On Sun, Apr 03, 2005 at 01:39:24PM -0700, Robert Fleming wrote: > Package: libjpeg-progs > Version: 6b-10 > Severity: grave > Justification: causes non-serious data loss
[[This is absolutly incorrect: there is no data loss at all: if you rotate the image back, you get exactly the original jpeg file!]] > See http://home.comcast.net/~fleminra/jpegtran-bug/ > > for example input and ouput. jpegtran incorrectly moves the bottom row of > blocks to the top of the image. This bug is also present in the JPEG > source distribution at > > http://www.ijg.org/files/jpegsrc.v6b.tar.gz > > This appears to happen when a dimension of the input JPEG is not a multiple > of the image block size, i.e. an image of height 256 is correctly rotated, but > an image of height 255 is not. Hello Robert, Yes, this is correct, but this is the documented behaviour. The JPEG format does not allow perfect rotation when the dimension is not a multiple of the image block size. Please read the jpegtran manual page, in particular: 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. For practical use, you may prefer to discard any untransformable edge pixels rather than having a strange-looking strip along the right and/or bottom edges of a transformed image. To do this, add the -trim switch: -trim Drop non-transformable edge blocks. Obviously, a transformation with -trim is not reversible, so strictly speaking jpegtran with this switch is not lossless. Also, the expected mathematical equivalences between the transformations no longer hold. For example, -rot 270 -trim trims only the bottom edge, but -rot 90 -trim followed by -rot 180 -trim trims both edges. If you are only interested by perfect transformation, add the -perfect switch: -perfect Fails with an error if the transformation is not perfect. For example you may want to do (jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg| pnmflip -r90 | cjpeg) to do a perfect rotation if available or an approximated one if not. Note: -perfect and lossless-crop are enhancements from http://syl- vana.net/jpegcrop/ that may not be available on non-Debian systems. Cheers, -- Bill. <[EMAIL PROTECTED]> Imagine a large red swirl here. -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
