Snow wavelet codec

[Tommy Carrot]

I think Snow codec has reached the point where it should deserve an own thread here. :) If someone wouldn't know what Snow is, it's an experimental wavelet codec made by the ffmpeg developers, and despite the "experimental" label, it's probably the most mature wavelet codec out there with very good performance, far surpassing other wavelet codecs (rududu, dirac) and imho mpeg4 ASP codecs (xvid, divx etc.) too, quality-wise. As it's part of the libavcodec codec library, there are several applications with Snow support, like ffmpeg, mplayer/mencoder and of course ffdshow.

Snow has a few settings which are probably not obvious at first glance, let's see them in mencoder, here is a command line example with the more important options:

mencoder in.avi -o out.avi -ovc lavc -lavcopts vcodec=snow:vstrict=-2:vqscale=3:pred=0:cmp=1:subcmp=1:mbcmp=1:qpel

Detailed descriptions of the various options are here.

Note: vqscale specifies the encoding quality, basicly it's the quantizer. If you leave it out from the command-line, snow will compress losslessly (lossless mode doesn't work with pred=0 currently!). 2-pass mode is also available, but in my experience it's far from perfect yet, it doesn't really hit the required bitrate, and it gives significantly worse quality than constant quantizer mode at comparable bitrate.

Recommended settings: I'd strongly suggest to set all cmp functions to 1 (sum of squared errors method) for minimalising the chroma (and other) artifacts. Enabling qpel is also recommended for better detail preservation. Theoretically using smaller motion partitions (:v4mv) should further improve the compression efficiency, but due to the immature state of snow, it has side effects (artifacts, less accurate ME etc), so i cannot recommend it yet.

Outside mencoder, FFdshow can also be used for encoding Snow, it's more user-friendly, but apparently not everything is right with the snow encoder implementation in it, because the output quality is inferior to mencoder (ffdshow adds a slight vibration to the video, the motions are not smooth, the finer details are jumping around... i can't explain it better, but the effect is obvious during playback. Enabling 4 mv option makes this phenomenon even worse.)

BTW, don't use Snow for archiving, experimental means that the bitstream might change during the development, so it's not guaranteed that current encodings will be decodeable with the future versions.