sub: do some timing postprocessing on preloaded subs

This fixes the -subfps option (which unfortunately is still useful),
and fixes minor annoying timing errors (which unfortunately still
happen).

Note that none of these affect ASS or image subtitles. ASS is specially
handled: libass loads subtitles as ASS_Track. There are no actual
packets passed around, and sd_ass just uses the ASS_Track.

Disable the --sub-no-text-pp option. It's misleading now and always was
completely useless.

DOCS/man/en/options.rst

@@ -1447,11 +1447,6 @@
     ``show_progress`` command (by default mapped to ``P``), or in some
     non-default cases when seeking. Expands properties. See property_expansion_.
 
---overlapsub
-    Allows the next subtitle to be displayed while the current one is still
-    visible (default is to enable the support only for specific formats). This
-    only matters for subtitles loaded with ``-sub``.
-
 --panscan=<0.0-1.0>
     Enables pan-and-scan functionality (cropping the sides of e.g. a 16:9
     movie to make it fit a 4:3 display without black bands). The range
@@ -1996,15 +1991,17 @@
     Use/display these subtitle files. Only one file can be displayed at the
     same time.
 
+--sub-fix-timing, --no-sub-fix-timing
+    By default, external text subtitles are preprocessed to remove minor gaps
+    or overlaps between subtitles (if the difference is smaller than 200 ms,
+    the gap or overlap is removed). This does not affect image subtitles,
+    subtitles muxed with audio/video, or subtitles in the ASS format.
+
 --sub-demuxer=<[+]name>
     Force subtitle demuxer type for ``--subfile``. Using a '+' before the name
     will force it, this will skip some checks! Give the demuxer name as
     printed by ``--sub-demuxer=help``.
 
---sub-no-text-pp
-    Disables any kind of text post processing done after loading the
-    subtitles. Used for debug purposes.
-
 --sub-paths=<path1:path2:...>
     Specify extra directories where to search for subtitles matching the
     video. Multiple directories can be separated by ":" (";" on Windows).

core/options.c

@@ -496,9 +496,7 @@ const m_option_t mp_opts[] = {
     OPT_FLAG("autosub", sub_auto, 0),
     OPT_FLAG("sub-visibility", sub_visibility, 0),
     OPT_FLAG("sub-forced-only", forced_subs_only, 0),
-    // enable Closed Captioning display
-    OPT_FLAG_CONSTANTS("overlapsub", suboverlap_enabled, 0, 0, 2),
-    OPT_FLAG_STORE("sub-no-text-pp", sub_no_text_pp, 0, 1),
+    OPT_FLAG_CONSTANTS("sub-fix-timing", suboverlap_enabled, 0, 1, 0),
     OPT_CHOICE("autosub-match", sub_match_fuzziness, 0,
                ({"exact", 0}, {"fuzzy", 1}, {"all", 2})),
     OPT_INTRANGE("sub-pos", sub_pos, 0, 0, 100),
@@ -804,7 +802,7 @@ const struct MPOpts mp_default_opts = {
     .ass_vsfilter_aspect_compat = 1,
     .ass_style_override = 1,
     .use_embedded_fonts = 1,
-    .suboverlap_enabled = 1,
+    .suboverlap_enabled = 0,
 
     .hwdec_codecs = "all",
 

sub/dec_sub.c

@@ -261,6 +261,43 @@ void sub_init_from_sh(struct dec_sub *sub, struct sh_sub *sh)
            sh->gsh->codec ? sh->gsh->codec : "<unknown>");
 }
 
+static void multiply_timings(struct packet_list *subs, double factor)
+{
+    for (int n = 0; n < subs->num_packets; n++) {
+        struct demux_packet *pkt = subs->packets[n];
+        if (pkt->pts != MP_NOPTS_VALUE)
+            pkt->pts *= factor;
+        if (pkt->duration > 0)
+            pkt->duration *= factor;
+    }
+}
+
+// Remove overlaps and fill gaps between adjacent subtitle packets. This is done
+// by adjusting the duration of the earlier packet. If the gaps or overlap are
+// larger than the threshold, or if the durations are close to the threshold,
+// don't change the events.
+// The algorithm is maximally naive and doesn't work if there are multiple
+// overlapping lines. (It's not worth the trouble.)
+static void fix_overlaps_and_gaps(struct packet_list *subs)
+{
+    double threshold = 0.2;     // up to 200 ms overlaps or gaps are removed
+    double keep = threshold * 2;// don't change timings if durations are smaller
+    for (int i = 0; i < subs->num_packets - 1; i++) {
+        struct demux_packet *cur = subs->packets[i];
+        struct demux_packet *next = subs->packets[i + 1];
+        if (cur->pts != MP_NOPTS_VALUE && cur->duration > 0 &&
+            next->pts != MP_NOPTS_VALUE && next->duration > 0)
+        {
+            double end = cur->pts + cur->duration;
+            if (fabs(next->pts - end) <= threshold && cur->duration >= keep &&
+                next->duration >= keep)
+            {
+                cur->duration = next->pts - cur->pts;
+            }
+        }
+    }
+}
+
 static void add_sub_list(struct dec_sub *sub, struct packet_list *subs)
 {
     struct sd *sd = sub_get_last_sd(sub);
@@ -285,6 +322,8 @@ static void add_sub_list(struct dec_sub *sub, struct packet_list *subs)
 // there are circumstances which makes this not possible.
 bool sub_read_all_packets(struct dec_sub *sub, struct sh_sub *sh)
 {
+    struct MPOpts *opts = sub->opts;
+
     if (!sub_accept_packets_in_advance(sub) || sh->track)
         return false;
 
@@ -301,6 +340,12 @@ bool sub_read_all_packets(struct dec_sub *sub, struct sh_sub *sh)
         MP_TARRAY_APPEND(tmp, subs.packets, subs.num_packets, pkt);
     }
 
+    if (opts->sub_fps && sub->video_fps)
+        multiply_timings(&subs, opts->sub_fps / sub->video_fps);
+
+    if (!opts->suboverlap_enabled)
+        fix_overlaps_and_gaps(&subs);
+
     add_sub_list(sub, &subs);
 
     talloc_free(tmp);