StarGAN v2 架构

    
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            代码实现解读
        
            GAN
        
    发布于：2021-03-29 21:31:35
    编辑于：2022-11-15 12:54:26
    浏览量：2030

概述

用于图片风格转换。

官方仓库：https://github.com/clovaai/stargan-v2

训练流程

首先取：
- 一张要转换的图片及其类别，
- 以及同类别的两张参考图片及其类别，
- 两个随机隐码。
如果 w_hpf 参数大于 0（高通滤波的参数，默认为 1），则调用 FAN 网络的 get_heatmap(x_real) 函数获取要转换的图片的 mask，供生成器使用。

The purpose of introducing masks is to accurately preserve face landmarks (i.e., eyes, nose) of a source image during translation. The hyperparameter w_hpf controls the strength of the preservation. If you increase the value of w_hpf, then the generator preserve the source landmarks more precisely, but it would decrease the reflection of reference styles.

From: https://github.com/clovaai/stargan-v2/issues/70

训练辨别器：
- 第一次使用单个隐码，第二次使用参考图片，之后都是利用相应的网络将其转换为样式码。
- 损失函数：loss = loss_real + loss_fake + args.lambda_reg * loss_reg
  - 对抗损失：将真实图片输入辨别器，得到辨别结果，据此构造对抗损失。
  - R1 正则化：基于真实图片和辨别器的输出构造 R1 正则化项。
训练生成器，映射网络以及样式编码器：
- 第一次使用单个隐码，第二次使用参考图片，之后都是利用相应的网络将其转换为样式码。
- 损失函数：loss = loss_adv + args.lambda_sty * loss_sty - args.lambda_ds * loss_ds + args.lambda_cyc * loss_cyc
  - 对抗损失：将生成图片输入辨别器，得到辨别结果，据此构造对抗损失。
  - 样式重建损失：将生成图片输入样式编码器，得到的样式码与原输入样式码之间的距离即样式重建损失。
  - 多样性敏感损失：使用第二个隐码或者第二个参考图片构造第二个样式码，基于此生成第二个生成图片，其与第一个生成图片之间的距离越大则多样性敏感损失越小。
  - 循环一致性损失：将生成图片以及原样式码输入生成器，得到原输入图片的重建图片，重建图片和原输入图片之间的距离即循环一致性损失。
更新 EMA 版本的网络参数：

moving_average(nets.generator, nets_ema.generator, beta=0.999)
moving_average(nets.mapping_network, nets_ema.mapping_network, beta=0.999)
moving_average(nets.style_encoder, nets_ema.style_encoder, beta=0.999)
            
def moving_average(model, model_test, beta=0.999):
    for param, param_test in zip(model.parameters(), model_test.parameters()):
        param_test.data = torch.lerp(param.data, param_test.data, beta)

面部对齐网络

初始化：

if args.w_hpf > 0:
    fan = FAN(fname_pretrained=args.wing_path).eval()
    nets.fan = fan
    nets_ema.fan = fan

虽然 FAN 是预训练好的模型，但是这里依然在 ema 版本的网络中加入了 FAN，原因是这样的话我们在加载模型的时候只需要加载一个模型文件就好。

注意这里的获取 heatmap 的操作：masks = nets.fan.get_heatmap(x_real) if args.w_hpf > 0 else None。

返回的是一个元组，其内有两个 mask，第一个 mask 是正常的 mask，第二个在第一个的基础之上移除了嘴巴和 faceline 部位的 mask。

这里有图和解释，https://github.com/clovaai/stargan-v2/issues/5

根据解释，

heatmap 部分是可选的，有的话源图片的眼睛，鼻子和嘴巴保留地更好。
x is for a coarse layer and x2 is for finer layer

生成器

# 高通滤波器，保留高频信息，主要为边缘信息
class HighPass(nn.Module):
    def __init__(self, w_hpf, device):
        super(HighPass, self).__init__()
        self.filter = torch.tensor([[-1, -1, -1],
                                    [-1, 8., -1],
                                    [-1, -1, -1]]).to(device) / w_hpf

    def forward(self, x):
        filter = self.filter.unsqueeze(0).unsqueeze(1).repeat(x.size(1), 1, 1, 1)
        return F.conv2d(x, filter, padding=1, groups=x.size(1))


class Generator(nn.Module):
    def __init__(self, img_size=256, style_dim=64, max_conv_dim=512, w_hpf=1):
        super().__init__()
        dim_in = 2**14 // img_size
        self.img_size = img_size
        self.from_rgb = nn.Conv2d(3, dim_in, 3, 1, 1)
        self.encode = nn.ModuleList()
        self.decode = nn.ModuleList()
        self.to_rgb = nn.Sequential(
            nn.InstanceNorm2d(dim_in, affine=True),
            nn.LeakyReLU(0.2),
            nn.Conv2d(dim_in, 3, 1, 1, 0))

