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import torch.nn.functional as F
import torch
import numpy as np
from torch.autograd import Function
import torch.nn as nn
from pdb import set_trace as breakpoint
import sys
import math
from torch.nn.parameter import Parameter
from torch.nn import init
import torch.nn as nn
from torch.autograd import Variable
class FocalLoss(nn.Module):
def __init__(self, gamma=2, alpha=0.25, size_average=True):
super(FocalLoss, self).__init__()
self.gamma = gamma
self.alpha = alpha
if isinstance(alpha, (float, int)): self.alpha = torch.Tensor([alpha, 1 - alpha])
if isinstance(alpha, list): self.alpha = torch.Tensor(alpha)
self.size_average = size_average
def forward(self, input, target):
if input.dim()>2:
input = input.view(input.size(0), input.size(1), -1) # N,C,H,W => N,C,H*W
input = input.transpose(1, 2) # N,C,H*W => N,H*W,C
input = input.contiguous().view(-1, input.size(2)) # N,H*W,C => N*H*W,C
target = target.view(-1, 1)
logpt = F.log_softmax(input, dim=1)
logpt = logpt.gather(1,target)
logpt = logpt.view(-1)
pt = logpt.exp()
if self.alpha is not None:
if self.alpha.type() != input.data.type():
self.alpha = self.alpha.type_as(input.data)
at = self.alpha.gather(0, target.data.view(-1))
logpt = logpt * at
loss = -1 * (1 - pt)**self.gamma * logpt
if self.size_average: return loss.mean()
else: return loss.sum()
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