物体检测——IOU系列（IOU/GIOU/DIOU/CIOU）

深度学习里非常重要的一块就是loss的设计，物体检测里怎么都逃不开IOU及其变种，一开始会使用bbox的距离作为loss，最近几年会将IOU变种直接作为loss训练，可以提点不少，让网络更容易学习到框的位置。

 

IOU（Intersection over Union）

• 交并比，两个rect的交集面积除以并集，很直观， 
• 不适合作为loss
• 在GIOU出现之前一般还是用bbox的距离作为loss

    def bbox_iou(self, boxes1, boxes2):

        boxes1_area = boxes1[..., 2] * boxes1[..., 3]
        boxes2_area = boxes2[..., 2] * boxes2[..., 3]

        boxes1 = tf.concat([boxes1[..., :2] - boxes1[..., 2:] * 0.5,
                            boxes1[..., :2] + boxes1[..., 2:] * 0.5], axis=-1)
        boxes2 = tf.concat([boxes2[..., :2] - boxes2[..., 2:] * 0.5,
                            boxes2[..., :2] + boxes2[..., 2:] * 0.5], axis=-1)

        left_up = tf.maximum(boxes1[..., :2], boxes2[..., :2])
        right_down = tf.minimum(boxes1[..., 2:], boxes2[..., 2:])

        inter_section = tf.maximum(right_down - left_up, 0.0)
        inter_area = inter_section[..., 0] * inter_section[..., 1]
        union_area = boxes1_area + boxes2_area - inter_area
        iou = 1.0 * inter_area / union_area

        return iou

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CVPR2019--GIOU:https://arxiv.org/pdf/1902.09630.pdf

• GIOU=IOU-外接矩形空隙面积/并集
• 利用1-GIOU替代bbox距离作为loss，数据集上表现更好
• 取值范围(-1,1]，当两个rect不重叠时候，IOU数值为0，但GIOU不为0，故而适合作为loss
• 比IOU更能反映直观感觉上两个Rect的接近程度，当然最主要的是能提点

    def bbox_giou(self, boxes1, boxes2):

        boxes1 = tf.concat([boxes1[..., :2] - boxes1[..., 2:] * 0.5,
                            boxes1[..., :2] + boxes1[..., 2:] * 0.5], axis=-1)
        boxes2 = tf.concat([boxes2[..., :2] - boxes2[..., 2:] * 0.5,
                            boxes2[..., :2] + boxes2[..., 2:] * 0.5], axis=-1)

        boxes1 = tf.concat([tf.minimum(boxes1[..., :2], boxes1[..., 2:]),
                            tf.maximum(boxes1[..., :2], boxes1[..., 2:])], axis=-1)
        boxes2 = tf.concat([tf.minimum(boxes2[..., :2], boxes2[..., 2:]),
                            tf.maximum(boxes2[..., :2], boxes2[..., 2:])], axis=-1)

        boxes1_area = (boxes1[..., 2] - boxes1[..., 0]) * (boxes1[..., 3] - boxes1[..., 1])
        boxes2_area = (boxes2[..., 2] - boxes2[..., 0]) * (boxes2[..., 3] - boxes2[..., 1])

        left_up = tf.maximum(boxes1[..., :2], boxes2[..., :2])
        right_down = tf.minimum(boxes1[..., 2:], boxes2[..., 2:])

        inter_section = tf.maximum(right_down - left_up, 0.0)
        inter_area = inter_section[..., 0] * inter_section[..., 1]
        union_area = boxes1_area + boxes2_area - inter_area
        iou = inter_area / union_area

        enclose_left_up = tf.minimum(boxes1[..., :2], boxes2[..., :2])
        enclose_right_down = tf.maximum(boxes1[..., 2:], boxes2[..., 2:])
        enclose = tf.maximum(enclose_right_down - enclose_left_up, 0.0)
        enclose_area = enclose[..., 0] * enclose[..., 1]
        giou = iou - 1.0 * (enclose_area - union_area) / enclose_area

        return giou

Rezatofighi H , Tsoi N , Gwak J Y , et al. Generalized Intersection over Union: A Metric and A Loss for Bounding Box Regression[J]. 2019.

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CVPR2020--DIOU/CIOU:https://arxiv.org/pdf/1911.08287.pdf

• DIOU=IOU-(中心点距离/并集外接矩形对角线距离)²
• 作为loss收敛得更快，可以进一步提点
• 用就对了

• 进一步考虑ratio，将长宽比作为罚项加进来（提出CIOU）

 

最后一行代表利用CIOU作为loss训练，利用DIOU来NMS

def bbox_diou_ciou(self, boxes1, boxes2):
    
    center_vec = boxes1[..., :2] - boxes2[..., :2]
    v_ = 0.4052847483961759*(tf.atan(boxes1[..., 2]/boxes1[..., 3]) - tf.atan(boxes2[..., 2]/boxes2[..., 3]))**2
    boxes1 = tf.concat([boxes1[..., :2] - boxes1[..., 2:] * 0.5,
                        boxes1[..., :2] + boxes1[..., 2:] * 0.5], axis=-1)
    boxes2 = tf.concat([boxes2[..., :2] - boxes2[..., 2:] * 0.5,
                        boxes2[..., :2] + boxes2[..., 2:] * 0.5], axis=-1)
    
    boxes1 = tf.concat([tf.minimum(boxes1[..., :2], boxes1[..., 2:]),
                        tf.maximum(boxes1[..., :2], boxes1[..., 2:])], axis=-1)
    boxes2 = tf.concat([tf.minimum(boxes2[..., :2], boxes2[..., 2:]),
                        tf.maximum(boxes2[..., :2], boxes2[..., 2:])], axis=-1)
    
    boxes1_area = (boxes1[..., 2] - boxes1[..., 0]) * (boxes1[..., 3] - boxes1[..., 1])
    boxes2_area = (boxes2[..., 2] - boxes2[..., 0]) * (boxes2[..., 3] - boxes2[..., 1])
    
    left_up = tf.maximum(boxes1[..., :2], boxes2[..., :2])
    right_down = tf.minimum(boxes1[..., 2:], boxes2[..., 2:])
    
    inter_section = tf.maximum(right_down - left_up, 0.0)
    inter_area = inter_section[..., 0] * inter_section[..., 1]
    union_area = boxes1_area + boxes2_area - inter_area
    iou = inter_area / union_area
    
    enclose_left_up = tf.minimum(boxes1[..., :2], boxes2[..., :2])
    enclose_right_down = tf.maximum(boxes1[..., 2:], boxes2[..., 2:])
    enclose_vec = enclose_right_down - enclose_left_up
    center_dis = tf.sqrt(center_vec[..., 0]**2+center_vec[..., 1]**2)
    enclose_dis = tf.sqrt(enclose_vec[..., 0]**2+enclose_vec[..., 1]**2)
    diou = iou - 1.0 * center_dis / enclose_dis
    a_ = v_/(1-IOU+v_)
    ciou= diou- a_*v_
    
    return diou,ciou

 

Zheng, Zhaohui, Wang, Ping, Liu, Wei. Distance-IoU Loss: Faster and Better Learning for Bounding Box Regression[J]. 2019.

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参考：

https://zhuanlan.zhihu.com/p/94799295

https://github.com/YunYang1994/tensorflow-yolov3