        # down/up-sampling blocks
        repeat_num = int(np.log2(img_size)) - 4
        if w_hpf > 0:
            repeat_num += 1
        for _ in range(repeat_num):
            dim_out = min(dim_in*2, max_conv_dim)
            self.encode.append(
                ResBlk(dim_in, dim_out, normalize=True, downsample=True))
            self.decode.insert(
                0, AdainResBlk(dim_out, dim_in, style_dim,
                               w_hpf=w_hpf, upsample=True))  # stack-like
            dim_in = dim_out

        # bottleneck blocks
        for _ in range(2):
            self.encode.append(
                ResBlk(dim_out, dim_out, normalize=True))
            self.decode.insert(
                0, AdainResBlk(dim_out, dim_out, style_dim, w_hpf=w_hpf))

        if w_hpf > 0:
            device = torch.device(
                'cuda' if torch.cuda.is_available() else 'cpu')
            self.hpf = HighPass(w_hpf, device)

    def forward(self, x, s, masks=None):
        # 卷积层处理
        x = self.from_rgb(x)
        cache = {}
        for block in self.encode:
            if (masks is not None) and (x.size(2) in [32, 64, 128]):
                cache[x.size(2)] = x
            # 残差块
            x = block(x)
        for block in self.decode:
            # 带有自适应实例归一化的残差块
            x = block(x, s)
            if (masks is not None) and (x.size(2) in [32, 64, 128]):
                # 对于大点的 feature map 使用第二个 mask
                mask = masks[0] if x.size(2) in [32] else masks[1]
                mask = F.interpolate(mask, size=x.size(2), mode='bilinear')
                # 注意这里的处理，首先将 encode 中对应层的 feature map 和 mask 相乘，之后对结果进行高通滤波
                x = x + self.hpf(mask * cache[x.size(2)])
        return self.to_rgb(x)

映射网络

class MappingNetwork(nn.Module):
    def __init__(self, latent_dim=16, style_dim=64, num_domains=2):
        super().__init__()
        layers = []
        layers += [nn.Linear(latent_dim, 512)]
        layers += [nn.ReLU()]
        for _ in range(3):
            layers += [nn.Linear(512, 512)]
            layers += [nn.ReLU()]
        # 共享层 4 层 MLP
        self.shared = nn.Sequential(*layers)

        self.unshared = nn.ModuleList()
        for _ in range(num_domains):
            # 每个 domain 有一个专门的头部，4 层 MLP，最后没有激活函数
            self.unshared += [nn.Sequential(nn.Linear(512, 512),
                                            nn.ReLU(),
                                            nn.Linear(512, 512),
                                            nn.ReLU(),
                                            nn.Linear(512, 512),
                                            nn.ReLU(),
                                            nn.Linear(512, style_dim))]

    def forward(self, z, y):
        h = self.shared(z)
        out = []
        for layer in self.unshared:
            out += [layer(h)]
        out = torch.stack(out, dim=1)  # (batch, num_domains, style_dim)
        idx = torch.LongTensor(range(y.size(0))).to(y.device)
        s = out[idx, y]  # (batch, style_dim)
        return s

样式编码器

class StyleEncoder(nn.Module):
    def __init__(self, img_size=256, style_dim=64, num_domains=2, max_conv_dim=512):
        super().__init__()
        dim_in = 2**14 // img_size
        blocks = []
        blocks += [nn.Conv2d(3, dim_in, 3, 1, 1)]

        repeat_num = int(np.log2(img_size)) - 2
        for _ in range(repeat_num):
            dim_out = min(dim_in*2, max_conv_dim)
            blocks += [ResBlk(dim_in, dim_out, downsample=True)]
            dim_in = dim_out

        blocks += [nn.LeakyReLU(0.2)]
        blocks += [nn.Conv2d(dim_out, dim_out, 4, 1, 0)]
        blocks += [nn.LeakyReLU(0.2)]
        self.shared = nn.Sequential(*blocks)

        self.unshared = nn.ModuleList()
        for _ in range(num_domains):
            # 这里的头部只有一层线性层，也没有激活层
            self.unshared += [nn.Linear(dim_out, style_dim)]

    def forward(self, x, y):
        h = self.shared(x)
        h = h.view(h.size(0), -1)
        out = []
        for layer in self.unshared:
            out += [layer(h)]
        out = torch.stack(out, dim=1)  # (batch, num_domains, style_dim)
        idx = torch.LongTensor(range(y.size(0))).to(y.device)
        s = out[idx, y]  # (batch, style_dim)
        return s

