player: redo --subs-fallback-forced

In the never ending quest of trying to satisfy every possible user
request for subtitle autoselection, I ended up redoing how
--subs-fallback-forced works. The old behavior had it as strictly a
fallback-type option when there were no lang matches, but now we can
make it an active part of compare_track and it works along with slang to
select the desired track. Since it's a three state option, the no option
still works to avoid selecting any forced subtitle tracks. The meaning
of always slightly changes to mean "only select forced subtitle tracks"
and yes remains essentially the same (no special priority given besides
the audio matching subtitle language case).

DOCS/interface-changes.rst

@@ -57,6 +57,7 @@ Interface changes
     - add `video-params/crop-[w,h,x,y]`
     - remove `--tone-mapping-mode`
     - add `auto` to `--sub-fix-timing` and make it the default
+    - change `--subs-fallback-forced` so that it works alongside `--slang`
  --- mpv 0.36.0 ---
     - add `--target-contrast`
     - Target luminance value is now also applied when ICC profile is used.

DOCS/man/options.rst

@@ -153,10 +153,11 @@ Track Selection
     Setting this to `default` means that only streams flagged as `default` will be selected.
 
 ``--subs-fallback-forced=<yes|no|always>``
-    When autoselecting a subtitle track, if no tracks match your preferred languages,
-    select a forced track that matches the language of the selected audio track (default: yes).
-    `always` will always select a forced track if possible, regardles if the language matches the
-    selected audio track or not.
+    When autoselecting a subtitle track, the default value of `yes` will prefer using a forced
+    subtitle track if the subtitle language matches the audio language and matches your list of
+    preferred languages. The special value `always` will only select forced subtitle tracks and
+    never fallback on a non-forced track. Conversely, `no` will never select a forced subtitle
+    track.
 
 
 Playback Control

player/loadfile.c

@@ -505,13 +505,16 @@ static bool compare_track(struct track *t1, struct track *t2, char **langs,
             (t2->program_id == preferred_program))
             return t1->program_id == preferred_program;
     }
+    int forced = t1->type == STREAM_SUB ? opts->subs_fallback_forced : 1;
+    bool force_match = forced == 1 || (t1->forced_track && forced == 2) ||
+                       (!t1->forced_track && !forced);
     int l1 = match_lang(langs, t1->lang), l2 = match_lang(langs, t2->lang);
     if (!os_langs && l1 != l2)
-        return l1 > l2;
+        return l1 > l2 && force_match;
     if (t1->default_track != t2->default_track)
-        return t1->default_track;
+        return t1->default_track && force_match;
     if (os_langs && l1 != l2)
-        return l1 > l2;
+        return l1 > l2 && force_match;
     if (t1->attached_picture != t2->attached_picture)
         return !t1->attached_picture;
     if (t1->stream && t2->stream && opts->hls_bitrate >= 0 &&
@@ -648,10 +651,11 @@ struct track *select_default_track(struct MPContext *mpctx, int order,
         // Autoselecting forced sub tracks requires the following:
         // 1. Matches the audio language or --subs-fallback-forced=always.
         // 2. Matches the users list of preferred languages or none were specified (i.e. slang was not set).
-        // 3. A track *wasn't* already selected by slang previously.
-        if (fallback_forced && track->forced_track &&
-            (os_langs || (match_lang(langs, track->lang) && !match_lang(langs, pick->lang))) &&
-            (mp_match_lang_single(audio_lang, track->lang) || opts->subs_fallback_forced == 2) &&
+        // 3. A track *wasn't* already selected by slang previously or the track->lang matches pick->lang and isn't forced.
+        bool valid_forced_slang = (os_langs || (mp_match_lang_single(pick->lang, track->lang) && !pick->forced_track) ||
+                                   (match_lang(langs, track->lang) && !match_lang(langs, pick->lang)));
+        bool audio_lang_match = mp_match_lang_single(audio_lang, track->lang);
+        if (fallback_forced && track->forced_track && valid_forced_slang && audio_lang_match &&
             (!forced_pick || compare_track(track, forced_pick, langs, os_langs, mpctx->opts, preferred_program)))
         {
             forced_pick = track;
@@ -671,8 +675,14 @@ struct track *select_default_track(struct MPContext *mpctx, int order,
     if (pick && !forced_pick && sub && (!match_lang(langs, pick->lang) || os_langs) && !sub_fallback)
         pick = NULL;
     // Handle this after matching langs and selecting a fallback.
-    if (pick && !forced_pick && sub && (!opts->subs_with_matching_audio && audio_matches))
+    if (pick && sub && (!opts->subs_with_matching_audio && audio_matches))
         pick = NULL;
+    // Handle edge cases if we picked a track that doesn't match the --subs-fallback-force value
+    if (pick && sub && ((!pick->forced_track && opts->subs_fallback_forced == 2) ||
+        (pick->forced_track && !opts->subs_fallback_forced)))
+    {
+        pick = NULL;
+    }
 
     if (pick && pick->attached_picture && !mpctx->opts->audio_display)
         pick = NULL;