nms 演算法演示（附程式碼）

import numpy as np

import matplotlib.pyplot as plt

def py_cpu_nms(nms_show, dets, thresh):# nms 操作

    "Pure Python NMS baseline"

    # x1 、 y1 、 x2 、 y2 以及 score 賦值

    x1 = dets[:,0]

    y1 = dets[:,1]

    x2 = dets[:,2]

    y2 = dets[:,3]

    scores = dets[:, 4]

    areas = (y2-y1+1) * (x2-x1+1) # 每一個檢測框的面積

    print("all_areas:",areas)

    order = scores.argsort()[::-1]# 按照 score 置信度降序排序

    print("order:",order)

    keep = [] # 保留的結果框集合

    k=0

    show_boxid = order

    print("show_boxid",show_boxid)

    while order.size > 0:

        i = order[0]       # every time the first is the biggst, and add it directly

        keep.append(i) # 保留該類剩餘 box 中得分最高的一個

        print("\nkeep( 被留下來的框 id):",keep)

        # 得到相交區域 , 比左上大和比右下小

        '''np.maximum(X, Y, out=None) X 和 Y 逐位進行比較 , 選擇最大值 '''

        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:]])

        print("xx1,yy1:",xx1,yy1)

        print("xx2,yy2:",xx2,yy2)

        print("w,h:",xx2-xx1+1,yy2-yy1+1)       

        #plt.scatter([xx1,xx2], [yy1,yy2], s=50, c='y')# 框左上角藍色

        # 計算相交的面積 ,外匯跟單gendan5.com 不重疊時面積為 0

        w = np.maximum(0, xx2-xx1+1)    # the weights of overlap

        h = np.maximum(0, yy2-yy1+1)    # the height of overlap

        inter = w*h

        print("w*h:",inter)

        # 計算 IoU ：重疊面積 / （面積 1+ 面積 2- 重疊面積）

        iou = inter / (areas[i]+areas[order[1:]] - inter)

        print("---iou:",iou)

        # 保留 IoU 小於閾值的 box

        ''' 去掉 keep 剩下的框按順序重新排序 '''

        indx = np.where(iou<=thresh)[0]

        dedx = np.where(iou>thresh)[0]

        print("order[0]:",order[0])

        show_boxid = np.delete(show_boxid,np.where(show_boxid==order[0])[0],axis = 0)

        print("show_boxid:",np.append(keep,show_boxid))

        delete_boxid = show_boxid[dedx]

        show_boxid = show_boxid[indx]

        #print("indx:",indx)

        print("after_iou_show_boxid:",np.append(keep,show_boxid))

        print("after_iou_delete_boxid:",delete_boxid,'\n')

        k=k+1

        ''' 繪製動圖 '''

        ax1 = nms_show.add_subplot(1,2,1)  

        ax1.set_title('begin_nms {} \nkeep:{} score:{}({})'.format(k,order[0],scores[order[0]],k))

        plot_bbox(dets, 'k', show_ids=np.arange(9) , keep_id = order[0])

        ax2 = nms_show.add_subplot(1,2,2)  

        ax2.set_title('after_nms {} \nkeep:{} delete:{}(iou={})'.format(k,order[0],delete_boxid,iou[dedx]))

        plot_bbox(dets[np.append(show_boxid,keep)], 'b', np.append(show_boxid,keep))

        plt.pause(5)

        ax1.remove()

        ax2.remove()

        ''' 置信度排前的數值給取出，剩下的數構成新的陣列 '''

        order = order[indx+1]   

        print("-----------afer_order:",order,'-----------')

    return keep      

def plot_bbox(dets, c='k', show_ids=[],keep_id=0):

    x1 = dets[:,0]

    y1 = dets[:,1]

    x2 = dets[:,2]

    y2 = dets[:,3]

    score=dets[:,4]

    #print(dets.shape)

    #plt.scatter(x1, y1, s=25, c='b', alpha=0.6)# 框左上角藍色

    #plt.scatter(x2, y2, s=25, c='r', alpha=0.6)# 框右下角紅色

    plt.plot([x1,x2], [y1,y1], c)

    plt.plot([x1,x1], [y1,y2], c)

    plt.plot([x1,x2], [y2,y2], c)

    plt.plot([x2,x2], [y1,y2], c)

    plt.xlim((60,450))

    plt.ylim((450,60))

    ''' 改變座標軸位置 '''

    ax = plt.gca()

    ax.spines["top"].set_color("k")

    ax.xaxis.set_ticks_position("top")

    for i in range(len(show_ids)):

        plt.text(x1[i], y1[i]+7, "(%d)%.2f"%(show_ids[i],score[i]), \

                 fontdict={'size': 10, 'color': 'r'},bbox={'facecolor':'blue', 'alpha':0.1})

    if keep_id != 0:

        ax.add_patch(plt.Rectangle((x1[keep_id], y1[keep_id]), x2[keep_id]-x1[keep_id]+1, y2[keep_id]-y1[keep_id]+1,

                                   color="y", fill=True, linewidth=2))

def main():

    boxes=np.array([

            [100,100,210,210,0.72],#0

            [280,290,420,420,0.8],#1

            [220,220,320,330,0.92],#2

            [105,90,220,210,0.71],#3

            [230,240,325,330,0.81],#4

            [305,300,420,420,0.9],#5

            [215,225,305,328,0.6],#6

            [150,260,290,400,0.99],#7

            [102,108,208,208,0.72]])#8  #9 個框

    plt.ion()

    fig = plt.figure(figsize=[14,9])

    ax1 = plt.subplot(1,2,1)

    ax1.set_title('before_nms')

    ax2 = plt.subplot(1,2,2)

    ax2.set_title('after_nms')

    plt.sca(ax1)# 選擇子圖 1

    plot_bbox(boxes,'k',show_ids=np.arange(9),keep_id=0)   # before nms

    keep = py_cpu_nms(fig, boxes, thresh=0.7)

    print("last_keep:",keep)

    plt.ioff()

    plt.pause(2)

    plt.close('all')

if __name__ =="__main__":

    main()