Display image as grayscale

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 it to be 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?

unutbu · Accepted Answer · 2019-04-03 13:36:13Z

522

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'.

edited Apr 3, 2019 at 13:36

answered Sep 29, 2010 at 16:40

unutbu

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3 Comments

unutbu Over a year ago

@jay: Per this doc string, "Note that any colormap listed here can be reversed by appending _r."

themadmax Over a year ago

Warning matplotlib adjust pixel scale intensity if you do not want use: plt.imshow(im_gray,cmap='gray', vmin = 0, vmax = 255)

prosti Over a year ago

You don't need to convert the image to single channel convert("L") if the only thing you need is to show the image as gray cmap="gray" is what you need in imshow. I have similar example in OpenCV for single channel image, but you can open any RGB image and show it with cmap='gray'.

janneb · Accepted Answer · 2016-01-26 10:50:56Z

43

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

edited Jan 26, 2016 at 10:50

answered Sep 29, 2010 at 16:39

janneb

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Comments

SherylHohman · Accepted Answer · 2017-06-19 11:18:04Z

31

import matplotlib.pyplot as plt

You can also run once in your code

plt.gray()

This will show the images in grayscale as default

im = array(Image.open('I_am_batman.jpg').convert('L'))
plt.imshow(im)
plt.show()

edited Jun 19, 2017 at 11:18

SherylHohman

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answered Jul 22, 2012 at 20:49

Eliel Van Hojman

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3 Comments

SherylHohman Over a year ago

Which library are you using for gray() or grey() ?

Cloud Cho Over a year ago

@SherylHohman, it is gray... gray is standard American spelling and grey is British spelling for the same color.

BenjaminWegener Over a year ago

@SherylHohman thats matplotlib, install with pip install matplotlib

Al Conrad · Accepted Answer · 2015-12-02 16:24:32Z

18

I would use the get_cmap method. Ex.:

import matplotlib.pyplot as plt

plt.imshow(matrix, cmap=plt.get_cmap('gray'))

answered Dec 2, 2015 at 16:24

Al Conrad

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Comments

Stanley · Accepted Answer · 2018-03-06 08:39:40Z

17

@unutbu's answer is quite close to the right answer.

By default, plt.imshow() will try to scale your (MxN) array data to 0.0~1.0. And then map to 0~255. For most natural taken images, this is fine, you won't see a different. But if you have narrow range of pixel value image, say the min pixel is 156 and the max pixel is 234. The gray image will looks totally wrong. The right way to show an image in gray is

from matplotlib.colors import NoNorm
...
plt.imshow(img,cmap='gray',norm=NoNorm())
...

Let's see an example:

this is the origianl image: original

this is using defaul norm setting,which is None: wrong pic

this is using NoNorm setting,which is NoNorm(): right pic

answered Mar 6, 2018 at 8:39

Stanley

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2 Comments

Axel Bregnsbo Over a year ago

In Colab I get error when using NoNorm, however norm=None works

BenjaminWegener Over a year ago

@AxelBregnsbo norm=NoNorm() works for me in colab

baco · Accepted Answer · 2012-03-09 15:00:54Z

10

try this:

import pylab
from scipy import misc

pylab.imshow(misc.lena(),cmap=pylab.gray())
pylab.show()

answered Mar 9, 2012 at 15:00

baco

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1 Comment

Frames Catherine White Over a year ago

there is no pylab.grey(), maybe it has been removed?

Ignatius · Accepted Answer · 2019-04-15 06:05:01Z

10

Use no interpolation and set to gray.

import matplotlib.pyplot as plt
plt.imshow(img[:,:,1], cmap='gray',interpolation='none')

edited Apr 15, 2019 at 6:05

Ignatius

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answered Apr 15, 2019 at 5:37

user3452134

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Comments

deepnmr · Accepted Answer · 2021-11-16 06:23:01Z

plt.imshow(img[:,:,0], cmap='gray')

plt.imshow(img[:,:,1], cmap='gray')

plt.imshow(img[:,:,2], cmap='gray')

should work. But, the issue with this approach is that it is not true gray. It only changes one of RGB channel to gray.

look below.

from sklearn.datasets import load_sample_image
flower = load_sample_image("flower.jpg")

plt.subplot(1,4,1)
plt.imshow(flower)
plt.axis("off")
plt.title("Original")

# R level to gray
plt.subplot(1,4,2)
plt.imshow(flower[:,:,0], cmap='gray')
plt.axis("off")
plt.title("R to gray")


# G leval to gray
plt.subplot(1,4,3)
plt.imshow(flower[:,:,1], cmap='gray')
plt.axis("off")
plt.title("R to gray")

# B leval to gray
plt.subplot(1,4,4)
plt.imshow(flower[:,:,2], cmap='gray')
plt.axis("off")
plt.title("R to gray")

plt.show()

[Result images]

niek tuytel · Accepted Answer · 2021-07-21 10:56:37Z

1

When the image has purple & yellow color.

change way of saving image:
plt.imsave(...., cmap='gray')

answered Jul 21, 2021 at 10:56

niek tuytel

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