Remove white traces around contours from tricontourf

Question

I'm trying to make a colormap in Python, and I have everything except this minor annoyance that is making the map look bad.

The code is straightforward. I am just using a matrix of values and plotting them using tricontourf. I am the looping over collections in my plot and changing the edgecolor and linewidth.

What I've noticed is the following. Say I want a thin white line around every contour, then I do.

CS = plt.tricontourf(X,Y,Z, 70, cmap=cm.seismic, antialiased=True)
print CS.collections

for c in CS.collections:
    c.set_edgecolor('white')
    c.set_linewidth(1)

plt.colorbar()  
plt.show()

and get

Now obviously we look at this and say, well, the white lines around the contours look pretty bad, lets get rid of them. You could do this in a number of ways, perhaps by setting the linewidth=0 or the color to 'none'. I'll just do both in the following code. We have

CS = plt.tricontourf(X,Y,Z, 70, cmap=cm.seismic, antialiased=True)
print CS.collections

for c in CS.collections:
    c.set_edgecolor('none')
    c.set_linewidth(0)

plt.colorbar()  
plt.show()

and get

Better, but do you still see the faint outlines of the contours? This is not just a shift in color from the colormap - this is clearly a very light line going through each contour.

Is there a way to somehow blend the colormap so that this doesn't happen? Thanks.

Answer 1

When you save a picture in pdf format, the problem becomes even more visible. With an increase in the number of contours, the picture is smoothed, but there are still problems with pdf.

For example:

import numpy as np
import matplotlib.pyplot as plt
from numpy.random import uniform
x = uniform(-2,2,200)
y = uniform(-2,2,200)
z = x*np.exp(-x**2-y**2)
plt.tricontourf(x,y,z, 300, cmap="seismic", antialiased=False)
plt.colorbar()
plt.savefig('stackoverflow/tricontourf.pdf')
plt.savefig('stackoverflow/tricontourf.png', dpi=300)

But very light lines running through each contour are still visible in pdf.

A partial solution is to use tricontour instead of tricontourf with some linewidths option:

import numpy as np
import matplotlib.pyplot as plt
from numpy.random import uniform
#Some Data
x = uniform(-2,2,200)
y = uniform(-2,2,200)
z = x*np.exp(-x**2-y**2)

plt.tricontour(x,y,z, 300, cmap="seismic", antialiased=False, linewidths=5)
plt.colorbar()  

plt.savefig('stackoverflow/tricontour.pdf')
plt.savefig('stackoverflow/tricontour.png', dpi=300)

tricontourf does not support this option.

Answer 2

You can overlap a tricontour plot with the same colormap used in your tricontourf. This will effectively get rid of the white traces.