[FFmpeg-devel] [PATCH] WebVTT demuxer - save cue id and settings as side data

[Clément Bœsch]

On Wed, May 29, 2013 at 04:26:13PM -0700, Matthew Heaney wrote:
> Currently the WebVTT demuxer parses the cues but throws away
> the cue id (the optional first line of the cue) and cue
> settings (the optional rendering instructions that follow
> the timestamp).
> 
> However, in order to write inband text tracks (to WebM
> files), the entire cue payload from the WebVTT source must
> be preserved.
> 
> This change makes no change to the data part of the output
> buffer packet (where the actual cue text is stored), but
> does add the cue id and settings as a single new side data
> item.  Existing code that cares only about the data part of
> the packet can continue to ignore the side data.
> ---
>  libavformat/webvttdec.c | 32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/libavformat/webvttdec.c b/libavformat/webvttdec.c
> index 694a020..a169d11 100644
> --- a/libavformat/webvttdec.c
> +++ b/libavformat/webvttdec.c
> @@ -74,8 +74,9 @@ static int webvtt_read_header(AVFormatContext *s)
>          int i;
>          int64_t pos;
>          AVPacket *sub;
> -        const char *p, *identifier;
> -        //const char *settings = NULL;
> +        const char *p, *identifier, *settings;
> +        int len, identifier_len, settings_len;
> +        uint8_t* buf;
>          int64_t ts_start, ts_end;
>  
>          ff_subtitles_read_chunk(s->pb, &cue);
> @@ -92,15 +93,15 @@ static int webvtt_read_header(AVFormatContext *s)
>              continue;
>  
>          /* optional cue identifier (can be a number like in SRT or some kind of
> -         * chaptering id), silently skip it */
> +         * chaptering id) */
>          for (i = 0; p[i] && p[i] != '\n'; i++) {
>              if (!strncmp(p + i, "-->", 3)) {
>                  identifier = NULL;
>                  break;
>              }
>          }
> -        if (identifier)
> -            p += strcspn(p, "\n");
> +        identifier_len = identifier ? strcspn(p, "\n") : 0;
> +        p += identifier_len;
>  
>          /* cue timestamps */
>          if ((ts_start = read_ts(p)) == AV_NOPTS_VALUE)
> @@ -112,16 +113,18 @@ static int webvtt_read_header(AVFormatContext *s)
>          if ((ts_end = read_ts(p)) == AV_NOPTS_VALUE)
>              break;
>  
> -        /* optional cue settings, TODO: store in side_data */
> +        /* optional cue settings */
>          p += strcspn(p, "\n\t ");
>          while (*p == '\t' || *p == ' ')
>              p++;
> +        settings = p;
> +        settings_len = strcspn(p, "\n");
> +        p += settings_len;
>          if (*p != '\n') {
> -            //settings = p;
> -            p += strcspn(p, "\n");
> +            av_log(s, AV_LOG_ERROR, "Bad WebVTT cue.\n");
> +            return AVERROR(EINVAL);
>          }
> -        if (*p == '\n')
> -            p++;
> +        p++;
>  
>          /* create packet */
>          sub = ff_subtitles_queue_insert(&webvtt->q, p, strlen(p), 0);
> @@ -132,6 +135,15 @@ static int webvtt_read_header(AVFormatContext *s)
>          sub->pos = pos;
>          sub->pts = ts_start;
>          sub->duration = ts_end - ts_start;
> +
> +        len = identifier_len + 1 + settings_len + 1;
> +        buf = av_packet_new_side_data(sub, AV_PKT_DATA_NEW_EXTRADATA, len);
> +        memcpy(buf, identifier, identifier_len);
> +        buf += identifier_len;
> +        *buf++ = '\n';
> +        memcpy(buf, settings, settings_len);
> +        buf += settings_len;
> +        *buf++ = '\n';

Exploiting this might be troublesome: won't you need to split this buffer
again in the webm muxer (and any potential other muxer such as... webvtt)?

While that may sound overkill, I believe the correct solution is to
introduce AV_PKT_DATA_WEBVTT_{IDENTIFIER,SETTINGS} side data types.

Those '\n' are likely unwanted too, BTW (of course the middle one looks
necessary for splitting).

-- 
Clément B.
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