Re: [Matplotlib-users] removing Figure background (or, making it transparent)

[John H. <jdh...@ac...>]

>>>>> "Andrew" == Andrew Straw <str...@as...> writes:

    Andrew> Hi all, I'd like use matplotlib to draw shapes on a
    Andrew> transparent background using the Agg backend to save .png
    Andrew> files. Is this possible?

    Andrew> I've tried

    Andrew> fig._figurePatch.set_alpha(0)

There is a vestigial limitation from the bad old days when we didn't
have an alpha channel.  For polyons, there is no place holder for a
separate alpha channel for the edge and face.  The way to do this
right is to simply replace all rgb instances throughout the library
with rgba, but this is a fair amount of work that touches the entire
library.  What I am doing currently is sharing the alpha attribute
between edge and face for polygons.

So setting alpha=0 *should work. What backend and version are you
using?  Rectangle calls gc.set_alpha before calling draw_polygon in
patches.Patch.  In _backend_agg.RendererAgg.draw_polygon, the alpha
attribute for the face is taken from the edge which gets it from the
gc (around line 204 in _backend_agg).  I say all this just because
from my read of the code *it should* work, and indeed, in my tests

  f = figure(1, facecolor='r')
  f._figurePatch.set_alpha(0.0)

does work for me using Agg and the latest matplotlib.

What backend and version are you using?

That said, the best way to do this is to add a frameon attribute for
figures, because this will be respected across backends including
those who don't understand alpha.  This way you can say

f = figure(1, frameon=False)

I checked this into CVS.

JDH





    Andrew> This seems to get halfway, but there's still (opaque)
    Andrew> white drawn in the background.

    Andrew> Is there a "recommended way", and, if so, what is it?
    Andrew> Otherwise, can we improve matplotlib to allow drawing on
    Andrew> transparent backgrounds?

    Andrew> Cheers!  Andrew


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