cascade r-cnn

本文探讨了Cascader-CNN检测模型在不同阈值下对正负样本界定的影响,分析了u=0.5和0.7阈值的优缺点,以及如何通过阶梯式的训练方式优化检测性能,避免过拟合。

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(a)中u=0.5是常用的正负样本界定值,但当u取0.5时会出现较多的误检。因为该阈值会使正负样本中有较多的背景。
(b)中用0.7的阈值会减小误检,但正样本的数量会呈指数级的减少,出现过拟合的现象。
(c)和(d)中的曲线是用来表示不听IOU阈值下的定位性能和检测性能,从C中可以看出,当检测模型采用某个阈值来界定正负样本时,当输入proposal的IOU阈值在这个值附近时,该检测模型比其他阈值训练的检测模型效果好。从D中可以看出来一味地使用较大的阈值来训练检测模型可以看出检测效果在下降,容易出现过拟合。
cascade r-cnn是由一系列的检测模型组成,每个检测模型都是根据不同的IOU阈值的正负样本训练得到,前一个检测模型的输出作为后一个检测模型的输入,因此是stage by stage的训练方式,而且越往后的检测模型的IOU阈值是不断上升的。在这里插入图片描述
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In object detection, an intersection over union (IoU) threshold is required to define positives and negatives. An object detector, trained with low IoU threshold, e.g. 0.5, usually produces noisy detections. However, detection per- formance tends to degrade with increasing the IoU thresh- olds. Two main factors are responsible for this: 1) over- fitting during training, due to exponentially vanishing pos- itive samples, and 2) inference-time mismatch between the IoUs for which the detector is optimal and those of the in- put hypotheses. A multi-stage object detection architecture, the Cascade R-CNN, is proposed to address these prob- lems. It consists of a sequence of detectors trained with increasing IoU thresholds, to be sequentially more selec- tive against close false positives. The detectors are trained stage by stage, leveraging the observation that the out- put of a detector is a good distribution for training the next higher quality detector. The resampling of progres- sively improved hypotheses guarantees that all detectors have a positive set of examples of equivalent size, reduc- ing the overfitting problem. The same cascade procedure is applied at inference, enabling a closer match between the hypotheses and the detector quality of each stage. A simple implementation of the Cascade R-CNN is shown to surpass all single-model object detectors on the challeng- ing COCO dataset. Experiments also show that the Cas- cade R-CNN is widely applicable across detector architec- tures, achieving consistent gains independently of the base- line detector strength. The code will be made available at https://github.com/zhaoweicai/cascade-rcnn.
### CASCADE R-CNN目标检测算法介绍 CASCADE R-CNN是一种多级联的目标检测框架,专为设计高质量的目标检测器而构建[^4]。该方法通过一系列逐步增加严格度的IoU(交并比)阈值来过滤候选框,从而有效地减少误报率并提高检测精度。 #### 主要特点 - **逐步加严的IoU阈值**:不同于传统的单阶段或多阶段检测器采用固定的IoU标准,CASCADE R-CNN引入了一个逐渐变严格的IoU序列,在每一层都应用更精确的标准来进行正负样本划分[^5]。 - **避免过拟合和质量不匹配**:此结构有助于防止训练过程中的过拟合并解决了推理过程中可能出现的质量差异问题。 - **广泛的适用性**:无论是Faster R-CNN还是其他基于区域建议网络(RPN)的方法,都可以利用CASCADE R-CNN的思想获得性能增益[^3]。 ### 实现流程 为了更好地理解CASCADE R-CNN的工作原理,下面给出一个简化版Python伪代码示例: ```python class CascadeRCNN(nn.Module): def __init__(self, backbone, num_classes=80): super(CascadeRCNN, self).__init__() # 定义骨干网和其他组件... self.stage_configs = [ {'iou_threshold': 0.5}, {'iou_threshold': 0.6}, {'iou_threshold': 0.7} ] def forward(self, images, targets=None): features = self.backbone(images) proposals = [] losses = {} for stage_config in self.stage_configs: roi_heads_output = self.roi_heads(features, proposals, stage_config['iou_threshold']) if self.training: loss_dict = compute_losses(roi_heads_output, targets) losses.update(loss_dict) else: detections = postprocess_detections(roi_heads_output) proposals.extend(detections) if not self.training: return detections return losses ``` 上述代码展示了如何在一个典型的PyTorch项目中定义CASCADE R-CNN类及其前向传播逻辑。注意这里只包含了核心概念,并未涉及具体细节如损失函数计算等。
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