RGB to Grayscale (Average Method) Python

Question

I'm supposed to write a method that converts an RGB image to Grayscale by using the "average method" where I take the average of the 3 colors (not the weighted method or luminosity method). I then must display the original RGB image and grayscale image next to each other (concatenated). The language I'm writing in is Python. This is what my code looks like currently.

import numpy as np
import cv2

def getRed(redVal):
    return '#%02x%02x%02x' % (redVal, 0, 0)

def getGreen(greenVal):
    return '#%02x%02x%02x' % (0, greenVal, 0)

def getBlue(blueVal):
    return '#%02x%02x%02x' % (0, 0, blueVal)

# Grayscale = (R + G + B / 3)
# For each pixel,
# 1- Get pixels red, green, and blue
# 2- Calculate the average value
# 3- Set each of red, green, and blue values to average value

def average_method(img):
    for p in img:
        red = p.getRed()
        green = p.getGreen()
        blue = p.getBlue()
        average = (red + green + blue) / 3
        p.setRed(average)
        p.setGreen(average)
        p.setBlue(average)

def main():
    img1 = cv2.imread('html/images/sun.jpeg')
    img1 = cv2.resize(img1, (0, 0), None, .50, .50)
    img2 = average_method(img1)
    img2 = np.stack(3 * [img2], axis=2)
    numpy_concat = np.concatenate((img1, img2), 1)
    cv2.imshow('Numpy Concat', numpy_concat)
    cv2.waitKey(0)
    cv2.destroyAllWindows

if __name__ =="__main__":
    main()

The portion that is commented within the average_method function is the steps that I must follow.

When I try to run the code, I get

  File "test.py", line 38, in <module>
    main()
  File "test.py", line 30, in main
    img2 = average_method(img1)
  File "test.py", line 15, in average_method
    red = p.getRed()
AttributeError: 'numpy.ndarray' object has no attribute 'getRed' 

I thought that defining the functions for getRed, getGreen, and getBlue up above would mean they'd become recognizable in my average_method function (I got those functions from online so I hope they're right). I'm also not sure what it has to do with numpy.ndarray. If anyone could help me fill in this average_method function with code that follows the commented steps correctly, I would really appreciate it.

EDIT::: New code looks like this:

import cv2
import numpy as np

def average_method(img):
    for p in img:
        gray = sum(p)/3
        for i in range(3):
            p[i] = gray

def main():
    img1 = cv2.imread('html/images/sun.jpeg')
    img1 = cv2.resize(img1, (0, 0), None, .50, .50)
    img2 = average_method(img1)
    img2 = np.stack(3 * [img2], axis=2)
    numpy_concat = np.concatenate((img1, img2), 1)
    cv2.imshow('Numpy Concat', numpy_concat)
    cv2.waitKey(0)
    cv2.destroyAllWindows

if __name__ =="__main__":
    main()

I now get the error

  File "test.py", line 50, in <module>
    main()
  File "test.py", line 43, in main
    img2 = np.stack(3 * [img2], axis=2)
  File "<__array_function__ internals>", line 5, in stack
  File "C:\Users\myname\AppData\Local\Programs\Python\Python38-32\lib\site-packages\numpy\core\shape_base.py", line 430, in stack
    axis = normalize_axis_index(axis, result_ndim)
numpy.AxisError: axis 2 is out of bounds for array of dimension 1

I have that line "img2 = np.stack(3 * [img2], axis=2)" since I was previously told on Stack Overflow I need it due to my img2 now being a greyscale (single-channel) image, when img1 is still color (three-channel). This line apparently fixes that. But now it seems like there is something wrong with that?

Answer 1

In Java, the for loop you highlighted is called an "enhanced for loop". Python doesn't have these because Python for loops pog (in terms of concision).

The Python equivalent of the line in question would be:

for p in img:

No need to state object class or anything like that.

EDIT: After OP changed question

The problem now is that you're not calling the functions correctly. p is an array containing the RGB values for that pixel. To call the function as you defined above do:

for p in img:
    red = getRed(p[0])
    green = getGreen(p[1])
    blue = getBlue(p[2])
    average = (red + green + blue) / 3
    p[0] = average
    p[1] = average
    p[2] = average

Remember when you moved the code to Python, you seem to no longer be working in Object Oriented Programming! Pixels don't come with methods that you can call like that anymore.

However, as pointed out by Guimoute in the comments, the code can be much simpler if you get rid of the get[Color] functions and do the following:

for p in img:
    gray = sum(p)/3
    for i in range(3):
        p[i] = gray