CODEWITHSUNDEEP
pythonnumpymatplotlibpython-imaging-librarycolor-mapping
heltonbiker
heltonbiker

Reputation: 27575

How to convert a NumPy array to PIL image applying matplotlib colormap

I want to take a NumPy 2D array which represents a grayscale image, and convert it to an RGB PIL image while applying some of the matplotlib colormaps.

I can get a reasonable PNG output by using the pyplot.figure.figimage command:

dpi = 100.0
w, h = myarray.shape[1]/dpi, myarray.shape[0]/dpi
fig = plt.figure(figsize=(w,h), dpi=dpi)
fig.figimage(sub, cmap=cm.gist_earth)
plt.savefig('out.png')

Although I could adapt this to get what I want (probably using StringIO do get the PIL image), I wonder if there is not a simpler way to do that, since it seems to be a very natural problem of image visualization. Let's say, something like this:

colored_PIL_image = magic_function(array, cmap)

Upvotes: 293

Views: 644508

Answers (3)

aravinda_gn
aravinda_gn

Reputation: 1360

  • input = numpy_image

  • np.uint8 -> converts to integers

  • convert('RGB') -> converts to RGB

  • Image.fromarray -> returns an image object

      from PIL import Image
  import numpy as np

  PIL_image = Image.fromarray(np.uint8(numpy_image)).convert('RGB')

  PIL_image = Image.fromarray(numpy_image.astype('uint8'), 'RGB')

Upvotes: 92

Shital Shah
Shital Shah

Reputation: 68728

The method described in the accepted answer didn't work for me even after applying changes mentioned in its comments. But the below simple code worked:

import matplotlib.pyplot as plt
plt.imsave(filename, np_array, cmap='Greys')

np_array could be either a 2D array with values from 0..1 floats o2 0..255 uint8, and in that case it needs cmap. For 3D arrays, cmap will be ignored.

Upvotes: 12

fraxel
fraxel

Reputation: 35269

Quite a busy one-liner, but here it is:

  1. First ensure your NumPy array, myarray, is normalised with the max value at 1.0.
  2. Apply the colormap directly to myarray.
  3. Rescale to the 0-255 range.
  4. Convert to integers, using np.uint8().
  5. Use Image.fromarray().

And you're done:

from PIL import Image
from matplotlib import cm
im = Image.fromarray(np.uint8(cm.gist_earth(myarray)*255))

with plt.savefig():

Enter image description here

with im.save():

Enter image description here

Upvotes: 399

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