[FFmpeg-devel] [PATCH v1] avformat/movenc: split empty text sample when duration overflow

[Jun Li]

On Fri, Sep 6, 2019 at 4:10 PM Jun Li <junli1026 at gmail.com> wrote:

> Fix #7637
> One empty/end sample is created and inserted between two caption lines
> when there is a gap.
> This patch is to split the sample into multiple ones when its duration is
> too long (>= INT_MAX).
> ---
>  libavformat/movenc.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
>
> diff --git a/libavformat/movenc.c b/libavformat/movenc.c
> index edddfeeb00..aeb7de351f 100644
> --- a/libavformat/movenc.c
> +++ b/libavformat/movenc.c
> @@ -5746,7 +5746,8 @@ static int mov_write_packet(AVFormatContext *s,
> AVPacket *pkt)
>           *
>           * 2) For each subtitle track, check if the current packet's
>           * dts is past the duration of the last subtitle sample. If
> -         * so, we now need to write an end sample for that subtitle.
> +         * so, we now need to write one or multiple end samples for
> +         * that subtitle.
>           *
>           * This must be done conditionally to allow for subtitles that
>           * immediately replace each other, in which case an end sample
> @@ -5760,11 +5761,22 @@ static int mov_write_packet(AVFormatContext *s,
> AVPacket *pkt)
>              int ret;
>
>              if (trk->par->codec_id == AV_CODEC_ID_MOV_TEXT &&
> -                trk->track_duration < pkt->dts &&
> -                (trk->entry == 0 || !trk->last_sample_is_subtitle_end)) {
> -                ret = mov_write_subtitle_end_packet(s, i,
> trk->track_duration);
> -                if (ret < 0) return ret;
> -                trk->last_sample_is_subtitle_end = 1;
> +                trk->track_duration < pkt->dts) {
> +                int max_duration = INT_MAX - 1;
> +                if (trk->entry == 0 || !trk->last_sample_is_subtitle_end)
> {
> +                    ret = mov_write_subtitle_end_packet(s, i,
> trk->track_duration);
> +                    if (ret < 0) return ret;
> +                    trk->last_sample_is_subtitle_end = 1;
> +                }
> +                if (trk->last_sample_is_subtitle_end &&
> +                    pkt->dts - trk->track_duration > max_duration) {
> +                    int64_t dts = trk->track_duration;
> +                    while(pkt->dts - dts > max_duration) {
> +                        dts += max_duration;
> +                        ret = mov_write_subtitle_end_packet(s, i, dts);
> +                        if (ret < 0) return ret;
> +                    }
> +                }
>              }
>          }
>
> --
> 2.17.1
>
>
Some details:
Current implementation will insert one empty sample whenever there is a gap
between two captions lines. But when the gap's duration is way too big, the
sample's duration will exceed MP4's limitation, thus mov muxer will fail on
function check_pkt.

This patch is to guarantee that, every empty samples' duration not exceed
INT_MAX-1, and insert multiple ones when the gap is bigger than INT_MAX-1.

Test results:
Fate test passed. Also tested with following SRT file with cmd:

ffmpeg -i input.mp4 -i input.srt -c copy -c:s mov_text test.mp4 -y

I used Apple's QuickTime player, confirmed that the caption shows up at the
correct time.

"input. srt" as follows
1
00:35:47.484 --> 00:35:50.000
Durations that exceed the signed
int max value break the program

2
00:40:47.484 --> 00:40:50.000
Durations2 that exceed the signed
int max value break the program

3
00:50:47.484 --> 00:50:50.000
Durations3 that exceed the signed
int max value break the program

4
02:50:47.484 --> 02:50:50.000
Durations4 that exceed the signed
int max value break the program

5
03:50:47.484 --> 03:50:50.000
Durations5 that exceed the signed
int max value break the program