DOC: Update color tutorial to explain alpha

jklymak · jklymak · commit 2f671e48c2a4 · 2017-12-08T14:44:51.000-08:00
diff --git a/tutorials/colors/colors.py b/tutorials/colors/colors.py
@@ -6,7 +6,7 @@
 Matplotlib recognizes the following formats to specify a color:
 
 * an RGB or RGBA tuple of float values in ``[0, 1]`` (e.g., ``(0.1, 0.2, 0.5)``
-  or  ``(0.1, 0.2, 0.5, 0.3)``);
+  or  ``(0.1, 0.2, 0.5, 0.3)``).  RGBA is short for Red, Green, Blue, Alpha;
 * a hex RGB or RGBA string (e.g., ``'#0F0F0F'`` or ``'#0F0F0F0F'``);
 * a string representation of a float value in ``[0, 1]`` inclusive for gray
   level (e.g., ``'0.5'``);
@@ -23,6 +23,19 @@
   the indexing occurs at artist creation time and defaults to black if the
   cycle does not include color.
 
+"Red", "Green" and "Blue", are the intensities of those colors, the combination
+of which span the colorspace.
+
+How "Alpha" behaves depends on the ``zorder`` of the Artist.  Higher
+`zorder` Artists are drawn on top of lower Artists, and "Alpha" determines
+whether the lower artist is covered by the higher.
+If the old RGB of a pixel is ``RGBold`` and the RGB of the
+pixel of the Artist being added is ``RGBnew`` with Alpha ``alpha``,
+then the RGB of the pixel is updated to:
+``RGB = RGBOld * (1 - Alpha) + RGBnew * Alpha``.  Alpha
+of 1 means the old color is completely covered by the new Artist, Alpha of 0
+means that pixel of the Artist is transparent.
+
 All string specifications of color, other than "CN", are case-insensitive.
 
 "CN" color selection
