Normalize params and calculate aspect.

def _normalize_params(image, width, height, crop):
    """
    Normalize params and calculate aspect.
    """
    if width is None and height is None:
        raise ValueError("Either width or height must be set. Otherwise "
                         "resizing is useless.")

    if width is None or height is None:
        aspect = float(image.width) / float(image.height)

        if crop:
            raise ValueError("Cropping the image would be useless since only "
                             "one dimention is give to resize along.")

        if width is None:
            width = int(round(height * aspect))
        else:
            height = int(round(width / aspect))

    return (width, height, crop)