Display image as grayscale using matplotlib


Question

I'm trying to display a grayscale image using matplotlib.pyplot.imshow(). My problem is that the grayscale image is displayed as a colormap. I need the grayscale because I want to draw on top of the image with color.

I read in the image and convert to grayscale using PIL's Image.open().convert("L")

image = Image.open(file).convert("L")

Then I convert the image to a matrix so that I can easily do some image processing using

matrix = scipy.misc.fromimage(image, 0)

However, when I do

figure()  
matplotlib.pyplot.imshow(matrix)  
show()

it displays the image using a colormap (i.e. it's not grayscale).

What am I doing wrong here?

1
202
11/28/2016 10:15:37 AM

Accepted Answer

The following code will load an image from a file image.png and will display it as grayscale.

import numpy as np
import matplotlib.pyplot as plt
from PIL import Image

fname = 'image.png'
image = Image.open(fname).convert("L")
arr = np.asarray(image)
plt.imshow(arr, cmap='gray', vmin=0, vmax=255)
plt.show()

If you want to display the inverse grayscale, switch the cmap to cmap='gray_r'.

312
4/3/2019 1:36:13 PM

Try to use a grayscale colormap?

E.g. something like

imshow(..., cmap=pyplot.cm.binary)

For a list of colormaps, see http://scipy-cookbook.readthedocs.org/items/Matplotlib_Show_colormaps.html


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