Archive for March, 2006

KMVC codec

Monday, March 27th, 2006

I have looked at about dozen of codecs and for me KMVC (codec used for FMV in Worms games) is surely the kinkiest.
Here is what I think is kinky:

  1. Usability – there are two version of codecs, one uses for decoding kmvidc32.dll, other can be played by DOS player (player and samples for second type can be found atMPlayer samples repository). DLL cannot play files played by DOS player and vice versa and all these two types differ is just extradata (another 12 bytes), frame format is the same!
  2. Strange bit reading policy: read 8 bits at once, when this bit buffer exhausts, read another byte (and a lot of data may be read between those two bitbuffer reads). I understand that was made to avoid bit reading issues, but it’s still very strange.
  3. Another credit goes straight to m$ visual C. I cannot make myself think that any more or less sane coder would write this:

    jmp L2
    ; load something into al
    jmp L3
    add al, al
    jz L1

    And that is the simplest example of this – sometimes block L2 occurs in the beginning of function and L1 is near the end of it! Is this some kind of disoptimization to run the code on modern CPUs (which don’t like branching) with the speed of ol’ good i486?

I hope to finish spec and decoder to May.

Another codec in FFmpeg

Wednesday, March 22nd, 2006

Yesterday I’ve added implementation of Smacker demuxer and decoder, so you can enjoy you favourite pieces of video from various games.

I cannot credit the person who did the actual reverse engineering as he prefers to stay anonymous. Anyway, thank you very much!

The most interesting things in Smacker are Huffman trees. Smacker has trees by principle “every byte must have a tree”. Video operates with 16-bits values from Huffman trees, every of this trees stores values as two codes from Huffman trees. So you must construct two smaller Huffman trees before decoding large one. And packed audio data has also 1,2 or 4 Huffman trees depending if we have 8-bit or 16-bit data and stereo or not.

And now spotted inefficiencies in Smacker:

  • Smacker stores header information for 7 audio streams (!) yet I’ve never seen Smacker with two audio streams. Was that intended for internationalization?
  • Due to weird Huffman tree decoding procedure one bit is constantly wasted on stream. That means global Huffman trees for video lack 12 bits of waste (two support Huffman trees, one large Huffman tree, repeat four times) and audio chunks waste 1-4 bits depending on size. Not a big deal though.

And the final thought – the main difference between Smacker and M$ Video 1 is the packing scheme, block structure is almost identical. Anyone volunteering to write convertor from one format to another?


Saturday, March 4th, 2006

I think there is no need to introduce DosBox. And to make it even more usable DosBox developer niknamed Harekiet added screen output recording capability with his own codec ZMBV – Zip Motion Blocks Video.

This codec employs intraframes and motion compensation (and XOR’ing instead of substracting/adding difference). ZLib is used for packing. For more detailed description go

For now, 8-bit, 16-bit and 32-bit depths are supported by codec as for recording. And FFmpeg decoder is already available – I could not miss that codec and not port it.