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demux: more backwards playback preroll packets for vorbis and mp3

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Together with the previous commit, this seems to make backward playback
work in files with vorbis and mp3 audio codecs.

For Vorbis (with libavcodec's decoder, didn't test libvorbis), the first
packet was just always completely discarded. This happened even though
we tell libavcodec that we do discarding of padding manually. It simply
happened inside the codec, not libavcodec's general initial padding
handling. In addition, the first output decoded frame seems to contain
partial data. (Unlike the opus decoder, it doesn't report any padding at
all.)

The Opus decoder (again libavcodec only tested) reports an initial
padding, but it appears to be too small, and it sounds right only with 2
packets discarded. So its status doesn't change.

I'm not sure why I need 2 frames for mp3, but with that value I had
success on the samples I tested.
This commit is contained in:
wm4 2019-05-31 16:24:06 +02:00
parent da6e862c4f
commit 8812530b31
2 changed files with 5 additions and 10 deletions
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11
DOCS/man/options.rst
View File

@ -493,13 +493,6 @@ Playback Control
framestep commands are transposed. Backstepping will perform very
expensive work to step forward by 1 frame.
- Backward playback with Vorbis does not work. libavcodec's decoder
discards the first Vorbis packet (after each decoder reset), and the
mechanism behind ``--audio-reversal-buffer`` assumes that it strictly
outputs a frame for each packet fed to it (the mechanism discards output
based on frame count, not timestamps). Since this tries to decode each
frame on its own, the player never gets a usable audio frame.
Tuning:
- Remove all ``--vf``/``--af`` filters you have set. Disable deinterlacing.
@ -565,8 +558,8 @@ Playback Control
The solution is to feed a previous packet to the decoder each time, and then
discard the output. This option controls how many packets to feed. The
``auto`` choice is currently hardcoded to 0 for video, and uses 1 for lossy
audio, 0 for lossless audio. As a hack, it sets it to 2 for Opus, which
requires this for unknown reasons.
audio, 0 for lossless audio. For some specific lossy audio codecs, this is
set to 2.
``--video-backward-overlap`` can potentially handle intra-refresh video,
depending on the exact conditions. You may have to use the

4
demux/demux.c
View File

@ -949,7 +949,9 @@ static void demux_add_sh_stream_locked(struct demux_internal *in,
ds->back_preroll = in->opts->audio_back_preroll;
if (ds->back_preroll < 0) { // auto
ds->back_preroll = mp_codec_is_lossless(sh->codec->codec) ? 0 : 1;
if (sh->codec->codec && strcmp(sh->codec->codec, "opus") == 0)
if (sh->codec->codec && (strcmp(sh->codec->codec, "opus") == 0 ||
strcmp(sh->codec->codec, "vorbis") == 0 ||
strcmp(sh->codec->codec, "mp3") == 0))
ds->back_preroll = 2;
}
break;