def get_baseline(fx, baseline=None, baseline_px=None):
    """
    Retrieve baseline / baseline(in pixels) using the focal length
    """
    assert(baseline is not None or baseline_px is not None)
    if baseline is not None:
        baseline_px = baseline * fx
    elif baseline_px is not None:
        baseline = baseline_px / fx
    else:
        raise AssertionError(
            'Ambiguous baseline and baseline_px values. Provide either, not both')
    return baseline, baseline_px