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

User · Answer

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

User · Answer

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

User · Answer

Use no interpolation and set to gray   import matplotlib pyplot as plt plt imshow img     1   cmap  gray  interpolation  none

User · 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

User · Answer

I would use the get cmap method  Ex    import matplotlib pyplot as plt  plt imshow matrix  cmap plt get cmap  gray

User · Answer

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

User · Answer

try this   import pylab from scipy import misc  pylab imshow misc lena   cmap pylab gray    pylab show

[python] Display image as grayscale using matplotlib

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