Restoration of compressed images

Post-processing can alleviate or remove artifacts introduced by compression. However without a priori information, image enhancement schemes may fail. What is noise in one image may be important data in another. Fortunately, in image compression, we have an advantage. Before an image is stored or transmitted, we have access to the original and the distorted versions. The enhanced codec is compared to the original block by block to determine which blocks have been improved by the enhancement. These blocks are then flagged for post-processing in a way that is compliant with the JPEG standard and adds nothing to the compressed images' bandwidth. A single JPEG coefficient is adjusted so that the sum of the coefficients contains the flag for post-processing as the parity of the block. Half of the blocks already have the correct parity. In the other blocks, a coefficient that is close to being half way between two values will be chosen and rounded in the other direction. This distorts the image by a very tiny amount. The end result is a compressed image that can be decompressed on any standard JPEG decompressor, but that can be enhanced by a sophisticated decompressor.