color: Always use parametric color spaces for raster

ui/gfx/color_space.h

 // Copyright (c) 2012 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.
 
 #ifndef UI_GFX_COLOR_SPACE_H_
 #define UI_GFX_COLOR_SPACE_H_
 
 #include <stdint.h>
 #include <ostream>
 
@@ skipping 24 matching lines @@
     BT470BG,
     SMPTE170M,
     SMPTE240M,
     FILM,
     BT2020,
     SMPTEST428_1,
     SMPTEST431_2,
     SMPTEST432_1,
     XYZ_D50,
     ADOBE_RGB,
+    ICC_PROFILE_REQUIRED,

[hubbe, 2017/03/13 17:59:09]

Why?

[ccameron, 2017/03/13 20:34:18]

In the CL description, I wrote
This enum denotes "the ICC profile is required to accurately represent this
color space".

Is there a more specific question here? It's hard for me to know what this
means.

[hubbe, 2017/03/13 20:38:56]

I don't understand why we don't use CUSTOM with the best possible approximation
here?

If there is a good explanation for that, I think it should be in the code, not
the CL description.

[ccameron, 2017/03/14 10:19:54]

Because we need to distinguish between when CUSTOM is accurately representing
the ICCProfile from which it came, and when it isn't. In particular:

1. For rasterization, it is necessary to use the parametric color space. We need
to know if this matches or does not match the display's true color space. Hence
why we need one space to be the approximation (used by raster) and one to be
precise (used by display).

2. For creating ColorTransforms this creates a simpler way of knowing if we can
use parametric-transform-in-a-shader that we've proven to be more efficient, or
if we must fall back to a LUT.
I've added more comments to the enums. Please also see the comments in
ICCProfile::GetParametricColorSpace and ICCProfile::GetColorSpace.

     CUSTOM,
     LAST = CUSTOM
   };
 
   enum class TransferID : uint16_t {
     INVALID,
     BT709,
     GAMMA22,
     GAMMA24,
     GAMMA28,
@@ skipping 12 matching lines @@
     ARIB_STD_B67,  // AKA hybrid-log gamma, HLG.
     // 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,
     // The same as IEC61966_2_1 on the interval [0, 1], with the nonlinear
     // segment continuing beyond 1 and point symmetry defining values below 0.
     IEC61966_2_1_HDR,
     // The same as LINEAR but is defined for all real values.
     LINEAR_HDR,
+    ICC_PROFILE_REQUIRED,

[hubbe, 2017/03/13 17:59:09]

Why?

     CUSTOM,
     LAST = CUSTOM,
   };
 
   enum class MatrixID : int16_t {
     INVALID,
     RGB,
     BT709,
     FCC,
     BT470BG,
@@ skipping 118 matching lines @@
   FRIEND_TEST_ALL_PREFIXES(SimpleColorSpace, GetColorSpace);
 };
 
 // Stream operator so ColorSpace can be used in assertion statements.
 GFX_EXPORT std::ostream& operator<<(std::ostream& out,
                                     const ColorSpace& color_space);
 
 }  // namespace gfx
 
 #endif  // UI_GFX_COLOR_SPACE_H_