pylab_examples example code: image_masked.py

(Source code, png, pdf)

"""
imshow with masked array input and out-of-range colors.

The second subplot illustrates the use of BoundaryNorm to
get a filled contour effect.
"""
from copy import copy
from numpy import ma
import matplotlib.colors as colors
import matplotlib.pyplot as plt
import matplotlib.mlab as mlab
import numpy as np

# compute some interesting data
delta = 0.025
x = y = np.arange(-3.0, 3.0, delta)
X, Y = np.meshgrid(x, y)
Z1 = mlab.bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)
Z2 = mlab.bivariate_normal(X, Y, 1.5, 0.5, 1, 1)
Z = 10*(Z2 - Z1)  # difference of Gaussians

# Set up a colormap:
# use copy so that we do not mutate the global colormap instance
palette = copy(plt.cm.gray)
palette.set_over('r', 1.0)
palette.set_under('g', 1.0)
palette.set_bad('b', 1.0)
# Alternatively, we could use
# palette.set_bad(alpha = 0.0)
# to make the bad region transparent.  This is the default.
# If you comment out all the palette.set* lines, you will see
# all the defaults; under and over will be colored with the
# first and last colors in the palette, respectively.
Zm = ma.masked_where(Z > 1.2, Z)

# By setting vmin and vmax in the norm, we establish the
# range to which the regular palette color scale is applied.
# Anything above that range is colored based on palette.set_over, etc.

# set up the axes
fig, (ax1, ax2) = plt.subplots(1, 2)

# plot using 'continuous' color map
im = ax1.imshow(Zm, interpolation='bilinear',
                cmap=palette,
                norm=colors.Normalize(vmin=-1.0, vmax=1.0, clip=False),
                origin='lower', extent=[-3, 3, -3, 3])
ax1.set_title('Green=low, Red=high, Blue=bad')
fig.colorbar(im, extend='both', orientation='horizontal', shrink=0.8, ax=ax1)

# plot using 'discrete' color map
im = ax2.imshow(Zm, interpolation='nearest',
                cmap=palette,
                norm=colors.BoundaryNorm([-1, -0.5, -0.2, 0, 0.2, 0.5, 1],
                                         ncolors=256, clip=False),
                origin='lower', extent=[-3, 3, -3, 3])
ax2.set_title('With BoundaryNorm')
fig.colorbar(im, extend='both', spacing='proportional',
             orientation='horizontal', shrink=0.8, ax=ax2)

plt.show()

Keywords: python, matplotlib, pylab, example, codex (see Search examples)