First cut at making HDR content viewable on SDR display with color management.

Without this, HDR10 content is almost entirely black when color management is
turned on.

Review-Url: https://codereview.chromium.org/2300813002
Cr-Commit-Position: refs/heads/master@{#416374}

ui/gfx/color_space.h

@@ -74,8 +74,13 @@ class GFX_EXPORT ColorSpace {
     // Chrome-specific values start at 1000.
     GAMMA24 = 1000,
 
+    // This is an ad-hoc transfer function that decodes SMPTE 2084 content
+    // into a 0-1 range more or less suitable for viewing on a non-hdr
+    // display.
+    SMPTEST2084_NON_HDR,
+
     // TODO(hubbe): Need to store an approximation of the gamma function(s).
-    CUSTOM = 1001,
+    CUSTOM,
     LAST = CUSTOM,
   };
 

ui/gfx/color_transform.cc

@@ -398,9 +398,39 @@ GFX_EXPORT float ToLinear(ColorSpace::TransferID id, float v) {
     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);
   }
 }
 
+namespace {
+// Assumes bt2020
+float Luma(const ColorTransform::TriStim& c) {
+  return c.x() * 0.2627f + c.y() * 0.6780f + c.z() * 0.0593f;
+}
+};
+
+GFX_EXPORT ColorTransform::TriStim ToLinear(ColorSpace::TransferID id,
+                                            ColorTransform::TriStim color) {
+  ColorTransform::TriStim ret(ToLinear(id, color.x()), ToLinear(id, color.y()),
+                              ToLinear(id, color.z()));
+
+  if (id == ColorSpace::TransferID::SMPTEST2084_NON_HDR) {
+    if (Luma(ret) > 0.0) {
+      ColorTransform::TriStim smpte2084(
+          ToLinear(ColorSpace::TransferID::SMPTEST2084, color.x()),
+          ToLinear(ColorSpace::TransferID::SMPTEST2084, color.y()),
+          ToLinear(ColorSpace::TransferID::SMPTEST2084, color.z()));
+      smpte2084.Scale(Luma(ret) / Luma(smpte2084));
+      ret = smpte2084;
+    }
+  }
+
+  return ret;
+}
+
 GFX_EXPORT Transform GetTransferMatrix(ColorSpace::MatrixID id) {
   float Kr = 0.0f, Kb = 0.0f;
   switch (id) {
@@ -509,6 +539,12 @@ class ColorSpaceToColorSpaceTransform : public ColorTransform {
           from_.transfer_ = ColorSpace::TransferID::GAMMA24;
           break;
 
+        case ColorSpace::TransferID::SMPTEST2084:
+          // We don't have an HDR display, so replace SMPTE 2084 with something
+          // that returns ranges more or less suitable for a normal display.
+          from_.transfer_ = ColorSpace::TransferID::SMPTEST2084_NON_HDR;
+          break;
+
         default:  // Do nothing
           break;
       }