Add a highlight_color util function.

qutebrowser/test/utils/test_misc.py

@@ -576,5 +576,36 @@ class NormalizeTests(unittest.TestCase):
             self.assertEqual(utils.normalize_keystr(orig), repl, orig)
 
 
+class HighlightColorTests(unittest.TestCase):
+
+    """Test highlight_color."""
+
+    def test_invalid(self):
+        """Test with an invalid Color."""
+        col = Color()
+        with self.assertRaises(QtValueError):
+            utils.highlight_color(col)
+
+    def test_large_factor(self):
+        """Test with a too large factor."""
+        col = Color('black')
+        with self.assertRaises(OverflowError):
+            utils.highlight_color(col, 2 ** 31)
+
+    def test_white(self):
+        """Test highlighting white."""
+        col = Color('white')
+        c1 = Color(utils.highlight_color(col, 1))
+        c2 = Color(127, 127, 127)
+        self.assertEqual(c1, c2)
+
+    def test_black(self):
+        """Test highlighting black."""
+        col = Color('black')
+        self.assertEqual(Color(utils.highlight_color(col, 0.5)),
+                         Color(204, 204, 204))
+
+
+
 if __name__ == '__main__':
     unittest.main()

qutebrowser/utils/misc.py

@@ -36,7 +36,8 @@ from pkg_resources import resource_string
 
 import qutebrowser
 import qutebrowser.utils.log as log
-from qutebrowser.utils.qt import qt_version_check, qt_ensure_valid
+from qutebrowser.utils.qt import (qt_version_check, qt_ensure_valid,
+                                  check_overflow)
 
 
 def elide(text, length):
@@ -489,3 +490,32 @@ def normalize_keystr(keystr):
     for mod in ('Ctrl', 'Meta', 'Alt', 'Shift'):
         keystr = keystr.replace(mod + '-', mod + '+')
     return keystr.lower()
+
+
+def highlight_color(color, factor=0.5):
+    """Get a highlighted color based on a base color.
+
+    If the passed color is dark, it will get lighter. If it is light, it will
+    get darker.
+
+    Args:
+        color: A QColor with the initial color.
+        factor: How much to lighten/darken the color (0.5: 50% lighter/darker).
+
+    Return:
+        A QColor with the highlighted color.
+    """
+    qt_ensure_valid(color)
+    qt_factor = 100 * (1 + factor)
+    check_overflow(qt_factor, 'int')
+    if color == QColor('black'):
+        # Black isn't handled by QColor::lighter because that just multiplies
+        # the HSV value by a factor.
+        # We also add an additional 30% because the effect would be way too
+        # subtle if we didn't.
+        return interpolate_color(QColor('black'), QColor('white'),
+                                 min((100 * factor) + 30, 100))
+    elif color.value() < 128:
+        return color.lighter(qt_factor)
+    else:
+        return color.darker(qt_factor)