model.py

import torch
import torch.nn as nn
import torch.nn.functional as F
from math import log2

"""
Factors is used in Discrmininator and Generator for how much
the channels should be multiplied and expanded for each layer,
so specifically the first 5 layers the channels stay the same,
whereas when we increase the img_size (towards the later layers)
we decrease the number of chanels by 1/2, 1/4, etc.
"""
factors = [1, 1, 1, 1, 1 / 2, 1 / 4, 1 / 8, 1 / 16, 1 / 32]


class WSConv2d(nn.Module):
    """
    Weight scaled Conv2d (Equalized Learning Rate)
    Note that input is multiplied rather than changing weights
    this will have the same result.


    """

    def __init__(
        self, in_channels, out_channels, kernel_size=3, stride=1, padding=1, gain=2
    ):
        super(WSConv2d, self).__init__()
        self.conv = nn.Conv2d(in_channels, out_channels, kernel_size, stride, padding)
        self.scale = (gain / (in_channels * (kernel_size ** 2))) ** 0.5
        self.bias = self.conv.bias
        self.conv.bias = None

        # initialize conv layer
        nn.init.normal_(self.conv.weight)
        nn.init.zeros_(self.bias)

    def forward(self, x):
        return self.conv(x * self.scale) + self.bias.view(1, self.bias.shape[0], 1, 1)


class PixelNorm(nn.Module):
    def __init__(self):
        super(PixelNorm, self).__init__()
        self.epsilon = 1e-8

    def forward(self, x):
        return x / torch.sqrt(torch.mean(x ** 2, dim=1, keepdim=True) + self.epsilon)


class ConvBlock(nn.Module):
    def __init__(self, in_channels, out_channels, use_pixelnorm=True):
        super(ConvBlock, self).__init__()
        self.use_pn = use_pixelnorm
        self.conv1 = WSConv2d(in_channels, out_channels)
        self.conv2 = WSConv2d(out_channels, out_channels)
        self.leaky = nn.LeakyReLU(0.2)
        self.pn = PixelNorm()

    def forward(self, x):
        x = self.leaky(self.conv1(x))
        x = self.pn(x) if self.use_pn else x
        x = self.leaky(self.conv2(x))
        x = self.pn(x) if self.use_pn else x
        return x


class Generator(nn.Module):
    def __init__(self, z_dim, in_channels, img_channels=3):
        super(Generator, self).__init__()

        # initial takes 1x1 -> 4x4
        self.initial = nn.Sequential(
            PixelNorm(),
            nn.ConvTranspose2d(z_dim, in_channels, 4, 1, 0),
            nn.LeakyReLU(0.2),
            WSConv2d(in_channels, in_channels, kernel_size=3, stride=1, padding=1),
            nn.LeakyReLU(0.2),
            PixelNorm(),
        )

        self.initial_rgb = WSConv2d(
            in_channels, img_channels, kernel_size=1, stride=1, padding=0
        )
        self.prog_blocks, self.rgb_layers = (
            nn.ModuleList([]),
            nn.ModuleList([self.initial_rgb]),
        )

        for i in range(
            len(factors) - 1
        ):  # -1 to prevent index error because of factors[i+1]
            conv_in_c = int(in_channels * factors[i])
            conv_out_c = int(in_channels * factors[i + 1])
            self.prog_blocks.append(ConvBlock(conv_in_c, conv_out_c))
            self.rgb_layers.append(
                WSConv2d(conv_out_c, img_channels, kernel_size=1, stride=1, padding=0)
            )

    def fade_in(self, alpha, upscaled, generated):
        # alpha should be scalar within [0, 1], and upscale.shape == generated.shape
        return torch.tanh(alpha * generated + (1 - alpha) * upscaled)

    def forward(self, x, alpha, steps):
        out = self.initial(x)

        if steps == 0:
            return self.initial_rgb(out)

        for step in range(steps):
            upscaled = F.interpolate(out, scale_factor=2, mode="nearest")
            out = self.prog_blocks[step](upscaled)

        # The number of channels in upscale will stay the same, while
        # out which has moved through prog_blocks might change. To ensure
        # we can convert both to rgb we use different rgb_layers
        # (steps-1) and steps for upscaled, out respectively
        final_upscaled = self.rgb_layers[steps - 1](upscaled)
        final_out = self.rgb_layers[steps](out)
        return self.fade_in(alpha, final_upscaled, final_out)


class Discriminator(nn.Module):
    def __init__(self, z_dim, in_channels, img_channels=3):
        super(Discriminator, self).__init__()
        self.prog_blocks, self.rgb_layers = nn.ModuleList([]), nn.ModuleList([])
        self.leaky = nn.LeakyReLU(0.2)

