networks/blocks/ifrnet.py

'''
    This code are partially borrowed from IFRNet (https://github.com/ltkong218/IFRNet). 
'''
import torch
import torch.nn as nn
import torch.nn.functional as F
from utils import warp

def resize(x, scale_factor):
    return F.interpolate(x, scale_factor=scale_factor, mode="bilinear", align_corners=False)

def convrelu(in_channels, out_channels, kernel_size=3, stride=1, padding=1, dilation=1, groups=1, bias=True):
    return nn.Sequential(
        nn.Conv2d(in_channels, out_channels, kernel_size, stride, padding, dilation, groups, bias=bias), 
        nn.PReLU(out_channels)
    )

class ResBlock(nn.Module):
    def __init__(self, in_channels, side_channels, bias=True):
        super(ResBlock, self).__init__()
        self.side_channels = side_channels
        self.conv1 = nn.Sequential(
            nn.Conv2d(in_channels, in_channels, kernel_size=3, stride=1, padding=1, bias=bias), 
            nn.PReLU(in_channels)
        )
        self.conv2 = nn.Sequential(
            nn.Conv2d(side_channels, side_channels, kernel_size=3, stride=1, padding=1, bias=bias), 
            nn.PReLU(side_channels)
        )
        self.conv3 = nn.Sequential(
            nn.Conv2d(in_channels, in_channels, kernel_size=3, stride=1, padding=1, bias=bias), 
            nn.PReLU(in_channels)
        )
        self.conv4 = nn.Sequential(
            nn.Conv2d(side_channels, side_channels, kernel_size=3, stride=1, padding=1, bias=bias), 
            nn.PReLU(side_channels)
        )
        self.conv5 = nn.Conv2d(in_channels, in_channels, kernel_size=3, stride=1, padding=1, bias=bias)
        self.prelu = nn.PReLU(in_channels)

    def forward(self, x):
        out = self.conv1(x)

        res_feat = out[:, :-self.side_channels, ...]
        side_feat = out[:, -self.side_channels:, :, :]
        side_feat = self.conv2(side_feat)
        out = self.conv3(torch.cat([res_feat, side_feat], 1))

        res_feat = out[:, :-self.side_channels, ...]
        side_feat = out[:, -self.side_channels:, :, :]
        side_feat = self.conv4(side_feat)
        out = self.conv5(torch.cat([res_feat, side_feat], 1))

        out = self.prelu(x + out)
        return out
    
class Encoder(nn.Module):
    def __init__(self, channels, large=False):
        super(Encoder, self).__init__()
        self.channels = channels        
        prev_ch = 3
        for idx, ch in enumerate(channels, 1):
            k = 7 if large and idx == 1 else 3
            p = 3 if k ==7 else 1
            self.register_module(f'pyramid{idx}', 
            nn.Sequential(
                convrelu(prev_ch, ch, k, 2, p),
                convrelu(ch, ch, 3, 1, 1)
            ))
            prev_ch = ch
                
    def forward(self, in_x):
        fs = []
        for idx in range(len(self.channels)):
            out_x = getattr(self, f'pyramid{idx+1}')(in_x)
            fs.append(out_x)
            in_x = out_x
        return fs
    
class InitDecoder(nn.Module):
    def __init__(self, in_ch, out_ch, skip_ch) -> None:
        super().__init__()
        self.convblock = nn.Sequential(
            convrelu(in_ch*2+1, in_ch*2), 
            ResBlock(in_ch*2, skip_ch), 
            nn.ConvTranspose2d(in_ch*2, out_ch+4, 4, 2, 1, bias=True)
        )
    def forward(self, f0, f1, embt):
        h, w = f0.shape[2:]
        embt = embt.repeat(1, 1, h, w)
        out = self.convblock(torch.cat([f0, f1, embt], 1))
        flow0, flow1 = torch.chunk(out[:, :4, ...], 2, 1)
        ft_ = out[:, 4:, ...]
        return flow0, flow1, ft_
    
class IntermediateDecoder(nn.Module):
    def __init__(self, in_ch, out_ch, skip_ch) -> None:
        super().__init__()
        self.convblock = nn.Sequential(
            convrelu(in_ch*3+4, in_ch*3), 
            ResBlock(in_ch*3, skip_ch), 
            nn.ConvTranspose2d(in_ch*3, out_ch+4, 4, 2, 1, bias=True)
        )
    def forward(self, ft_, f0, f1, flow0_in, flow1_in):
        f0_warp = warp(f0, flow0_in)
        f1_warp = warp(f1, flow1_in)
        f_in = torch.cat([ft_, f0_warp, f1_warp, flow0_in, flow1_in], 1)
        out = self.convblock(f_in)
        flow0, flow1 = torch.chunk(out[:, :4, ...], 2, 1)
        ft_ = out[:, 4:, ...]
        flow0 = flow0 + 2.0 * resize(flow0_in, scale_factor=2.0)
        flow1 = flow1 + 2.0 * resize(flow1_in, scale_factor=2.0)
        return flow0, flow1, ft_