[x264-devel] Optimized multiple rendition encoding

[Eran Kornblau]

> On Fri, 11 Nov 2016 21:23:39 +0000, Eran Kornblau wrote:
> > Hi all,
> >
> > (I apologize if this has been asked before, tried to google about it 
> > and found nothing…)
> >
> > My question is – is it possible to encode several different renditions 
> > of the same video so that the knowledge  gained while encoding one 
> > rendition will be used to boost the encoding time of another 
> > rendition?
> > For example, first encode a 4mpbs/1080p rendition, then encode a 
> > 2.5mbps/720p rendition faster thanks  to the work performed on the 
> > first rendition.
> >
> > From my limited knowledge on the subject, as I understand, the 
> > calculation of the motion vectors is the most  computationally 
> > intensive part of the encoding. And the motion vectors are probably 
> > the same for different  bitrates of the same resolution. So it should 
> > (at least in theory) be possible to reuse them to generate different  
> > bitrates more efficiently.
> > For different resolutions, it may be possible to scale the motion 
> > vectors, though I’m not sure that would  provide adequate results.
> >
> > Any info you can share on this will be appreciated
> >
> > Thank you!
> >
> > Eran
> 
> Hi. If you use 2-pass encoding than you can use same 1-st pass stats-file from one of the target bitrates to speed up encoding of other bitrates. My guess would be better to go from high to low bitrates. So make `--pass 1` with highest bitrate and then `--pass 2` for all bitrates. Or use `--pass 3` instead of `--pass 2` to update stats-file and go lower and lower.
> And no, encodes at different bitrate use different MVs so they can't be reused directly (may be as predictors but this still will nead ME and I am dunno if it would be faster).
> 

Thanks for your reply! However, as far as I know, 2 pass is about 50% slower than 1 pass. So if the highest rendition 
(which is the heaviest to transcode) will be transcoded with 2 pass, the whole process will probably take more time
than it does now. To put it differently - I don't think the speed gain in the lower transcodes will compensate for that. 
Yes, I may get higher quality thanks to the 2 pass, but my goal here is to shorten the transcoding time/reduce the 
resource usage.

> _______________________________________________
> x264-devel mailing list
> x264-devel at videolan.org
> https://mailman.videolan.org/listinfo/x264-devel
>