scale.py

# -*- coding: utf-8 -*-
"""
Scale a PNG by an integer factor, keeping it pixelated.
"""

import png
from sys import argv

# We're assuming rgba for for ints per pixel. Looking at the pypng docs there
# are other options this might be, but this is all I needed.
VAL_PER_PIXEL = 4  # rgba

if __name__ == '__main__':
    # we expect scale_factor, in_file, out_file
    if len(argv) != 4:
        raise ValueError('Usage: scale_factor, in_file, out_file')

    factor = int(argv[1])
    in_file = argv[2]
    out_file = argv[3]

    reader = png.Reader(in_file)
    content = reader.asDirect()
    rows = content[2]

    # content[2][0] assumes at least one row. There are original_width pixels in
    # this row, and each pixel has VAL_PER_PIXEL ints for each pixel.
    original_width = len(content[2][0]) / VAL_PER_PIXEL
    original_height = len(content[2])

    out_width = original_width * factor
    out_height = original_height * factor

    all_rows = []

    # Iterate over each row. Each pixel is represented by several ints (the
    # exact number depends on the file and is defined by VAL_PER_PIXEL). Each
    # pixel must be grown by factor, and each row must then be appended factor
    # number of times to all_rows.
    for row in rows:
        new_row = []
        for num_pixel in range(len(row) / VAL_PER_PIXEL):
            # / 4 for rgba
            # if a single pixel row was [255, 128, 25, 0], we would want to
            # expand that times 4.
            pixel_start = num_pixel * VAL_PER_PIXEL
            pixel_values = row[pixel_start:pixel_start + VAL_PER_PIXEL]
            larger_values = list(pixel_values) * factor
            new_row = new_row + larger_values
        for i in range(factor):
            # we've enlarged the row, now add it.
            all_rows.append(new_row)

    with open(out_file, 'w') as f:
        # alpha = True for rgba. This will have to be updated for PNGs without
        # alpha.
        w = png.Writer(out_width, out_height, alpha=True)
        w.write(f, all_rows)