Re: [Matplotlib-users] imshow() and a transparent colormap?

[Benjamin R. <ben...@ou...>]

On Thu, Sep 30, 2010 at 4:59 PM, Nicolas Bigaouette
<nbi...@gm...>wrote:

> Hi all,
>
> I'm plotting some data with imshow(). On the screen it looks good, but when
> printed (it's part of a paper) it's hard to see because it all looks blue or
> black.
>
> I'm attaching an example.
>
> What I would like to do it have a colormap which is transparent for lower
> values of data and opaque for high values, so the blue on the figure would
> appear white/transparent.
>
> Is it possible? How to achieve this? alpha options seems to accept only a
> float for a uniform transparency.
>
> Thanx!
>
> N
>
>
Correct, most plotting functions in mpl assume a scalar alpha channel, but
the backend of mpl does not have this restriction.

The colormap object itself is not restricted to a single value for the alpha
channel.  I would imagine, although I have not tried this myself, that it
would be possible to modify the colormap object's alpha values in the
self._lut rgba array.  Note that self._lut for a Colormap object is not
created right away, only when it needs to make a color from a value.

I don't know if that helps at all, but good luck!

Ben Root