lightning/pl_examples/domain_templates/unet.py

# Copyright The PyTorch Lightning team.
#
# 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 torch
import torch.nn as nn
import torch.nn.functional as F


class UNet(nn.Module):
    """Architecture based on U-Net: Convolutional Networks for Biomedical Image Segmentation.

    Link - https://arxiv.org/abs/1505.04597

    >>> UNet(num_classes=2, num_layers=3)  # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
    UNet(
      (layers): ModuleList(
        (0): DoubleConv(...)
        (1): Down(...)
        (2): Down(...)
        (3): Up(...)
        (4): Up(...)
        (5): Conv2d(64, 2, kernel_size=(1, 1), stride=(1, 1))
      )
    )
    """

    def __init__(self, num_classes: int = 19, num_layers: int = 5, features_start: int = 64, bilinear: bool = False):
        """
        Args:
            num_classes: Number of output classes required (default 19 for KITTI dataset)
            num_layers: Number of layers in each side of U-net
            features_start: Number of features in first layer
            bilinear: Whether to use bilinear interpolation or transposed convolutions for upsampling.
        """
        super().__init__()
        self.num_layers = num_layers

        layers = [DoubleConv(3, features_start)]

        feats = features_start
        for _ in range(num_layers - 1):
            layers.append(Down(feats, feats * 2))
            feats *= 2

        for _ in range(num_layers - 1):
            layers.append(Up(feats, feats // 2, bilinear))
            feats //= 2

        layers.append(nn.Conv2d(feats, num_classes, kernel_size=1))

        self.layers = nn.ModuleList(layers)

    def forward(self, x):
        xi = [self.layers[0](x)]
        # Down path
        for layer in self.layers[1 : self.num_layers]:
            xi.append(layer(xi[-1]))
        # Up path
        for i, layer in enumerate(self.layers[self.num_layers : -1]):
            xi[-1] = layer(xi[-1], xi[-2 - i])
        return self.layers[-1](xi[-1])


class DoubleConv(nn.Module):
    """Double Convolution and BN and ReLU (3x3 conv -> BN -> ReLU) ** 2.

    >>> DoubleConv(4, 4)  # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
    DoubleConv(
      (net): Sequential(...)
    )
    """

    def __init__(self, in_ch: int, out_ch: int):
        super().__init__()
        self.net = nn.Sequential(
            nn.Conv2d(in_ch, out_ch, kernel_size=3, padding=1),
            nn.BatchNorm2d(out_ch),
            nn.ReLU(inplace=True),
            nn.Conv2d(out_ch, out_ch, kernel_size=3, padding=1),
            nn.BatchNorm2d(out_ch),
            nn.ReLU(inplace=True),
        )

    def forward(self, x):
        return self.net(x)


class Down(nn.Module):
    """Combination of MaxPool2d and DoubleConv in series.

    >>> Down(4, 8)  # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
    Down(
      (net): Sequential(
        (0): MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False)
        (1): DoubleConv(
          (net): Sequential(...)
        )
      )
    )
    """

    def __init__(self, in_ch: int, out_ch: int):
        super().__init__()
        self.net = nn.Sequential(nn.MaxPool2d(kernel_size=2, stride=2), DoubleConv(in_ch, out_ch))

    def forward(self, x):
        return self.net(x)


class Up(nn.Module):
    """Upsampling (by either bilinear interpolation or transpose convolutions) followed by concatenation of feature
    map from contracting path, followed by double 3x3 convolution.

    >>> Up(8, 4)  # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
    Up(
      (upsample): ConvTranspose2d(8, 4, kernel_size=(2, 2), stride=(2, 2))
      (conv): DoubleConv(
        (net): Sequential(...)
      )
    )
    """

    def __init__(self, in_ch: int, out_ch: int, bilinear: bool = False):
        super().__init__()
        self.upsample = None
        if bilinear:
            self.upsample = nn.Sequential(
                nn.Upsample(scale_factor=2, mode="bilinear", align_corners=True),
                nn.Conv2d(in_ch, in_ch // 2, kernel_size=1),
            )
        else:
            self.upsample = nn.ConvTranspose2d(in_ch, in_ch // 2, kernel_size=2, stride=2)

        self.conv = DoubleConv(in_ch, out_ch)

    def forward(self, x1, x2):
        x1 = self.upsample(x1)

        # Pad x1 to the size of x2
        diff_h = x2.shape[2] - x1.shape[2]
        diff_w = x2.shape[3] - x1.shape[3]

        x1 = F.pad(x1, [diff_w // 2, diff_w - diff_w // 2, diff_h // 2, diff_h - diff_h // 2])

        # Concatenate along the channels axis
        x = torch.cat([x2, x1], dim=1)
        return self.conv(x)