vo_opengl: Add :icc-approx-gamma suboption to approximate BT.709 gamma

This uses the value of 1.95 as an approximation for the exact gamma
curve, which replicates the behavior of popular video software including
anything in the Apple ecosystem, as per issue #534.

2 files changed, 17 insertions, 6 deletions

diff --git a/video/out/gl_lcms.c b/video/out/gl_lcms.c
index a6f30e0126..6fd86d9874 100644
--- a/video/out/gl_lcms.c
+++ b/video/out/gl_lcms.c
@@ -73,6 +73,7 @@ const struct m_sub_options mp_icc_conf = {
         OPT_STRING("icc-profile", profile, 0),
         OPT_STRING("icc-cache", cache, 0),
         OPT_INT("icc-intent", intent, 0),
+        OPT_FLAG("icc-approx-gamma", approx, 0),
         OPT_STRING_VALIDATE("3dlut-size", size_str, 0, validate_3dlut_size_opt),
         {0}
     },
@@ -164,13 +165,22 @@ struct lut3d *mp_load_icc(struct mp_icc_opts *opts, struct mp_log *log,
         .Blue  = {0.15, 0.06, 1.0},
     };
 
-    /* Rec BT.709 defines the tone curve as:
-       V = 1.099 * L^0.45 - 0.099 for L >= 0.018
-       V = 4.500 * L              for L <  0.018
-
-       The 0.081 parameter comes from inserting 0.018 into the function */
-    cmsToneCurve *tonecurve = cmsBuildParametricToneCurve(NULL, 4,
+    cmsToneCurve *tonecurve;
+    if (opts->approx) {
+        /* Apple's CMS, among other programs that rely on it, uses 1.95 as a
+           faster approximation of this curve. It's not quite correct, but the
+           option is provided for compatibility with such incorrect clips. */
+        tonecurve = cmsBuildGamma(NULL, 1.95);
+    } else {
+        /* Rec BT.709 defines the tone curve as:
+           V = 1.099 * L^0.45 - 0.099 for L >= 0.018
+           V = 4.500 * L              for L <  0.018
+
+           The 0.081 parameter comes from inserting 0.018 into the function */
+        tonecurve = cmsBuildParametricToneCurve(NULL, 4,
             (cmsFloat64Number[5]){1/0.45, 1/1.099, 0.099/1.099, 1/4.5, 0.081});
+    }
+
     cmsHPROFILE vid_profile = cmsCreateRGBProfile(&d65, &bt709prim,
                         (cmsToneCurve*[3]){tonecurve, tonecurve, tonecurve});
     cmsFreeToneCurve(tonecurve);
diff --git a/video/out/gl_lcms.h b/video/out/gl_lcms.h
index a579b78f43..4a8e0cfb6a 100644
--- a/video/out/gl_lcms.h
+++ b/video/out/gl_lcms.h
@@ -8,6 +8,7 @@ struct mp_icc_opts {
     char *cache;
     char *size_str;
     int intent;
+    int approx;
 };
 
 struct lut3d;