Move helper functions to more public location

Change-Id: I12a2000bfc82c194d12fb5c25302a06f782aa605
Reviewed-by: David Schulz <david.schulz@qt.io>

src/libs/utils/stylehelper.cpp

@@ -573,4 +573,39 @@ QList<int> StyleHelper::availableImageResolutions(const QString &fileName)
     return result;
 }
 
+static double luminance(const QColor &color)
+{
+    // calculate the luminance based on
+    // https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
+    auto val = [](const double &colorVal) {
+        return colorVal < 0.03928 ? colorVal / 12.92 : std::pow((colorVal + 0.055) / 1.055, 2.4);
+    };
+
+    static QHash<QRgb, double> cache;
+    QHash<QRgb, double>::iterator it = cache.find(color.rgb());
+    if (it == cache.end()) {
+        it = cache.insert(color.rgb(), 0.2126 * val(color.redF())
+                          + 0.7152 * val(color.greenF())
+                          + 0.0722 * val(color.blueF()));
+    }
+    return it.value();
+}
+
+static double contrastRatio(const QColor &color1, const QColor &color2)
+{
+    // calculate the contrast ratio based on
+    // https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
+    auto contrast = (luminance(color1) + 0.05) / (luminance(color2) + 0.05);
+    if (contrast < 1)
+        return 1 / contrast;
+    return contrast;
+}
+
+bool StyleHelper::isReadableOn(const QColor &background, const QColor &foreground)
+{
+    // following the W3C Recommendation on contrast for large Text
+    // https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
+    return contrastRatio(background, foreground) > 3;
+}
+
 } // namespace Utils