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@darka
Last active August 31, 2024 01:01
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Generating a PNG in Python
import struct
import zlib
from typing import BinaryIO, List, Tuple
Pixel = Tuple[int, int, int]
Image = List[List[Pixel]]
BLACK_PIXEL: Pixel = (0, 0, 0)
WHITE_PIXEL: Pixel = (255, 255, 255)
HEADER = b'\x89PNG\r\n\x1A\n'
def generate_checkerboard_pattern(width: int, height: int) -> Image:
out = []
for i in range(height):
# Generate a single row of white/black pixels
row = []
for j in range(width):
if (i + j) % 2 == 0:
row.append(WHITE_PIXEL)
else:
row.append(BLACK_PIXEL)
out.append(row)
return out
def get_checksum(chunk_type: bytes, data: bytes) -> int:
checksum = zlib.crc32(chunk_type)
checksum = zlib.crc32(data, checksum)
return checksum
def chunk(out: BinaryIO, chunk_type: bytes, data: bytes) -> None:
out.write(struct.pack('>I', len(data)))
out.write(chunk_type)
out.write(data)
checksum = get_checksum(chunk_type, data)
out.write(struct.pack('>I', checksum))
def make_ihdr(width: int, height: int, bit_depth: int, color_type: int) -> bytes:
return struct.pack('>2I5B', width, height, bit_depth, color_type, 0, 0, 0)
def encode_data(img: Image) -> List[int]:
ret = []
for row in img:
ret.append(0)
color_values = [
color_value
for pixel in row
for color_value in pixel
]
ret.extend(color_values)
return ret
def compress_data(data: List[int]) -> bytes:
data_bytes = bytearray(data)
return zlib.compress(data_bytes)
def make_idat(img: Image) -> bytes:
encoded_data = encode_data(img)
compressed_data = compress_data(encoded_data)
return compressed_data
def dump_png(out: BinaryIO, img: Image) -> None:
out.write(HEADER) # start by writing the header
assert len(img) > 0 # assume we were not given empty image data
width = len(img[0])
height = len(img)
bit_depth = 8 # bits per pixel
color_type = 2 # pixel is RGB triple
ihdr_data = make_ihdr(width, height, bit_depth, color_type)
chunk(out, b'IHDR', ihdr_data)
compressed_data = make_idat(img)
chunk(out, b'IDAT', data=compressed_data)
chunk(out, b'IEND', data=b'')
def save_png(img: Image, filename: str) -> None:
with open(filename, 'wb') as out:
dump_png(out, img)
if __name__ == '__main__':
width = 100
height = 100
img = generate_checkerboard_pattern(width, height)
save_png(img, 'out.png')
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