def get_square_centre_crop(img):
    """
    This function calculates a centered square crop bounding box
    as big as the image's smaller dimension allows.

    :param img: PIL Image object.
    :return: Tuple with bounding box of the square crop.
    """
    width, height = img.size
    if width > height:
        x_min = int((width - height) / 2)
        x_max = int(width - (width - height) / 2)
        y_min = 0
        y_max = height
    else:
        x_min = 0
        x_max = width
        y_min = int((height - width) / 2)
        y_max = int(height - (height - width) / 2)

    return x_min, y_min, x_max, y_max