1.configs

有的时候写在head中作为参数，有的时候head内部进行默认调用。

我们以为例（这里没有直接写loss相关参数）：

https://github.com/open-mmlab/mmdetection/blob/master/configs/yolox/yolox_s_8x8_300e_coco.py#L17

2.具体实现

从head文件中，写了4个loss。

以IoULoss为例，具体实现：

mmdetection/iou_loss.py at master · open-mmlab/mmdetection · GitHub

@LOSSES.register_module()
class IoULoss(nn.Module):
    """IoULoss.
    Computing the IoU loss between a set of predicted bboxes and target bboxes.
    Args:
        linear (bool): If True, use linear scale of loss else determined
            by mode. Default: False.
        eps (float): Eps to avoid log(0).
        reduction (str): Options are "none", "mean" and "sum".
        loss_weight (float): Weight of loss.
        mode (str): Loss scaling mode, including "linear", "square", and "log".
            Default: 'log'
    """

    def __init__(self,
                 linear=False,
                 eps=1e-6,
                 reduction='mean',
                 loss_weight=1.0,
                 mode='log'):
        super(IoULoss, self).__init__()
        assert mode in ['linear', 'square', 'log']
        if linear:
            mode = 'linear'
            warnings.warn('DeprecationWarning: Setting "linear=True" in '
                          'IOULoss is deprecated, please use "mode=`linear`" '
                          'instead.')
        self.mode = mode
        self.linear = linear
        self.eps = eps
        self.reduction = reduction
        self.loss_weight = loss_weight

    def forward(self,
                pred,
                target,
                weight=None,
                avg_factor=None,
                reduction_override=None,
                **kwargs):
        """Forward function.
        Args:
            pred (torch.Tensor): The prediction.
            target (torch.Tensor): The learning target of the prediction.
            weight (torch.Tensor, optional): The weight of loss for each
                prediction. Defaults to None.
            avg_factor (int, optional): Average factor that is used to average
                the loss. Defaults to None.
            reduction_override (str, optional): The reduction method used to
                override the original reduction method of the loss.
                Defaults to None. Options are "none", "mean" and "sum".
        """
        assert reduction_override in (None, 'none', 'mean', 'sum')
        reduction = (
            reduction_override if reduction_override else self.reduction)
        if (weight is not None) and (not torch.any(weight > 0)) and (
                reduction != 'none'):
            if pred.dim() == weight.dim() + 1:
                weight = weight.unsqueeze(1)
            return (pred * weight).sum()  # 0
        if weight is not None and weight.dim() > 1:
            # TODO: remove this in the future
            # reduce the weight of shape (n, 4) to (n,) to match the
            # iou_loss of shape (n,)
            assert weight.shape == pred.shape
            weight = weight.mean(-1)
        loss = self.loss_weight * iou_loss(
            pred,
            target,
            weight,
            mode=self.mode,
            eps=self.eps,
            reduction=reduction,
            avg_factor=avg_factor,
            **kwargs)
        return loss

3.调用

3.1 注册

注册创建类名字典。

@LOSSES.register_module()

3.2 调用

实例化在YOLOXHead该类中：

https://github.com/open-mmlab/mmdetection/blob/master/mmdet/models/dense_heads/yolox_head.py#L109

self.loss_cls = build_loss(loss_cls)
self.loss_bbox = build_loss(loss_bbox)
self.loss_obj = build_loss(loss_obj)

self.use_l1 = False  # This flag will be modified by hooks.
self.loss_l1 = build_loss(loss_l1)

代码通过调用YOLOX的基类中的forward_train函数调用到head的forward_train：

https://github.com/open-mmlab/mmdetection/blob/31c84958f54287a8be2b99cbf87a6dcf12e57753/mmdet/models/detectors/single_stage.py#L57

    def forward_train(self,
                      img,
                      img_metas,
                      gt_bboxes,
                      gt_labels,
                      gt_bboxes_ignore=None):
        """
        Args:
            img (Tensor): Input images of shape (N, C, H, W).
                Typically these should be mean centered and std scaled.
            img_metas (list[dict]): A List of image info dict where each dict
                has: 'img_shape', 'scale_factor', 'flip', and may also contain
                'filename', 'ori_shape', 'pad_shape', and 'img_norm_cfg'.
                For details on the values of these keys see
                :class:`mmdet.datasets.pipelines.Collect`.
            gt_bboxes (list[Tensor]): Each item are the truth boxes for each
                image in [tl_x, tl_y, br_x, br_y] format.
            gt_labels (list[Tensor]): Class indices corresponding to each box
            gt_bboxes_ignore (None | list[Tensor]): Specify which bounding
                boxes can be ignored when computing the loss.
        Returns:
            dict[str, Tensor]: A dictionary of loss components.
        """
        super(SingleStageDetector, self).forward_train(img, img_metas)
        x = self.extract_feat(img)
        losses = self.bbox_head.forward_train(x, img_metas, gt_bboxes,
                                              gt_labels, gt_bboxes_ignore)
        return losses

