The resulting image is not lossless as compared to the BMP or PNG file which might have come before it. However all of the images in my zip file contain identical data when uncompressed. A simple experiment:
Code:
mangix@Mangix ~/arith-jpeg
$ jpegtran -arithmetic < original.jpg | md5sum
8f67c18ea05cb25a7fa3532bf3778f96 *-
mangix@Mangix ~/arith-jpeg
$ cat original-ac.jpg | md5sum
8f67c18ea05cb25a7fa3532bf3778f96 *-
mangix@Mangix ~/arith-jpeg
$ jpegtran.exe -optimize < original-ac.jpg | md5sum
44974c16964219b8c22130c96caf2868 *-
mangix@Mangix ~/arith-jpeg
$ cat original.jpg | md5sum
44974c16964219b8c22130c96caf2868 *-
The first example turns original.jpg to original-ac.jpg and md5sums it. The second just reads original-ac.jpg and md5sums it. The third and fourth are the same but backwards.