matplotlib-devel Mailing List for matplotlib

On 12/30/2011 01:57 PM, Paul Ivanov wrote:
> Eric Firing, on 2011年12月27日 15:31, wrote:
>> It looks like this is something I can fix by modifying ListedColormap.
>> It is discarding the alpha values, and I don't think there is any reason
>> it needs to do so.
>
> One of my first attempts at a contribution to matplotlib three
> years ago was related to this. It was in reply to a similar
> question on list, and I wrote a patch, but never saw it through
> to inclusion because it wasn't something I needed.
>
> http://www.mail-archive.com/mat...@li.../msg09216.html
>
> I think it's a helpful starting point, as I include a discussion
> on the limitation of mpl colormaps there.
I'm switching this to the devel list.
Please try
https://github.com/efiring/matplotlib/tree/colormap_alpha
which has changes similar to yours so that alpha is fully changeable in 
colormaps.
I think this is going to be OK as far as the colormap end of things is 
concerned, but it turns up a new problem related to alpha in images, and 
reminds us of an old problem with alpha in agg, at least. The problems 
are illustrated in the attached modification of the custom_cmap.py 
example. I added a fourth panel for testing alpha. Look at the 
comments on the code for that panel, and try switching between 
pcolormesh and imshow. Pcolormesh basically works as expected, except 
for the prominent artifacts on patch boundaries (visible also in the 
colorbar for that panel). These boundary artifacts are the old problem. 
The new problem is that imshow with alpha in the colormap is completely 
wonky with a white background, but looks more normal with a black 
background--which is not so good if what you really want is a white 
background showing through the transparency.
Eric