[FFmpeg-devel] [PATCH] avformat/mp3dec: prefer "fast_seek" to TOC seek for CBR files.

[Chris Cunningham]

The new patch incorporates the "3 use-cases" feedback from wm4.

1. default: use generic (accurate but slow) seeking
2. fast: set -fflags fastseek to get fast accurate scaled seek for CBR, TOC
seek for VBR falling back to scaling if no TOC exists
3. custom: set -usetoc 1 to always use TOC for VBR & CBR. Will override
fastseek flag. This is very inaccurate for large files.

fastseek is really very close to generic seek for CBR files. for VBR its
pretty rough.

The fate-seek-extra-mp3 test is still failing. Its easy to fix, but I need
some guidance on what the test is supposed to focus on. It was originally
going through the av_rescale path of mp3_seek. With my patch it goes
through the return -1 (slow generic index) path. IIUC, the generic index
path is covered elsewhere (
https://github.com/FFmpeg/FFmpeg/blob/a62178be80dd6a643973f62002fc0ed42495c358/tests/fate-run.sh#L229).
I think I can restore the behavior of this test to use the av_rescale path
if I replace -usetoc 0 with -fflags fastseek (and update args parsing)...
is that what we want?

Thanks,
Chris

On Tue, Nov 24, 2015 at 4:55 PM, <chcunningham at chromium.org> wrote:

> From: Chris Cunningham <chcunningham at chromium.org>
>
> "Fast seek" uses linear interpolation to find the position of the
> requested seek time. For CBR this is more direct than using the
> mp3 TOC and bypassing the TOC avoids problems with TOC precision.
> (see https://crbug.com/545914#c13)
>
> For VBR, fast seek is not precise, so continue to prefer the TOC
> when available (the lesser of two evils).
>
> Also, some re-ordering of the logic in mp3_seek to simplify and
> give usetoc=1 precedence over fastseek flag.
> ---
>  libavformat/mp3dec.c | 45 +++++++++++++++++++++++----------------------
>  1 file changed, 23 insertions(+), 22 deletions(-)
>
> diff --git a/libavformat/mp3dec.c b/libavformat/mp3dec.c
> index 32ca00c..a1f21b7 100644
> --- a/libavformat/mp3dec.c
> +++ b/libavformat/mp3dec.c
> @@ -115,7 +115,8 @@ static void read_xing_toc(AVFormatContext *s, int64_t
> filesize, int64_t duration
>  {
>      int i;
>      MP3DecContext *mp3 = s->priv_data;
> -    int fill_index = mp3->usetoc == 1 && duration > 0;
> +    int fast_seek = (s->flags & AVFMT_FLAG_FAST_SEEK) ? 1 : 0;
> +    int fill_index = (mp3->usetoc || fast_seek) && duration > 0;
>
>      if (!filesize &&
>          !(filesize = avio_size(s->pb))) {
> @@ -344,9 +345,6 @@ static int mp3_read_header(AVFormatContext *s)
>      int ret;
>      int i;
>
> -    if (mp3->usetoc < 0)
> -        mp3->usetoc = (s->flags & AVFMT_FLAG_FAST_SEEK) ? 1 : 2;
> -
>      st = avformat_new_stream(s, NULL);
>      if (!st)
>          return AVERROR(ENOMEM);
> @@ -501,40 +499,43 @@ static int mp3_seek(AVFormatContext *s, int
> stream_index, int64_t timestamp,
>      MP3DecContext *mp3 = s->priv_data;
>      AVIndexEntry *ie, ie1;
>      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;
>
> -    if (mp3->usetoc == 2)
> -        return -1; // generic index code
> -
>      if (filesize <= 0) {
>          int64_t size = avio_size(s->pb);
>          if (size > 0 && size > s->internal->data_offset)
>              filesize = size - s->internal->data_offset;
>      }
>
> -    if (   (mp3->is_cbr || fast_seek)
> -        && (mp3->usetoc == 0 || !mp3->xing_toc)
> -        && st->duration > 0
> -        && filesize > 0) {
> -        ie = &ie1;
> -        timestamp = av_clip64(timestamp, 0, st->duration);
> -        ie->timestamp = timestamp;
> -        ie->pos       = av_rescale(timestamp, filesize, st->duration) +
> s->internal->data_offset;
> -    } else if (mp3->xing_toc) {
> +    if (mp3->xing_toc && (mp3->usetoc || (fast_seek && !mp3->is_cbr))) {
> +        av_log(s, AV_LOG_ERROR, "XING seeking. filesize = %"PRId64"\n",
> filesize);
> +        // NOTE: The MP3 TOC is not a precise lookup table. The precision
> is
> +        // worse closer to the end of the file, especially for large
> files.
> +        // Seeking to 90% of duration in file of size 4M will be off by
> roughly 1 second.
> +        if (filesize > 4000000)
> +            av_log(s, AV_LOG_WARNING, "Using MP3 TOC to seek; may be
> imprecise.\n");
> +
> +        int64_t ret  = av_index_search_timestamp(st, timestamp, flags);
>          if (ret < 0)
>              return ret;
>
>          ie = &st->index_entries[ret];
> +    } else if (fast_seek && st->duration > 0 && filesize > 0) {
> +        if (!mp3->is_cbr)
> +            av_log(s, AV_LOG_WARNING, "Using scaling to seek VBR MP3; may
> be imprecise.\n");
> +
> +        ie = &ie1;
> +        timestamp = av_clip64(timestamp, 0, st->duration);
> +        ie->timestamp = timestamp;
> +        ie->pos       = av_rescale(timestamp, filesize, st->duration) +
> s->internal->data_offset;
>      } else {
> -        return -1;
> +        return -1; // generic index code
>      }
>
> -    best_pos = mp3_sync(s, ie->pos, flags);
> +    int64_t best_pos = mp3_sync(s, ie->pos, flags);
>      if (best_pos < 0)
> -        return best_pos;
> +      return best_pos;
>
>      if (mp3->is_cbr && ie == &ie1 && mp3->frames) {
>          int frame_duration = av_rescale(st->duration, 1, mp3->frames);
> @@ -546,7 +547,7 @@ static int mp3_seek(AVFormatContext *s, int
> stream_index, int64_t timestamp,
>  }
>
>  static const AVOption options[] = {
> -    { "usetoc", "use table of contents", offsetof(MP3DecContext, usetoc),
> AV_OPT_TYPE_INT, {.i64 = -1}, -1, 2, AV_OPT_FLAG_DECODING_PARAM},
> +    { "usetoc", "use table of contents", offsetof(MP3DecContext, usetoc),
> AV_OPT_TYPE_INT, {.i64 = 0}, 0, 1, AV_OPT_FLAG_DECODING_PARAM},
>      { NULL },
>  };
>
> --
> 2.6.0.rc2.230.g3dd15c0
>
>