[FFmpeg-devel] [PATCH 1/2] libavcodec/zmbvenc: block scoring improvements/bug fixes

[Tomas Härdin]

fre 2019-02-22 klockan 00:11 +0100 skrev Michael Niedermayer:
> On Thu, Feb 21, 2019 at 09:26:44PM +0000, Matthew Fearnley wrote:
> > - To clear up the effects: the change from 'i=1' to 'i=0' was the only
> > change that should make any difference in the output.
> > The rest of the changes were to improve the speed of an inner loop, and to
> > fix two bugs that happily cancelled each other out, but would have broken
> > if the code were changed to emit larger blocks.  These should not lead to
> > changes in the blocks chosen or the final compression size.
> > 
> > - Regarding the worse compression: let me start by stating that scoring on
> > byte frequency alone will not always predict well how Deflate will work,
> > since Deflate also uses pattern matching as well.
> > Frequency-based block scoring will only ever be a heuristic.  There may be
> > scope for improving the scoring, but it would add a lot of complexity, and
> > I think it would be easy to make things worse rather than better.  (I found
> > this in my brief experiments with including the frequencies from the
> > previous block.)
> 
> implementing the exact scoring by using Deflate itself would allow us to
> understand how much there is between the heuristic and what is achievable.
> If there is a gap that is significant then it may be interresting to
> look at improving the heuristic if the gap is small than the whole
> heuristic improvment path can be discarded.
> The obvious improvment for the heuristic would be to move to higher
> order entropy estimation. ATM IIUC this uses a 0th order. 1st order could
> be tried or any fast compression algorithm could also be used to estimate
> entropy.

We tried a sliding window order-0 model, which did worse. An order-1
model might be useful, but would need quite a bit of context for it to
be meaningful

> It also would be interresting to see how a higher order heuristic or
> some simple fast LZ like compressor would perform with the previous block.
> That is if the issue you saw with the previous block inclusion was because
> of the very simplistic entropy estimation.

zlib at level 1 maybe? You could also have a multi-pass thing where it
first does a coarse pass then tries to jiggle MVs and greedily accept
any change that results in smaller output.

/Tomas