[matplotlib-devel] graphics context: use alpha value from foreground color if present

[Michiel de H. <mjl...@ya...>]

Dear all,

I am currently modifying the MacOSX backend to make its interactive/non-interactive behavior consistent with the other backends in matplotlib.
When I was testing the backend, I found a new bug that seems to be related to a recent change in backend_bases.py:

https://github.com/matplotlib/matplotlib/commit/4c078ddf68cc0ecc1a5f36009a3e3a8b4921b037#lib/matplotlib/backend_bases.py

After this commit, GraphicsContextBase.set_alpha has no effect if alpha==None; previously it would set alpha to 1.0.

The bug appears here in Text.draw in text.py:

        gc = renderer.new_gc()
        gc.set_foreground(self.get_color())
        gc.set_alpha(self.get_alpha())

In this code, self is a Text object, which derives from the Artist class, which initializes its _alpha member to None. So self.get_alpha() returns None, and gc.set_alpha has no effect. The alpha value used then depends on whatever was present in the gc before the call to new_gc, which is backend-dependent; the MacOSX and cairo backends end up with an incorrect alpha value.

I guess the easiest solution is to initialize _alpha in the Artist class to 1.0 instead of to None.

Thanks,
--Michiel