def subImage(Img, FracBox):
    """return a subImage from Image based on coordinates in dimensionless units"""
    width, height = Img.size
    left = round(width * FracBox[0])
    right = round(width * FracBox[2])
    bottom = round(height * FracBox[1])
    top = round(height * FracBox[3])
    return Img.crop((left, bottom, right, top))
