def get_stretch(image_name, sample_name):
    """extract stretch value from filename

    ex: hpr10p100031.TIF --> 0.1%
    """
    image_name = image_name.replace(sample_name, '')
    image_name = image_name.split('.')[0]
    u, d = image_name.split('p')
    d = d[:1]
    s = float(u) + float(d)/10
    return s