Use float math in color_utils.cc

Switch from double to float in color_utils.cc. The HSL struct and
CalculateBoringScore still use double in their API because changing
those requires enough changes to justify being done in a different CL.

Note that SkColorToRgbaString intentionally uses a double precision
constant for 255 so that the calculation is done to double precision
so that spurious rounding in the result string is avoided.

Bug: 863135
Change-Id: Id5b678ed6e1c40475ec516bde2c902ba27bb783d
Reviewed-on: https://chromium-review.googlesource.com/1135550
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578543}

chrome/browser/search/ntp_icon_source.cc

@@ -198,7 +198,7 @@ std::vector<unsigned char> RenderIconBitmap(const GURL& icon_url,
     // If luminance is too high, the white text will become unreadable. Invert
     // the background color to achieve better constrast. The constant comes
     // W3C Accessibility standards.
-    if (color_utils::GetRelativeLuminance(background_color) > 0.179)
+    if (color_utils::GetRelativeLuminance(background_color) > 0.179f)
       background_color = color_utils::InvertColor(background_color);
 
     DrawCircleInCanvas(&canvas, size, background_color);

chrome/browser/themes/theme_service.cc

@@ -673,16 +673,16 @@ SkColor ThemeService::GetSeparatorColor(SkColor tab_color,
   // However, if the frame is already very dark or very light, respectively,
   // this won't contrast sufficiently with the frame color, so we'll need to
   // reverse when we're lightening and darkening.
-  const double tab_luminance = color_utils::GetRelativeLuminance(tab_color);
-  const double frame_luminance = color_utils::GetRelativeLuminance(frame_color);
+  const float tab_luminance = color_utils::GetRelativeLuminance(tab_color);
+  const float frame_luminance = color_utils::GetRelativeLuminance(frame_color);
   const bool lighten = tab_luminance < frame_luminance;
   SkColor separator_color = lighten ? SK_ColorWHITE : SK_ColorBLACK;
-  double separator_luminance = color_utils::GetRelativeLuminance(
+  float separator_luminance = color_utils::GetRelativeLuminance(
       color_utils::AlphaBlend(separator_color, frame_color, kAlpha));
   // The minimum contrast ratio here is just under the ~1.1469 in the default MD
   // incognito theme.  We want the separator to still darken the frame in that
   // theme, but that's about as low of contrast as we're willing to accept.
-  const double kMinContrastRatio = 1.1465;
+  const float kMinContrastRatio = 1.1465f;
   if (color_utils::GetContrastRatio(separator_luminance, frame_luminance) >=
       kMinContrastRatio)
     return SkColorSetA(separator_color, kAlpha);
@@ -700,7 +700,7 @@ SkColor ThemeService::GetSeparatorColor(SkColor tab_color,
   // The reversed separator doesn't contrast enough with the tab.  Compute the
   // resulting luminance from adjusting the tab color, instead of the frame
   // color, by the separator color.
-  const double target_luminance = color_utils::GetRelativeLuminance(
+  const float target_luminance = color_utils::GetRelativeLuminance(
       color_utils::AlphaBlend(separator_color, tab_color, kAlpha));
 
   // Now try to compute an alpha for the separator such that, when blended with
@@ -709,7 +709,7 @@ SkColor ThemeService::GetSeparatorColor(SkColor tab_color,
   // possible range of alpha values.
   SkAlpha alpha = 128;
   for (int delta = lighten ? 64 : -64; delta != 0; delta /= 2) {
-    const double luminance = color_utils::GetRelativeLuminance(
+    const float luminance = color_utils::GetRelativeLuminance(
         color_utils::AlphaBlend(separator_color, frame_color, alpha));
     if (luminance == target_luminance)
       break;

chrome/browser/themes/theme_service_unittest.cc

@@ -456,7 +456,7 @@ TEST_F(ThemeServiceTest, SeparatorColor) {
     EXPECT_EQ(theme_color, separator_color);
 