而head的forward_train，继承的其基类实现：

https://github.com/open-mmlab/mmdetection/blob/master/mmdet/models/dense_heads/base_dense_head.py#L303

def forward_train(self,
                      x,
                      img_metas,
                      gt_bboxes,
                      gt_labels=None,
                      gt_bboxes_ignore=None,
                      proposal_cfg=None,
                      **kwargs):
        """
        Args:
            x (list[Tensor]): Features from FPN.
            img_metas (list[dict]): Meta information of each image, e.g.,
                image size, scaling factor, etc.
            gt_bboxes (Tensor): Ground truth bboxes of the image,
                shape (num_gts, 4).
            gt_labels (Tensor): Ground truth labels of each box,
                shape (num_gts,).
            gt_bboxes_ignore (Tensor): Ground truth bboxes to be
                ignored, shape (num_ignored_gts, 4).
            proposal_cfg (mmcv.Config): Test / postprocessing configuration,
                if None, test_cfg would be used
        Returns:
            tuple:
                losses: (dict[str, Tensor]): A dictionary of loss components.
                proposal_list (list[Tensor]): Proposals of each image.
        """
        outs = self(x)
        if gt_labels is None:
            loss_inputs = outs + (gt_bboxes, img_metas)
        else:
            loss_inputs = outs + (gt_bboxes, gt_labels, img_metas)
        losses = self.loss(*loss_inputs, gt_bboxes_ignore=gt_bboxes_ignore)
        if proposal_cfg is None:
            return losses
        else:
            proposal_list = self.get_bboxes(
                *outs, img_metas=img_metas, cfg=proposal_cfg)
            return losses, proposal_list

其中调用的self.loss函数，基类中未实现，所以loss函数调用的head的：

losses = self.loss(*loss_inputs, gt_bboxes_ignore=gt_bboxes_ignore)

mmdetection/yolox_head.py at master · open-mmlab/mmdetection · GitHub

@force_fp32(apply_to=('cls_scores', 'bbox_preds', 'objectnesses'))
    def loss(self,
             cls_scores,
             bbox_preds,
             objectnesses,
             gt_bboxes,
             gt_labels,
             img_metas,
             gt_bboxes_ignore=None):
        """Compute loss of the head.
        Args:
            cls_scores (list[Tensor]): Box scores for each scale level,
                each is a 4D-tensor, the channel number is
                num_priors * num_classes.
            bbox_preds (list[Tensor]): Box energies / deltas for each scale
                level, each is a 4D-tensor, the channel number is
                num_priors * 4.
            objectnesses (list[Tensor], Optional): Score factor for
                all scale level, each is a 4D-tensor, has shape
                (batch_size, 1, H, W).
            gt_bboxes (list[Tensor]): Ground truth bboxes for each image with
                shape (num_gts, 4) in [tl_x, tl_y, br_x, br_y] format.
            gt_labels (list[Tensor]): class indices corresponding to each box
            img_metas (list[dict]): Meta information of each image, e.g.,
                image size, scaling factor, etc.
            gt_bboxes_ignore (None | list[Tensor]): specify which bounding
                boxes can be ignored when computing the loss.
        """
        num_imgs = len(img_metas)
        featmap_sizes = [cls_score.shape[2:] for cls_score in cls_scores]
        mlvl_priors = self.prior_generator.grid_priors(
            featmap_sizes,
            dtype=cls_scores[0].dtype,
            device=cls_scores[0].device,
            with_stride=True)

        flatten_cls_preds = [
            cls_pred.permute(0, 2, 3, 1).reshape(num_imgs, -1,
                                                 self.cls_out_channels)
            for cls_pred in cls_scores
        ]
        flatten_bbox_preds = [
            bbox_pred.permute(0, 2, 3, 1).reshape(num_imgs, -1, 4)
            for bbox_pred in bbox_preds
        ]
        flatten_objectness = [
            objectness.permute(0, 2, 3, 1).reshape(num_imgs, -1)
            for objectness in objectnesses
        ]

        flatten_cls_preds = torch.cat(flatten_cls_preds, dim=1)
        flatten_bbox_preds = torch.cat(flatten_bbox_preds, dim=1)
        flatten_objectness = torch.cat(flatten_objectness, dim=1)
        flatten_priors = torch.cat(mlvl_priors)
        flatten_bboxes = self._bbox_decode(flatten_priors, flatten_bbox_preds)

