Reinventing BGR to Grayscale OpenCV convert function in Python

Question

For academic purposes I want to reinvent Blue Green Red to Grayscale function in Python. I am new to Python so I believe my code below can still be optimized.

import cv2
import numpy as np


data = np.array([[[255, 0, 0], [0, 255, 0], [0, 0, 255]], [
    [0, 0, 0], [128, 128, 128], [255, 255, 255], ]], dtype=np.uint8)

rows = len(data)
cols = len(data[0])


grayed = []
for i in range(rows):
    row = []
    for j in range(cols):
        blue, green, red = data[i, j]
        gray = int(0.114 * blue + 0.587 * green + 0.299 * red)
        row.append(gray)
    grayed.append(row)
grayed = np.array(grayed, dtype=np.uint8)

print(data)
print(grayed)

wndData = "data"
wndGrayed = "greyed"

cv2.namedWindow(wndData, cv2.WINDOW_NORMAL)
cv2.imshow(wndData, data)
cv2.namedWindow(wndGrayed, cv2.WINDOW_NORMAL)
cv2.imshow(wndGrayed, grayed)
cv2.waitKey()

Could you review my code above and make it much better?

Answer 1

It appears that you are calculating linear combinations. If you are already using numpy, then the same can be achieved by broadcasting the dot product (with @):

import numpy as np

data = np.array([[[255, 0, 0], [0, 255, 0], [0, 0, 255]], 
                 [[0, 0, 0], [128, 128, 128], [255, 255, 255]]], dtype=np.uint8)

coefficients = np.array([0.114,0.587, 0.299])

greyed = (data @ coefficients).astype(np.uint8)