目标检测面试-nms

1、python语言下：

def nms(self, dets, scores):
        # 这一代码来源于Faster RCNN项目
        """"Pure Python NMS baseline."""
        x1 = dets[:, 0]  #xmin
        y1 = dets[:, 1]  #ymin
        x2 = dets[:, 2]  #xmax
        y2 = dets[:, 3]  #ymax

        areas = (x2 - x1) * (y2 - y1)                    # bbox的宽w和高h
        order = scores.argsort()[::-1]                   # 按照降序对bbox的得分进行排序

        keep = []                                        # 用于保存经过筛的最终bbox结果
        while order.size > 0:
            i = order[0]                                 # 得到最高的那个bbox
            keep.append(i)                               
            xx1 = np.maximum(x1[i], x1[order[1:]])
            yy1 = np.maximum(y1[i], y1[order[1:]])
            xx2 = np.minimum(x2[i], x2[order[1:]])
            yy2 = np.minimum(y2[i], y2[order[1:]])

            w = np.maximum(1e-28, xx2 - xx1)
            h = np.maximum(1e-28, yy2 - yy1)
            inter = w * h

            # Cross Area / (bbox + particular area - Cross Area)
            ovr = inter / (areas[i] + areas[order[1:]] - inter)
            #reserve all the boundingbox whose ovr less than thresh
            inds = np.where(ovr <= self.nms_thresh)[0]
            order = order[inds + 1]

        return keep

2、C++下：

#include<bits/stdc++.h>
using namespace std;

typedef struct Bbox
{
    int xmin;
    int ymin;
    int xmax;
    int ymax;
    float score;
}Bbox;

bool sort_score(Bbox box1,Bbox box2)
{
    return (box1.score > box2.score);
}

float iou(Bbox box1,Bbox box2)
{
    int x1 = max(box1.xmin,box2.xmin);
    int y1 = max(box1.ymin,box2.ymin);
    int x2 = min(box1.xmax,box2.xmax);
    int y2 = min(box1.ymax,box2.ymax);
    int w1 = box1.xmax - box1.xmin, h1 = box1.ymax - box1.ymin;
    int w2 = box2.xmax - box2.xmin, h2 = box2.ymax - box2.ymin;
    float inter_area = (x2 - x1) * (y2 - y1);
    float iou = inter_area/(w1*h1 + w2*h2-inter_area);
    return iou;
}
//方法1
vector<Bbox> nms(vector<Bbox>&vec_boxs,float threshold)
{
    sort(vec_boxs.begin(),vec_boxs.end(),sort_score);
    vector<bool>del(vec_boxs.size(),false);
    for(int i =0; i<vec_boxs.size();i++)
    {
        for (int j =0;j<vec_boxs.size();j++)
        {
            float iou_value =iou(vec_boxs[i],vec_boxs[j]);
            if(iou_value>threshold) del[j]=true;
        }
    }
	vector<Bbox>results;
    for(const auto i :del)
    {
        if(!del[i]) results.push_back(vec_box[i]);
    }
    return results;
}
//方法2  这种执行效率更高
vector<Bbox> nms(vector<Bbox>&vec_boxs,float threshold)
{
    vector<Bbox>results;
    while(vec_boxs.size() > 0)
    {
        sort(vec_boxs.begin(),vec_boxs.end(),sort_score);
        results.push_back(vec_boxs[0]);
        for(int i =0;i <vec_boxs.size()-1;i++)
        {
            float iou_value =iou(vec_boxs[0],vec_boxs[i+1]);
            if (iou_value >threshold) vec_boxs.erase(vec_boxs[i+1]);
        }
        vec_boxs.erase(vec_boxs[0]);
    }
    return results;
}

1-python版：https://www.zhihu.com/column/c_1364967262269693952

2-C++版：https://blog.csdn.net/qq_32582681/article/details/81352758