Drawing Histogram in OpenCV-Python

Question

I was just trying to draw histogram using new OpenCV Python interface ( cv2 ).

Below is the code i tried:

import cv2
import numpy as np
import time

img = cv2.imread('zzz.jpg')
h = np.zeros((300,256,3))
b,g,r = cv2.split(img)
bins = np.arange(256).reshape(256,1)
color = [ (255,0,0),(0,255,0),(0,0,255) ]

for item,col in zip([b,g,r],color):
    hist_item = cv2.calcHist([item],[0],None,[256],[0,255])
    cv2.normalize(hist_item,hist_item,0,255,cv2.NORM_MINMAX)
    hist=np.int32(np.around(hist_item))
    pts = np.column_stack((bins,hist))
    cv2.polylines(h,[pts],False,col)

h=np.flipud(h)

cv2.imshow('colorhist',h)
cv2.waitKey(0)

And it works fine. Below is the resulting histogram i obtained.

---

Then i modified the code a little bit.

ie changed the sixth line in code b,g,r = cv2.split(img) to b,g,r = img[:,:,0], img[:,:,1], img[:,:,2] (because it works a little faster than cv2.split).

Now the output is something different. Below is the output.

---

I checked the values of b,g,r from both the codes. They are same.

Difference lies in the output of cv2.calcHist. Result of hist_item is different in both the cases.

Question:

How does it happen? Why the result of cv2.calcHist is different when inputs are same?

EDIT

I tried a different code. Now, a numpy version of my first code.

import cv2
import numpy as np

img = cv2.imread('zzz.jpg')
h = np.zeros((300,256,3))
b,g,r = img[:,:,0],img[:,:,1],img[:,:,2]
bins = np.arange(257)
bin = bins[0:-1]
color = [ (255,0,0),(0,255,0),(0,0,255) ]

for item,col in zip([b,g,r],color):
    N,bins = np.histogram(item,bins)
    v=N.max()
    N = np.int32(np.around((N*255)/v))
    N=N.reshape(256,1)
    pts = np.column_stack((bin,N))
    cv2.polylines(h,[pts],False,col,2)

h=np.flipud(h)

cv2.imshow('img',h)
cv2.waitKey(0)

And the output is same as first one.

You can get my original image here: zzz.jpg

Thank you.

Solution

You should copy the array:

b,g,r = img[:,:,0].copy(), img[:,:,1].copy(), img[:,:,2].copy()

But, since calcHist() can accept channels parameter, you need not to split your img to three array.

import cv2
import numpy as np

img = cv2.imread('zzzyj.jpg')
h = np.zeros((300,256,3))

bins = np.arange(256).reshape(256,1)
color = [ (255,0,0),(0,255,0),(0,0,255) ]

for ch, col in enumerate(color):
    hist_item = cv2.calcHist([img],[ch],None,[256],[0,255])
    cv2.normalize(hist_item,hist_item,0,255,cv2.NORM_MINMAX)
    hist=np.int32(np.around(hist_item))
    pts = np.column_stack((bins,hist))
    cv2.polylines(h,[pts],False,col)

h=np.flipud(h)

cv2.imshow('colorhist',h)
cv2.waitKey(0)

Comments

[Abid Rahman K]

What is the need for copy? And what is meant by calcHist() accept channel parameter? What does it actually denote?

[HYRY]

you can call cv2.calcHist([img],[CH],None,[256],[0,255]) to calculate the histogram of channel CH of img, that is img[:, :, CH]. You need to copy the array because data in img[:, :, 0] is not continuous.

[Abid Rahman K]

you mean c_contiguous? What is its significance?

[Neon22]

numpy is more efficent at manipulating arrays and is used in cv2. the python split operator doesn't keep all the data together. the copy operator collects them neatly.

[user601836]

but is it me or the image size is not 300 * 256?

[letmaik]

In the end, it's really the task of OpenCV's Python wrapper to make sure that the array is C-contiguous, by calling np.ascontiguousarray. Just my 2 cents..

[dorbodwolf]

@user601836 I have the same problems