[x264-devel] Re: bit allocation algorithms among frames

[Tuukka Toivonen]

On Thu, 13 Jul 2006, Loren Merritt wrote:

> Interesting idea, but it requires random access in the encoder, which makes it
> O(movie length * # of QPs * GOP size).

Nah, just run the algorithm in smaller (sliding?) windows several times 
until the end of the movie, so you just need to buffer some frames (just 
like now).

The question is just how much the window size affects the results.

> Also, there must be some way to start with an approximation of a good bit
> allocation, rather than decrementing from qp=51.

That is what I was thinking. Maybe you could first pick good guesses for 
all QPs in all frames, then pick a frame where changing the QP improves the 
result the most, then either increment or decrement the QP, depending 
whether the current bitrate is too low or high. I'm not sure if it is 
possible to modify the algorithm to also increase QPs when necessary, 
probably there is some reason to disallow that, but maybe the limitation 
could be overcome?

Heck, this is almost like motion estimation: in ME you try to minimize 
distortion+lambda*rate by changing mvx and mvy, why not trying to minimize 
distortion+lambda*rate among frames by changing a vector of QPs?

> Of course I could just pick
> some overestimate of QP, but I'd like to refine an estimate at the same
> bitrate.

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