JPEG is a lossy format, and naive rotation results in a loss of quality. JPEG does allow some lossless operations, such as rotation by 90 degrees and flipping, on the basic blocks (MCUs) that compromise the image. It also allows re-arranging those blocks. Using this lossless operation, it is possible to preform a lossless JPEG rotation. To do so, the rotated image mus meet some basic criteria like having it size a multiple of the MCU size (usually 16×16).
Not all programs preform a lossless JPEG rotation, so it is useful to be aware which does. I check a couple of commonly used program to see if they indeed preform lossless rotation. The testing procedure was:
- Start with the original JPEG photo.
- Rotate it once to the right using each program.
- Rotate a copy of the rotated photo back to the right using the same program.
- Compare using ImageMagick (
compare -metric ae) the results.
Results
Gnome’s Image Viewer 3.14.1 is lossless
Digikam (4.4.0) is lossless, however rotating with Digikam’s Image Editor is lossy.
Shotwell (0.20.1) does lossy rotation.