Add docstring to _get_scalar_alpha.

tillahoffmann · tillahoffmann · commit c778bd945beb · 2019-09-09T10:19:01.000+01:00
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
@@ -293,6 +293,15 @@ def set_alpha(self, alpha):
         self._imcache = None
 
     def _get_scalar_alpha(self):
+        """
+        Get a scalar alpha value to be applied to the artist as a whole.
+
+        If the alpha value is a matrix, the method returns 1.0 because pixels
+        have individual alpha values (see `~._ImageBase._make_image` for
+        details). If the alpha value is a scalar, the method returns said value
+        to be applied to the artist as a whole because pixels do not have
+        individual alpha values.
+        """
         return 1.0 if self._alpha is None or np.ndim(self._alpha) > 0 \
             else self._alpha
 
