diff mbox series

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

Message ID 20200704194543.1738-1-mstamat@gmail.com
State Superseded, archived
Headers show
Series [FFmpeg-devel] avfilter/vf_subtitles: add shift option
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Commit Message

Manolis Stamatogiannakis July 4, 2020, 7:45 p.m. UTC
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@gmail.com>
---
 doc/filters.texi           |  8 ++++++++
 libavfilter/vf_subtitles.c | 23 +++++++++++++++++++++--
 2 files changed, 29 insertions(+), 2 deletions(-)
diff mbox series

Patch

diff --git a/doc/filters.texi b/doc/filters.texi
index ad2448acb2..c962ac55b0 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -17935,6 +17935,9 @@  The filter accepts the following options:
 @item filename, f
 Set the filename of the subtitle file to read. It must be specified.
 
+@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
@@ -17991,6 +17994,11 @@  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:
+@example
+subtitles=filename=sub.srt:shift='-20\:20'
+@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 1bd42391e0..125fbd9ac7 100644
--- a/libavfilter/vf_subtitles.c
+++ b/libavfilter/vf_subtitles.c
@@ -52,6 +52,7 @@  typedef struct AssContext {
     char *filename;
     char *fontsdir;
     char *charenc;
+    int64_t shift;
     char *force_style;
     int stream_index;
     int alpha;
@@ -103,6 +104,11 @@  static av_cold int init(AVFilterContext *ctx)
         return AVERROR(EINVAL);
     }
 
+    if (ass->shift != 0) {
+        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");
@@ -267,6 +273,7 @@  static const AVOption subtitles_options[] = {
     {"stream_index", "set stream index",             OFFSET(stream_index), AV_OPT_TYPE_INT,    { .i64 = -1 }, -1,       INT_MAX,  FLAGS},
     {"si",           "set stream index",             OFFSET(stream_index), AV_OPT_TYPE_INT,    { .i64 = -1 }, -1,       INT_MAX,  FLAGS},
     {"force_style",  "force subtitle style",         OFFSET(force_style),  AV_OPT_TYPE_STRING, {.str = NULL}, 0, 0, FLAGS},
+    {"shift",        "shift subtitles timing",       OFFSET(shift),        AV_OPT_TYPE_DURATION, {.i64 = 0},  INT64_MIN, INT64_MAX, FLAGS },
     {NULL},
 };
 
@@ -297,7 +304,7 @@  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 +455,7 @@  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 +466,17 @@  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 +489,8 @@  static av_cold int init_subtitles(AVFilterContext *ctx)
                 }
             }
         }
+
+pkt_end:
         av_packet_unref(&pkt);
         avsubtitle_free(&sub);
     }