[FFmpeg-devel] [PATCH] fftools/ffmpeg: fix -t inaccurate recording time

[Shiwang Xie]

Have tested several cases, will push after tomorrow if without objection, 
thanks.

On Thu, 29 Apr 2021, Shiwang.Xie wrote:

> if input start time is not 0 -t is inaccurate doing stream copy,
> will record extra duration according to input start time.
> it should base on following cases:
>
> input video start time from 60s, duration is 300s,
> 1. stream copy:
>       ffmpeg -ss 40 -t 60 -i in.mp4 -c copy -y out.mp4
>   open_input_file() will seek to 100 and set ts_offset to -100,
>   process_input() will offset pkt->pts with ts_offset to make it 0,
>   so when do_streamcopy() with -t, exits when ist->pts >= recording_time.
>
> 2. stream copy with -copyts:
>       ffmpeg -ss 40 -t 60 -copyts -i in.mp4 -c copy -y out.mp4
>   open_input_file() will seek to 100 and set ts_offset to 0,
>   process_input() will keep raw pkt->pts as ts_offset is 0,
>   so when do_streamcopy() with -t, exits when
>   ist->pts >= (recording_time+f->start_time+f->ctx->start_time).
>
> 3. stream copy with -copyts -start_at_zero:
>       ffmpeg -ss 40 -t 60 -copyts -start_at_zero -i in.mp4 -c copy -y out.mp4
>   open_input_file() will seek to 120 and set ts_offset to -60 as start_to_zero option,
>   process_input() will offset pkt->pts with input file start time,
>   so when do_streamcopy() with -t, exits when ist->pts >= (recording_time+f->start_time).
>
> 0      60     40      60                     360
> |_______|_____|_______|_______________________|
>      start   -ss     -t
>
> This fixes ticket #9141.
>
> Signed-off-by: Shiwang.Xie <shiwang.xie666 at outlook.com>
> ---
> fftools/ffmpeg.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
> index 3ad11452da..fac9f67462 100644
> --- a/fftools/ffmpeg.c
> +++ b/fftools/ffmpeg.c
> @@ -2082,9 +2082,11 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
>     }
>
>     if (f->recording_time != INT64_MAX) {
> -        start_time = f->ctx->start_time;
> -        if (f->start_time != AV_NOPTS_VALUE && copy_ts)
> -            start_time += f->start_time;
> +        start_time = 0;
> +        if (copy_ts) {
> +            start_time += f->start_time != AV_NOPTS_VALUE ? f->start_time : 0;
> +            start_time += start_at_zero ? 0 : f->ctx->start_time;
> +        }
>         if (ist->pts >= f->recording_time + start_time) {
>             close_output_stream(ost);
>             return;
> -- 
> 2.24.3 (Apple Git-128)
>
>