Re: [Matplotlib-users] Single color transparent colormap

[Eric F. <ef...@ha...>]

On 2014/11/19, 1:03 PM, Benjamin Root wrote:
> What you are seeing is the fact that the adjacent cells share the same
> coordinates, so neighboring cells overlap by one pixel. This is only
> visible when alpha != 1. This is a tricky issue to solve, but I could
> have sworn we made some progress on that front by setting "snap" to
> False somewhere. There have been past discussions about it, for sure...

I don't think we ever made any progress; it seems like a problem with 
the renderer itself, agg in this case, and one that differs from one 
renderer to another (e.g., if the plot is saved as pdf and then rendered 
by different libraries).  Try turning off antialiasing.

Eric

>
> Ben Root
>
> On Wed, Nov 19, 2014 at 12:57 PM, Loïc Estève <loi...@in...
> <mailto:loi...@in...>> wrote:
>
>     Thanks for the suggestions, I have tried the easiest one for now,
>     namely pcolormesh, see attached plot. The alpha colormap look great
>     but I can't seem to figure out how to prevent the edges of the cells
>     from being visible. I tried using edgecolors='none' to no avail. I
>     guess retrospectively that is similar to the lines we see in the
>     colormap on the right.
>
>     The snippet I am using:
>
>     import numpy as np
>
>     import matplotlib.pyplot as plt
>     from matplotlib.colors import LinearSegmentedColormap
>
>     import matplotlib
>
>     matplotlib.rcParams['figure.__facecolor'] = 'white'
>
>     cm_dict = {'red':  ((0.0, 1.0, 1.0),
>                          (1.0, 1.0, 1.0)),
>                 'green': ((0.0, 0.0, 0.0),
>                           (1.0, 0.0, 0.0)),
>                 'blue':  ((0.0, 0.0, 0.0),
>                           (1.0, 0.0, 0.0)),
>                 'alpha': ((0.0, 0.0, 0.0),
>                           (1.0, 1.0, 1.0))
>                }
>
>     my_cm = LinearSegmentedColormap('my___cm', cm_dict)
>
>     vals = np.tile(np.linspace(-1, 1, 30), (20, 1))
>
>     fig = plt.figure()
>     ax = plt.pcolormesh(vals, cmap=my_cm)
>     plt.colorbar()
>     plt.show()
>
>     Cheers,
>     Loïc
>
>
>
>
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