options: add --sub-ass-colorspace option

Option to control what colorspace subtitle should be converted to before
drawing them. Upstream ASS specification says that all subtitles should
be rendered with color primaries and transfer matching their associated
video.

This has certain limitations in the case of HDR/WCG content. See the
discussion in libass/libass#297.

To mitigate current situation add an option for users to control the
render target of subtitles.

This could be also accomplished with --blend-subtitles to some degree,
but I don't want to mux options that are not really clear to affect this
certain case.

See-Also: https://github.com/libass/libass/blob/649a7c2e1fc6f4188ea1a89968560715800b883d/libass/ass_types.h#L233-L237
See-Also: libass/libass#297
See-Also: mpv-player#13381
Fixes: mpv-player#13673

DOCS/interface-changes.rst

@@ -63,6 +63,7 @@ Interface changes
     - add `--border-background` option
     - add `video-target-params` property
     - add `hdr10plus` sub-parameter to `format` video filter
+    - add `--sub-ass-colorspace` option
  --- mpv 0.37.0 ---
     - `--save-position-on-quit` and its associated commands now store state files
       in %LOCALAPPDATA% instead of %APPDATA% directory by default on Windows.

DOCS/man/options.rst

@@ -2528,6 +2528,21 @@ Subtitles
 
     Default: yes.
 
+``--sub-ass-colorspace=<auto|video|sdr>``
+    Specify the RGB primaries and transfer function to use when rendering
+    ASS subtitles.
+
+    :auto:  Follow the ASS specification. This is currently equivalent to
+            ``video``. This behavior may change in the future. (default)
+    :video: Assume that ASS subtitles are in the video's colorspace. Match the
+            color primaries and transfer function of the ASS subtitles to those
+            of the associated video.
+    :sdr:   Assume that ASS subtitles are SDR. Match them to the video's
+            colorspace in SDR mode. If the video is HDR/WCG, fall back to sRGB.
+
+    This option affects ``--vo=gpu-next``, ``--vo=gpu`` always assume sRGB
+    colorspace for subtitles.
+
 ``--sub-ass-vsfilter-aspect-compat=<yes|no>``
     Stretch SSA/ASS subtitles when playing anamorphic videos for compatibility
     with traditional VSFilter behavior. This switch has no effect when the

options/options.c

@@ -325,6 +325,8 @@ const struct m_sub_options mp_subtitle_sub_opts = {
         {"sub-past-video-end", OPT_BOOL(sub_past_video_end)},
         {"sub-ass-force-style", OPT_REPLACED("sub-ass-style-overrides")},
         {"sub-lavc-o", OPT_KEYVALUELIST(sub_avopts)},
+        {"sub-ass-colorspace", OPT_CHOICE(ass_colorspace,
+            {"auto", 0}, {"video", 1}, {"sdr", 2})},
         {0}
     },
     .size = sizeof(OPT_BASE_STRUCT),

options/options.h

@@ -113,6 +113,7 @@ struct mp_subtitle_opts {
     bool sub_clear_on_seek;
     int teletext_page;
     bool sub_past_video_end;
+    int ass_colorspace;
     char **sub_avopts;
 };
 

video/out/vo_gpu_next.c

@@ -141,6 +141,8 @@ struct priv {
     struct m_config_cache *opts_cache;
     struct m_config_cache *next_opts_cache;
     struct gl_next_opts *next_opts;
+    struct m_config_cache *subtitle_opts_cache;
+    struct mp_subtitle_opts *subtitle_opts;
     struct cache shader_cache, icc_cache;
     struct mp_csp_equalizer_state *video_eq;
     struct scaler_params scalers[SCALER_COUNT];
@@ -378,8 +380,15 @@ static void update_overlays(struct vo *vo, struct mp_osd_res res,
                 }
             }
             break;
-        case SUBBITMAP_LIBASS:
-            if (src && item->video_color_space)
+        case SUBBITMAP_LIBASS:;
+            bool video_color_space = item->video_color_space;
+            if (p->subtitle_opts->ass_colorspace == 2 &&
+                (pl_color_space_is_hdr(&src->params.color) ||
+                 pl_color_primaries_is_wide_gamut(src->params.color.primaries)))
+            {
+                video_color_space = false;
+            }
+            if (src && video_color_space)
                 ol->color = src->params.color;
             ol->mode = PL_OVERLAY_MONOCHROME;
             ol->repr.alpha = PL_ALPHA_INDEPENDENT;
@@ -768,6 +777,8 @@ static void update_options(struct vo *vo)
     if (changed)
         update_render_options(vo);
 
+    m_config_cache_update(p->subtitle_opts_cache);
+
     update_lut(p, &p->next_opts->lut);
     pars->params.lut = p->next_opts->lut.lut;
     pars->params.lut_type = p->next_opts->lut.type;
@@ -1820,6 +1831,8 @@ static int preinit(struct vo *vo)
     p->opts_cache = m_config_cache_alloc(p, vo->global, &gl_video_conf);
     p->next_opts_cache = m_config_cache_alloc(p, vo->global, &gl_next_conf);
     p->next_opts = p->next_opts_cache->opts;
+    p->subtitle_opts_cache = m_config_cache_alloc(p, vo->global, &mp_subtitle_sub_opts);
+    p->subtitle_opts = p->subtitle_opts_cache->opts;
     p->video_eq = mp_csp_equalizer_create(p, vo->global);
     p->global = vo->global;
     p->log = vo->log;