# Source code for monai.losses.deform

```
# Copyright 2020 - 2021 MONAI Consortium
# 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.
from typing import Union
import torch
from torch.nn.modules.loss import _Loss
from monai.utils import LossReduction
def spatial_gradient(x: torch.Tensor, dim: int) -> torch.Tensor:
"""
Calculate gradients on single dimension of a tensor using central finite difference.
It moves the tensor along the dimension to calculate the approximate gradient
dx[i] = (x[i+1] - x[i-1]) / 2.
Adapted from:
DeepReg (https://github.com/DeepRegNet/DeepReg)
Args:
x: the shape should be BCH(WD).
dim: dimension to calculate gradient along.
Returns:
gradient_dx: the shape should be BCH(WD)
"""
slice_1 = slice(1, -1)
slice_2_s = slice(2, None)
slice_2_e = slice(None, -2)
slice_all = slice(None)
slicing_s, slicing_e = [slice_all, slice_all], [slice_all, slice_all]
while len(slicing_s) < x.ndim:
slicing_s = slicing_s + [slice_1]
slicing_e = slicing_e + [slice_1]
slicing_s[dim] = slice_2_s
slicing_e[dim] = slice_2_e
return (x[slicing_s] - x[slicing_e]) / 2.0
[docs]class BendingEnergyLoss(_Loss):
"""
Calculate the bending energy based on second-order differentiation of pred using central finite difference.
Adapted from:
DeepReg (https://github.com/DeepRegNet/DeepReg)
"""
def __init__(
self,
reduction: Union[LossReduction, str] = LossReduction.MEAN,
) -> None:
"""
Args:
reduction: {``"none"``, ``"mean"``, ``"sum"``}
Specifies the reduction to apply to the output. Defaults to ``"mean"``.
- ``"none"``: no reduction will be applied.
- ``"mean"``: the sum of the output will be divided by the number of elements in the output.
- ``"sum"``: the output will be summed.
"""
super(BendingEnergyLoss, self).__init__(reduction=LossReduction(reduction).value)
[docs] def forward(self, pred: torch.Tensor) -> torch.Tensor:
"""
Args:
pred: the shape should be BCH(WD)
Raises:
ValueError: When ``self.reduction`` is not one of ["mean", "sum", "none"].
"""
if pred.ndim not in [3, 4, 5]:
raise ValueError(f"expecting 3-d, 4-d or 5-d pred, instead got pred of shape {pred.shape}")
for i in range(pred.ndim - 2):
if pred.shape[-i - 1] <= 4:
raise ValueError("all spatial dimensions must > 4, got pred of shape {pred.shape}")
# first order gradient
first_order_gradient = [spatial_gradient(pred, dim) for dim in range(2, pred.ndim)]
energy = torch.tensor(0)
for dim_1, g in enumerate(first_order_gradient):
dim_1 += 2
energy = spatial_gradient(g, dim_1) ** 2 + energy
for dim_2 in range(dim_1 + 1, pred.ndim):
energy = 2 * spatial_gradient(g, dim_2) ** 2 + energy
if self.reduction == LossReduction.MEAN.value:
energy = torch.mean(energy) # the batch and channel average
elif self.reduction == LossReduction.SUM.value:
energy = torch.sum(energy) # sum over the batch and channel dims
elif self.reduction == LossReduction.NONE.value:
pass # returns [N, n_classes] losses
else:
raise ValueError(f'Unsupported reduction: {self.reduction}, available options are ["mean", "sum", "none"].')
return energy
```