[FFmpeg-devel] [PATCH 2/2] aadec: add chapters and seeking

[wm4]

On Sun, 28 Jan 2018 00:01:14 +0100
Karsten Otto <ottoka at posteo.de> wrote:

> read_packet reads content in chunks. Thus seek must be clamped to valid
> chunk positions in the file, which in turn are relative to chapter start
> positions.
> 
> So in read_header, scan for chapter headers once by skipping through the
> content. Set stream time_base to bitrate in bytes/s, for easy timestamp to
> position conversion.
> 
> Then in read_seek, find the chapter containing the seek position, calculate
> the nearest chunk position, and reinit the read_seek state accordingly.
> ---
>  libavformat/aadec.c | 75 ++++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 72 insertions(+), 3 deletions(-)
> 
> diff --git a/libavformat/aadec.c b/libavformat/aadec.c
> index d6b6c125da..2480781df0 100644
> --- a/libavformat/aadec.c
> +++ b/libavformat/aadec.c
> @@ -35,6 +35,7 @@
>  #define MAX_TOC_ENTRIES 16
>  #define MAX_DICTIONARY_ENTRIES 128
>  #define TEA_BLOCK_SIZE 8
> +#define CHAPTER_HEADER_SIZE 8
>  
>  typedef struct AADemuxContext {
>      AVClass *class;
> @@ -46,6 +47,7 @@ typedef struct AADemuxContext {
>      struct AVTEA *tea_ctx;
>      uint8_t file_key[16];
>      int64_t current_chapter_size;
> +    int64_t content_start;
>      int64_t content_end;
>  } AADemuxContext;
>  
> @@ -70,7 +72,7 @@ static int aa_read_header(AVFormatContext *s)
>      uint32_t nkey, nval, toc_size, npairs, header_seed = 0, start;
>      char key[128], val[128], codec_name[64] = {0};
>      uint8_t output[24], dst[8], src[8];
> -    int64_t largest_size = -1, current_size = -1;
> +    int64_t largest_size = -1, current_size = -1, chapter_pos;
>      struct toc_entry {
>          uint32_t offset;
>          uint32_t size;
> @@ -172,19 +174,23 @@ static int aa_read_header(AVFormatContext *s)
>      if (!strcmp(codec_name, "mp332")) {
>          st->codecpar->codec_id = AV_CODEC_ID_MP3;
>          st->codecpar->sample_rate = 22050;
> +        st->time_base = av_make_q(8, 32000);
>          st->need_parsing = AVSTREAM_PARSE_FULL_RAW;
> -        st->start_time = 0;
>      } else if (!strcmp(codec_name, "acelp85")) {
>          st->codecpar->codec_id = AV_CODEC_ID_SIPR;
>          st->codecpar->block_align = 19;
>          st->codecpar->channels = 1;
>          st->codecpar->sample_rate = 8500;
> +        st->codecpar->bit_rate = 8500;
> +        st->time_base = av_make_q(8, 8500);
>          st->need_parsing = AVSTREAM_PARSE_FULL_RAW;
>      } else if (!strcmp(codec_name, "acelp16")) {
>          st->codecpar->codec_id = AV_CODEC_ID_SIPR;
>          st->codecpar->block_align = 20;
>          st->codecpar->channels = 1;
>          st->codecpar->sample_rate = 16000;
> +        st->codecpar->bit_rate = 16000;
> +        st->time_base = av_make_q(8, 16000);
>          st->need_parsing = AVSTREAM_PARSE_FULL_RAW;
>      }
>  
> @@ -198,7 +204,30 @@ static int aa_read_header(AVFormatContext *s)
>      }
>      start = TOC[largest_idx].offset;
>      avio_seek(pb, start, SEEK_SET);
> +
> +    // extract chapter positions. since all formats have constant bit rate, use it
> +    // as time base in bytes/s, for easy stream position <-> timestamp conversion
> +    st->start_time = 0;
> +    c->content_start = start;
>      c->content_end = start + largest_size;
> +
> +    while ((chapter_pos = avio_tell(pb)) >= 0 && chapter_pos < c->content_end) {
> +        int chapter_idx, chapter_size;
> +        chapter_idx = s->nb_chapters;
> +        chapter_pos -= start + CHAPTER_HEADER_SIZE * chapter_idx;
> +        chapter_size = avio_rb32(pb);
> +        if (chapter_size == 0) break;
> +        avio_skip(pb, 4);
> +        avio_skip(pb, chapter_size);
> +        if (!avpriv_new_chapter(s, chapter_idx, st->time_base,
> +            chapter_pos, chapter_pos + chapter_size, NULL))
> +                return AVERROR(ENOMEM);
> +    }
> +
> +    st->duration = largest_size - CHAPTER_HEADER_SIZE * s->nb_chapters;
> +
> +    ff_update_cur_dts(s, st, 0);
> +    avio_seek(pb, start, SEEK_SET);
>      c->current_chapter_size = 0;
>  
>      return 0;
> @@ -267,6 +296,45 @@ static int aa_read_packet(AVFormatContext *s, AVPacket *pkt)
>      return 0;
>  }
>  
> +static int aa_read_seek(AVFormatContext *s,
> +                        int stream_index, int64_t timestamp, int flags)
> +{
> +    AADemuxContext *c = s->priv_data;
> +    AVChapter *ch;
> +    int64_t chapter_pos, chapter_start;
> +    int chapter_idx = 0;
> +
> +    // find chapter containing seek timestamp
> +    if (timestamp < 0)
> +        timestamp = 0;
> +
> +    while (chapter_idx < s->nb_chapters && timestamp >= s->chapters[chapter_idx]->end) {
> +        ++chapter_idx;
> +    }
> +
> +    if (chapter_idx >= s->nb_chapters) {
> +        chapter_idx = s->nb_chapters - 1;
> +        if (chapter_idx < 0) return -1; // there is no chapter.
> +        timestamp = s->chapters[chapter_idx]->end;
> +    }
> +
> +    ch = s->chapters[chapter_idx];
> +
> +    // sync by clamping timestamp to nearest valid block position in its chapter
> +    chapter_pos = (timestamp - ch->start) / c->codec_second_size * c->codec_second_size;
> +    chapter_start = c->content_start + ch->start + CHAPTER_HEADER_SIZE * (1 + chapter_idx);

Can't comment on most of the patches, but how you need to "clamp" the
seek target to a valid position depends on the AVSEEK_FLAG_BACKWARD bit
in flags. If that flag is set, you need to clamp backwards; if it's not
set, clamp forwards.

> +    avio_seek(s->pb, chapter_start + chapter_pos, SEEK_SET);
> +    c->current_codec_second_size = c->codec_second_size;
> +    c->current_chapter_size = ch->end - ch->start;
> +    c->current_chapter_size -= chapter_pos;
> +    c->chapter_idx = 1 + chapter_idx;
> +
> +    ff_update_cur_dts(s, s->streams[0], ch->start + chapter_pos);
> +
> +    return 1;
> +}
> +
>  static int aa_probe(AVProbeData *p)
>  {
>      uint8_t *buf = p->buf;
> @@ -312,6 +380,7 @@ AVInputFormat ff_aa_demuxer = {
>      .read_probe     = aa_probe,
>      .read_header    = aa_read_header,
>      .read_packet    = aa_read_packet,
> +    .read_seek      = aa_read_seek,
>      .read_close     = aa_read_close,
> -    .flags          = AVFMT_GENERIC_INDEX,
> +    .flags          = AVFMT_NO_BYTE_SEEK | AVFMT_NOGENSEARCH,
>  };