@staticmethod
def _dim(n, start, w0):
    """Calculate 'wiped' position and size in one dimension"""
    if start is not None:
        w = round(n * w0)
        dw = w0 - w
        x = dw // 2 if start & 1 else dw if start & 2 else 0
        return x, w
    return 0, w0
