[FFmpeg-user] Why are these different?

Mark Filipak markfilipak.imdb at gmail.com
Mon Feb 12 03:23:59 EET 2024

On 11/02/2024 17.39, Paul B Mahol wrote:
> On Sun, Feb 11, 2024 at 11:08 PM Mark Filipak <markfilipak.imdb at gmail.com>
> wrote:
>> On 11/02/2024 16.42, Paul B Mahol wrote:
>>> Codec copy with -ss before -i and after -i is not documented that it gives
>>> different results
>> Does that make sense? I think it's not documented because no one knows how
>> to explain the results.
> It is self explanatory.

Hahaha... That's funny. You're a comedian, Paul.

>>> because once you do not decode everything from start you basically can
>>> not expect to get always (for any container and for any input)
>>> frame-accurate results in both cases.
>> I don't think decoding is being done. The cutting/trimming can be by
>> dropping packets & remuxing.
>> The packets have to be 'opened' to read the PTSs, but I don't think
>> there's actual decoding.
> Packets here are just demuxed, not decoded. So -c copy will demux, so
> sample accurate audio seeking is not possible.
>> '-ss'-before-'-i' is cutting at the right point.
> That one actually seeks without demuxing each packet from start.

I think we're getting closer to my problem. So '-ss'-before-'-i' looks at the packet headers and the 
PES headers, but only individually. It doesn't form streams until it gets to the cut point. Then it 
begins forming the various streams. Is that what you mean?

>> '-i'-before-'-ss' is cutting at the wrong point.
> This one does not seek at all, it just demux from start and drops packets
> up to certain point.

So, wouldn't the end result be the same? Many people have said that but that's not been my 
experience. Not at all.

> For -c copy, that one demuxes from start (without any seeking, but just
> skipping packets processing) but gives output only after timeline point.

Sorry. "That one"? Which one? They both have '-c copy' of course.

> Which is kind of logical because it does not decode at all. And for complex
> codec that have different forward/backward frame predictions it will give
> incorrect results because PTS may not match DTS - presentation timestamp vs
> demux timestamp.

DTS is not demux timestamp. And what do you mean by "it"? What is the antecedent of "it"? I'm lost 
in your explanation.

> So for '-c copy'  -ss after -i is mostly not useful, exp. for complex
> video/audio codecs with complex interframe coding.

As I wrote earlier in this thread, I did '-i'-before-'-ss' on a lark to see if it would be better. 
It was worse.

So, it seems like I have the operations right and that they should be the same but they're not.

Correct? Are you saying that '-ss'-before-'-i' & '-i'-before-'-ss' should be identical?

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