src/models/crnn.py

import torch
import torch.nn as nn

class CRNN(nn.Module):
    def __init__(self, img_channel, img_height, img_width, num_class, 
                 map_to_seq_hidden=64, rnn_hidden=256, leaky_relu=False):
        super(CRNN, self).__init__()
        
        self.cnn, (output_channel, output_height, output_width) = \
            self._cnn_backbone(img_channel, img_height, img_width, leaky_relu)
        
        self.map_to_seq = nn.Linear(output_channel * output_height, map_to_seq_hidden)
        
        self.rnn1 = nn.LSTM(map_to_seq_hidden, rnn_hidden, bidirectional=True, batch_first=True)
        self.rnn2 = nn.LSTM(rnn_hidden * 2, rnn_hidden, bidirectional=True, batch_first=True)
        
        self.dense = nn.Linear(rnn_hidden * 2, num_class)
        
    def _cnn_backbone(self, img_channel, img_height, img_width, leaky_relu):
        assert img_height % 16 == 0
        assert img_width % 4 == 0
        
        channels = [img_channel, 64, 128, 256, 256, 512, 512, 512]
        kernel_sizes = [3, 3, 3, 3, 3, 3, 2]
        strides = [1, 1, 1, 1, 1, 1, 1]
        paddings = [1, 1, 1, 1, 1, 1, 0]
        
        cnn = nn.Sequential()
        
        def conv_relu(i, batch_normalization=False):
            n_in = channels[i]
            n_out = channels[i+1]
            cnn.add_module(f'conv{i}', nn.Conv2d(n_in, n_out, kernel_sizes[i], strides[i], paddings[i]))
            if batch_normalization:
                cnn.add_module(f'batchnorm{i}', nn.BatchNorm2d(n_out))
            if leaky_relu:
                cnn.add_module(f'relu{i}', nn.LeakyReLU(0.2, inplace=True))
            else:
                cnn.add_module(f'relu{i}', nn.ReLU(inplace=True))
                
        conv_relu(0)
        cnn.add_module('pooling0', nn.MaxPool2d(kernel_size=2, stride=2)) # 64x16x64
        conv_relu(1)
        cnn.add_module('pooling1', nn.MaxPool2d(kernel_size=2, stride=2)) # 128x8x32
        conv_relu(2, True)
        conv_relu(3)
        cnn.add_module('pooling2', nn.MaxPool2d(kernel_size=(2, 2), stride=(2, 1), padding=(0, 1))) # 256x4x33
        conv_relu(4, True)
        conv_relu(5)
        cnn.add_module('pooling3', nn.MaxPool2d(kernel_size=(2, 2), stride=(2, 1), padding=(0, 1))) # 512x2x34
        conv_relu(6, True) # 512x1x33
        
        output_channel, output_height, output_width = channels[-1], img_height // 16 - 1, img_width // 4 + 1
        return cnn, (output_channel, output_height, output_width)
        
    def forward(self, images):
        # shape of images: (batch, channel, height, width)
        conv = self.cnn(images)
        batch, channel, height, width = conv.size()
        
        conv = conv.view(batch, channel * height, width)
        conv = conv.permute(0, 2, 1) # (batch, width, channel*height)
        
        seq = self.map_to_seq(conv)
        
        recurrent, _ = self.rnn1(seq)
        recurrent, _ = self.rnn2(recurrent)
        
        output = self.dense(recurrent)
        
        # Log softmax for CTC loss
        # Note: PyTorch's CTCLoss expects inputs of shape (input_length, batch_size, num_classes)
        # So we permute it if we are returning it for CTC loss calculation directly
        return output.log_softmax(2)

if __name__ == '__main__':
    # Test model
    dummy_input = torch.randn(1, 1, 32, 1024)
    model = CRNN(img_channel=1, img_height=32, img_width=1024, num_class=80)
    output = model(dummy_input)
    print(f"Output shape: {output.shape}") # Expected: (1, 33, 80)