        (pos_masks, cls_targets, obj_targets, bbox_targets, l1_targets,
         num_fg_imgs) = multi_apply(
             self._get_target_single, flatten_cls_preds.detach(),
             flatten_objectness.detach(),
             flatten_priors.unsqueeze(0).repeat(num_imgs, 1, 1),
             flatten_bboxes.detach(), gt_bboxes, gt_labels)

        # The experimental results show that ‘reduce_mean’ can improve
        # performance on the COCO dataset.
        num_pos = torch.tensor(
            sum(num_fg_imgs),
            dtype=torch.float,
            device=flatten_cls_preds.device)
        num_total_samples = max(reduce_mean(num_pos), 1.0)

        pos_masks = torch.cat(pos_masks, 0)
        cls_targets = torch.cat(cls_targets, 0)
        obj_targets = torch.cat(obj_targets, 0)
        bbox_targets = torch.cat(bbox_targets, 0)
        if self.use_l1:
            l1_targets = torch.cat(l1_targets, 0)

        loss_bbox = self.loss_bbox(
            flatten_bboxes.view(-1, 4)[pos_masks],
            bbox_targets) / num_total_samples
        loss_obj = self.loss_obj(flatten_objectness.view(-1, 1),
                                 obj_targets) / num_total_samples
        loss_cls = self.loss_cls(
            flatten_cls_preds.view(-1, self.num_classes)[pos_masks],
            cls_targets) / num_total_samples

        loss_dict = dict(
            loss_cls=loss_cls, loss_bbox=loss_bbox, loss_obj=loss_obj)

        if self.use_l1:
            loss_l1 = self.loss_l1(
                flatten_bbox_preds.view(-1, 4)[pos_masks],
                l1_targets) / num_total_samples
            loss_dict.update(loss_l1=loss_l1)

        return loss_dict

而对应的里面的loss函数，来自：

https://github.com/open-mmlab/mmdetection/blob/master/mmdet/models/dense_heads/yolox_head.py#L327

self.loss_cls = build_loss(loss_cls)
self.loss_bbox = build_loss(loss_bbox)
self.loss_obj = build_loss(loss_obj)

self.use_l1 = False  # This flag will be modified by hooks.
self.loss_l1 = build_loss(loss_l1)

loss的具体实现在：https://github.com/open-mmlab/mmdetection/blob/master/mmdet/models/losses/iou_loss.py#L16

@LOSSES.register_module()
class IoULoss(nn.Module):
    """IoULoss.
    Computing the IoU loss between a set of predicted bboxes and target bboxes.
    Args:
        linear (bool): If True, use linear scale of loss else determined
            by mode. Default: False.
        eps (float): Eps to avoid log(0).
        reduction (str): Options are "none", "mean" and "sum".
        loss_weight (float): Weight of loss.
        mode (str): Loss scaling mode, including "linear", "square", and "log".
            Default: 'log'
    """

    def __init__(self,
                 linear=False,
                 eps=1e-6,
                 reduction='mean',
                 loss_weight=1.0,
                 mode='log'):
        super(IoULoss, self).__init__()
        assert mode in ['linear', 'square', 'log']
        if linear:
            mode = 'linear'
            warnings.warn('DeprecationWarning: Setting "linear=True" in '
                          'IOULoss is deprecated, please use "mode=`linear`" '
                          'instead.')
        self.mode = mode
        self.linear = linear
        self.eps = eps
        self.reduction = reduction
        self.loss_weight = loss_weight

    def forward(self,
                pred,
                target,
                weight=None,
                avg_factor=None,
                reduction_override=None,
                **kwargs):
        """Forward function.
        Args:
            pred (torch.Tensor): The prediction.
            target (torch.Tensor): The learning target of the prediction.
            weight (torch.Tensor, optional): The weight of loss for each
                prediction. Defaults to None.
            avg_factor (int, optional): Average factor that is used to average
                the loss. Defaults to None.
            reduction_override (str, optional): The reduction method used to
                override the original reduction method of the loss.
                Defaults to None. Options are "none", "mean" and "sum".
        """
        assert reduction_override in (None, 'none', 'mean', 'sum')
        reduction = (
            reduction_override if reduction_override else self.reduction)
        if (weight is not None) and (not torch.any(weight > 0)) and (
                reduction != 'none'):
            if pred.dim() == weight.dim() + 1:
                weight = weight.unsqueeze(1)
            return (pred * weight).sum()  # 0
        if weight is not None and weight.dim() > 1:
            # TODO: remove this in the future
            # reduce the weight of shape (n, 4) to (n,) to match the
            # iou_loss of shape (n,)
            assert weight.shape == pred.shape
            weight = weight.mean(-1)
        loss = self.loss_weight * iou_loss(
            pred,
            target,
            weight,
            mode=self.mode,
            eps=self.eps,
            reduction=reduction,
            avg_factor=avg_factor,
            **kwargs)
        return loss