Drawing a histogram in opencv python

Question

I want to detect lane departures of vehicles by processing front camera video.For that I want to select one pixel line of each frame horizontally and analyze how those pixels colors change. For that I want to draw histograms for R,G,B channels which has pixel number from left to right in x axis and the corresponding pixel value in y axis. Since I'm new to this area, I would like to know how can I draw this histogram for each frame in video by using opencv and python ? Thank you.

Answer 1

This is for histogram of image (opencv+matplotlib):

#!/usr/bin/python3
# 2017.12.20 14:00:12 CST
# 2018.01.06 19:07:57 CST
import matplotlib.pyplot as plt
import numpy as np
import cv2

img = cv2.imread("test.png")
b,g,r = cv2.split(img)
fig = plt.figure(figsize=(8,4))

ax = fig.add_subplot(121)
ax.imshow(img[...,::-1])

ax = fig.add_subplot(122)
for x, c in zip([b,g,r], ["b", "g", "r"]):
    xs = np.arange(256)
    ys = cv2.calcHist([x], [0], None, [256], [0,256])
    ax.plot(xs, ys.ravel(), color=c)

ax.set_xlabel('X')
ax.set_ylabel('Y')
plt.show()

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Here are two examples about draw 3D histogram, you can take for reference.

(1) How to plot 3D histogram of an image in OpenCV

(2) 3d plot of list of (hist, bin_edges) where histogram bar chart or lines are in the z-y plane