亚像素卷积网络(ESPCN)学习与Pytorch复现

2023-05-16

论文内容

论文地址：Real-Time Single Image and Video Super-Resolution Using an Efficient Sub-Pixel Convolutional Neural Network | IEEE Conference Publication | IEEE Xplore

或者：[1609.05158] Real-Time Single Image and Video Super-Resolution Using an Efficient Sub-Pixel Convolutional Neural Network (arxiv.org)

ESPCN是2016年提出的，是一篇经典的超分辨率重建算法文章，虽然它的效果和现在的文章相比不算好，但是它所提出的Efficient Sub-pixel Convolution，也叫亚像素卷积/子像素卷积为后面网络PSNR的提升做出了很大贡献，关键这个Sub-pixel Convolution比插值，反卷积，反池化这些上采样方法计算量要更少，因此网络的运行速度会有很大提升，如下图所示。

那么接下来看看这个Sub-pixel Convolution的结构，正常情况下，卷积操作会使feature map的高和宽变小，但当 $stride=\frac{1}{r}<1$ 时，可以让卷积后的feature map的高和宽变大，就实现了分辨率的提升也就是超分辨重建，这个操作叫做sub-pixel convolution。

对于sub-pixel convolution，作者将一个H × W的低分辨率输入图像（Low Resolution）作为输入，低分辨率图像特征提取完毕后，生成n1个特征图，然后经过中间一堆操作等，不管有多少，只要到该上采样的时候，在最后一个卷积调整成 $r^{2}C$ 就可以通过Sub-pixel操作将其变为rH x rW的高分辨率图像（High Resolution）。但是其实现过程不是直接通过插值等方式产生这个高分辨率图像，而是通过卷积先得到 $r^{2}C$ 个通道的特征图（特征图大小和输入低分辨率图像一致），然后通过周期筛选（periodic shuffing）的方法得到这个高分辨率的图像，其中r rr为上采样因子（upscaling factor），也就是图像的扩大倍率。

Pytorch复现

sub-pixel convolution这个操作在pytorch里面提供的有接口，只需要调用就可以了。

输入: $(n,channels\cdot upscalefactor^{2},height,width)$
输出: $(n,channels,upscalefactor\cdot height,upscalefactor\cdot width)$
比如:

sub= nn.PixelShuffle(4)
input = torch.tensor(1, 4**2, 4, 4)
output = sub(input)
torch.Size为[1, 1, 16, 16]

SRCNN使用sub-pixel convolution

主干网络：

import torch.nn as nn
import torch.nn.init as init


class Net(nn.Module):
    def __init__(self, upscale_factor):
        super(Net, self).__init__()

        self.relu = nn.ReLU()
        self.conv1 = nn.Conv2d(1, 64, kernel_size=5, stride=1, padding=2)
        self.conv2 = nn.Conv2d(64, 64, kernel_size=3, stride=1, padding=1)
        self.conv3 = nn.Conv2d(64, 32, kernel_size=3, stride=1, padding=1)
        self.conv4 = nn.Conv2d(32, upscale_factor ** 2, kernel_size=3, stride=1, padding=1)
        self.pixel_shuffle = nn.PixelShuffle(upscale_factor)

        self._initialize_weights()

    def _initialize_weights(self):
        init.orthogonal_(self.conv1.weight, init.calculate_gain('relu'))
        init.orthogonal_(self.conv2.weight, init.calculate_gain('relu'))
        init.orthogonal_(self.conv3.weight, init.calculate_gain('relu'))
        init.orthogonal_(self.conv4.weight)

    def forward(self, x):
        x = self.conv1(x)
        x = self.relu(x)
        x = self.conv2(x)
        x = self.relu(x)
        x = self.conv3(x)
        x = self.relu(x)
        x = self.conv4(x)
        x = self.pixel_shuffle(x)
        return x

训练：

from __future__ import print_function

from math import log10

import torch
import torch.backends.cudnn as cudnn

from SubPixelCNN.model import Net
from progress_bar import progress_bar


class SubPixelTrainer(object):
    def __init__(self, config, training_loader, testing_loader):
        super(SubPixelTrainer, self).__init__()
        self.CUDA = torch.cuda.is_available()
        self.device = torch.device('cuda' if self.CUDA else 'cpu')
        self.model = None
        self.lr = config.lr
        self.nEpochs = config.nEpochs
        self.criterion = None
        self.optimizer = None
        self.scheduler = None
        self.seed = config.seed
        self.upscale_factor = config.upscale_factor
        self.training_loader = training_loader
        self.testing_loader = testing_loader

    def build_model(self):
        self.model = Net(upscale_factor=self.upscale_factor).to(self.device)
        self.criterion = torch.nn.MSELoss()
        torch.manual_seed(self.seed)

        if self.CUDA:
            torch.cuda.manual_seed(self.seed)
            cudnn.benchmark = True
            self.criterion.cuda()

