In one of the previous posts I said that this encoder will likely keep me occupied for a long time. Considering how bad was that estimation I must be a programmer.
Anyway, there were four main issues to be resolved: compatibility with the reference player, B-frame selection and performing motion estimation for interpolated macroblocks in them, and rate control.
I gave up on the compatibility. The reference player is unwieldy and I’d rather not run it at all let alone debug it. Nowadays the majority of players use my decoder anyway and the produced videos seem to play fine with it.
The question of motion vector search for interpolated macroblocks was discusses in the previous post. The solution is there but it slows down encoding by several times. As a side note, by omitting intra 4×4 mode in B-frames I’ve got a significant speed-up (ten to thirty percent depending on quantiser) so I decided to keep it this way by default.
The last two issues were resolved with the same trick: estimating frame complexity. This is done in a relatively simple way: calculate SATD (sum of absolute values of Hadamard-transformed block) of the differences between current and some previous frame with motion compensation applied. For speed reasons you can downsample those frames and use a simpler motion search (like with pixel-precision only). And then you can use calculated value to estimate some frame properties.
For example, if the difference between frames 0 and 1 is about the same as the difference between frames 1 and 2 then frame 1 should probably be coded as B-frame. I’ve implemented it as a simple dynamic frame selector that allows one B-frame between reference frames (it can be extended to allow several B-frames but I didn’t bother) and it improved coding compared to the fixed frame order.
Additionally there seems to be a correlation between frame complexity and output frame size (also depending on the quantiser of course). So I reworked rate control system to rely on those factors to select the quantiser for I- and P-frames (adjusting them if the predicted and the actual sizes differ too much). B-frames simply use P-frame quantiser plus constant offset. The system seems to work rather well except that it tends to assign too high quantisers for some frames, resulting in rather crisp I-frame followed by more and more blurry frames.
I suppose I’ll play with it for a week or two, hopefully improving it a bit, and then I shall commit it and move to something else.
P.S. the main goal of NihAV is to provide me with a playground for learning and testing new ideas. If it becomes useful beside that, that’s a bonus (for example, I’m mostly using nihav-sndplay to play audio nowadays). So RealVideo 4 encoder has served its purpose by allowing me to play more with various concepts related to B-frames and rate control (plus there were some other tricks). Even if its output makes RealPlayer hang, even if it’s slow—that does not matter much as I’m not going to use it myself and nobody else is going to use it either (VP6 encoder had some initial burst of interest from some people but none afterwards, and nobody cares about RV4 from the start).
Now the challenge is to find myself an interesting task, because most of the tasks I can think about involve improving some encoder or decoder or—shudder—writing a MOV/MP4 muxer. Oh well, I hope I’ll come with something regardless.
