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# Copyright (c) OpenMMLab. All rights reserved.
import torch.nn as nn
import torch.nn.functional as F
from mmpretrain.registry import MODELS
from .utils import convert_to_one_hot, weight_reduce_loss
def sigmoid_focal_loss(pred,
target,
weight=None,
gamma=2.0,
alpha=0.25,
reduction='mean',
avg_factor=None):
r"""Sigmoid focal loss.
Args:
pred (torch.Tensor): The prediction with shape (N, \*).
target (torch.Tensor): The ground truth label of the prediction with
shape (N, \*).
weight (torch.Tensor, optional): Sample-wise loss weight with shape
(N, ). Defaults to None.
gamma (float): The gamma for calculating the modulating factor.
Defaults to 2.0.
alpha (float): A balanced form for Focal Loss. Defaults to 0.25.
reduction (str): The method used to reduce the loss.
Options are "none", "mean" and "sum". If reduction is 'none' ,
loss is same shape as pred and label. Defaults to 'mean'.
avg_factor (int, optional): Average factor that is used to average
the loss. Defaults to None.
Returns:
torch.Tensor: Loss.
"""
assert pred.shape == \
target.shape, 'pred and target should be in the same shape.'
pred_sigmoid = pred.sigmoid()
target = target.type_as(pred)
pt = (1 - pred_sigmoid) * target + pred_sigmoid * (1 - target)
focal_weight = (alpha * target + (1 - alpha) *
(1 - target)) * pt.pow(gamma)
loss = F.binary_cross_entropy_with_logits(
pred, target, reduction='none') * focal_weight
if weight is not None:
assert weight.dim() == 1
weight = weight.float()
if pred.dim() > 1:
weight = weight.reshape(-1, 1)
loss = weight_reduce_loss(loss, weight, reduction, avg_factor)
return loss
@MODELS.register_module()
class FocalLoss(nn.Module):
"""Focal loss.
Args:
gamma (float): Focusing parameter in focal loss.
Defaults to 2.0.
alpha (float): The parameter in balanced form of focal
loss. Defaults to 0.25.
reduction (str): The method used to reduce the loss into
a scalar. Options are "none" and "mean". Defaults to 'mean'.
loss_weight (float): Weight of loss. Defaults to 1.0.
"""
def __init__(self,
gamma=2.0,
alpha=0.25,
reduction='mean',
loss_weight=1.0):
super(FocalLoss, self).__init__()
self.gamma = gamma
self.alpha = alpha
self.reduction = reduction
self.loss_weight = loss_weight
def forward(self,
pred,
target,
weight=None,
avg_factor=None,
reduction_override=None):
r"""Sigmoid focal loss.
Args:
pred (torch.Tensor): The prediction with shape (N, \*).
target (torch.Tensor): The ground truth label of the prediction
with shape (N, \*), N or (N,1).
weight (torch.Tensor, optional): Sample-wise loss weight with shape
(N, \*). Defaults to None.
avg_factor (int, optional): Average factor that is used to average
the loss. Defaults to None.
reduction_override (str, optional): The method used to reduce the
loss into a scalar. Options are "none", "mean" and "sum".
Defaults to None.
Returns:
torch.Tensor: Loss.
"""
assert reduction_override in (None, 'none', 'mean', 'sum')
reduction = (
reduction_override if reduction_override else self.reduction)
if target.dim() == 1 or (target.dim() == 2 and target.shape[1] == 1):
target = convert_to_one_hot(target.view(-1, 1), pred.shape[-1])
loss_cls = self.loss_weight * sigmoid_focal_loss(
pred,
target,
weight,
gamma=self.gamma,
alpha=self.alpha,
reduction=reduction,
avg_factor=avg_factor)
return loss_cls
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