text: // 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_UTILS_H_
  #define UI_GFX_COLOR_UTILS_H_
  #pragma once
  
  #include "base/basictypes.h"
  #include "third_party/skia/include/core/SkColor.h"
  #include "ui/base/ui_export.h"
  
  class SkBitmap;
  
  namespace color_utils {
  
  // Represents an HSL color.
  struct HSL {
    double h;
    double s;
    double l;
  };
  
  UI_EXPORT unsigned char GetLuminanceForColor(SkColor color);
  
  // Calculated according to http://www.w3.org/TR/WCAG20/#relativeluminancedef
  UI_EXPORT double RelativeLuminance(SkColor color);
  
  // Note: these transformations assume sRGB as the source color space
  UI_EXPORT void SkColorToHSL(SkColor c, HSL* hsl);
  UI_EXPORT SkColor HSLToSkColor(const HSL& hsl, SkAlpha alpha);
  
  // HSL-Shift an SkColor. The shift values are in the range of 0-1, with the
  // option to specify -1 for 'no change'. The shift values are defined as:
  // hsl_shift[0] (hue): The absolute hue value - 0 and 1 map
  //    to 0 and 360 on the hue color wheel (red).
  // hsl_shift[1] (saturation): A saturation shift, with the
  //    following key values:
  //    0 = remove all color.
  //    0.5 = leave unchanged.
  //    1 = fully saturate the image.
  // hsl_shift[2] (lightness): A lightness shift, with the
  //    following key values:
  //    0 = remove all lightness (make all pixels black).
  //    0.5 = leave unchanged.
  //    1 = full lightness (make all pixels white).
  UI_EXPORT SkColor HSLShift(SkColor color, const HSL& shift);
  
  // Determine if a given alpha value is nearly completely transparent.
  bool IsColorCloseToTransparent(SkAlpha alpha);
  
  // Determine if a color is near grey.
  bool IsColorCloseToGrey(int r, int g, int b);
  
  // Gets a color representing a bitmap. The definition of "representing" is the
  // average color in the bitmap. The color returned is modified to have the
  // specified alpha.
  SkColor GetAverageColorOfFavicon(SkBitmap* bitmap, SkAlpha alpha);
  
  // Builds a histogram based on the Y' of the Y'UV representation of
  // this image.
  UI_EXPORT void BuildLumaHistogram(const SkBitmap& bitmap, int histogram[256]);
  
  // Returns a blend of the supplied colors, ranging from |background| (for
  // |alpha| == 0) to |foreground| (for |alpha| == 255). The alpha channels of
  // the supplied colors are also taken into account, so the returned color may
  // be partially transparent.
  UI_EXPORT SkColor AlphaBlend(SkColor foreground, SkColor background,
                               SkAlpha alpha);
  
  // Given a foreground and background color, try to return a foreground color
  // that is "readable" over the background color by luma-inverting the foreground
  // color and then picking whichever foreground color has higher contrast against
  // the background color.
  //
  // NOTE: This won't do anything but waste time if the supplied foreground color
  // has a luma value close to the midpoint (0.5 in the HSL representation).
  UI_EXPORT SkColor GetReadableColor(SkColor foreground, SkColor background);
  
  // Invert a color.
  UI_EXPORT SkColor InvertColor(SkColor color);
  
  // Gets a Windows system color as a SkColor
  UI_EXPORT SkColor GetSysSkColor(int which);
  
  }  // namespace color_utils
  
  #endif  // UI_GFX_COLOR_UTILS_H_
  