Calculate intersect areas, normalized.

def _intersect(self, label, xmin, ymin, xmax, ymax):
        """Calculate intersect areas, normalized."""
        left = np.maximum(label[:, 0], xmin)
        right = np.minimum(label[:, 2], xmax)
        top = np.maximum(label[:, 1], ymin)
        bot = np.minimum(label[:, 3], ymax)
        invalid = np.where(np.logical_or(left >= right, top >= bot))[0]
        out = label.copy()
        out[:, 0] = left
        out[:, 1] = top
        out[:, 2] = right
        out[:, 3] = bot
        out[invalid, :] = 0
        return out