Re: [Matplotlib-users] colormap from blue to transparent to red

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

On Tue, Oct 12, 2010 at 9:25 AM, Benjamin Root <ben...@ou...> wrote:

> On Tue, Oct 12, 2010 at 7:57 AM, Stefan Mauerberger <
> ste...@mn...> wrote:
>
>> Hi everyone,
>>
>> I am having trouble with colormaps unsing pcolormesh. I would like to
>> plot and colorise a seismic wave field above a map. Plotting works fine
>> but I do not know how to bring transparency into colormaps. For negative
>> values I want the coloration being blue then it should become
>> transparent and the greatest value should be drawn red. I have tried a
>> lot but without any success. As far as I can see, the keyarg alpha does
>> not fit my needs at all.
>>
>> Do you have any suggestions for me?
>>
>> Regards
>>
>> Stefan
>>
>>
> Stefan,
>
> If you mean using the keyword 'alpha' in the pcolormesh function, then yes,
> that isn't the right place for it.  In your case, the right place to specify
> the alpha channel is in the colormap itself.  Unfortunately, it is currently
> designed around the idea of a scalar value specifying the transparency.
>
> What you can do is a little trick.  First, get the colormap object and then
> initialize it.  This will cause it to internally create an array called
> "_lut" which holds rgba values.
>
> >>> theCM = cm.get_cmap('somemap')
> >>> theCM._init()
>
> Then, you need to fill in the alpha values for yourself.  For this, I am
> going to use a triangle function:
>
> >>> alphas = np.abs(np.linspace(-1.0, 1.0, theCM.N))
> >>> theCM._lut[:,-1] = alphas
>
> Now that your colormap is set up, you can pass it into your pcolormesh
> call.
>
> >>> plt.pcolormesh(X, Y, C, cmap=theCM)
>
> That should work, but I have not fully tested it.  I hope that helps!
> Ben Root
>
>

Slight correction to my code example:

>>> theCM._lut[:-3,-1] = alphas

Sorry for any confusion.

Ben Root