options: add -sub-speed option

Should we actually get into trouble for unproper handling of
frame-based subtitle formats, this might be the simplest way to
work this around. Also is a bit more intuitive than -subfps, which
might use an unknown, misdetected, or non-sense video FPS.
Still pretty silly, though.

DOCS/man/en/options.rst

@@ -2062,6 +2062,8 @@
     *NOTE*: <rate> > movie fps speeds the subtitles up for frame-based
     subtitle files and slows them down for time-based ones.
 
+    Also see ``--sub-speed`` option.
+
 --sub-gauss=<0.0-3.0>
     Apply gaussian blur to image subtitles (default: 0). This can help making
     pixelated DVD/Vobsubs look nicer. A value other than 0 also switches to
@@ -2088,6 +2090,16 @@
     *NOTE*: this affects ASS subtitles as well, and may lead to incorrect
     subtitle rendering. Use with care, or use ``--sub-text-font-size`` instead.
 
+--sub-speed=<0.1-10.0>
+    Multiply the subtitle event timestamps with the given value. Can be used
+    to fix the playback speed for frame-based subtitle formats. Works for
+    external text subtitles only.
+
+    *EXAMPLE*:
+
+    - ``--sub-speed=25/23.976`` play frame based subtitles, which have been
+      loaded assuming a framerate of 23.976, at 25 FPS.
+
 --sws=<n>
     Specify the software scaler algorithm to be used with ``--vf=scale``. This
     also affects video output drivers which lack hardware acceleration,

core/options.c

@@ -492,6 +492,7 @@ const m_option_t mp_opts[] = {
     OPT_STRING("subcp", sub_cp, 0),
     OPT_FLOAT("sub-delay", sub_delay, 0),
     OPT_FLOAT("subfps", sub_fps, 0),
+    OPT_FLOAT("sub-speed", sub_speed, 0),
     OPT_FLAG("autosub", sub_auto, 0),
     OPT_FLAG("sub-visibility", sub_visibility, 0),
     OPT_FLAG("sub-forced-only", forced_subs_only, 0),
@@ -786,6 +787,7 @@ const struct MPOpts mp_default_opts = {
     .audio_display = 1,
     .sub_visibility = 1,
     .sub_pos = 100,
+    .sub_speed = 1.0,
     .extension_parsing = 1,
     .audio_output_channels = MP_CHMAP_INIT_STEREO,
     .audio_output_format = -1,  // AF_FORMAT_UNKNOWN

core/options.h

@@ -141,6 +141,7 @@ typedef struct MPOpts {
     int sub_pos;
     float sub_delay;
     float sub_fps;
+    float sub_speed;
     int forced_subs_only;
     char *quvi_format;
 

sub/dec_sub.c

@@ -401,12 +401,19 @@ bool sub_read_all_packets(struct dec_sub *sub, struct sh_sub *sh)
     if (sub->charset)
         mp_msg(MSGT_OSD, MSGL_INFO, "Using subtitle charset: %s\n", sub->charset);
 
+    double sub_speed = 1.0;
+
     // 23.976 FPS is used as default timebase for frame based formats
     if (sub->video_fps && sh->frame_based)
-        multiply_timings(subs, sub->video_fps / 23.976);
+        sub_speed *= sub->video_fps / 23.976;
 
     if (opts->sub_fps && sub->video_fps)
-        multiply_timings(subs, opts->sub_fps / sub->video_fps);
+        sub_speed *= opts->sub_fps / sub->video_fps;
+
+    sub_speed *= opts->sub_speed;
+
+    if (sub_speed != 1.0)
+        multiply_timings(subs, sub_speed);
 
     if (!opts->suboverlap_enabled)
         fix_overlaps_and_gaps(subs);