On 09/02/2010 10:23 AM, Benjamin Root wrote:
On Thu, Sep 2, 2010 at 9:11 AM, Ryan May <rma...@gmail.com
<mailto:rma...@gmail.com>> wrote:
On Thu, Sep 2, 2010 at 2:15 AM, Mitesh Patel <qed...@gmail.com
<mailto:qed...@gmail.com>> wrote:
> Hello,
>
> Is it possible to specify both an alpha level and a background
color so
> that an entire saved image has a uniform transparency and color?
For
> example, with matplotlib 1.0.0, this script yields the attached
image:
>
> from matplotlib.pyplot import figure, savefig, show
>
> fig = figure()
> ax = fig.add_subplot(111)
> ax.plot([1,2,3])
>
> fig.patch.set_alpha(0.5)
> for ax in fig.axes:
> ax.patch.set_alpha(0.5)
>
> fig.patch.set_facecolor('red')
> for ax in fig.axes:
> ax.patch.set_facecolor('red')
>
> savefig('test.png', facecolor='red')
>
>
> In particular, the areas inside and outside the axes have different
> transparency level and color. Perhaps I'm over/mis/ab-using the
options
> here?
It's not that they're not uniform--you're seeing alpha blending
between the figure patch and the axes patch. Within the axes, both are
being rendered and blended together. This is more readily apparent if
you use blue for the axes patch, as I did for the attached image. When
the red and blue are blended together, you end up with purple. If you
want it all uniform, you'd be better off setting the axes patch to an
alpha of 0.0.
Ryan
This also raises another pet peeve of mine. The Agg backend seems to
use linear blending for alpha. This is inconsistent with how the
world works. It is more realistic for logarithmic blending, or at
least, a piece-wise linear blending.
Imagine I have two overlapping objects with alpha set to .5 (a_1 and
a_2). What is rendered in matplotlib is completely opaque. A more
realistic result would have a final alpha setting of .75 (i.e. - the
first item takes away half the transparency, then the second item
takes away half of the remaining transparency.
I am not nearly familiar enough with the Agg backend to know how to
implement this. Is this at all feasible?
The problem is that then the other backends (PDF, SVG etc) would have
different alpha blending behavior. If that can be resolved, then I
think getting Agg to handle logarithmic blending is the easy part.
Mike
--
Michael Droettboom
Science Software Branch
Space Telescope Science Institute
Baltimore, Maryland, USA
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