Re: [Matplotlib-users] Alpha not working from imshow?

[Paul I. <piv...@gm...>]

Tim Burgess, on 2011-01-18 13:32,  wrote:
> I'm generating images that I want to use as overlays in Google
> Earth. Thus, I would like the masked portion of the numpy
> arrays I am using to appear as transparent.  However, I seem to
> consistently get white instead of a transparent point.
> 
> To clarify the issue, I wrote this short piece of code. I am
> setting both the figure background to transaprent and the
> masked value to transparent but I get white.
> 
> I have pulled the current svn code and built matplotlib. I get
> the same issue. 
> 
> from numpy import ma
> import matplotlib.pyplot as plt
> from pylab import colorbar, imshow, show, cm
> 
> 
> def main():
>   a = ma.array([[1,2,3],[4,5,6]],mask=[[0,0,1],[0,0,0]])
>   fig = plt.figure()
>   fig.patch.set_alpha(0.0)
>   cmap = cm.jet
>   cmap.set_bad(alpha=0.0)
>   imshow(a, interpolation='nearest', cmap=cmap)
>   colorbar()
>   show()

Hi Tim,

sorry to hear of your troubles - add these two lines to get your
desired effect:

  ax = plt.gca()
  ax.patch.set_alpha(0.0)

The figure has a patch associated with it, but so does each axes 
subplot, which is what you were seeing through the transparent
portion of your masked array image.

best,
-- 
Paul Ivanov
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