def bbox_center_to_lowerleft(bbox):
    """convert a bbox of form [cx, cy, w, h] to [minx, miny, w, h]. Works for numpy array. """
    cx, cy, w, h = bbox
    out_box    = [[]]*4
    out_box[0] = cx - 0.5*w 
    out_box[1] = cy - 0.5*h
    out_box[2] = w
    out_box[3] = h        
    return out_box