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# Copyright (c) OpenMMLab. All rights reserved.
import numpy as np
import torch
import torch.nn as nn
from mmdet.registry import MODELS
from .utils import weighted_loss
@weighted_loss
def balanced_l1_loss(pred,
target,
beta=1.0,
alpha=0.5,
gamma=1.5,
reduction='mean'):
"""Calculate balanced L1 loss.
Please see the `Libra R-CNN <https://arxiv.org/pdf/1904.02701.pdf>`_
Args:
pred (torch.Tensor): The prediction with shape (N, 4).
target (torch.Tensor): The learning target of the prediction with
shape (N, 4).
beta (float): The loss is a piecewise function of prediction and target
and ``beta`` serves as a threshold for the difference between the
prediction and target. Defaults to 1.0.
alpha (float): The denominator ``alpha`` in the balanced L1 loss.
Defaults to 0.5.
gamma (float): The ``gamma`` in the balanced L1 loss.
Defaults to 1.5.
reduction (str, optional): The method that reduces the loss to a
scalar. Options are "none", "mean" and "sum".
Returns:
torch.Tensor: The calculated loss
"""
assert beta > 0
if target.numel() == 0:
return pred.sum() * 0
assert pred.size() == target.size()
diff = torch.abs(pred - target)
b = np.e**(gamma / alpha) - 1
loss = torch.where(
diff < beta, alpha / b *
(b * diff + 1) * torch.log(b * diff / beta + 1) - alpha * diff,
gamma * diff + gamma / b - alpha * beta)
return loss
@MODELS.register_module()
class BalancedL1Loss(nn.Module):
"""Balanced L1 Loss.
arXiv: https://arxiv.org/pdf/1904.02701.pdf (CVPR 2019)
Args:
alpha (float): The denominator ``alpha`` in the balanced L1 loss.
Defaults to 0.5.
gamma (float): The ``gamma`` in the balanced L1 loss. Defaults to 1.5.
beta (float, optional): The loss is a piecewise function of prediction
and target. ``beta`` serves as a threshold for the difference
between the prediction and target. Defaults to 1.0.
reduction (str, optional): The method that reduces the loss to a
scalar. Options are "none", "mean" and "sum".
loss_weight (float, optional): The weight of the loss. Defaults to 1.0
"""
def __init__(self,
alpha=0.5,
gamma=1.5,
beta=1.0,
reduction='mean',
loss_weight=1.0):
super(BalancedL1Loss, self).__init__()
self.alpha = alpha
self.gamma = gamma
self.beta = beta
self.reduction = reduction
self.loss_weight = loss_weight
def forward(self,
pred,
target,
weight=None,
avg_factor=None,
reduction_override=None,
**kwargs):
"""Forward function of loss.
Args:
pred (torch.Tensor): The prediction with shape (N, 4).
target (torch.Tensor): The learning target of the prediction with
shape (N, 4).
weight (torch.Tensor, optional): Sample-wise loss weight with
shape (N, ).
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.
Options are "none", "mean" and "sum".
Returns:
torch.Tensor: The calculated loss
"""
assert reduction_override in (None, 'none', 'mean', 'sum')
reduction = (
reduction_override if reduction_override else self.reduction)
loss_bbox = self.loss_weight * balanced_l1_loss(
pred,
target,
weight,
alpha=self.alpha,
gamma=self.gamma,
beta=self.beta,
reduction=reduction,
avg_factor=avg_factor,
**kwargs)
return loss_bbox
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