Kostya's Boring Codec World

To answer the obvious question with the obvious answer, brute force searching for a decent motion vector takes insanely large time. For example, VP6 motion search area be up to 63×63 pixels and checking all possible positions there requires a lot of tries. And if you remember that VP6 has quarterpel motion compensation precision, you should multiply that number by 16 possible sub-pixel positions. Obviously in order to reduce the number of tries various tricks are employed.

While by itself fast motion search methods I describe here are not that complex, it was rather hard to locate books where such details of developing video encoders are presented. At last I’ve found two or three books with the chapters dedicated to motion compensation plus the papers referenced there. The results of this mini-research are given below.
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I admit that I haven’t spent much time on writing encoder but I still have some progress to report.
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It is no secret (not even to me) that I suck at writing encoders. But with NihAV being developed exactly for trying new things and concepts, why not go ahead and try writing an encoder? It is not for having an encoder per se but rather a way to learn how things work (the best way to learn things is to try them yourself after all).

There are several reasons why I picked VP6:

• it is complex enough to have different concept of encoding to try on it;
• in the same time it’s not that complex (just DCT, MC, bool coder and no B-frames, complex block partitioning or complex context-adaptive coding);
• there’s no opensource encoders for it;
• there’s a decoder for it in NihAV already;
• this is not a toy format so it may be of some use for me later.

Of course I’m aware of other attempts to bring us an opensource VP6 encoder and that they all failed, but nothing prevents me from failing at it myself and documenting my path so others might fail at it faster and better.

Speaking of documenting, here’s a roadmap of things I want to play with (or played with already) and report how it went:

1. DCT;
2. bool coder;
3. simple intraframe coding;
4. motion estimation (including fast search and subpixel precision);
5. rate distortion optimisation;
6. rate control.

Hopefully the post about DCT will come tomorrow.

P.S. Why I declare this in public? So that I won’t chicken out immediately.