[FFmpeg-devel] [PATCH] avformat/mp3dec: Make MP3 seek fast

[Tsung-Hung Wu]

Hi FFmpeg developers,

Sorry for the spam. I know you are busy. Please take your time to review
the patch. I don't mean to rush at all.
Since the is my first patch, I just want to make sure I was doing the right
thing. Please kindly let me know if I posted to a wrong mail group or made
the patch wrong. Thank you so much for all your great contributions.

Best Regards,
Andy


On Tue, Sep 1, 2015 at 11:35 AM, Tsung-Hung Wu <tsunghung at chromium.org>
wrote:

> Hi FFmpeg developers,
>
> I am trying to solve an issue: seeking a large MP3 file is slow. This
> happens when play Podcast audio, for example
>
> http://podcastdownload.npr.org/anon.npr-podcasts/podcast/344098539/430765119/npr_430765119.mp3
> It works not so good in less computing power mobile devices, especially
> under slow network environment.
> My goal is to improve the usability, although it may sacrifice the
> accuracy.
>
> I noticed that FFmpeg has AVFMT_FLAG_FAST_SEEK flag. That's exactly what I
> need, sacrifices the accuracy of playback time in exchange for
> responsiveness. However, it does not work as my expectation. I made some
> changes to fulfill my needs. Please find the attachment for the patch.
> Just to be clear, I put some comments below for discussions, which are not
> in the patch.
>
> Could you please kindly review the attached patch? Let me know if you have
> any comments. Thank you so much for your help.
>
> Best Regards,
> Andy
>
> ---
>  libavformat/mp3dec.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/libavformat/mp3dec.c b/libavformat/mp3dec.c
> index 007c6ea..7f675ab 100644
> --- a/libavformat/mp3dec.c
> +++ b/libavformat/mp3dec.c
> @@ -342,7 +342,7 @@ static int mp3_read_header(AVFormatContext *s)
>      int i;
>
>      if (mp3->usetoc < 0)
> -        mp3->usetoc = (s->flags & AVFMT_FLAG_FAST_SEEK) ? 0 : 2;
> +        mp3->usetoc = (s->flags & AVFMT_FLAG_FAST_SEEK) ? 1 : 2;
>
> /*
> Should usetoc be 0 when AVFMT_FLAG_FAST_SEEK is enabled? When usetoc is 0,
> seeking a VBR mp3 is slow, even if it contains a TOC.
> Thus, I think 1 is a more proper selection.
> */
>
>      st = avformat_new_stream(s, NULL);
>      if (!st)
> @@ -489,19 +489,21 @@ static int mp3_seek(AVFormatContext *s, int
> stream_index, int64_t timestamp,
>      AVStream *st = s->streams[0];
>      int64_t ret  = av_index_search_timestamp(st, timestamp, flags);
>      int64_t best_pos;
> +    int fast_seek = (s->flags & AVFMT_FLAG_FAST_SEEK) ? 1 : 0;
> +    int64_t filesize = mp3->header_filesize > 0 ? mp3->header_filesize
> +                            : avio_size(s->pb) - s->internal->data_offset;
>
> /*
> Many MP3 files in Podcast does not have VBRI, LAME, or XING tag, so
> header_filesize is not always available. (file size - data offset) could be
> a good estimation.
> */
>
>
>      if (mp3->usetoc == 2)
>          return -1; // generic index code
>
> -    if (   mp3->is_cbr
> +    if (   (mp3->is_cbr || fast_seek)
>          && (mp3->usetoc == 0 || !mp3->xing_toc)
>          && st->duration > 0
> -        && mp3->header_filesize > s->internal->data_offset
> -        && mp3->frames) {
> +        && filesize > 0) {
> /*
> Not sure why (mp3->header_filesize > s->internal->data_offset) has to be
> true. What if a MP3 file contains a short audio and a large picture?
> Again, mp3->frames is not always available. Do we really need it here? I
> move the check to below.
> */
>          ie = &ie1;
>          timestamp = av_clip64(timestamp, 0, st->duration);
>          ie->timestamp = timestamp;
> -        ie->pos       = av_rescale(timestamp, mp3->header_filesize,
> st->duration) + s->internal->data_offset;
> +        ie->pos       = av_rescale(timestamp, filesize, st->duration) +
> s->internal->data_offset;
>      } else if (mp3->xing_toc) {
>          if (ret < 0)
>              return ret;
> @@ -515,7 +517,7 @@ static int mp3_seek(AVFormatContext *s, int
> stream_index, int64_t timestamp,
>      if (best_pos < 0)
>          return best_pos;
>
> -    if (mp3->is_cbr && ie == &ie1) {
> +    if (mp3->is_cbr && ie == &ie1 && mp3->frames) {
>          int frame_duration = av_rescale(st->duration, 1, mp3->frames);
>          ie1.timestamp = frame_duration * av_rescale(best_pos -
> s->internal->data_offset, mp3->frames, mp3->header_filesize);
>      }
> --
> 2.5.0.457.gab17608
>
>