alpha is not set correctly when using eps format

[ppurka]

Consider the following piece of code:

from matplotlib.figure import Figure
figure=Figure()
subplot = figure.add_subplot(111)
import matplotlib.patches as patches
p = patches.Polygon([(0,0), (1,2), (0,1), (-1,2)], fill=True, alpha=0.1)
patches.Patch.get_facecolor(p)
Out[6]: (0.0, 0.0, 1.0, 0.1)

subplot.add_patch(p)
from matplotlib.backends.backend_agg import FigureCanvasAgg
figure.set_canvas(FigureCanvasAgg(figure))
figure.savefig('a.eps')
figure.savefig('a.pdf')

The image a.eps gets a solid fill that would be obtained if we gave alpha=1 (but alpha=0.0 gives an empty fill, as it should). And the image a.pdf gets the correct alpha 0.1 fill.

[mdboom]

Unfortunately, the postscript format does not support alpha, so there's little we can do there. Some libraries, such as Cairo, fake it by rendering some things as images, but matplotlib doesn't do that.

[ppurka]

According to wikipedia postscript does support transparency. Also, I created this using inkscape and it does render alpha channels: http://ompldr.org/vaGViZg/drawing.eps (However, I think inkscape uses cairo and poppler, which is why it is able to use transparency).

Take away from this is - matplotlib (and ps in general) does not support writing alpha channels. I guess, you can close this ticket, unless you want to keep it open for some far-off future implementation.

[mdboom]

The important note in the Wikipedia article is (emphasis mine): The PostScript language has limited support for full (not partial) transparency.

Inkscape does indeed emulate alpha blending by converting parts of the output to an image. The downside of this is that the Postscript file is no longer infinitely scalable, and will start to look blocky when zoomed in. It's a tradeoff, and I understand why it was made. If that behavior is ok for you, you can always use matplotlib's Cairo backend to generate the postscript file.

I'm going to close this bug, but feel free to reopen if any implementation ideas or solutions occur in the future.

[darkdragon-001]

In 764227f, the following warning was added

matplotlib/lib/matplotlib/backends/backend_ps.py

Line 783 in 43e7d3b

"The PostScript backend does not support transparency; "

How can I explicitly remove transparency from my figure and avoid this warning?

[tacaswell]

@darkdragon-001 To answer that we would probably have to see your code. This question is probably best handled on the mailing list.

My complete knee-jerk guess is that you have a legend in your figure which now defaults to a semi-transparent background.