from PIL import Image

def legal( v ) :
    ''' converts v to a legal RGB level; if v is negative, 0 is returned.
        if v exceeds 255, 255 is returned; otherwise, v is returned
    '''
    
    pass

def offset( pixel, x, y, z ) :
    ''' returns a new pixel by first decrementing the RGB values of pixel by
        x, y, and z respectively. to ensure the new pixel, the RGB values
        are run through legal()
    '''

    pass

def create_color_shift( original, x, y, z ) :
    ''' returns a new image that is a copy of the original, where a
        pixel in the new image is produced by using offset() on the 
        corresponding original pixel with respect to x, y, and z
    '''
    ow, oh = original.size                       # get dimensions of original
    nw, nh = ow, oh                              # new image has same dimensions

    new_image = Image.new( 'RGB', ( nw, nh ) )   # create new image

    for nx in range( 0, nw ) :                   # fill in every pixel of new image
        for ny in range( 0, nh ) :

            nspot = (nx, ny )                    # set spot to be filled in new image

            ospot = nspot                        # use corresponding spot in original

            opixel = original.getpixel( ospot )  # get pixel at corresponding spot

            npixel = offset( opixel, x, y, z )   # shift that pixel using x, y, z offsets

            new_image.putpixel( nspot, npixel )  # set spot in new image

    return new_image                             # return color_shift

if ( __name__ == '__main__' ) :

    import url

    TEST_URL = 'http://www.cs.virginia.edu/~cs1112/images/ml.png'

    original = url.get_image( TEST_URL )

    result   = create_color_shift( original, 25, -50, 75 )

    result.save( "color-shift-ml.png" )