sub: add option to not scale subtitles with window

--sub-scale-by-window=no attempts to keep subs always at the same pixel
size.

The implementation is a bit all over the place, because it compensates
already done scaling by an inverse scale factor, but it will probably do
its job.

Fixes #1424. (The semantics and name of --sub-scale-with-window are
kept, and this adds a new option - the name is confusingly similar, but
it's actually analogue to --osd-scale-by-window.)

DOCS/man/options.rst

@@ -1123,12 +1123,23 @@ Subtitles
         This affects ASS subtitles as well, and may lead to incorrect subtitle
         rendering. Use with care, or use ``--sub-text-font-size`` instead.
 
+``--sub-scale-by-window=<yes|no>``
+    Whether to scale subtitles with the window size (default: yes). If this is
+    disabled, changing the window size won't change the subtitle font size.
+
+    Like ``--sub-scale``, this can break ASS subtitles.
+
 ``--sub-scale-with-window=<yes|no>``
     Make the subtitle font size relative to the window, instead of the video.
     This is useful if you always want the same font size, even if the video
     doesn't covert the window fully, e.g. because screen aspect and window
     aspect mismatch (and the player adds black bars).
 
+    This option is misnamed. The difference to the confusingly similar sounding
+    option ``--sub-scale-by-window`` is that ``--sub-scale-with-window`` still
+    scales with the approximate window size, while the other option disables
+    this scaling.
+
     Like ``--sub-scale``, this can break ASS subtitles.
 
 ``--embeddedfonts``, ``--no-embeddedfonts``

options/options.c

@@ -347,6 +347,7 @@ const m_option_t mp_opts[] = {
                ({"simple", 0}, {"complex", 1})),
     OPT_CHOICE("ass-style-override", ass_style_override, 0,
                ({"no", 0}, {"yes", 1}, {"force", 3}, {"signfs", 4})),
+    OPT_FLAG("sub-scale-by-window", sub_scale_by_window, 0),
     OPT_FLAG("sub-scale-with-window", sub_scale_with_window, 0),
     OPT_FLAG("osd-bar", osd_bar_visible, 0),
     OPT_FLOATRANGE("osd-bar-align-x", osd_bar_align_x, 0, -1.0, +1.0),
@@ -699,6 +700,7 @@ const struct MPOpts mp_default_opts = {
     .osd_bar_h = 3.125,
     .osd_scale = 1,
     .osd_scale_by_window = 1,
+    .sub_scale_by_window = 1,
     .use_text_osd = 1,
 #if HAVE_LUA
     .lua_load_osc = 1,

options/options.h

@@ -235,6 +235,8 @@ typedef struct MPOpts {
     float osd_bar_h;
     float osd_scale;
     int osd_scale_by_window;
+    int sub_scale_by_window;
+    int sub_scale_with_window;
     struct osd_style_opts *osd_style;
     struct osd_style_opts *sub_text_style;
     float sub_scale;
@@ -252,7 +254,6 @@ typedef struct MPOpts {
     int ass_style_override;
     int ass_hinting;
     int ass_shaper;
-    int sub_scale_with_window;
     int sub_clear_on_seek;
 
     int hwdec_api;

sub/ass_mp.c

@@ -146,6 +146,11 @@ void mp_ass_configure(ASS_Renderer *priv, struct MPOpts *opts,
             int vidh = dim->h - (dim->mt + dim->mb);
             set_font_scale *= dim->h / (float)MPMAX(vidh, 1);
         }
+        if (!opts->sub_scale_by_window) {
+            double factor = dim->h / 720.0;
+            if (factor != 0.0)
+                set_font_scale /= factor;
+        }
     }
 
     ass_set_use_margins(priv, set_use_margins);