Invert colors when plotting a PNG file using matplotlib

Question

I'm trying to display a PNG file using matplotlib and of course, python. For this test, I've generated the following image:

Now, I load and transform the image into a multidimensional numpy matrix:

import numpy as np
import cv2
from matplotlib import pyplot as plt

cube = cv2.imread('Graphics/Display.png')
plt.imshow(cube)
plt.ion()

When I try to plot that image in matplotlib, the colors are inverted:

If the matrix does not have any modifications, why the colors in the plot are wrong?

Thanks in advance.

Solution 1

It appears that you may somehow have RGB switched with BGR. Notice that your greens are retained but all the blues turned to red. If cube has shape (M,N,3), try swapping cube[:,:,0] with cube[:,:,2]. You can do that with numpy like so:

rgb = numpy.fliplr(cube.reshape(-1,3)).reshape(cube.shape)

From the OpenCV documentation:

Note: In the case of color images, the decoded images will have the channels stored in B G R order.

Solution 2

Try:

plt.imshow(cv2.cvtColor(cube, cv2.COLOR_BGR2RGB))

Solution 3

As others have pointed out, the problem is that numpy arrays are in BGR format, but matplotlib expects the arrays to be ordered in a different way.

You are looking for scipy.misc.toimage:

import scipy.misc
rgb = scipy.misc.toimage(cube)

Alternatively, you can use scipy.misc.imshow().

Comments

[Mark Ransom]

Good workaround, but the question still remains: why did it happen?

[pyan]

Nice elegant solution!

[Ryan Soklaski]

There is an easier way to flip the color channels: cube = cube[..., ::-1]

[Kaaf]

@Mark Ransom OpenCV is quite old, and these color orders were chosen way back then before RGB became the de facto standard order, BGR made more sense since it is in alphabetical order, and was widely used at the time.

[Kaaf]

@Mark Ransom Well, a lot of products used BGR back then, RGB was not as universal as it is today, products like the Game Gear, the wonderSwan and more importantly for this subject, camera makers, image software providers and Windows itself all used BGR, and I don't need to explain to you why the last three were an important decision point for a library like OpenCV. For more details see this: learnopencv.com/why-does-opencv-use-bgr-color-format:

[Mark Ransom]

@Kaaf thanks for the link. I knew that BGR was used by Windows, but that wasn't until the '80's. I still don't have a single example of it being used before then.

[Mr-Programs]

wonder if there isnt a cmap kwarg solution to this, such as cmap=bgr