辨别器

一层卷积层，几个残差块，再接两个卷积层。

class Discriminator(nn.Module):
    def __init__(self, img_size=256, num_domains=2, max_conv_dim=512):
        super().__init__()
        dim_in = 2**14 // img_size
        blocks = []
        blocks += [nn.Conv2d(3, dim_in, 3, 1, 1)]

        repeat_num = int(np.log2(img_size)) - 2
        for _ in range(repeat_num):
            dim_out = min(dim_in*2, max_conv_dim)
            blocks += [ResBlk(dim_in, dim_out, downsample=True)]
            dim_in = dim_out

        blocks += [nn.LeakyReLU(0.2)]
        blocks += [nn.Conv2d(dim_out, dim_out, 4, 1, 0)]
        blocks += [nn.LeakyReLU(0.2)]
        blocks += [nn.Conv2d(dim_out, num_domains, 1, 1, 0)]
        self.main = nn.Sequential(*blocks)

    def forward(self, x, y):
        out = self.main(x)
        out = out.view(out.size(0), -1)  # (batch, num_domains)
        idx = torch.LongTensor(range(y.size(0))).to(y.device)
        out = out[idx, y]  # (batch)
        return out

残差块

class ResBlk(nn.Module):
    def __init__(self, dim_in, dim_out, actv=nn.LeakyReLU(0.2),
                 normalize=False, downsample=False):
        super().__init__()
        self.actv = actv
        self.normalize = normalize
        self.downsample = downsample
        self.learned_sc = dim_in != dim_out
        self._build_weights(dim_in, dim_out)

    def _build_weights(self, dim_in, dim_out):
        self.conv1 = nn.Conv2d(dim_in, dim_in, 3, 1, 1)
        self.conv2 = nn.Conv2d(dim_in, dim_out, 3, 1, 1)
        if self.normalize:
            self.norm1 = nn.InstanceNorm2d(dim_in, affine=True)
            self.norm2 = nn.InstanceNorm2d(dim_in, affine=True)
        if self.learned_sc:
            # 如果输入维度和输出维度不同，则需要一个 1x1 卷积层来改变维度。
            self.conv1x1 = nn.Conv2d(dim_in, dim_out, 1, 1, 0, bias=False)

    def _shortcut(self, x):
        if self.learned_sc:
            x = self.conv1x1(x)
        if self.downsample:
            x = F.avg_pool2d(x, 2)
        return x
        
    def _residual(self, x):
        if self.normalize:
            x = self.norm1(x)
        x = self.actv(x)
        x = self.conv1(x)
        if self.downsample:
            x = F.avg_pool2d(x, 2)
        if self.normalize:
            x = self.norm2(x)
        x = self.actv(x)
        x = self.conv2(x)
        return x

    def forward(self, x):
        x = self._shortcut(x) + self._residual(x)
        return x / math.sqrt(2)  # unit variance

带有自适应样式归一化的残差块

class AdainResBlk(nn.Module):
    def __init__(self, dim_in, dim_out, style_dim=64, w_hpf=0,
                 actv=nn.LeakyReLU(0.2), upsample=False):
        super().__init__()
        self.w_hpf = w_hpf
        self.actv = actv
        self.upsample = upsample
        self.learned_sc = dim_in != dim_out
        self._build_weights(dim_in, dim_out, style_dim)

    def _build_weights(self, dim_in, dim_out, style_dim=64):
        self.conv1 = nn.Conv2d(dim_in, dim_out, 3, 1, 1)
        self.conv2 = nn.Conv2d(dim_out, dim_out, 3, 1, 1)
        self.norm1 = AdaIN(style_dim, dim_in)
        self.norm2 = AdaIN(style_dim, dim_out)
        if self.learned_sc:
            self.conv1x1 = nn.Conv2d(dim_in, dim_out, 1, 1, 0, bias=False)

    def _shortcut(self, x):
        if self.upsample:
            x = F.interpolate(x, scale_factor=2, mode='nearest')
        if self.learned_sc:
            x = self.conv1x1(x)
        return x

    def _residual(self, x, s):
        x = self.norm1(x, s)
        x = self.actv(x)
        if self.upsample:
            x = F.interpolate(x, scale_factor=2, mode='nearest')
        x = self.conv1(x)
        x = self.norm2(x, s)
        x = self.actv(x)
        x = self.conv2(x)
        return x

    def forward(self, x, s):
        out = self._residual(x, s)
        if self.w_hpf == 0:
            out = (out + self._shortcut(x)) / math.sqrt(2)
        return out

class AdaIN(nn.Module):
    def __init__(self, style_dim, num_features):
        super().__init__()
        self.norm = nn.InstanceNorm2d(num_features, affine=False)
        self.fc = nn.Linear(style_dim, num_features*2)

    def forward(self, x, s):
        # 拿一层线性层先对均值和方差做一下仿射变换
        h = self.fc(s)
        h = h.view(h.size(0), h.size(1), 1, 1)
        gamma, beta = torch.chunk(h, chunks=2, dim=1)
        return (1 + gamma) * self.norm(x) + beta

Links: stargan-v2-arch