The region retains the exact same data it had in the main image with the restriction that the image can only be cropped along DCT block boundaries. My main use for this feature is modifying digital camera images which come out of the camera already JPEG compressed and need to be rotated and cropped for storage.
Negative offsets are relative to the lower and righthand sides.
If the image ends in a partial DCT block that part of the image will be lost, even though it would be possible to include it. Perhaps an enterprising reader will fix that deficiency. Many thanks to Guido Vollbeding for implementing the lossless rotation in jpegtran . I use it regularly to rotate digital camera images and the code framework made the addition of cropping easy. His Online JPEG Lab will let you test-drive jpegtran from the comfort of your web browser.