Re: [Matplotlib-users] Two circles with “exponential decay” coloring, and alpha < 1, having trouble with color mixing?

[Thomas C. <tca...@gm...>]

The way we do the alpha blending, the output value is (alpha * v1) +
((alpha-1) * v2).  All of the artists are compsited down on top of a white
background so the compositing is not commutative.

(a * .5) + (.5 * (b * .5 + .5))  =/= (b * .5) + (.5 * (a * .5 + .5))


On Mon, Nov 23, 2015 at 1:55 PM Sterling Smith <sm...@fu...>
wrote:

> Maybe the issue is with the colormap not having an alpha?  Does this
>
> http://stackoverflow.com/questions/10127284/overlay-imshow-plots-in-matplotlib
> help?
>
> Otherwise, you might file a bug at
> https://github.com/matplotlib/matplotlib/issues/new
>
> -Sterling
>
> On Nov 20, 2015, at 4:46PM, Brian Merchant <bhm...@gm...> wrote:
>
> > Hi all,
> >
> > In order to get circles such that their coloring is radially symmetric,
> with center being the darkest, and exponential decay in color as one moves
> farther away from the center along the radius, I used imshow with clip_path
> using Circle patches.
> >
> > Here's a toy script that overlaps two such circles:
> https://gist.github.com/bmer/7063cc2dd09f1b80a252
> >
> > As you can see if you run the script (or, if you follow this link:
> http://i.imgur.com/H9jEAZ3.png), even though the alpha is set at 0.5,
> there doesn't seem to be proper "color mixing" occurring (we should see a
> result that is symmetric along the x-axis).
> >
> > Why is that, and what could I do to fix this issue?
> >
> > Kind regards,
> > Brian
> >
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