Save matplotlib plot in grayscale

Question

I have some color plots that I need to be saved in grayscale. Is there an easy way to do this without changing the plotting formats?

There were some discussion around that, see convert figure from color to BW/grayscale. Don't know whether the set_gray attribute is available or not in the latest release of matplotlib. — chl
– chl, Commented May 16, 2011 at 18:29
You could use imagemagick afterward: convert -type Grayscale old.png new.png I think there is already something on SO cf link — Pierre Guilbert
– Pierre Guilbert, Commented May 17, 2011 at 20:53

Jake Zhang · Accepted Answer · 2018-04-11 23:22:15Z

15

There is an easy solution:

plt.imsave(filename, image, cmap='gray')

answered Apr 11, 2018 at 23:22

Jake Zhang

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Mu Mind · Accepted Answer · 2011-05-23 17:45:17Z

9

It's currently awkward to do directly from matplotlib, but in "the future" they plan to support a set_gray(True) call on the figure (see the mailing list thread here).

Your best options will be to save it in color and convert it, either in python with PIL:

import Image
Image.open('color.png').convert('L').save('bw.png')

or from the command line with imagemagick:

convert -type Grayscale color.png bw.png

answered May 23, 2011 at 17:45

Mu Mind

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1 Comment

Ian Goodfellow Over a year ago

Like the other answers, this answer is not very satisfactory because it rasterizes the plot.

Tot Zam · Accepted Answer · 2017-08-21 16:38:12Z

5

Actually, this was asked before. Here is a pretty good answer, which comes 2nd on google (as of today):
Display image as grayscale using matplotlib

And the solution is VERY similar to Suki's...

Oh, well, I am bored, so I post here also a full blown code:

import numpy as np
import pylab as p
xv=np.ones(4)*.5
yv=np.arange(0,4,1)
xv1=np.ones(4)*-.5
yv1=np.arange(0,4,1)

#red vertical line on the right
yv2=np.arange(0,1.5,0.1)
xv2=np.ones_like(yv2)*.7

#red vertical line on the left
yv3=np.arange(0,2,0.01)
xv3=np.ones_like(yv3)*-0.7

###
xc=np.arange(-1.4,2,0.05)
yc=np.ones_like(xc)*1

fig = p.figure()
ax1 = fig.add_subplot(111)
#adjustprops = dict(left=0.12, bottom=0.2, right=0.965, top=0.96, wspace=0.13, hspace=0.37)
#fig.subplots_adjust(**adjustprops)
ax1.plot(xv,yv, color='blue', lw=1, linestyle='dashed')
ax1.plot(xv1,yv1, 'green', linestyle='dashed')
ax1.plot(np.r_[-1:1:0.2],np.r_[-1:1:0.2],'red')
ax1.plot(xc,yc, 'k.', markersize=3)

p.savefig('colored_image.png')

import matplotlib.image as mpimg
import matplotlib.cm as cm
import Image

figprops = dict(figsize=(10,10), dpi=100)
fig1 = p.figure(**figprops)
#fig1 = p.figure()
#ax1 = fig.add_subplot(111)
adjustprops = dict()
image=Image.open('colored_image.png').convert("L")
arr=np.asarray(image)
p.figimage(arr,cmap=cm.Greys_r)
p.savefig('grayed.png')
p.savefig('grayed.pdf',papertype='a4',orientation='portrait')

This will produce a graph in color, than read it, convert it to gray scale, and will save a png and pdf.

edited Aug 21, 2017 at 16:38

Tot Zam

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answered Jun 29, 2011 at 9:25

oz123

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2 Comments

Ian Goodfellow Over a year ago

This answer is not very good because it requires rasterizing the plot and any fonts in it. The rasterization occurs when you convert to .png. It would be better to write a pdf to the filesystem and then convert that to grayscale, perhaps using ghostscript.

warunapww Over a year ago

"import Image" no longer works. Instead use "from PIL import Image"

jez · Accepted Answer · 2018-09-07 23:29:02Z

4

I am struggling with this issue too. As far as I can tell, matplotlib doesn't support direct conversion to grayscale, but you can save a color pdf and then convert it to grayscale with ghostscript:

gs -sOutputFile=gray.pdf -sDEVICE=pdfwrite -sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray -dNOPAUSE -dBATCH -dAutoRotatePages=/None color.pdf

edited Sep 7, 2018 at 23:29

jez

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answered Apr 25, 2015 at 5:13

Ian Goodfellow

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5 Comments

Paul H Over a year ago

why not just use the grayscale style sheet? matplotlib.org/examples/style_sheets/plot_grayscale.html

Ian Goodfellow Over a year ago

That doesn't actually convert a color plot to grayscale. It just makes some elements gray by default. I have plots that I want to export as both color for a web page and convert to gray for printing.

Paul H Over a year ago

You can use the style sheets within a context manager. Wrap your calls to a non-data processing plotting function with those.

Ian Goodfellow Over a year ago

I don't understand this suggestion. Why does a context manager help? How is data processing relevant?

Paul H Over a year ago

if you plotting function does a lot of data processing, making multiple calls to it will slow down the flow of your program. But pre-processing the data and then calling a simpler plotting function within a colorful then grayscale style context manager should be pretty manageable.

suki · Accepted Answer · 2011-06-25 10:49:08Z

0

And to add to Mu Mind's solution

If for whatever reason you want to avoid writing it to a file you can use StringIO like a file:


 import Image
 import pylab
 from StringIO import StringIO

 pylab.plot(range(10),[x**2 for x in range(10)])

 IO = StringIO()
 pylab.savefig(IO,format='png')
 IO.seek(0)

 #this, I stole from Mu Mind solution
 Image.open(IO).convert('L').show()

answered Jun 25, 2011 at 10:49

suki

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1 Comment

Ian Goodfellow Over a year ago

Like the other answers, this answer is not very satisfactory because it rasterizes the plot.

yeeking · Accepted Answer · 2023-07-30 13:29:23Z

Developed from Ian Goodfellow's answer, here is a python script that generates and runs a ghostscript command that converts a PDF into grayscale. It is preferable to the answers that rasterise to PNG as it retains the vector representation of the plot.

import subprocess
import sys 

def pdf_to_grayscale(input_pdf, output_pdf):
    try:
        # Ghostscript command to convert PDF to grayscale
        ghostscript_cmd = [
            "gs",
            "-sDEVICE=pdfwrite",
            "-sColorConversionStrategy=Gray",
            "-sProcessColorModel=DeviceGray",
            "-dCompatibilityLevel=1.4",
            "-dNOPAUSE",
            "-dQUIET",
            "-dBATCH",
            f"-sOutputFile={output_pdf}",
            input_pdf
        ]

        # Execute Ghostscript command using subprocess
        subprocess.run(ghostscript_cmd, check=True)

        print("PDF converted to grayscale successfully.")
    except subprocess.CalledProcessError:
        print("Error occurred during PDF conversion to grayscale.")

if __name__ == "__main__":
    assert len(sys.argv) == 3, "Two args: input pdf and output pdf"    
    pdf_to_grayscale(sys.argv[1], sys.argv[2])

Save it as to_gray.py and run it like this:

python to_gray.py test.pdf gray.pdf

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