[Ffmpeg-devel] Bug in multithreaded h/x264 encoding

Loren Merritt lorenm
Tue Jun 21 10:37:56 CEST 2005

On Tue, 21 Jun 2005, Tuukka Toivonen wrote:
> On Mon, 20 Jun 2005, Erik Slagter wrote:
>> Still it's interesting that the output of the encoder apparently is not
>> bit exact regarding to nr of threads.
> x264? Yes, not at all, x264 splits frames into as many number of
> independent slices as there are threads. Makes parallelization
> easy (even for decoding) but hurts a little bit compression
> efficiency.
> ffmpeg with multiple threads isn't bit exact either, not at all
> (even though obviously it doesn't use slices).

It's not called "slices" in MPEG-4 ASP, but it's really the same idea 
(AFAICT from both 14496-2 and lavc's implementation).
Yes, H.264 allows a lot more flexibility in changing settings between 
slices, FMO, FSO, redundant slices, etc. But those aren't relevant to 
multithreading, and x264 doesn't do them.

--Loren Merritt

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