Paul:
On 2024-02-11 13:42, Paul B Mahol wrote:
Codec copy with -ss before -i and after -i is not documented that it gives
different results because
once you do not decode everything from start you basically can notexpect to get
always (for any
container and for any input) frame-accurate results in both cases.
I think you are saying something important here, but I am having a hard
time understanding what you mean. This sentence uses the word "not"
three times. I get lost.
Let me see if I can figure it out. Is this what you mean, Paul?
[Definition of "codec copy operation": the copying of content from one
file to another by demuxing packets from the input container (file),
then muxing them unchanged into the output container (file). The "-c
copy" option tells FFmpeg to perform a codec copy operation.]
The FFmpeg documentation does not say that FFmpeg gives different
results if you do a codec copy operation with a "-ss" option before
the input argument, compared to a code copy operation with the same
"-ss" option after the input argument. Nor does it say that FFmpeg
gives the same results.
It is unreasonable to expect FFmpeg to give frame-accurate results
from a given "-ss" option both when the option is before and is after
the input argument, for every container and for every input. Instead,
one should expect that for some containers and for some inputs, and
for some cases of the "-ss" option before or after the input argument,
FFmpeg will give results which are inexact in the number of frames
passed to the output.
But one should not expect the FFmpeg documentation to be clear about
which cases have frame-accurate results and which do not. The FFmpeg
documentation leaves any statement of results being the same or
different based on the position of the "-ss" option. The result of
this is that the FFmpeg documentation does not need to engage with the
task of clearly describing which cases give frame-accurate results and
which do not.
I think Mark Filipak asked a reasonable and (eventually) well-defined
question. This thread has so far been 30 messages. I get the impression
that the answer to his question is:
Because the FFmpeg project includes different implementations for
different containers, input types, and "-ss" option positions, but:
1. The project does not require any of the implementations to give
frame-accurate results for the codec copy operation, (though it is
allowed), and
2. The project does not require the documentation to describe whether
the results are frame-accurate or not, and
3. The project does not require the documentation to confess either
#1 or #2.
4. Because of #1, any answer to whether or not a particular case is
frame-accurate, or the the way in which it is inaccurate, or why,
must come from studying the code.
So one should not expect a person on this list to have the answer
easily at hand. Mark's answer must come from studying the code.
I suspect Mark will not be delighted with that answer.
—Jim DeLaHunt
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