maevit.py

from torch.utils.data import Dataset
import torch.nn as nn
from PIL import Image
import json
import os
import random
import torch
import numpy as np
from transformer import TransformerEncoder, TransformerEncoderLayer, TransformerDecoder, TransformerDecoderLayer
    
#includes both MAE and Vision Transformer for pretraining
class MAEViT(nn.Module):
    """
    Masked Autoencoder (MAE) for Vision Transformer.
    Encoder sees only a fraction of patches; decoder reconstructs all patches.
    """
    def __init__(
        self,
        # default values for ViT-B-16
        image_size: int = 224,
        patch_size: int = 16,
        in_chans: int = 3,
        embed_dim: int = 768,
        encoder_layers: int = 12,
        encoder_heads: int = 12,
        mlp_ratio: float = 4.0,
        mask_ratio: float = 0.75,
        decoder_embed_dim: int = 512, 
        decoder_layers: int = 8,
        decoder_heads: int = 16,
        dropout: float = 0.0,
    ):
        super().__init__()
        assert image_size % patch_size == 0, "Image size must be divisible by patch size"
        self.in_chans = in_chans
        self.image_size = image_size
        self.patch_size = patch_size

        #Conv2d trick to PATCHIFY AND EMBED (DIFFERENT FROM THE PATCHIFY Function
        #which is used in validation)
        self.conv_proj = nn.Conv2d(
            in_channels = in_chans,
            out_channels = embed_dim, #embed_dim is for the TOTAL; this is patch_dimen^2 * 3 (# of color channels)
            kernel_size = patch_size, #this is so that the kernel is basically the patch (a square)
            stride = patch_size #this ensures that the kernel moves so that the patches do not overlap
        )
        num_patches = (image_size // patch_size) ** 2 #just the number of patches since image_size // patch_size deals with only the dimension
        self.mask_ratio = mask_ratio #75% is masked for best results with MAE

        #set CLS token, a class token that contains a learnable vector that will eventually contain embeddings for the whole image
        self.cls_token = nn.Parameter(torch.zeros(1, 1, embed_dim)) 
        nn.init.normal_(self.cls_token, std = 0.02) #normal distribution

        #Transformer encoder: learns contextual relationships b/t patches, generates embeddings
        enc_layer = TransformerEncoderLayer(
            embed_dim = embed_dim,
            num_heads = encoder_heads, #for multihead attn
            mlp_dim = int(embed_dim * mlp_ratio), 
            dropout = dropout #used in MLP
        )
        self.encoder = TransformerEncoder(enc_layer, encoder_layers, embed_dim) #does self attn & feed forward

        #Encoder -> Decoder (Linear Projection)
        self.enc_to_dec = nn.Linear(embed_dim, decoder_embed_dim, bias = False)

        #Decoder mask token (learnable placeholder token at each masked patch, helps decoder reconstruct those patches) and positional embedding generated
        self.dec_mask_token = nn.Parameter(torch.zeros(1, 1, decoder_embed_dim)) #mask tokens originally set to zero
        self.dec_pos_embed = nn.Parameter(torch.empty(1, num_patches + 1, decoder_embed_dim)) #num_patches + 1 includes cls token
        self.enc_pos_embed = nn.Parameter(torch.empty(1, num_patches + 1, embed_dim))
        
        nn.init.normal_(self.dec_mask_token, std=0.02)
        nn.init.normal_(self.enc_pos_embed, std=0.02)
        nn.init.normal_(self.dec_pos_embed, std=0.02)

        #Decoder for the transformer: predicts the masked patches
        dec_layer = TransformerDecoderLayer(
            encoder_embed_dim = embed_dim,
            decoder_embed_dim = decoder_embed_dim,
            num_heads = decoder_heads,
            mlp_dim = int(decoder_embed_dim * mlp_ratio),
            dropout = dropout #a regularizer to prevent model from overfitting and possibly making decisions based on noise
        )
        self.decoder = TransformerDecoder(dec_layer, decoder_layers, embed_dim = decoder_embed_dim)

        #Reconstruction for masked patches
        self.pred = nn.Linear(decoder_embed_dim, (patch_size ** 2) * in_chans)

        self.norm = nn.LayerNorm(embed_dim)

    #patchify: converts image into tensors for patches
    def patchify(self, imgs):
        """
        imgs: (B, C, H, W)
        returns: (B, N, patch_size * patch_size * C)
        """
        B, C, H, W = imgs.shape
        p = self.patch_size
        assert H % p == 0 and W % p == 0, "Image dimensions must be divisible by the patch size."

        h = H // p
        w = W // p
        patches = imgs.reshape(B, C, h, p, w, p)
        patches = torch.einsum('nchawb->nhwabc', patches)
        patches = patches.reshape(B, h * w, p * p * C)
        #print("size of patches: ")
        #print(patches.size())
        return patches

