[FFmpeg-devel] [PATCH] doc/filters: Add double-pass example for loudnorm

[Marvin Scholz]

On 19 Aug 2018, at 23:27, Carl Eugen Hoyos wrote:

> 2018-08-19 21:33 GMT+02:00, Marvin Scholz <epirat07 at gmail.com>:
>> ---
>>  doc/filters.texi | 23 +++++++++++++++++++++++
>>  1 file changed, 23 insertions(+)
>>
>> diff --git a/doc/filters.texi b/doc/filters.texi
>> index 32c95b591c..2078ee0f6f 100644
>> --- a/doc/filters.texi
>> +++ b/doc/filters.texi
>> @@ -3659,6 +3659,29 @@ Set print format for stats. Options are 
>> summary,
>> json, or none.
>>  Default value is none.
>>  @end table
>>
>> + at subsection Examples
>> +
>> +For double pass processing you need to first run the filter with
>> + at code{print_format} set to either @code{json} or @code{summary}, 
>> then read
>> +the values in the output and pass it to the next run of the filter:
>> +
>> + at example
>> +$ ffmpeg -i file.mov -map 0:a -af 
>> loudnorm=I=-13:TP=-1:print_format=summary
>> -f null -
>> +
>> +[...]
>> +Input Integrated:     -9.0 LUFS
>> +Input True Peak:      +1.5 dBTP
>> +Input LRA:             9.4 LU
>> +Input Threshold:     -19.5 LUFS
>> + at end example
>> +
>> +Then pass the input measurements to the next run that produces the 
>> result:
>> +
>> + at example
>> +ffmpeg -i file.mov -af
>> loudnorm=I=-13:TP=-1:measured_I=-9.0:measured_TP=1.5:measured_LRA=9.4:
>> measured_thresh=-19.5:print_format=summary out.mov
>
> I believe it should be "ffmpeg -i input"

Ok

> Why does the first run output to null but the second to mov?

If I understood correctly, the output of the first run is not relevant
as the only interesting thing is the filter "Input" measurements for
the whole file, which on second pass is then given to it so the filter 
can
know those in advance and behave based on those and this output with the
filter applied is the one that is actually the "good" result.

But maybe I misunderstand something here.

>
> Carl Eugen
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