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I would like to present a histogram from an image in Python. Doing some research, I found a way of doing it using matplotlib. So, I just do this:

im = plt.array(Image.open('Mean.png').convert('L'))
plt.figure()
plt.hist(im, facecolor='green', alpha=0.75)
plt.savefig("Histogram.png")

But I didn't like what I got:

Histogram

The bars are not green, and it's kind of complicated to read the histogram. I could not even figure out if the x axis is the number of points and y axis the rgb color or the inverse... :S I would like to know if somebody knows how could I turn this histogram more readable.

Thanks in advance.

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To get the histogram you have to flatten your image:

img = np.asarray(Image.open('your_image').convert('L'))
plt.hist(img.flatten(), facecolor='green', alpha=0.75)

Since you converted the image to grayscale with convert('L') the x axis is the grayscale level from 0-255 and the y axis is the number of pixels.

You can also control the number of bins using the bins parameter:

plt.hist(img.flatten(), bins=100, facecolor='green', alpha=0.75)
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2 Comments

Do you know how could I do this passing an list of list? In this case, I can't use the flatten method, but doing this plt.hist(self.data, bins=10, facecolor='green', alpha=0.5), I got something like the image that I posted before. I don't get it. :S
@pceccon You have to pass in a one dimensional structure. If you have a list of lists you have to flatten in somehow... Many possible solutions here. The easiest, but not the fastest would be just to do plt.hist(sum(self.data), ...)

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