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          


        

Answer 1

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)