使用 OpenCV 和 Python 获取图像中颜色的百分比

Question

So I have this python script that detects and print a range of HSV color in an image but i want to add another functionality to it.

It want it to be able to print the percentage of that color.

My Script:

import cv2
import numpy as np
import matplotlib.pyplot as plt


img = cv2.imread('C:/Users/Vishu Rana/Documents/PY/test_cases/image.jpg')


grid_RGB = cv2.cvtColor(img, cv2.COLOR_BGR2RGB)
plt.figure(figsize=(20,8))
plt.imshow(grid_RGB) # Printing the original picture after converting to RGB


grid_HSV = cv2.cvtColor(grid_RGB, cv2.COLOR_RGB2HSV) # Converting to HSV
lower_green = np.array([25,52,72])
upper_green = np.array([102,255,255])

mask= cv2.inRange(grid_HSV, lower_green, upper_green)
res = cv2.bitwise_and(img, img, mask=mask) # Generating image with the green part

print("Green Part of Image")
plt.figure(figsize=(20,8))
plt.imshow(res)

What do i need to add in this code to make it also print the percentage of green color.

Answer 1

OpenCV arrays create a mask that uses the value 255. A simple way to get the percentage of green is simply implement the following code after you generate the mask.

    green_perc = (mask>0).mean()

A more thorough explanation was asked about why this works. When OpenCV creates a mask it will create a 2 dimensional array with values of 0s and values of 255. In the context of this question the values of 255 are the parts of the mask that indicate the picture is green.

The reason that (mask>0).mean() works is because we only have values of 255 and 0. Mask > 0 will create a boolean array of True/False for every value in our mask.

The value True will indicate that the part of the array is green and the value of False will indicate it is not green. Taking the mean of this boolean array will give us the percentage of the array that is green. (when taking the mean of a boolean array True will take the value of 1 and False will take the value of 0).

Another way to get the same result is to implement code like this.

green_perc = (mask==255).mean()

A comment above also mentions the solution of np.sum(mask)/np.size(mask). This does not work right because OpenCV uses the value 255. You could tweak this to get the same percentage by dividing this value by 255.

green_perc = (np.sum(mask) / np.size(mask))/255