    #unpatchify: helps reconstruct image from patches (tensors -> images)
    #is not actually needed, maybe for debugging
    def unpatchify(self, x):
        #x is a tensor of shape (B, num_patches, patch_size*patch_size*in_chans)
        #x represents flattened pixel values
        #imgs: (returned) has shape (B, in_chans, img_size, img_size)
        patch_dimen = self.patch_size
        h = int(x.shape[1]**0.5)
        w = h
        assert h * w == x.shape[1]
        x = x.reshape(x.shape[0], h, w, patch_dimen, patch_dimen, self.in_chans)
        x = torch.einsum('nhwpqc->nchpwq', x)
        imgs = x.reshape([x.shape[0], self.in_chans, self.image_size, self.image_size])
        return imgs

    def random_masking(self, x):
        """
        Perform per-sample random masking by shuffling.
        returns:
            x_masked: Tensor with visible patches
            mask: Tensor indicating which patches are visible (0) or masked (1)
            ids_restore: Tensor to restore original order of patches
        """
        B, L, D = x.shape
        #number of patches to keep
        len_keep = int(L*(1 - self.mask_ratio))

        #indices for visible patches by generating noise
        noise = torch.rand(B, L, device=x.device)
        ids_shuffle = torch.argsort(noise, dim=1)

        #restore indices for unshuffling patches
        ids_restore = torch.argsort(ids_shuffle, dim=1)

        #indices of kept patches
        ids_keep = ids_shuffle[:, :len_keep]
        #visible patches gathered
        x_masked = torch.gather(x, dim=1, index = ids_keep.unsqueeze(-1).repeat(1, 1, D))
        
        #binary mask for patch embedding (1 is for masked, 0 is for visible)
        mask = torch.ones(B, L, device=x.device)
        #mask is unshuffled back into original patch order
        mask[:, :len_keep] = 0 ## DONGHEE: THIS PART WAS MISSING IN THE ORIGINAL CODE
        mask = torch.gather(mask, 1, ids_restore)

        return x_masked, mask, ids_restore, ids_keep  
    
    def forward_encoder(self, imgs):

        # 1. Patch embedding
        x = self.conv_proj(imgs)            # [B, embed_dim, H/ps, W/ps]
        x = x.flatten(2).transpose(1, 2)    # [B, N, embed_dim]
        x = self.norm(x)                  # [B, N, embed_dim]
        B, N, D = x.shape

        # 2. Add positional embeddings (w/o class token)
        #print(x.shape)
        #print(self.enc_pos_embed.shape)
        x = x + self.enc_pos_embed[:, 1:, :] 

        # 3. Random masking
        x_masked, mask, ids_restore, ids_keep = self.random_masking(x)

        # 4. Encoder input (cls token + visible patches)
        cls_token = self.cls_token + self.enc_pos_embed[:, :1, :]  # class token with positional embedding
        cls_tokens = cls_token.expand(B, -1, -1)  # repeat for batch size
        x_enc = torch.cat([cls_tokens, x_masked], dim=1)

        # 5. Encoder forward
        x_enc = self.encoder(x_enc) # TO DO

        return x_enc, mask, ids_restore
    
    def forward_decoder(self, x_enc, ids_restore):
        # encoder output needs to be projected to decoder embedding space
        x_dec = self.enc_to_dec(x_enc)

        #sequence unshuffled to original order
        B, L, D = x_dec.shape
        mask_tokens = self.dec_mask_token.repeat(B, ids_restore.shape[1] + 1 - x_dec.shape[1], 1)
        
        #concatenate output from heads?
        x_no_cls = torch.cat([x_dec[:, 1:, :], mask_tokens], dim=1)
        x_no_cls = torch.gather(x_no_cls, 1, ids_restore.unsqueeze(-1).repeat(1, 1, D))
        x_dec = torch.cat([x_dec[:, :1, :], x_no_cls], dim=1)

        #add positional embeddings
        x_dec = x_dec + self.dec_pos_embed[:, :x_dec.size(1), :]

        #decoder forward
        x_dec = self.decoder(x_dec, x_enc) 

        #predict pixels (without class token)
        x_rec = self.pred(x_dec)

        return x_rec
    
    def compute_mae_loss(self, imgs, pred, mask):
        """
        Mean Squared Error loss for masked patches
        imgs: [N, 3, H, W]
        pred: [N, L, p*p*3]
        mask: [N, L], 0 is keep, 1 is remove, 
        """

        #mask: binary mask tensor
        target = self.patchify(imgs) 
        #print("target size: ")
        #print(target.size())
        #print("pred size: ")
        #print(pred.size())

        pred = pred[:, 1:, :]
        loss = (pred - target)**2
        loss = loss.mean(dim=-1)
        #we don't want to calculate loss on visible patches, only masked patches
        loss = (loss * mask).sum() / (mask.sum() + 1e-6)

        return loss
    
    def forward(self, imgs):
        """
        Forward pass for MAE: encode, decode, and compute reconstruction loss.
        imgs: [B, 3, H, W]
        returns: reconstruction loss
        """

        # 1. Forward encoder
        x_enc, mask, ids_restore = self.forward_encoder(imgs)

        #x_enc = self.enc_to_dec(x_enc)

        # 2. Forward decoder
        x_rec = self.forward_decoder(x_enc, ids_restore)

        loss = self.compute_mae_loss(imgs, x_rec, mask)

        return loss