转:非极大值抑制(Non-Maximum Suppression,NMS)
非极大值抑制(Non-Maximum Suppression,NMS)
单类别NMS的numpy实现
def py_cpu_nms(dets, thresh): """Pure Python NMS baseline.""" #x1、y1、x2、y2、以及score赋值 x1 = dets[:, 0] y1 = dets[:, 1] x2 = dets[:, 2] y2 = dets[:, 3] scores = dets[:, 4] #每一个检测框的面积 areas = (x2 - x1 + 1) * (y2 - y1 + 1) #按照score置信度降序排序 order = scores.argsort()[::-1] kept_bboxes = [] #保留的结果框集合 while order.size > 0: i = order[0] kept_bboxes.append(i) #保留该类剩余box中得分最高的一个 #得到相交区域,左上及右下 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:]]) #计算相交的面积,不重叠时面积为0 w = np.maximum(0.0, xx2 - xx1 + 1) h = np.maximum(0.0, yy2 - yy1 + 1) inter = w * h #计算IoU:重叠面积 /(面积1+面积2-重叠面积) ovr = inter / (areas[i] + areas[order[1:]] - inter) #保留IoU小于阈值的box inds = np.where(ovr <= thresh)[0] order = order[inds + 1] #因为ovr数组的长度比order数组少一个,所以这里要将所有下标后移一位 return kept_bboxes
单类别NMS的pytorch实现
def _nms(self, bboxes, scores, threshold=0.5): x1 = bboxes[:,0] y1 = bboxes[:,1] x2 = bboxes[:,2] y2 = bboxes[:,3] areas = (x2-x1)*(y2-y1) # [N,] 每个bbox的面积 _, order = scores.sort(0, descending=True) # 降序排列 kept_bboxes = [] while order.numel() > 0: # torch.numel()返回张量元素个数 if order.numel() == 1: # 保留框只剩一个 i = order.item() kept_bboxes.append(i) break else: i = order[0].item() # 保留scores最大的那个框box[i] kept_bboxes.append(i) # 计算box[i]与其余各框的IOU(思路很好) xx1 = x1[order[1:]].clamp(min=x1[i]) # [N-1,] yy1 = y1[order[1:]].clamp(min=y1[i]) xx2 = x2[order[1:]].clamp(max=x2[i]) yy2 = y2[order[1:]].clamp(max=y2[i]) inter = (xx2-xx1).clamp(min=0) * (yy2-yy1).clamp(min=0) # [N-1,] iou = inter / (areas[i]+areas[order[1:]]-inter) # [N-1,] idx = (iou <= threshold).nonzero().squeeze() # 注意此时idx为[N-1,] 而order为[N,] if idx.numel() == 0: break order = order[idx+1] # 修补索引之间的差值 return torch.LongTensor(kept_bboxes) # Pytorch的索引值为LongTensor