and target2.jpg
Cheers,
--
Bill. <[EMAIL PROTECTED]>
Imagine a large red swirl here.
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