'Remove all boxes with any side smaller than min_size'

@staticmethod
def _filter_boxes(boxes, min_size):

    ws = ((boxes[:, 2] - boxes[:, 0]) + 1)
    hs = ((boxes[:, 3] - boxes[:, 1]) + 1)
    keep = np.where(((ws >= min_size) & (hs >= min_size)))[0]
    return keep
