[FFmpeg-devel] WIP: Loudness analysis filter with EBU R.128

[Clément Bœsch]

Hi folks,

First, I'm sorry it's again a WIP... I again need some comments to finalize it
properly.

The main point of this filter is to graph the loudness (related:
http://tech.ebu.ch/loudness, https://en.wikipedia.org/wiki/Loudness_war). As a
consequence, a video output is available, but is optional: at some point this
filter could be used just to inject the loudness metadata in the filtergraph
for further processing (like normalization), and thus the video output wouldn't
be required. Concerning the audio, it's passed through unchanged to the next
filter in both cases (because you likely want to hear what's being graph in
real time, and it would be simpler to have a "ebur128,normalize" filtergraph
than splitting the audio).

Preview:

    ./ffmpeg -nostats -i ~/ebur128/samples/seq-3341-2011-8_seq-3342-6-24bit-v02.wav -filter_complex ebur128=video=1 -y loudness.webm
    => http://lucy.pkh.me/loudness.webm

There is one main problem I still can't figure out: there are some A/V sync
issues with ffplay (but not with ffmpeg as shown in the above loudness.webm
output):

    ./ffplay -f lavfi -i "amovie=input.mp3,ebur128=video=1 [out0][out1]

I'm not sure where the problem is, but since no real-time is possible with the
filter at the moment because of this issue.

Just so things are clear, here is how audio and video are pushed in the ebur128
filter:

    - input is always 48kHz
    - every 100ms (4800 samples) a video frame is pushed (within
      filter_samples() callback), with pts computed based on insamples->pts and
      the number of insamples processed
    - at the end of the filter_samples() call, the insamples are sent to the
      next filter

Concerning the possible improvements on the filter, it should be stated in the
commit description, so feel free to comment there. I also need to do some
extended testing to check if the results are actually correct.

About the first patch of the patch set, it moves the xGA stuff to lavu so I can
use it in the filter instead of depending on freetype and stuff (it really is
way more simpler in this context). Comments are welcome about how to expose
this API in its best shape (see commit description).

Regards,