        # here we work back ways from factors because the discriminator
        # should be mirrored from the generator. So the first prog_block and
        # rgb layer we append will work for input size 1024x1024, then 512->256-> etc
        for i in range(len(factors) - 1, 0, -1):
            conv_in = int(in_channels * factors[i])
            conv_out = int(in_channels * factors[i - 1])
            self.prog_blocks.append(ConvBlock(conv_in, conv_out, use_pixelnorm=False))
            self.rgb_layers.append(
                WSConv2d(img_channels, conv_in, kernel_size=1, stride=1, padding=0)
            )

        # perhaps confusing name "initial_rgb" this is just the RGB layer for 4x4 input size
        # did this to "mirror" the generator initial_rgb
        self.initial_rgb = WSConv2d(
            img_channels, in_channels, kernel_size=1, stride=1, padding=0
        )
        self.rgb_layers.append(self.initial_rgb)
        self.avg_pool = nn.AvgPool2d(
            kernel_size=2, stride=2
        )  # down sampling using avg pool

        # this is the block for 4x4 input size
        self.final_block = nn.Sequential(
            # +1 to in_channels because we concatenate from MiniBatch std
            WSConv2d(in_channels + 1, in_channels, kernel_size=3, padding=1),
            nn.LeakyReLU(0.2),
            WSConv2d(in_channels, in_channels, kernel_size=4, padding=0, stride=1),
            nn.LeakyReLU(0.2),
            WSConv2d(
                in_channels, 1, kernel_size=1, padding=0, stride=1
            ),  # we use this instead of linear layer
        )

    def fade_in(self, alpha, downscaled, out):
        """Used to fade in downscaled using avg pooling and output from CNN"""
        # alpha should be scalar within [0, 1], and upscale.shape == generated.shape
        return alpha * out + (1 - alpha) * downscaled

    def minibatch_std(self, x):
        batch_statistics = (
            torch.std(x, dim=0).mean().repeat(x.shape[0], 1, x.shape[2], x.shape[3])
        )
        # we take the std for each example (across all channels, and pixels) then we repeat it
        # for a single channel and concatenate it with the image. In this way the discriminator
        # will get information about the variation in the batch/image
        return torch.cat([x, batch_statistics], dim=1)

    def forward(self, x, alpha, steps):
        # where we should start in the list of prog_blocks, maybe a bit confusing but
        # the last is for the 4x4. So example let's say steps=1, then we should start
        # at the second to last because input_size will be 8x8. If steps==0 we just
        # use the final block
        cur_step = len(self.prog_blocks) - steps

        # convert from rgb as initial step, this will depend on
        # the image size (each will have it's on rgb layer)
        out = self.leaky(self.rgb_layers[cur_step](x))

        if steps == 0:  # i.e, image is 4x4
            out = self.minibatch_std(out)
            return self.final_block(out).view(out.shape[0], -1)

        # because prog_blocks might change the channels, for down scale we use rgb_layer
        # from previous/smaller size which in our case correlates to +1 in the indexing
        downscaled = self.leaky(self.rgb_layers[cur_step + 1](self.avg_pool(x)))
        out = self.avg_pool(self.prog_blocks[cur_step](out))

        # the fade_in is done first between the downscaled and the input
        # this is opposite from the generator
        out = self.fade_in(alpha, downscaled, out)

        for step in range(cur_step + 1, len(self.prog_blocks)):
            out = self.prog_blocks[step](out)
            out = self.avg_pool(out)

        out = self.minibatch_std(out)
        return self.final_block(out).view(out.shape[0], -1)


if __name__ == "__main__":
    Z_DIM = 100
    IN_CHANNELS = 256
    gen = Generator(Z_DIM, IN_CHANNELS, img_channels=3)
    critic = Discriminator(Z_DIM, IN_CHANNELS, img_channels=3)

    for img_size in [4, 8, 16, 32, 64, 128, 256, 512, 1024]:
        num_steps = int(log2(img_size / 4))
        x = torch.randn((1, Z_DIM, 1, 1))
        z = gen(x, 0.5, steps=num_steps)
        assert z.shape == (1, 3, img_size, img_size)
        out = critic(z, alpha=0.5, steps=num_steps)
        assert out.shape == (1, 1)
        print(f"Success! At img size: {img_size}")