[FFmpeg-devel] [PATCH v5 3/3] aadec: improve seeking in mp3 content

[Michael Niedermayer]

On Wed, Jul 11, 2018 at 11:53:25PM +0200, Karsten Otto wrote:
> MP3 frames may not be aligned to aa chunk boundaries. When seeking,
> calculate the expected frame offset in the target chunk. Adjust the
> timestamp and truncate the next packet accordingly.
> 
> This solution works for the majority of tested audio material. For
> some rare encodings with mp3 padding or embedded id3 tags, it will
> mispredict the correct offset, and at worst skip an extra frame.
> ---
>  libavformat/aadec.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/libavformat/aadec.c b/libavformat/aadec.c
> index 17ad20686b..2d78def6e8 100644
> --- a/libavformat/aadec.c
> +++ b/libavformat/aadec.c
> @@ -37,6 +37,7 @@
>  #define TEA_BLOCK_SIZE 8
>  #define CHAPTER_HEADER_SIZE 8
>  #define TIMEPREC 1000
> +#define MP3_FRAME_SIZE 104
>  
>  typedef struct AADemuxContext {
>      AVClass *class;
> @@ -50,6 +51,7 @@ typedef struct AADemuxContext {
>      int64_t current_chapter_size;
>      int64_t content_start;
>      int64_t content_end;
> +    int seek_offset;
>  } AADemuxContext;
>  
>  static int get_second_size(char *codec_name)
> @@ -228,6 +230,7 @@ static int aa_read_header(AVFormatContext *s)
>      ff_update_cur_dts(s, st, 0);
>      avio_seek(pb, start, SEEK_SET);
>      c->current_chapter_size = 0;
> +    c->seek_offset = 0;
>  
>      return 0;
>  }
> @@ -266,6 +269,8 @@ static int aa_read_packet(AVFormatContext *s, AVPacket *pkt)
>      // is this the last block in this chapter?
>      if (c->current_chapter_size / c->current_codec_second_size == 0) {
>          c->current_codec_second_size = c->current_chapter_size % c->current_codec_second_size;
> +        if (c->seek_offset > c->current_codec_second_size)
> +            c->seek_offset = 0;
>      }
>  
>      // decrypt c->current_codec_second_size bytes
> @@ -293,12 +298,13 @@ static int aa_read_packet(AVFormatContext *s, AVPacket *pkt)
>      if (c->current_chapter_size <= 0)
>          c->current_chapter_size = 0;
>  
> -    ret = av_new_packet(pkt, written);
> +    ret = av_new_packet(pkt, written - c->seek_offset);

Please document how the existing checks gurantee that written >= c->seek_offset


thanks

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

When you are offended at any man's fault, turn to yourself and study your
own failings. Then you will forget your anger. -- Epictetus
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