[FFmpeg-devel] [PATCH] avfilter/vf_subtitles: add shift option

[Manolis Stamatogiannakis]

Bumping this up after 10 days without a comment.

On Fri, 8 May 2020 at 13:14, Manolis Stamatogiannakis <mstamat at gmail.com>
wrote:

> Allows shifting of subtitle display times to align them with the video.
> This avoids having to rewrite the subtitle file in order to display
> subtitles correctly when input is seeked (-ss).
> Also handy for minor subtitle timing corrections without rewriting the
> subtitles file.
>
> Signed-off-by: Manolis Stamatogiannakis <mstamat at gmail.com>
> ---
>  doc/filters.texi           |  8 ++++++++
>  libavfilter/vf_subtitles.c | 29 +++++++++++++++++++++++++++--
>  2 files changed, 35 insertions(+), 2 deletions(-)
>
> diff --git a/doc/filters.texi b/doc/filters.texi
> index d19fd346ae..94323495f0 100644
> --- a/doc/filters.texi
> +++ b/doc/filters.texi
> @@ -17851,6 +17851,9 @@ The filter accepts the following options:
>  @item filename, f
>  Set the filename of the subtitle file to read. It must be specified.
>
> + at item shift
> +Shift subtitles timings by the specified amount.
> +
>  @item original_size
>  Specify the size of the original video, the video for which the ASS file
>  was composed. For the syntax of this option, check the
> @@ -17907,6 +17910,11 @@ To make the subtitles stream from @file{sub.srt}
> appear in 80% transparent blue
>  subtitles=sub.srt:force_style='FontName=DejaVu
> Serif,PrimaryColour=&HCCFF0000'
>  @end example
>
> +To re-sync subtitles after seeking the input e.g. with @code{-ss 20:20},
> use:
> + at example
> +subtitles=filename=sub.srt:shift=-20\:20
> + at end example
> +
>  @section super2xsai
>
>  Scale the input by 2x and smooth using the Super2xSaI (Scale and
> diff --git a/libavfilter/vf_subtitles.c b/libavfilter/vf_subtitles.c
> index a3b4029af4..47a38b55b1 100644
> --- a/libavfilter/vf_subtitles.c
> +++ b/libavfilter/vf_subtitles.c
> @@ -52,6 +52,8 @@ typedef struct AssContext {
>      char *filename;
>      char *fontsdir;
>      char *charenc;
> +    int64_t shift;
> +    char *shift_opt;
>      char *force_style;
>      int stream_index;
>      int alpha;
> @@ -68,6 +70,7 @@ typedef struct AssContext {
>  #define COMMON_OPTIONS \
>      {"filename",       "set the filename of file to read",
>          OFFSET(filename),   AV_OPT_TYPE_STRING,     {.str = NULL},  0, 0,
> FLAGS }, \
>      {"f",              "set the filename of file to read",
>          OFFSET(filename),   AV_OPT_TYPE_STRING,     {.str = NULL},  0, 0,
> FLAGS }, \
> +    {"shift",          "shift the timing of the subtitles",
>           OFFSET(shift_opt),  AV_OPT_TYPE_STRING,     {.str = NULL},  0, 0,
> FLAGS }, \
>      {"original_size",  "set the size of the original video (used to scale
> fonts)", OFFSET(original_w), AV_OPT_TYPE_IMAGE_SIZE, {.str = NULL},  0, 0,
> FLAGS }, \
>      {"fontsdir",       "set the directory containing the fonts to read",
>          OFFSET(fontsdir),   AV_OPT_TYPE_STRING,     {.str = NULL},  0, 0,
> FLAGS }, \
>      {"alpha",          "enable processing of alpha channel",
>          OFFSET(alpha),      AV_OPT_TYPE_BOOL,       {.i64 = 0   },
>  0,        1, FLAGS }, \
> @@ -103,6 +106,16 @@ static av_cold int init(AVFilterContext *ctx)
>          return AVERROR(EINVAL);
>      }
>
> +    if (ass->shift_opt) {
> +        if (av_parse_time(&ass->shift, ass->shift_opt, 1) < 0) {
> +            av_log(ctx, AV_LOG_ERROR, "Invalid subtitles shift: %s\n",
> +                   ass->shift_opt);
> +            return AVERROR(EINVAL);
> +        }
> +        ass->shift = av_rescale_q(ass->shift, AV_TIME_BASE_Q,
> av_make_q(1, 1000));
> +        av_log(ctx, AV_LOG_DEBUG, "Shifting subtitles by %0.3fsec.\n",
> ass->shift/1000.0);
> +    }
> +
>      ass->library = ass_library_init();
>      if (!ass->library) {
>          av_log(ctx, AV_LOG_ERROR, "Could not initialize libass.\n");
> @@ -297,7 +310,7 @@ AVFILTER_DEFINE_CLASS(subtitles);
>
>  static av_cold int init_subtitles(AVFilterContext *ctx)
>  {
> -    int j, ret, sid;
> +    int j, ret, sid, nskip;
>      int k = 0;
>      AVDictionary *codec_opts = NULL;
>      AVFormatContext *fmt = NULL;
> @@ -448,6 +461,7 @@ static av_cold int init_subtitles(AVFilterContext *ctx)
>      av_init_packet(&pkt);
>      pkt.data = NULL;
>      pkt.size = 0;
> +    nskip = 0;
>      while (av_read_frame(fmt, &pkt) >= 0) {
>          int i, got_subtitle;
>          AVSubtitle sub = {0};
> @@ -458,8 +472,17 @@ static av_cold int init_subtitles(AVFilterContext
> *ctx)
>                  av_log(ctx, AV_LOG_WARNING, "Error decoding: %s
> (ignored)\n",
>                         av_err2str(ret));
>              } else if (got_subtitle) {
> -                const int64_t start_time = av_rescale_q(sub.pts,
> AV_TIME_BASE_Q, av_make_q(1, 1000));
> +                const int64_t start_time = av_rescale_q(sub.pts,
> AV_TIME_BASE_Q, av_make_q(1, 1000)) + ass->shift;
>                  const int64_t duration   = sub.end_display_time;
> +
> +                if (start_time + duration < 0) {
> +                    nskip++;
> +                    goto pkt_end;
> +                } else if (nskip > 0) {
> +                    av_log(ctx, AV_LOG_INFO, "Skipped %d subtitles out of
> time range.\n", nskip);
> +                    nskip = 0;
> +                }
> +
>                  for (i = 0; i < sub.num_rects; i++) {
>                      char *ass_line = sub.rects[i]->ass;
>                      if (!ass_line)
> @@ -472,6 +495,8 @@ static av_cold int init_subtitles(AVFilterContext *ctx)
>                  }
>              }
>          }
> +
> +pkt_end:
>          av_packet_unref(&pkt);
>          avsubtitle_free(&sub);
>      }
> --
> 2.17.1
>
>