Recently I’ve been contacted by some guy working on a mod campaign for Heroes of Might and Magic III. The question was about the encoder for videos there. And since the original one is not likely to exist, I just wrote a simple one that would take PGMYUV image sequence and encode it. Here’s the gzipped source.
It took a couple of evenings to do that mostly because I still have weak symptoms of creeping perfectionism (thankfully it’s treated with my laziness). BIKb does not have Huffman-coded bundles, so the simplest straightforward encoding would be: write block type bundle (13-bit size and 4-bit elements), write empty other bundles, write several bundles containing pixels and you’re done. There’s a proper approach: write a full-featured encoder that takes input in several formats and that encodes using all possible features selecting the best quality for the target bitrate. There’s a hacky approach—translate later versions of Bink into BIKb (and then you remember that it has different motion compensation scheme so this approach won’t work). I’ve chosen something simple yet with some effectiveness: write an encoder that employs only vector quantisation and motion compensation for non-overlapped blocks plus add a quality setting so users can play with output size/quality if they really need it.
So how does the block encoding work? Block truncation coding, the fast and good way to quantise block into two colours (many video codecs back in the day used it and only some dared to use vector quantisation for more than two different values per block). Essentially you just calculate average pixel value and select two values depending on how many pixels in the block are larger than average and by how much they deviate. And here’s where quality parameter comes into play: depending on it encoder sets the threshold above which block is coded as is (aka full mode) instead as two colours and pattern in which they occur (of course if it’s a solid-colour fill it’s always coded as such). As I said, it’s simple but quite effective. Motion compensation is currently lossless i.e. encoder will try to find only the block that matches exactly (again, it can be improved but that would only lead to longer implementation times and even longer debugging times). This makes me appreciate the work on Smacker and Bink 1 video codecs and encoders for them even more.
Overall, it was a nice diversion from implementing Duck decoders for NihAV but I should probably return to it. The sooner it’s done the sooner I can move to something more exciting like finally experimenting with vector quantisation, or trying to write a player, or something else entirely. I avoid making plans but there are many possibilities at hoof so I just need to pick one.