     // For the default theme, the separator should darken the frame.
-    double frame_luminance = color_utils::GetRelativeLuminance(frame_color);
+    float frame_luminance = color_utils::GetRelativeLuminance(frame_color);
     EXPECT_LT(color_utils::GetRelativeLuminance(separator_color),
               frame_luminance);
 
@@ -466,8 +466,8 @@ TEST_F(ThemeServiceTest, SeparatorColor) {
     // "tab" (frame color) since otherwise the contrast the contrast with the
     // "tab color" would be too minimal.
     separator_color = GetSeparatorColor(frame_color, tab_color);
-    double tab_luminance = color_utils::GetRelativeLuminance(tab_color);
-    double separator_luminance =
+    float tab_luminance = color_utils::GetRelativeLuminance(tab_color);
+    float separator_luminance =
         color_utils::GetRelativeLuminance(separator_color);
     EXPECT_LT(separator_luminance, tab_luminance);
     EXPECT_LT(separator_luminance, frame_luminance);
@@ -513,7 +513,7 @@ TEST_F(ThemeServiceTest, SeparatorColor) {
     // And if we reverse the colors, the separator should lighten the "frame"
     // (tab color).
     separator_color = GetSeparatorColor(frame_color, tab_color);
-    double tab_luminance = color_utils::GetRelativeLuminance(tab_color);
+    float tab_luminance = color_utils::GetRelativeLuminance(tab_color);
     EXPECT_GT(color_utils::GetRelativeLuminance(separator_color),
               tab_luminance);
 
@@ -521,7 +521,7 @@ TEST_F(ThemeServiceTest, SeparatorColor) {
     // should also be lighter than the tab color since otherwise the contrast
     // with the tab would be too minimal.
     separator_color = GetSeparatorColor(tab_color, SK_ColorBLACK);
-    double separator_luminance =
+    float separator_luminance =
         color_utils::GetRelativeLuminance(separator_color);
     EXPECT_GT(separator_luminance, 0);
     EXPECT_GT(separator_luminance, tab_luminance);

chrome/browser/ui/views/payments/payment_request_views_util.cc

@@ -414,7 +414,7 @@ std::unique_ptr<views::View> CreateShippingOptionLabel(
 }
 
 SkColor GetForegroundColorForBackground(SkColor background_color) {
-  constexpr double kLightForegroundRatioThreshold = 3;
+  constexpr float kLightForegroundRatioThreshold = 3;
   if (background_color != 0 &&
       color_utils::GetContrastRatio(background_color, SK_ColorWHITE) >=
           kLightForegroundRatioThreshold) {

ui/gfx/color_utils.h

@@ -23,18 +23,18 @@ struct HSL {
 
 // The minimum contrast between text and background that is still readable.
 // This value is taken from w3c accessibility guidelines.
-constexpr double kMinimumReadableContrastRatio = 4.5;
+constexpr float kMinimumReadableContrastRatio = 4.5f;
 
 // Determines the contrast ratio of two colors or two relative luminance values
 // (as computed by RelativeLuminance()), calculated according to
 // http://www.w3.org/TR/WCAG20/#contrast-ratiodef .
-GFX_EXPORT double GetContrastRatio(SkColor color_a, SkColor color_b);
-GFX_EXPORT double GetContrastRatio(double luminance_a, double luminance_b);
+GFX_EXPORT float GetContrastRatio(SkColor color_a, SkColor color_b);
+GFX_EXPORT float GetContrastRatio(float luminance_a, float luminance_b);
 
 // The relative luminance of |color|, that is, the weighted sum of the
 // linearized RGB components, normalized to 0..1, per BT.709.  See
 // http://www.w3.org/TR/WCAG20/#relativeluminancedef .
-GFX_EXPORT double GetRelativeLuminance(SkColor color);
+GFX_EXPORT float GetRelativeLuminance(SkColor color);
 
 // The luma of |color|, that is, the weighted sum of the gamma-compressed R'G'B'
 // components, per BT.601, a.k.a. the Y' in Y'UV.  See