vo_opengl: add gamma-auto option

This automatically sets the gamma option depending on lighting conditions
measured from the computer's ambient light sensor.

sRGB – arguably the “sibling” to BT.709 for still images – has a reference
viewing environment defined in its specification (IEC 61966-2-1:1999, see
http://www.color.org/chardata/rgb/srgb.xalter). According to this data, the
assumed ambient illuminance is 64 lux. This is the illuminance where the gamma
that results from ICC color management is correct.

On the other hand, BT.1886 formalizes that the gamma level for dim environments
to be 2.40, and Apple resources (WWDC12: 2012 Session 523: Best practices for
color management) define the BT.1886 dim at 16 lux.

So the logic we apply is:

  * >= 64lux -> 1.961 gamma
  * =< 16lux -> 2.400 gamma
  * 16lux < x < 64lux -> logaritmic rescale of lux to gamma. The human
    perception of illuminance roughly follows a logaritmic scale of lux [1].

[1]: https://msdn.microsoft.com/en-us/library/windows/desktop/dd319008%28v=vs.85%29.aspx

DOCS/man/vo.rst

@@ -543,6 +543,15 @@ Available video output drivers are:
         0.8
             Pitch black room
 
+    ``gamma-auto``
+        Automatically corrects the gamma value depending on ambient lighting
+        conditions (adding a gamma boost for dark rooms).
+
+        With ambient illuminance of 64lux, mpv will pick the 1.0 gamma value
+        (no boost), and slightly increase the boost up until 0.8 for 16lux.
+
+        NOTE: Only implemented on OS X.
+
     ``icc-profile=<file>``
         Load an ICC profile and use it to transform linear RGB to screen output.
         Needs LittleCMS 2 support compiled in. This option overrides the ``srgb``

test/gl_video.c

@@ -0,0 +1,42 @@
+#include "test_helpers.h"
+#include "video/out/gl_video.h"
+
+static void test_scale_ambient_lux_limits(void **state) {
+    float x;
+    x = gl_video_scale_ambient_lux(16.0, 64.0, 2.40, 1.961, 16.0);
+    assert_double_equal(x, 2.40f);
+
+    x = gl_video_scale_ambient_lux(16.0, 64.0, 2.40, 1.961, 64.0);
+    assert_double_equal(x, 1.961f);
+}
+
+static void test_scale_ambient_lux_sign(void **state) {
+    float x;
+    x = gl_video_scale_ambient_lux(16.0, 64.0, 1.961, 2.40, 64.0);
+    assert_double_equal(x, 2.40f);
+}
+
+static void test_scale_ambient_lux_clamping(void **state) {
+    float x;
+    x = gl_video_scale_ambient_lux(16.0, 64.0, 2.40, 1.961, 0.0);
+    assert_double_equal(x, 2.40f);
+}
+
+static void test_scale_ambient_lux_log10_midpoint(void **state) {
+    float x;
+    // 32 corresponds to the the midpoint after converting lux to the log10 scale
+    x = gl_video_scale_ambient_lux(16.0, 64.0, 2.40, 1.961, 32.0);
+    float mid_gamma = (2.40 - 1.961) / 2 + 1.961;
+    assert_double_equal(x, mid_gamma);
+}
+
+int main(void) {
+    const UnitTest tests[] = {
+        unit_test(test_scale_ambient_lux_limits),
+        unit_test(test_scale_ambient_lux_sign),
+        unit_test(test_scale_ambient_lux_clamping),
+        unit_test(test_scale_ambient_lux_log10_midpoint),
+    };
+    return run_tests(tests);
+}
+

test/test_helpers.h

@@ -7,5 +7,9 @@
 #include <cmocka.h>
 
 #include <stdio.h>
+#include <math.h>
+#include <float.h>
+
+#define assert_double_equal(a, b) assert_true(fabs(a - b) <= DBL_EPSILON)
 
