matplotlib.pyplot.imshow does not work as expected

[Kulbear]

Bug report

Bug summary

matplotlib.pyplot.imshow does not work as expected.

Actual outcome

You can see I actually add 4 times mean (which is 24 for this image), and there is nothing changed.

Expected outcome

After uncommenting the line I assigned one pixel to value 0. It changed to:

Matplotlib version

• Operating system: Window 10
• Matplotlib version: 2.2.2
• Matplotlib backend: module://ipykernel.pylab.backend_inline
• Python version: 3.6.5
• Jupyter version (if applicable): jupyter==1.0.0
  jupyter-client==5.2.3
  jupyter-console==5.2.0
  jupyter-core==4.4.0
  jupyterlab==0.32.1
  jupyterlab-launcher==0.10.5

pip

[QuLogic]

You are passing a 2D array (MxN), which gets colour mapped. The default normalization for the mapping is the data limits, so if you add a constant to all elements, the result will not change. If you want the change to be visible, then you should specify some limits for the normalization with vmin and/or vmax.

Closing as this is not a bug.

[tacaswell]

@Kulbear is https://matplotlib.org/tutorials/colors/colormapnorms.html useful for understanding the behavior?

[Kulbear]

@tacaswell @QuLogic Thanks the documentation is helpful.