Support ARIB STD B67, aka hybrid-log gamma (HLG) transfer function.

ui/gfx/color_transform.cc

 // Copyright (c) 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/color_transform.h"
 
 #include <vector>
 
 #include "base/logging.h"
 #include "ui/gfx/color_space.h"
@@ skipping 269 matching lines @@
       float c1 = 3424.0f / 4096.0f;
       float c2 = (2413.0f / 4096.0f) * 32.0f;
       float c3 = (2392.0f / 4096.0f) * 32.0f;
       return powf((c1 + c2 * powf(v, m1)) / (1.0f + c3 * powf(v, m1)), m2);
     }
 
     case ColorSpace::TransferID::SMPTEST428_1:
       v = fmax(0.0f, v);
       return powf(48.0f * v + 52.37f, 1.0f / 2.6f);
 
+    // Spec: http://www.arib.or.jp/english/html/overview/doc/2-STD-B67v1_0.pdf
+    case ColorSpace::TransferID::ARIB_STD_B67: {
+      float a = 0.17883277f;
+      float b = 0.28466892f;
+      float c = 0.55991073f;
+      float Lmax = 12.0f;
+      v = Lmax * fmax(0.0f, v);
+      if (v <= 1)
+        return 0.5f * sqrtf(v);
+      else
+        return a * log(v - b) + c;
+    }
+
     // Chrome-specific values below
     case ColorSpace::TransferID::GAMMA24:
       v = fmax(0.0f, v);
       return powf(v, 1.0f / 2.4f);
   }
 }
 
 GFX_EXPORT float ToLinear(ColorSpace::TransferID id, float v) {
   switch (id) {
     default:
@@ skipping 95 matching lines @@
       return (powf(v, 2.6f) - 52.37f) / 48.0f;
 
     // Chrome-specific values below
     case ColorSpace::TransferID::GAMMA24:
       v = fmax(0.0f, v);
       return powf(v, 2.4f);
 
     case ColorSpace::TransferID::SMPTEST2084_NON_HDR:
       v = fmax(0.0f, v);
       return fmin(2.3f * pow(v, 2.8f), v / 5.0f + 0.8f);
+
+    // Spec: http://www.arib.or.jp/english/html/overview/doc/2-STD-B67v1_0.pdf
+    case ColorSpace::TransferID::ARIB_STD_B67: {
+      v = fmax(0.0f, v);
+      float a = 0.17883277f;
+      float b = 0.28466892f;
+      float c = 0.55991073f;
+      float Lmax = 12.0f;
+      float v_ = 0.0f;
+      if (v <= 0.5f) {
+        v_ = (v * 2.0f) * (v * 2.0f);
+      } else {
+        v_ = exp(v - c) / a + b;

[hubbe, 2016/09/15 20:58:06]

Hmm, shouldn't this be exp((v - c) / a) + b ?

[servolk, 2016/09/15 21:12:50]

Ah, right, that was it, gfx_unittests pass now. Thanks!

+      }
+      return v_ / Lmax;
+    }
   }
 }
 
 namespace {
 // Assumes bt2020
 float Luma(const ColorTransform::TriStim& c) {
   return c.x() * 0.2627f + c.y() * 0.6780f + c.z() * 0.0593f;
 }
 };
 
@@ skipping 284 matching lines @@
           std::unique_ptr<ColorTransform>(
               new QCMSColorTransform(GetXYZD50Profile(), to_profile))));
     } else {
       return std::unique_ptr<ColorTransform>(
           new ColorSpaceToColorSpaceTransform(from, to, intent));
     }
   }
 }
 
 }  // namespace gfx