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# Copyright (c) 2021 PaddlePaddle Authors. All Rights Reserve.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import numpy as np
import paddle
import paddle.nn as nn
import paddle.nn.functional as F
from paddleseg.cvlibs import manager
@manager.LOSSES.add_component
class FocalLoss(nn.Layer):
"""
The implement of focal loss.
The focal loss requires the label is 0 or 1 for now.
Args:
alpha (float, list, optional): The alpha of focal loss. alpha is the weight
of class 1, 1-alpha is the weight of class 0. Default: 0.25
gamma (float, optional): The gamma of Focal Loss. Default: 2.0
ignore_index (int64, optional): Specifies a target value that is ignored
and does not contribute to the input gradient. Default ``255``.
"""
def __init__(self, alpha=0.25, gamma=2.0, ignore_index=255):
super().__init__()
self.alpha = alpha
self.gamma = gamma
self.ignore_index = ignore_index
self.EPS = 1e-10
def forward(self, logit, label):
"""
Forward computation.
Args:
logit (Tensor): Logit tensor, the data type is float32, float64. Shape is
(N, C, H, W), where C is number of classes.
label (Tensor): Label tensor, the data type is int64. Shape is (N, W, W),
where each value is 0 <= label[i] <= C-1.
Returns:
(Tensor): The average loss.
"""
assert logit.ndim == 4, "The ndim of logit should be 4."
assert logit.shape[1] == 2, "The channel of logit should be 2."
assert label.ndim == 3, "The ndim of label should be 3."
class_num = logit.shape[1] # class num is 2
logit = paddle.transpose(logit, [0, 2, 3, 1]) # N,C,H,W => N,H,W,C
mask = label != self.ignore_index # N,H,W
mask = paddle.unsqueeze(mask, 3)
mask = paddle.cast(mask, 'float32')
mask.stop_gradient = True
label = F.one_hot(label, class_num) # N,H,W,C
label = paddle.cast(label, logit.dtype)
label.stop_gradient = True
loss = F.sigmoid_focal_loss(
logit=logit,
label=label,
alpha=self.alpha,
gamma=self.gamma,
reduction='none')
loss = loss * mask
avg_loss = paddle.sum(loss) / (
paddle.sum(paddle.cast(mask != 0., 'int32')) * class_num + self.EPS)
return avg_loss
@manager.LOSSES.add_component
class MultiClassFocalLoss(nn.Layer):
"""
The implement of focal loss for multi class.
Args:
alpha (float, list, optional): The alpha of focal loss. alpha is the weight
of class 1, 1-alpha is the weight of class 0. Default: 0.25
gamma (float, optional): The gamma of Focal Loss. Default: 2.0
ignore_index (int64, optional): Specifies a target value that is ignored
and does not contribute to the input gradient. Default ``255``.
"""
def __init__(self, num_class, alpha=1.0, gamma=2.0, ignore_index=255):
super().__init__()
self.num_class = num_class
self.alpha = alpha
self.gamma = gamma
self.ignore_index = ignore_index
self.EPS = 1e-10
def forward(self, logit, label):
"""
Forward computation.
Args:
logit (Tensor): Logit tensor, the data type is float32, float64. Shape is
(N, C, H, W), where C is number of classes.
label (Tensor): Label tensor, the data type is int64. Shape is (N, W, W),
where each value is 0 <= label[i] <= C-1.
Returns:
(Tensor): The average loss.
"""
assert logit.ndim == 4, "The ndim of logit should be 4."
assert label.ndim == 3, "The ndim of label should be 3."
logit = paddle.transpose(logit, [0, 2, 3, 1])
label = label.astype('int64')
ce_loss = F.cross_entropy(
logit, label, ignore_index=self.ignore_index, reduction='none')
pt = paddle.exp(-ce_loss)
focal_loss = self.alpha * ((1 - pt)**self.gamma) * ce_loss
mask = paddle.cast(label != self.ignore_index, 'float32')
focal_loss *= mask
avg_loss = paddle.mean(focal_loss) / (paddle.mean(mask) + self.EPS)
return avg_loss
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