missing some bins values in matplotlib histogram

Question

With the following lines of code, I want to print the rgb histograms of a given image. The overall result is good, but comparing the three histograms with the ones obtained with gimp I noticed that some bins inside the histogram are missing, I mean that there is a white value where there should be a number different from 0 of pixel associated to that tone.

here is my code:

im = np.array(Image.open('myimage.jpg'))

plt.figure()
plt.hist(im[:,:,0].flatten(), 256, color='red', label='Histogram Red')

plt.figure()
plt.hist(im[:,:,1].flatten(), 256, color='green', label='Histogram Green')

plt.figure()
plt.hist(im[:,:,2].flatten(), 256, color='blue', label='Histogram Blue')

plt.show()

tacaswell · Accepted Answer · 2013-01-21 17:16:40Z

3

You haven't really given enough information to answer your question, but my guess is that you are having issues with bin edges. Giving a number of bins like you did will divide up the range of your data in to 256 bins, but your data is in the range [12,230] (that is np.min(img[:,:0]) == 12 and np.max(img[:,:,0]) == 230), then your bins will be less than 1 wide and you may get strange aliasing. I would instead use

 plot.hist(...,bins=arange(0,257) - .5,...)

Which explicitly passes in the bin edges to use (I am infering that you data in 8 bit ints). This will give you 256 bars, centered on the integers.

For aesthetic reasons I also would add lw=0 to get rid of the black outline around each bar.

edited Jan 21, 2013 at 17:16

answered Jan 18, 2013 at 15:47

tacaswell

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3 Comments

user601836 Over a year ago

when you say: "but your data is in the range [12,230]" you actually mean "if" right?because my values are in range [0,255]

tacaswell Over a year ago

@user601836 see edit, I mean the actual range of your data, not the range of int8.

user601836 Over a year ago

i found another way: plt.hist(im[:,:,0].flatten(), bins=256, range=[0,255], histtype='stepfilled', color='red')

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