def function(self,p):
        """Selects and returns one of the patterns in the list."""
        pg = p.generator
        motion_orientation=p.orientation+pi/2.0

        new_x = p.x+p.size*pg.x
        new_y = p.y+p.size*pg.y

        image_array = pg(xdensity=p.xdensity,ydensity=p.ydensity,bounds=p.bounds,
                         x=new_x + p.speed*p.step*np.cos(motion_orientation),
                         y=new_y + p.speed*p.step*np.sin(motion_orientation),
                         orientation=p.orientation,
                         scale=pg.scale*p.scale,offset=pg.offset+p.offset)

        return image_array
Selects and returns one of the patterns in the list.