 #endif

video/out/gl_video.c

@@ -355,6 +355,7 @@ static int validate_scaler_opt(struct mp_log *log, const m_option_t *opt,
 const struct m_sub_options gl_video_conf = {
     .opts = (const m_option_t[]) {
         OPT_FLOATRANGE("gamma", gamma, 0, 0.1, 2.0),
+        OPT_FLAG("gamma-auto", gamma_auto, 0),
         OPT_FLAG("srgb", srgb, 0),
         OPT_FLAG("npot", npot, 0),
         OPT_FLAG("pbo", pbo, 0),
@@ -2564,6 +2565,31 @@ void gl_video_resize_redraw(struct gl_video *p, int w, int h)
     gl_video_render_frame(p, 0, NULL);
 }
 
+float gl_video_scale_ambient_lux(float lmin, float lmax,
+                                 float rmin, float rmax, float lux)
+{
+    assert(lmax > lmin);
+
+    float num = (rmax - rmin) * (log10(lux) - log10(lmin));
+    float den = log10(lmax) - log10(lmin);
+    float result = num / den + rmin;
+
+    // clamp the result
+    float max = MPMAX(rmax, rmin);
+    float min = MPMIN(rmax, rmin);
+    return MPMAX(MPMIN(result, max), min);
+}
+
+void gl_video_set_ambient_lux(struct gl_video *p, int lux)
+{
+    if (p->opts.gamma_auto) {
+        float gamma = gl_video_scale_ambient_lux(16.0, 64.0, 2.40, 1.961, lux);
+        MP_INFO(p, "ambient light changed: %dlux (gamma: %f)\n", lux, gamma);
+        p->opts.gamma = MPMIN(1.0, 1.961 / gamma);
+        gl_video_eq_update(p);
+    }
+}
+
 void gl_video_set_hwdec(struct gl_video *p, struct gl_hwdec *hwdec)
 {
     p->hwdec = hwdec;

video/out/gl_video.h

@@ -35,6 +35,7 @@ struct gl_video_opts {
     float scaler_radius[2];
     float scaler_antiring[2];
     float gamma;
+    int gamma_auto;
     int srgb;
     int linear_scaling;
     int fancy_downscaling;
@@ -83,6 +84,10 @@ void gl_video_eq_update(struct gl_video *p);
 void gl_video_set_debug(struct gl_video *p, bool enable);
 void gl_video_resize_redraw(struct gl_video *p, int w, int h);
 
+float gl_video_scale_ambient_lux(float lmin, float lmax,
+                                 float rmin, float rmax, float lux);
+void gl_video_set_ambient_lux(struct gl_video *p, int lux);
+
 void gl_video_set_gl_state(struct gl_video *p);
 void gl_video_unset_gl_state(struct gl_video *p);
 void gl_video_reset(struct gl_video *p);

video/out/vo_opengl.c

@@ -276,6 +276,19 @@ static bool get_and_update_icc_profile(struct gl_priv *p, int *events)
     return true;
 }
 
+static void get_and_update_ambient_lighting(struct gl_priv *p, int *events)
+{
+    int lux;
+    int r = p->glctx->vo_control(p->vo, events, VOCTRL_GET_AMBIENT_LUX, &lux);
+    if (r == VO_TRUE) {
+        gl_video_set_ambient_lux(p->renderer, lux);
+    }
+    if (r != VO_TRUE && p->renderer_opts->gamma_auto) {
+        MP_ERR(p, "gamma_auto option provided, but querying for ambient"
+                  " lighting is not supported on this platform\n");
+    }
+}
+
 static bool reparse_cmdline(struct gl_priv *p, char *args)
 {
     struct m_config *cfg = NULL;
@@ -390,6 +403,10 @@ static int control(struct vo *vo, uint32_t request, void *data)
         get_and_update_icc_profile(p, &events);
         vo->want_redraw = true;
     }
+    if (events & VO_EVENT_AMBIENT_LIGHTING_CHANGED) {
+        get_and_update_ambient_lighting(p, &events);
+        vo->want_redraw = true;
+    }
     if (events & VO_EVENT_RESIZE)
         resize(p);
     if (events & VO_EVENT_EXPOSE)