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#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Usage: python palfix.py image.png
Fix the palette format of the input image to two-bit grayscale.
Colored images will have their palette sorted {white, light
color, dark color, black}.
"""
import sys
import png
def rgb8_to_rgb5(c):
r, g, b = c
return (r // 8, g // 8, b // 8)
def invert(c):
r, g, b = c
return (31 - r, 31 - g, 31 - b)
def luminance(c):
r, g, b = c
return 0.299 * r**2 + 0.587 * g**2 + 0.114 * b**2
def rgb5_pixels(row):
yield from (rgb8_to_rgb5(row[x:x+3]) for x in range(0, len(row), 4))
def fix_pal(filename):
with open(filename, 'rb') as file:
width, height, rows = png.Reader(file).asRGBA8()[:3]
rows = list(rows)
b_and_w = {(0, 0, 0), (31, 31, 31)}
colors = {c for row in rows for c in rgb5_pixels(row)} - b_and_w
if not colors:
colors = {(21, 21, 21), (10, 10, 10)}
elif len(colors) == 1:
c = colors.pop()
colors = {c, invert(c)}
elif len(colors) != 2:
return False
palette = tuple(sorted(colors | b_and_w, key=luminance, reverse=True))
assert len(palette) == 4
rows = [[3 - palette.index(c) for c in rgb5_pixels(row)] for row in rows]
writer = png.Writer(width, height, greyscale=True, bitdepth=2, compression=9)
with open(filename, 'wb') as file:
writer.write(file, rows)
return True
def main():
if len(sys.argv) < 2:
print(f'Usage: {sys.argv[0]} pic.png', file=sys.stderr)
sys.exit(1)
for filename in sys.argv[1:]:
if not filename.lower().endswith('.png'):
print(f'{filename} is not a .png file!', file=sys.stderr)
elif not fix_pal(filename):
print(f'{filename} has too many colors!', file=sys.stderr)
if __name__ == '__main__':
main()
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