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

[Paul B Mahol]

On 9/4/15, Tsung-Hung Wu <tsunghung at chromium.org> wrote:
> 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
>

First do not top post, second you already received review.

>
> 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
>>
>>
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel at ffmpeg.org
> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>