def calculate_width_and_height(url_parts, options):
    """Appends width and height information to url"""
    width = options.get('width', 0)
    has_width = width
    height = options.get('height', 0)
    has_height = height
    flip = options.get('flip', False)
    flop = options.get('flop', False)
    if flip:
        width = width * -1
    if flop:
        height = height * -1
    if not has_width and not has_height:
        if flip:
            width = '-0'
        if flop:
            height = '-0'
    if width or height:
        url_parts.append('%sx%s' % (width, height))
