vo_opengl: add a gamut warning feature

This clearly highlights all out-of-gamut/clipped pixels. (Either too
bright or too saturated)

Has some (documented) caveats. Also make TONE_MAPPING_CLIP stop actually
clamping the value range (it's unnecessary and breaks this feature).

DOCS/man/options.rst

@@ -4865,6 +4865,18 @@ The following video options are currently all specific to ``--vo=opengl`` and
     The default of 2.0 is somewhat conservative and will mostly just apply to
     skies or directly sunlit surfaces. A setting of 0.0 disables this option.
 
+``--gamut-warning``
+    If enabled, mpv will mark all clipped/out-of-gamut pixels that exceed a
+    given threshold (currently hard-coded to 101%). The affected pixels will be
+    inverted to make them stand out. Note: This option applies after the
+    effects of all of mpv's color space transformation / tone mapping options,
+    so it's a good idea to combine this with ``--tone-mapping=clip`` and use
+    ``--target-gamut`` to set the gamut to simulate. For example,
+    ``--target-gamut=bt.709`` would make mpv highlight all pixels that exceed the
+    gamut of a standard gamut (sRGB) display. This option also does not work
+    well with ICC profiles, since the 3DLUTs are always generated against the
+    source color space and have chromatically-accurate clipping built in.
+
 ``--use-embedded-icc-profile``
     Load the embedded ICC profile contained in media files such as PNG images.
     (Default: yes). Note that this option only works when also using a display

video/out/opengl/video.c

@@ -363,6 +363,7 @@ const struct m_sub_options gl_video_conf = {
         OPT_FLAG("hdr-compute-peak", compute_hdr_peak, 0),
         OPT_FLOAT("tone-mapping-param", tone_mapping_param, 0),
         OPT_FLOAT("tone-mapping-desaturate", tone_mapping_desat, 0),
+        OPT_FLAG("gamut-warning", gamut_warning, 0),
         OPT_FLAG("opengl-pbo", pbo, 0),
         SCALER_OPTS("scale",  SCALER_SCALE),
         SCALER_OPTS("dscale", SCALER_DSCALE),
@@ -2460,7 +2461,7 @@ static void pass_colormanage(struct gl_video *p, struct mp_colorspace src, bool
     // Adapt from src to dst as necessary
     pass_color_map(p->sc, src, dst, p->opts.tone_mapping,
                    p->opts.tone_mapping_param, p->opts.tone_mapping_desat,
-                   detect_peak, p->use_linear && !osd);
+                   detect_peak, p->opts.gamut_warning, p->use_linear && !osd);
 
     if (p->use_lut_3d) {
         gl_sc_uniform_texture(p->sc, "lut_3d", p->lut_3d_texture);

video/out/opengl/video.h

@@ -115,6 +115,7 @@ struct gl_video_opts {
     int compute_hdr_peak;
     float tone_mapping_param;
     float tone_mapping_desat;
+    int gamut_warning;
     int linear_scaling;
     int correct_downscaling;
     int sigmoid_upscaling;

video/out/opengl/video_shaders.c

@@ -616,7 +616,7 @@ static void pass_tone_map(struct gl_shader_cache *sc, float ref_peak,
 
     switch (algo) {
     case TONE_MAPPING_CLIP:
-        GLSLF("sig = clamp(%f * sig, 0.0, 1.0);\n", isnan(param) ? 1.0 : param);
+        GLSLF("sig = %f * sig;\n", isnan(param) ? 1.0 : param);
         break;
 
     case TONE_MAPPING_MOBIUS:
@@ -683,7 +683,7 @@ void pass_color_map(struct gl_shader_cache *sc,
                     struct mp_colorspace src, struct mp_colorspace dst,
                     enum tone_mapping algo, float tone_mapping_param,
                     float tone_mapping_desat, bool detect_peak,
-                    bool is_linear)
+                    bool gamut_warning, bool is_linear)
 {
     GLSLF("// color mapping\n");
 
@@ -748,6 +748,12 @@ void pass_color_map(struct gl_shader_cache *sc,
     if (src.light != dst.light)
         pass_inverse_ootf(sc, dst.light, mp_trc_nom_peak(dst.gamma));
 
+    // Warn for remaining out-of-gamut colors is enabled
+    if (gamut_warning) {
+        GLSL(if (any(greaterThan(color.rgb, vec3(1.01)))))
+            GLSL(color.rgb = vec3(1.0) - color.rgb;) // invert
+    }
+
     if (is_linear)
         pass_delinearize(sc, dst.gamma);
 }

video/out/opengl/video_shaders.h

@@ -46,7 +46,7 @@ void pass_color_map(struct gl_shader_cache *sc,
                     struct mp_colorspace src, struct mp_colorspace dst,
                     enum tone_mapping algo, float tone_mapping_param,
                     float tone_mapping_desat, bool use_detected_peak,
-                    bool is_linear);
+                    bool gamut_warning, bool is_linear);
 
 void pass_sample_deband(struct gl_shader_cache *sc, struct deband_opts *opts,
                         AVLFG *lfg, enum mp_csp_trc trc);