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binary_cross_entropy_loss.py
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# Copyright (c) 2020 PaddlePaddle Authors. All Rights Reserved.
#
# 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 paddle
import paddle.nn as nn
import paddle.nn.functional as F
class BCELoss(nn.Layer):
"""
This operator combines the sigmoid layer and the :ref:`api_nn_loss_BCELoss` layer.
Also, we can see it as the combine of ``sigmoid_cross_entropy_with_logits``
layer and some reduce operations.
This measures the element-wise probability error in classification tasks
in which each class is independent.
This can be thought of as predicting labels for a data-point, where labels
are not mutually exclusive. For example, a news article can be about
politics, technology or sports at the same time or none of these.
First this operator calculate loss function as follows:
.. math::
Out = -Labels * \\log(\\sigma(Logit)) - (1 - Labels) * \\log(1 - \\sigma(Logit))
We know that :math:`\\sigma(Logit) = \\frac{1}{1 + \\e^{-Logit}}`. By substituting this we get:
.. math::
Out = Logit - Logit * Labels + \\log(1 + \\e^{-Logit})
For stability and to prevent overflow of :math:`\\e^{-Logit}` when Logit < 0,
we reformulate the loss as follows:
.. math::
Out = \\max(Logit, 0) - Logit * Labels + \\log(1 + \\e^{-\|Logit\|})
Then, if ``weight`` or ``pos_weight`` is not None, this operator multiply the
weight tensor on the loss `Out`. The ``weight`` tensor will attach different
weight on every items in the batch. The ``pos_weight`` will attach different
weight on the positive label of each class.
Finally, this operator applies reduce operation on the loss.
If :attr:`reduction` set to ``'none'``, the operator will return the original loss `Out`.
If :attr:`reduction` set to ``'mean'``, the reduced mean loss is :math:`Out = MEAN(Out)`.
If :attr:`reduction` set to ``'sum'``, the reduced sum loss is :math:`Out = SUM(Out)`.
Note that the target labels ``label`` should be numbers between 0 and 1.
Args:
weight (Tensor | str, optional): A manual rescaling weight given to the loss of each
batch element. If given, it has to be a 1D Tensor whose size is `[N, ]`,
The data type is float32, float64. If type is str, it should equal to 'dynamic'.
It will compute weight dynamically in every step.
Default is ``'None'``.
pos_weight (float|str, optional): A weight of positive examples. If type is str,
it should equal to 'dynamic'. It will compute weight dynamically in every step.
Default is ``'None'``.
ignore_index (int64, optional): Specifies a target value that is ignored
and does not contribute to the input gradient. Default ``255``.
edge_label (bool, optional): Whether to use edge label. Default: False
Shapes:
logit (Tensor): The input predications tensor. 2-D tensor with shape: [N, *],
N is batch_size, `*` means number of additional dimensions. The ``logit``
is usually the output of Linear layer. Available dtype is float32, float64.
label (Tensor): The target labels tensor. 2-D tensor with the same shape as
``logit``. The target labels which values should be numbers between 0 and 1.
Available dtype is float32, float64.
Returns:
A callable object of BCEWithLogitsLoss.
Examples:
.. code-block:: python
import paddle
paddle.disable_static()
logit = paddle.to_tensor([5.0, 1.0, 3.0], dtype="float32")
label = paddle.to_tensor([1.0, 0.0, 1.0], dtype="float32")
bce_logit_loss = paddle.nn.BCEWithLogitsLoss()
output = bce_logit_loss(logit, label)
print(output.numpy()) # [0.45618808]
"""
def __init__(self,
weight=None,
pos_weight=None,
ignore_index=255,
edge_label=False):
super().__init__()
self.weight = weight
self.pos_weight = pos_weight
self.ignore_index = ignore_index
self.edge_label = edge_label
self.EPS = 1e-10
if self.weight is not None:
if isinstance(self.weight, str):
if self.weight != 'dynamic':
raise ValueError(
"if type of `weight` is str, it should equal to 'dynamic', but it is {}"
.format(self.weight))
elif isinstance(self.weight, paddle.VarBase):
raise TypeError(
'The type of `weight` is wrong, it should be Tensor or str, but it is {}'
.format(type(self.weight)))
if self.pos_weight is not None:
if isinstance(self.pos_weight, str):
if self.pos_weight != 'dynamic':
raise ValueError(
"if type of `pos_weight` is str, it should equal to 'dynamic', but it is {}"
.format(self.pos_weight))
elif isinstance(self.pos_weight, float):
self.pos_weight = paddle.to_tensor(
self.pos_weight, dtype='float32')
else:
raise TypeError(
'The type of `pos_weight` is wrong, it should be float or str, but it is {}'
.format(type(self.pos_weight)))
def forward(self, logit, label):
"""
Forward computation.
Args:
logit (Tensor): Logit tensor, the data type is float32, float64. Shape is
(N, C), where C is number of classes, and if shape is more than 2D, this
is (N, C, D1, D2,..., Dk), k >= 1.
label (Tensor): Label tensor, the data type is int64. Shape is (N, C), where each
value is 0 or 1, and if shape is more than 2D, this is
(N, C, D1, D2,..., Dk), k >= 1.
"""
if len(label.shape) != len(logit.shape):
label = paddle.unsqueeze(label, 1)
mask = (label != self.ignore_index)
mask = paddle.cast(mask, 'float32')
# label.shape should equal to the logit.shape
if label.shape[1] != logit.shape[1]:
label = label.squeeze(1)
label = F.one_hot(label, logit.shape[1])
label = label.transpose((0, 3, 1, 2))
if isinstance(self.weight, str):
pos_index = (label == 1)
neg_index = (label == 0)
pos_num = paddle.sum(pos_index.astype('float32'))
neg_num = paddle.sum(neg_index.astype('float32'))
sum_num = pos_num + neg_num
weight_pos = 2 * neg_num / (sum_num + self.EPS)
weight_neg = 2 * pos_num / (sum_num + self.EPS)
weight = weight_pos * label + weight_neg * (1 - label)
else:
weight = self.weight
if isinstance(self.pos_weight, str):
pos_index = (label == 1)
neg_index = (label == 0)
pos_num = paddle.sum(pos_index.astype('float32'))
neg_num = paddle.sum(neg_index.astype('float32'))
sum_num = pos_num + neg_num
pos_weight = 2 * neg_num / (sum_num + self.EPS)
else:
pos_weight = self.pos_weight
label = label.astype('float32')
loss = paddle.nn.functional.binary_cross_entropy_with_logits(
logit,
label,
weight=weight,
reduction='none',
pos_weight=pos_weight)
loss = loss * mask
loss = paddle.mean(loss) / (paddle.mean(mask) + self.EPS)
label.stop_gradient = True
mask.stop_gradient = True
return loss