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

1 files changed, 26 insertions, 0 deletions

diff --git a/video/out/gl_video.c b/video/out/gl_video.c
index f1ea03ed80..7420d9d4d1 100644
--- a/video/out/gl_video.c
+++ b/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;