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On 2013/12/01 5:11 AM, lin...@eu... wrote:
> Z = X**2 + Y**2
> Z[(X**2+Y**2 < r**2) | (X**2+Y**2 > R**2)] = np.ma.masked

The problem is that Z is not a masked array.  Change the first line 
above to Z = np.ma.array(X**2 + Y**2).

Also, use pcolormesh instead of pcolor.  Pcolormesh is much faster.

Eric

see http://matplotlib.1069221.n5.nabble.com/cmap-set-bad-not-showing-any-effect-with-pcolor-td27530.html

On Dec 1, 2013, at 10:11 AM, lin...@eu... wrote:

> Hi,
> 
> I m trying to plot some data with pcolor. The data should be plotted on a ring. Inside and outside of the ring should be white area. However, now I have black area. Where is my mistake? Minimal working example:
> 
> from matplotlib import cm
> import matplotlib.pyplot as plt
> from pylab import *
> 
> import numpy as np
> import scipy as sp
>    
> X = np.arange(-1.0, 1.01, 0.01)
> Y = np.arange(-1.0, 1.01, 0.01)
> X, Y = np.meshgrid(X, Y)
> 
> [r, R] = [0.25, 1.0]
> an = sp.linspace(0,2*sp.pi,100)
> 
> fig = plt.figure(1, figsize=(6,6))
> 
> ax = fig.add_subplot(111, aspect = 'equal')
> Z = X**2 + Y**2
> Z[(X**2+Y**2 < r**2) | (X**2+Y**2 > R**2)] = np.ma.masked
> cm.hot.set_bad('white', alpha=None)
> plot = pcolor(X, Y, Z, cmap=cm.hot)


I think part of the problem is that Z is not a masked_array.  This works for me.  I set_bad to green to show the effect.  

Z = np.ma.masked_array(X**2 + Y**2, 
                       mask = (X**2+Y**2 <= r**2) | (X**2+Y**2 > R**2))
cm.hot.set_bad('green', alpha=None)
plot = pcolormesh(X, Y, Z, cmap=cm.hot)

> plt.plot(r*sp.cos(an), r*sp.sin(an), 'k')
> plt.plot(R*sp.cos(an), R*sp.sin(an), 'k')
> axis([-1.0, 1.0, -1.0, 1.0])
> 
> plt.show()

Hi,

I m trying to plot some data with pcolor. The data should be plotted on a ring. Inside and outside of the ring should be white area. However, now I have black area. Where is my mistake? Minimal working example:

from matplotlib import cm
import matplotlib.pyplot as plt
from pylab import *

import numpy as np
import scipy as sp
    
X = np.arange(-1.0, 1.01, 0.01)
Y = np.arange(-1.0, 1.01, 0.01)
X, Y = np.meshgrid(X, Y)

[r, R] = [0.25, 1.0]
an = sp.linspace(0,2*sp.pi,100)

fig = plt.figure(1, figsize=(6,6))

ax = fig.add_subplot(111, aspect = 'equal')
Z = X**2 + Y**2
Z[(X**2+Y**2 < r**2) | (X**2+Y**2 > R**2)] = np.ma.masked
cm.hot.set_bad('white', alpha=None)
plot = pcolor(X, Y, Z, cmap=cm.hot)
plt.plot(r*sp.cos(an), r*sp.sin(an), 'k')
plt.plot(R*sp.cos(an), R*sp.sin(an), 'k')
axis([-1.0, 1.0, -1.0, 1.0])

plt.show()