GNOME Bugzilla – Bug 141588
gThumb does not rotate jpeg images correctly
Last modified: 2006-09-20 18:55:57 UTC
When I rotate a jpeg image via the menu entry Tools --> Rotate afterwards clicking: left 90°(graphical button) --> Aply the image is not preserved. It loooks like scrolled up with the upper part of the image shown at the bottom.
Dupicate of 143813
*** Bug 143813 has been marked as a duplicate of this bug. ***
I've done some research into this and it is the expected behaviour for a loss-less JPEG rotation. The two alternatives are: 1) unexpected corruption on lower edge of image 2) crop image to remove the row of partial blocks From the jpegtran man page: The other transformations operate rather oddly if the image dimensions are not a multiple of the iMCU size (usually 8 or 16 pixels), because they can only transform complete blocks of DCT coefficient 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. Hori-zontal 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 consistency, 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.
Marking as a duplicate of bug 329129, where there is a bit more discussion. *** This bug has been marked as a duplicate of 329129 ***