I recently had two posts, one about compression of CCTV archives and the other about image stabilization. These two posts actually have something in common. In case while reading the one about CCTV you thought "If the image is not clear, then they can just filter it", you should understand that it won't work very well.
The reason is that filtering compressed images generally causes the compression artifacts (little 'errors' left behind after removing the image detail) to get amplified, thereby making the filtered image worse than the original compressed one. My attempts (on real-life video, not simulated images) have shown that image stabilization, brightness and contrast are about the only filters that can reliably work on (lossy) compressed video. Edge contrast enhancement and other such techniques that work very well on uncompressed images do not work on compressed ones (at least none of the ones I've seen so far).