Creating a chessboard in PIL

Question

Background

I have been trying to create a chessboard in the PIL module and I have got the general pattern for the first two rows, but can't figure out how to apply this to the entire board. As you can see, I have created an image:

from PIL import Image

img = Image.new("RGB", (15,15), "white") # create a new 15x15 image
pixels = img.load() # create the pixel map

My solution for the first two rows

Note - I am still learning Python so this code may seem very inefficient, but feel free to suggest improvements.

The second row:

Code:

black_2 = []
for i in range(img.size[0]):
    if i % 2 == 0:
        black_2.append(i)

This gives me all the horizontal index positions on where to put a black pixel. Therefore, for the 15x15 board I created, it returns [0, 2, 4, 6, 8, 10, 12, 14]

The first row:

Code:

I then use the second row to work out the horizontal index positions for the first row

black_1 = [i-1 for i in black_2 if i > 0]
if img.size[0] % 2 == 0: # 'that' if statement
    black_1.append(img.size[0]-1)

For the 15x15 pixel board I created, it returns [1, 3, 5, 7, 9, 11, 13]. I created that if statement because I realised that the last black pixel was not showing if the board had an even length, and that seemed to fix it.

Changing the pixels to black:

# hardcoded to check patterns
for i in black_1:
    pixels[i,0] = (0,0,0)

for k in black_2:
    pixels[k,1] = (0,0,0)

img.show()

How can I apply both patterns to the rest of the board, regardless of its size?

I would suspect a for var in range() loop is needed, but I am not sure how it would change depending on if the height(img.size[1]) of the board is odd or even.

Overall pattern so far:

black_1 applies to first row

black_2 applies to second row

Answer 1

A chess board has 64 squares instead of 256. Firstly you need (8,8) and then you can use double for loops to assign the color to all the 8 rows.

General example for any size

from PIL import Image

size = 16
img = Image.new("RGB", (size,size), "white") # create a new 15x15 image
pixels = img.load() # create the pixel map

black_2 = []
for i in range(img.size[0]):
    if i % 2 == 0:
        black_2.append(i)

black_1 = [i-1 for i in black_2 if i > 0]
if img.size[0] % 2 == 0: # 'that' if statement
    black_1.append(img.size[0]-1)


for i in black_1:
    for j in range(0, size, 2):
        pixels[i,j] = (0,0,0)

for k in black_2:
    for l in range(1, size+1, 2):
        pixels[k,l] = (0,0,0)

img.show()