Bug report

Bug summary

Artifacts appear when plotting periodic data with discontinuities, i.e. something like x mod 1.0
The bug appears at when render resolution is relatively small comparing to the real resolution of the data (downsampling problem?).

Code for reproduction

import numpy as np
import matplotlib.pyplot as plt

xx, yy = np.meshgrid(np.linspace(-5, 5, 201), np.linspace(-5, 5, 201))
psi = xx % 1.0

plt.figure()
plt.imshow(psi, cmap="twilight")
plt.savefig("small.png", dpi=100)
plt.savefig("large.png", dpi=200)

Outcome

(large.png, small.png)

Matplotlib version

The issue is repeatable locally and in CoLab:

• Operating system: Linux
• Matplotlib version: 3.3.3, 3.2.2
• Matplotlib backend (): TkAgg, ipykernel.pylab.backend_inline
• Python version: 3.8.5, 3.7