apple_png_converter.py

import sys
import zlib

PNG_MAGIC = b"\x89PNG\r\n\x1a\n"


def parse_chunks(stream, out):
	magic = stream.read(8)
	assert(magic == PNG_MAGIC)
	out.write(magic)

	chunks = [] # (type, body)
	while True:
		chunk_len = int.from_bytes(stream.read(4), "big")
		chunk_type = stream.read(4)
		if not chunk_type:
			raise Exception("Unexpected EOF")
		chunk_body = stream.read(chunk_len)
		chunk_crc = int.from_bytes(stream.read(4), "big")
		
		assert(chunk_crc == zlib.crc32(chunk_type + chunk_body))
		
		if chunk_type == b"IHDR":
			width  = int.from_bytes(chunk_body[0:4], "big")
			height = int.from_bytes(chunk_body[4:8], "big")
		
		if chunk_type == b"CgBI":
			# drop CgBI chunks
			pass
		elif chunk_type == b"IDAT" and chunks[-1][0] == b"IDAT":
			# consolidate consecutive IDATs
			chunks[-1] = (b"IDAT", chunks[-1][1] + chunk_body)
		else:
			# passthru
			chunks.append((chunk_type, chunk_body))
		
		if chunk_type == b"IEND":
			break
	
	for chunk_type, chunk_body in chunks:
		if chunk_type == b"IDAT":
			body = zlib.decompress(chunk_body, -15)
			fixed = []
			for y in range(height):
				fixed.append(body[(width*4+1)*y])
				for x in range(width):
					i = (width*4+1)*y+x*4
					fixed += [body[i+3], body[i+2], body[i+1], body[i+0]]
			chunk_body = zlib.compress(bytes(fixed))
		
		out.write(len(chunk_body).to_bytes(4, "big"))
		out.write(chunk_type)
		out.write(chunk_body)
		out.write(zlib.crc32(chunk_type+chunk_body).to_bytes(4, "big"))


data = open(sys.argv[1], "rb")
out = open(sys.argv[2], "wb")
parse_chunks(data, out)