Recolor the default tab favicon based on minimum contrast.

The existing code bases the decision on whether the toolbar button
icons have a custom color, which causes accessibility problems.

Bug: 1153079
Change-Id: I0f9996891a7327ffed3d45b77944fa33481b8b0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587578
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836793}

chrome/browser/ui/views/tabs/tab_icon.cc

@@ -22,7 +22,9 @@
 #include "ui/gfx/animation/linear_animation.h"
 #include "ui/gfx/animation/tween.h"
 #include "ui/gfx/canvas.h"
+#include "ui/gfx/color_analysis.h"
 #include "ui/gfx/color_palette.h"
+#include "ui/gfx/color_utils.h"
 #include "ui/gfx/favicon_size.h"
 #include "ui/gfx/image/image_skia_operations.h"
 #include "ui/gfx/scoped_canvas.h"
@@ -432,11 +434,32 @@ void TabIcon::RefreshLayer() {
 }
 
 gfx::ImageSkia TabIcon::ThemeImage(const gfx::ImageSkia& source) {
-  if (!GetThemeProvider()->HasCustomColor(
-          ThemeProperties::COLOR_TOOLBAR_BUTTON_ICON))
-    return source;
-
-  return gfx::ImageSkiaOperations::CreateColorMask(
-      source,
-      GetThemeProvider()->GetColor(ThemeProperties::COLOR_TOOLBAR_BUTTON_ICON));
+  // Choose between leaving the image as-is or changing to the toolbar button
+  // icon color.
+  const SkColor original_color =
+      color_utils::CalculateKMeanColorOfBitmap(*source.bitmap());
+  const ui::ThemeProvider* tp = GetThemeProvider();
+  const SkColor alternate_color =
+      tp->GetColor(ThemeProperties::COLOR_TOOLBAR_BUTTON_ICON);
+
+  // Compute the minimum contrast of each color against foreground and
+  // background tabs (for active windows).
+  const SkColor active_tab_background =
+      tp->GetColor(ThemeProperties::COLOR_TAB_BACKGROUND_ACTIVE_FRAME_ACTIVE);
+  const SkColor inactive_tab_background =
+      tp->GetColor(ThemeProperties::COLOR_TAB_BACKGROUND_INACTIVE_FRAME_ACTIVE);
+  const float original_contrast = std::min(
+      color_utils::GetContrastRatio(original_color, active_tab_background),
+      color_utils::GetContrastRatio(original_color, inactive_tab_background));
+  const float alternate_contrast = std::min(
+      color_utils::GetContrastRatio(alternate_color, active_tab_background),
+      color_utils::GetContrastRatio(alternate_color, inactive_tab_background));
+
+  // Recolor the image if the original has low minimum contrast and recoloring
+  // will improve it.
+  return ((original_contrast < color_utils::kMinimumReadableContrastRatio) &&
+          (alternate_contrast > original_contrast))
+             ? gfx::ImageSkiaOperations::CreateColorMask(source,
+                                                         alternate_color)
+             : source;
 }

chrome/browser/ui/views/tabs/tab_icon.h

@@ -111,8 +111,6 @@ class TabIcon : public views::View, public views::AnimationDelegateViews {
   // whether a layer can be used.
   void RefreshLayer();
 
-  void UpdateThemedFavicon();
-
   gfx::ImageSkia ThemeImage(const gfx::ImageSkia& source);
 
   const base::TickClock* clock_;