        self.optimizer = torch.optim.Adam(self.model.parameters(), lr=self.lr)
        self.scheduler = torch.optim.lr_scheduler.MultiStepLR(self.optimizer, milestones=[50, 75, 100], gamma=0.5)  # lr decay

    def save(self):
        model_out_path = "model_path.pth"
        torch.save(self.model, model_out_path)
        print("Checkpoint saved to {}".format(model_out_path))

    def train(self):
        self.model.train()
        train_loss = 0
        for batch_num, (data, target) in enumerate(self.training_loader):
            data, target = data.to(self.device), target.to(self.device)
            self.optimizer.zero_grad()
            loss = self.criterion(self.model(data), target)
            train_loss += loss.item()
            loss.backward()
            self.optimizer.step()
            progress_bar(batch_num, len(self.training_loader), 'Loss: %.4f' % (train_loss / (batch_num + 1)))

        print("    Average Loss: {:.4f}".format(train_loss / len(self.training_loader)))

    def test(self):
        self.model.eval()
        avg_psnr = 0

        with torch.no_grad():
            for batch_num, (data, target) in enumerate(self.testing_loader):
                data, target = data.to(self.device), target.to(self.device)
                prediction = self.model(data)
                mse = self.criterion(prediction, target)
                psnr = 10 * log10(1 / mse.item())
                avg_psnr += psnr
                progress_bar(batch_num, len(self.testing_loader), 'PSNR: %.4f' % (avg_psnr / (batch_num + 1)))

        print("    Average PSNR: {:.4f} dB".format(avg_psnr / len(self.testing_loader)))

    def run(self):
        self.build_model()
        for epoch in range(1, self.nEpochs + 1):
            print("\n===> Epoch {} starts:".format(epoch))
            self.train()
            self.test()
            self.scheduler.step(epoch)
            if epoch == self.nEpochs:
                self.save()

EDSR使用sub-pixel convolution

import math

import torch
import torch.nn as nn


class Net(nn.Module):
    def __init__(self, num_channels, base_channel, upscale_factor, num_residuals):
        super(Net, self).__init__()

        self.input_conv = nn.Conv2d(num_channels, base_channel, kernel_size=3, stride=1, padding=1)

        resnet_blocks = []
        for _ in range(num_residuals):
            resnet_blocks.append(ResnetBlock(base_channel, kernel=3, stride=1, padding=1))
        self.residual_layers = nn.Sequential(*resnet_blocks)

        self.mid_conv = nn.Conv2d(base_channel, base_channel, kernel_size=3, stride=1, padding=1)

        upscale = []
        for _ in range(int(math.log2(upscale_factor))):
            upscale.append(PixelShuffleBlock(base_channel, base_channel, upscale_factor=2))
        self.upscale_layers = nn.Sequential(*upscale)

        self.output_conv = nn.Conv2d(base_channel, num_channels, kernel_size=3, stride=1, padding=1)

    def weight_init(self, mean=0.0, std=0.02):
        for m in self._modules:
            normal_init(self._modules[m], mean, std)

    def forward(self, x):
        x = self.input_conv(x)
        residual = x
        x = self.residual_layers(x)
        x = self.mid_conv(x)
        x = torch.add(x, residual)
        x = self.upscale_layers(x)
        x = self.output_conv(x)
        return x


def normal_init(m, mean, std):
    if isinstance(m, nn.ConvTranspose2d) or isinstance(m, nn.Conv2d):
        m.weight.data.normal_(mean, std)
        if m.bias is not None:
            m.bias.data.zero_()


class ResnetBlock(nn.Module):
    def __init__(self, num_channel, kernel=3, stride=1, padding=1):
        super(ResnetBlock, self).__init__()
        self.conv1 = nn.Conv2d(num_channel, num_channel, kernel, stride, padding)
        self.conv2 = nn.Conv2d(num_channel, num_channel, kernel, stride, padding)
        self.bn = nn.BatchNorm2d(num_channel)
        self.activation = nn.ReLU(inplace=True)

    def forward(self, x):
        residual = x
        x = self.bn(self.conv1(x))
        x = self.activation(x)
        x = self.bn(self.conv2(x))
        x = torch.add(x, residual)
        return x


class PixelShuffleBlock(nn.Module):
    def __init__(self, in_channel, out_channel, upscale_factor, kernel=3, stride=1, padding=1):
        super(PixelShuffleBlock, self).__init__()
        self.conv = nn.Conv2d(in_channel, out_channel * upscale_factor ** 2, kernel, stride, padding)
        self.ps = nn.PixelShuffle(upscale_factor)

    def forward(self, x):
        x = self.ps(self.conv(x))
        return x

EDSR这个网络后面得单独写一篇。

实验结果

论文中给出的结果如下表，我实际跑出来比原文要略低，应该是因为训练不到位和训练数据集不一样的原因，论文的训练数据集是Image，我是用BSD300训练的。

从左到右分别是原图，Bicubic，ESPCN，效果还说的过去。

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