Tech Recipies/Improving MJPEG movies taken by digital cameras

Motion JPEG format

• Low compression. Because each frame is independent, inter-frame similarities cannot be "factored out" and compressed away.
• Exaggerated noise. JPEG compression results in small noise-like artifacts that are hard to notice in a single frame but jump out in movies, where inter-frame differences are easy to see.

Re-encoding MJPEG movies to improve compression and perceived quality

To get around these problems, I re-encode my digicam's MJPEG movies using the following mencoder recipe:

mencoder input.avi -vf eq2=1.6,hqdn3d -oac copy -ovc lavc -o output.avi

The magic occurs in the video-filter specification (in bold), which comprises two post-decoding processing steps:

• eq2=1.6 – Increases the gamma setting to 1.6, brightening the movie. (I find that movies captured indoors are often too dark.)
• hqdn3d – Filters the video using a high-quality version of the denoise3d filter, which "aims to reduce image noise, producing smooth images and making still images really still."

To preview the effects of these filters before re-encoding with them, you can try them with mplayer:

mplayer input.avi -vf eq2=1.6,hqdn3d

QuickTime compatibility

The ffmpeg tool can be used to re-encode movies for QuickTime compatibility:

ffmpeg -i input.avi output.mp4

To avoid re-encoding artifacts, use a lossless codec in the initial mencoder step:

mencoder input.avi -vf eq2=1.6,hqdn3d -oac copy -ovc lavc -lavcopts vcodec=ffvhuff:vstrict=-1 -o temp.avi
ffmpeg -i temp.avi output.mp4
rm -f temp.avi  # the temp file can be large