Does loss accumulate with repeated compression/decompression?
It would be nice if, having compressed an image with JPEG, you could decompress it, manipulate it (crop off a border, say), and recompress it without any further image degradation beyond what you lost initially. Unfortunately THIS IS NOT THE CASE. In general, recompressing an altered image loses more information. Hence it’s important to minimize the number of generations of JPEG compression between initial and final versions of an image. There are a few specialized operations that can be done on a JPEG file without decompressing it, and thus without incurring the generational loss that you’d normally get from loading and re-saving the image in a regular image editor. In particular it is possible to do 90-degree rotations and flips losslessly, if the image dimensions are a multiple of the file’s block size (typically 16×16, 16×8, or 8×8 pixels for color JPEGs). This fact used to be just an academic curiosity, but it has assumed practical importance recently because many users of digital
