GUI programming/arrays in Python -

i need little more elaboration on arrays in gui programming. image processing, given following piece of code example of image processing:

def grayscale(im):     height=len(im)     width = len(im[0])     row in range(height):         col in range(width):             average = sum(im[row][col])/3             im[row][col]=[average,average,average]     return im 

the final line before code returns--what mean? code supposed running through pixel pixel , averaging out rgb values grayscale value--how sum of each pixel/3 average? how code know red, blue, , green values are?

each color in pixel defined variation of 3 components : red, green , blue, oscillating between 0 , 255, creating 16 millions colors (255^3).

a shade of gray represented 3 identical values red, green , blue.

there several methods achieve grayscale convertion, including quick n' dirty average method (see others here).

since grayscale needs 1 scale (from white black), can average 3 components formula (r + g + b) / 3 make new color value each color component, creating gray shade.