kivy/kivy — pull request #686 by tito (2012-09-18 23:07:19+00:00)
Title: graphics/widget: introduce opacity attribute on the canvas and widget.

You can use the opacity attribute to control the opacity of the widget
and its children. The attribute is cumulative, and multiplied to the
current context color. By default, the opacity is 1.0, and doesn't act
at all.

This is part of the issue #682.