pixels

Lag N Crash 2025

misc easy

For some reason, it seems as though this folder of pixels are not what they seem. Can you find out what is going on?

Solution

we are provided with a zip file, pixels.zip. unzipping gives a pixels/ directory with 625 subfolders, each named pixel_<row>_<col> (rows and cols going from 0 to 24). every folder holds a single 500x500 png.

opening a few of the pngs, they’re all just solid colour, either fully black or fully white. 625 = 25*25, so the folder name basically tells you where that tile goes in a 25x25 grid. one image = one qr module.

from here it’s just reconstruction. sample the centre pixel of each tile (since each png is uniform), stitch them into a 25x25 grid, scale up so it’s actually scannable, and slap on a quiet zone so pyzbar doesn’t complain.

from PIL import Image
from pyzbar.pyzbar import decode

N = 25
MODULE = 20   # px per module
BORDER = 4    # quiet zone in modules

size = (N + 2 * BORDER) * MODULE
img = Image.new('L', (size, size), 255)

for r in range(N):
    for c in range(N):
        tile = Image.open(f'pixels/pixel_{r}_{c}/pixel_{r}_{c}.png').convert('L')
        v = tile.getpixel((250, 250))
        colour = 0 if v < 128 else 255
        for dy in range(MODULE):
            for dx in range(MODULE):
                x = (c + BORDER) * MODULE + dx
                y = (r + BORDER) * MODULE + dy
                img.putpixel((x, y), colour)

img.save('qr.png')
print(decode(img))

run it, scan the qr, get the flag!

View challenge source on GitHub