MobileNetV2-SSDLite代码分析-7 loss

train时定义如下。

criterion = MultiboxLoss(config.priors, iou_threshold=0.5, neg_pos_ratio=3, center_variance=0.1, size_variance=0.2, device=DEVICE)

获取时使用如下。

regression_loss, classification_loss = criterion(confidence, locations, labels, boxes)

MultiboxLoss则是另写了一个类来实现
初始化就是设置了一些参数，包括：

事先规定好生成的anchor框
iou阈值
neg_pos_ratio，正负样本的比例
中心点偏差
尺寸偏差
计算设备

class MultiboxLoss(nn.Module):
    def __init__(self, priors, iou_threshold, neg_pos_ratio,
                 center_variance, size_variance, device):
        """Implement SSD Multibox Loss.

        Basically, Multibox loss combines classification loss
         and Smooth L1 regression loss.
        """
        super(MultiboxLoss, self).__init__()
        self.iou_threshold = iou_threshold
        self.neg_pos_ratio = neg_pos_ratio
        self.center_variance = center_variance
        self.size_variance = size_variance
        self.priors = priors
        self.priors.to(device)

计算过程如下。
参数包括四个：计算得到的class confidence和predicted locations，真实标注的labels和gt_locations.

confidence符合(img, -1, classes)的格式，location符合(img,-1, 4)的格式

首先hard_negative_mining用来确定哪些正负样本用来计算loss

classification_loss 通过confidence和labels[mask]计算cross entropy

location_loss通过smooth_l1_loss来计算的

最后要除以数量算平均值

Smooth L1

相比于L1损失函数，可以收敛得更快。
相比于L2损失函数，对离群点、异常值不敏感，梯度变化相对更小，训练时不容易跑飞。

image.png

    def forward(self, confidence, predicted_locations, labels, gt_locations):
        """Compute classification loss and smooth l1 loss.

        Args:
            confidence (batch_size, num_priors, num_classes): class predictions.
            locations (batch_size, num_priors, 4): predicted locations.
            labels (batch_size, num_priors): real labels of all the priors.
            boxes (batch_size, num_priors, 4): real boxes corresponding all the priors.
        """
        num_classes = confidence.size(2)
        with torch.no_grad():# 不需要反向传播
            # derived from cross_entropy=sum(log(p))
            loss = -F.log_softmax(confidence, dim=2)[:, :, 0]
            mask = box_utils.hard_negative_mining(loss, labels, self.neg_pos_ratio)

        confidence = confidence[mask, :]
        classification_loss = F.cross_entropy(confidence.reshape(-1, num_classes), labels[mask], size_average=False)
        pos_mask = labels > 0
        predicted_locations = predicted_locations[pos_mask, :].reshape(-1, 4)
        gt_locations = gt_locations[pos_mask, :].reshape(-1, 4)
        smooth_l1_loss = F.smooth_l1_loss(predicted_locations, gt_locations, size_average=False)
        num_pos = gt_locations.size(0)
        return smooth_l1_loss/num_pos, classification_loss/num_pos

box_utils.hard_negative_mining定义如下。SSD里面有正负样本1：3的说法，不然负样本就太多了。这个就是用来抑制负样本数量的。对于每张图片，做1：3的抑制。这里有点问题，注释说是对每张图做1：3，但实际看输入应该还是基于batch做的

def hard_negative_mining(loss, labels, neg_pos_ratio):
    """
    It used to suppress the presence of a large number of negative prediction.
    It works on image level not batch level.
    For any example/image, it keeps all the positive predictions and
     cut the number of negative predictions to make sure the ratio
     between the negative examples and positive examples is no more
     the given ratio for an image.

    Args:
        loss (N, num_priors): the loss for each example.
        labels (N, num_priors): the labels.
        neg_pos_ratio:  the ratio between the negative examples and positive examples.
    """
    pos_mask = labels > 0 # 选取的正样本，格式为(batch_size, num_priors)
    num_pos = pos_mask.long().sum(dim=1, keepdim=True)
    num_neg = num_pos * neg_pos_ratio

    loss[pos_mask] = -math.inf
    _, indexes = loss.sort(dim=1, descending=True)
    _, orders = indexes.sort(dim=1)
    neg_mask = orders < num_neg
    return pos_mask | neg_mask #只要你在我划分的正样本或负样本中，我就返回1，其余就返回0，返回的是一个01的串，标志着到底该样本用不用来计算loss