from cImage import *

# possible improvements:
#  foreground image could be a different size
#  foreground image could be positioned arbitrarily
#  screen color could be set flexibly

def composite(foreground, background):
    w=background.getWidth()
    h=background.getHeight()
    image = EmptyImage(w,h)
    for row in range(h):
        for col in range(w):
            pixel = foreground.getPixel(col, row)
            if isGreen(pixel):
                image.setPixel(col,row,background.getPixel(col,row))
            else:
                image.setPixel(col,row,pixel)
    return image

def isGreen(pixel):
    '''returns True if the pixel is green'''
    return pixel.getGreen() - pixel.getRed() - pixel.getBlue() > 200

def show(image):
    win = ImageWin('Image', image.getWidth(), image.getHeight())
    image.draw(win)
    win.exitOnClick()