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#!/usr/bin/env python3

'''
Converts images with up to 4 indexed colors into the UXN 2-bits-per-pixel CHR format.

Usage:

- input: path to raw image input, where each pixel is one byte between 0x00 and 0x03 inclusive
- output: path to chr output file
- width: image width in number of 8x8 sprites (not pixels)

./indexed-to-chr.py [input] [output] [width]
./indexed-to-chr.py input.data output.chr 12
'''

import sys

WIDTH = int(sys.argv[3])

with open(sys.argv[1], "rb") as input_file, open(sys.argv[2], "wb") as output_file:
	data = input_file.read()
	sprite = 0

	while sprite * 8 * 8 < len(data):
		sprite_y = sprite // WIDTH
		sprite_x = sprite %  WIDTH

		pixel_y = sprite_y * 8
		pixel_x = sprite_x * 8

		# write a sprite
		low = bytearray(8)
		high = bytearray(8)

		for y in range(8):
			offset = (pixel_y + y) * WIDTH * 8 + pixel_x
			row = data[slice(offset, offset+8)]
			for x in range(8): # mask, shift, and combine
				low[y]  |= (row[x] & 0b00000001) << (7-x)
				high[y] |= ((row[x] & 0b00000010) >> 1) << (7-x)

		output_file.write(bytes(low))
		output_file.write(bytes(high))

		sprite += 1