# requires PySerial and PIL!
# http://pyserial.sourceforge.net/
# http://www.pythonware.com/products/pil/

import sys
import time
import serial
import PIL.Image

threshold = 128*[0] + 128*[0xFF]

def main():

    ser = serial.Serial('/dev/tty.usbserial-A1001MPC', 9600)
    
    grays = range(0, 256, 4) + range(255, -1, -4)
    
    try:
        for i in xrange(0x7FFFFFFF):
            gray = grays[i % len(grays)]
            ser.write('o')
            ser.write(image2frame(PIL.Image.new('L', (6, 6), gray)))
            time.sleep(0.02)

    except KeyboardInterrupt:
        print ''

    ser.close()
    return 0

def dither(original):
    """ Return a monohrome, Atkinson-dithered version of an image.
    """
    dithered = original.copy()

    for y in range(dithered.size[1]):
        for x in range(dithered.size[0]):
    
            old = dithered.getpixel((x, y))
            new = threshold[old]
            err = (old - new) >> 3 # divide by 8
                
            dithered.putpixel((x, y), new)
            
            for nxy in [(x+1, y), (x+2, y), (x-1, y+1), (x, y+1), (x+1, y+1), (x, y+2)]:
                try:
                    dithered.putpixel(nxy, dithered.getpixel(nxy) + err)
                except IndexError:
                    pass

    return dithered

def image2frame(image):
    """ Convert an image to a 1-bit representation, using no more than the top-left 8x8 pixels.
    """
    image = dither(image)
    bytes = []
    
    for row in range(min(8, image.size[1])):
    
        bytes.append(0x00)
    
        for col in range(min(8, image.size[0])):

            # use bitwise-or to assign each pixel on this row from left to right.
            # trust that getpixel() will return either 0x00 or 0xFF, thanks to dither().
            bytes[-1] |= (image.getpixel((col, row)) & (1 << (7 - col)))

    return ''.join(map(chr, bytes))

if __name__ == '__main__':

    sys.exit(main())