How to convert a NumPy array to PIL image applying matplotlib colormap
Question:
I have a simple problem, but I cannot find a good solution to it.
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)
Answers:
Quite a busy one-liner, but here it is:
- First ensure your NumPy array,
myarray, is normalised with the max value at 1.0.
- Apply the colormap directly to
myarray.
- Rescale to the
0-255 range.
- Convert to integers, using
np.uint8().
- 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():
with im.save():
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.
-
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')
I have a simple problem, but I cannot find a good solution to it.
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)
Quite a busy one-liner, but here it is:
- First ensure your NumPy array,
myarray, is normalised with the max value at1.0. - Apply the colormap directly to
myarray. - Rescale to the
0-255range. - Convert to integers, using
np.uint8(). - 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():
with im.save():
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.
-
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')