Alpha not working from imshow?

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()

Any thoughts?

Tim B

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
314 address only used for lists, off-list direct email at:
http://pirsquared.org | GPG/PGP key id: 0x0F3E28F7

Thanks, Paul! A bit of tweaking in my actual app but it now works beautifully!

···

On 18/01/2011, at 4:08 PM, Paul Ivanov wrote:

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
314 address only used for lists, off-list direct email at:
http://pirsquared.org | GPG/PGP key id: 0x0F3E28F7
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