As I mentioned in the previous post, one of the problems is to find a good motion vector for B-frame interpolated macroblock. Since I had nothing better to do I’ve decided to try motion vector search in the same style as the usual motion estimation: start from the candidate motion vector pair and try adjusting both vectors using diamond pattern (since it’s the simplest one).
The results are not exciting: while it slightly improves PSNR and reduces file size (on lower quantisers), encoding time explodes. My 17-second 320×240 test clip encoded with quant=16 and two B-frames between I/P-frames takes 40 seconds without that option and 136 seconds with it. And while average PSNR improves from 38.0446 to 38.0521, the size decreases from 1511843 bytes to 1507224.
That’s the law of diminishing returns in action. Of course it can be made significantly faster by e.g. using pre-interpolated set of reference frames but why bother in this case? I’ve put this under an option (i.e. be satisfied with the initial guess or try to search for a better pair of motion vectors) but I doubt anybody will ever use it (the same applies to the whole encoder as well).