Browsing through the MultimediaWiki I found a codec called Mimic that I previously had not paid too much attention to. I remember someone on the ffmpeg-devel list inquiring about some reverse engineering advice regarding the codec with the FourCC ML20, which is a.k.a. Mimic. The FourCC was apparently a bit frustrating to work with because it only occurs on the wire– it is only used for teleconferencing app and there are no apps to easily save the encoded data into a handy container format.
Anyway, someone persisted (one Ole André Vadla Ravnås according to the source headers) and eventually reverse engineered the complete codec and created an open source library called libmimic, according to the Wiki page. The link on the Wiki page for libmimic does not seem to work. However, another open source teleconferencing app called Farsight incorporates the full library, which even knows how to encode video.
It’s a very simple codec for those of us — ahem — skilled in the art. I’ve done my best to document the details based on the RE’d source code. Stop me if you’ve heard this one before: Transform 8×8 blocks of data with discrete cosine transform -> quantize DCT coefficients -> zigzag coefficients -> Huffman code the non-zero coefficients and zero-runs between the non-zero coefficients. The only major intraframe coding concepts the codec is missing are macroblocks and delta coding. Interframes feature frame differencing– if a block is unchanged from the block at the same position in the previous frame, leave it alone. Good design decision for a teleconferencing video codec.
