[FFmpeg-user] FFmpeg Architecture

Mark Filipak markfilipak.imdb at gmail.com
Tue Feb 13 20:21:05 EET 2024

On 13/02/2024 03.24, Paul B Mahol wrote:
> On Tue, Feb 13, 2024 at 12:37 AM Carl Zwanzig <cpz at tuunq.com> wrote:

Paul, Carl, I assume you are both familiar with the source code. I'm not.

Fact: '-ss'-before-'-i' and '-i'-before-'-ss' _do_ give different results. I suggested that the 
difference might result from the way that FFmpeg is designed.

I mentioned dynamic versus static architectures. When thinking about dynamic versus static think 
"build up/tear down" versus "call". "Build up/tear down" versus "call" has to do with implementation 
-- how FFmpeg runs jobs -- not whether the job is streaming or real-time. In H.222, ISO & ITU imply 
(to me) that they had "build up/tear down" in mind when they created the system model they created.

The question is this: Should '-ss'-before-'-i' and '-i'-before-'-ss' give different results? They 
_do_, but _should_ they?

Set aside for the moment that '-ss'-before-'-i' and '-i'-before-'-ss' make differing cuts (at 
differing PTSs) that are hundreds of frames different.
Set aside for the moment that '-ss'-before-'-i' honors '-muxdelay 0' but '-i'-before-'-ss' doesn't.
Set aside for the moment that both of them cut the 'earliest' audio packets but both leave some 
'too-early' audio packets in their results.
Set aside for the moment that both of them include video packets that have 'too-early' PTSs that 
should have been cut.

None of that matters for now.

I can't submit a bug ticket unless I know whether '-ss'-before-'-i' and '-i'-before-'-ss' should 
give the same results. After that is established and I submit a ticket, assuming the answer is that 
they should be the same, then all the other details will be hashed out and all the fighting will 
occur in ... github? Is it github? I think so.

Please help with this if you can.


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