Reference code for iterating through a 2d array (not literally, in this sense) via either Row/Col, and optionally in a zigzag pattern.

python_2d_iteration.py

pixels_array = [
    0, 1, 2, 3,
    4, 5, 6, 7,
    8, 9, 10, 11
]
width, height = (4, 3)


order = "ROW"
zigzag = True
zigzag_start = "RIGHT"

start_a = 0
start_b = 0
step_a = 1
step_b = 1
if order == "ROW":
    end_a = height
    end_b = width
else:
    end_a = width
    end_b = height

for a in range(start_a, end_a, step_a):
    if zigzag:
        if zigzag_start == "RIGHT":
            zigzag_flip = 0
        elif zigzag_start == "LEFT":
            zigzag_flip = 1
        if a % 2 == zigzag_flip:
            start_b = end_b - 1
            end_b = -1
            step_b = -1
        else:
            start_b = 0
            step_b = 1
            if order == "ROW":
                end_b = width
            else:
                end_b = height
    for b in range(start_b, end_b, step_b):
        if order == "ROW":
            index = (a * width) + b
        else:
            index = (b * width) + a
        print(pixels_array[index])