Python snippets to create an image alpha mask from a PNG and a PNG from the resulting mask.

circle.py

import png

mask = [
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 7, 12, 7, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 34, 135, 252, 255, 255, 255, 251, 134, 34, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 11, 153, 255, 255, 255, 255, 255, 255, 255, 255, 255, 153, 11, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 25, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 25, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 12, 249, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 249, 12, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 156, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 157, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 33, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 33, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 133, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 133, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 250, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 249, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 6, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 6, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 11, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 11, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 6, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 6, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 249, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 250, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 133, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 133, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 33, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 33, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 157, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 157, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 11, 249, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 249, 12, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 25, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 25, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 11, 153, 255, 255, 255, 255, 255, 255, 255, 255, 255, 153, 11, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 34, 134, 251, 255, 255, 255, 252, 134, 34, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 7, 12, 7, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
]

def write_circle(filename, color):
    """Write a 32x32 PNG filled with a color and our alpha mask to a file."""
    color.ljust(6, '0')
    r = int(color[0:2], 16)
    g = int(color[2:4], 16)
    b = int(color[4:6], 16)
    if len(color) > 6:
        color.ljust(8, '0')
        a = int(color[6:8], 16)
    else:
        a = 255

    a /= 255.0

    pixels = [[] for i in xrange(len(mask))]
    for y, row in enumerate(mask):
        for alpha in row:
            alpha = int(alpha * a)
            pixels[y] += [r, g, b, alpha]

    writer = png.Writer(len(mask[0]), len(mask), alpha=True)
    with open(filename, 'wb') as f:
        writer.write(f, pixels)

pngtomask.py

import png

reader = png.Reader('icon.png')
pixels = reader.asRGBA8()[2]

mask = [[0] * 32 for i in xrange(32)]
for y, row in enumerate(pixels):
    for pos in xrange(0, len(row), 4):
        x = pos / 4
        mask[y][x] = row[pos + 3]

print mask