Re: [FFmpeg-user] problem with fast seeking and splitting

[Alexander Solonsky via ffmpeg-user]

> On 5/1/2022 10:27 PM, Alexander Solonsky via ffmpeg-user wrote:
>> I'm trying to split an input file into 2 second chunks, transcode and
>> assemble using a concat muxer.
> I hate to question the question, but what's the purpose of this process? 
> Generally speaking, you're better off transcoding in a single pass. If that 
> process is too slow, can you use hardware to accelerate the process? Cutting 
> into 2-second chunks to recode seems like a poor approach.

> If you really want to recode chunks, which may have their own issues when 
> concatenated together, decode the entire stream and select chunks from the 
> uncompressed data.

Hi Carl,

the original idea was to split accelerate video encoding by splitting on scene 
changes or much larger chunks than 2 seconds to make it more efficient, for 
sure. 2 seconds were selected to isolate the problem and make the debugging 
easier and faster.
VP9 and AV1 encoders are not thread efficient yet and splitting to several 
independent encoding processes speeds up things significantly. Or the lambda 
encoding also requires accurate splitting and concatenation. And none of that 
seems to be working. Decoding into intermediate uncompressed is not an option 
because of complications and additional cost (think about 4K movie in 10 bits). 
And why would we do that? - when we have input stream with all offsets, frame 
types, pts times etc. And neither seeking (I couldn't actually find a 
compressed file for which the fast seek would work more or less accurately) nor 
concatenation seems to be working properly.
And yet there is https://trac.ffmpeg.org/wiki/Seeking with quotes  "-ss is now 
also "frame-accurate" even when used as an input option", "The input will be 
parsed using keyframes, which is very fast", "-ss as an input option is now 
both fast and accurate" @Paul B Mahol <one...@gmail.com>

Thanks,
Alexander
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-- 
Best regards,
Alexander Solonsky

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