pytorch 笔记： Swin-Transformer 代码

Posted 2022-09-26 UQI-LIUWJ

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理论部分：论文笔记：Swin Transformer: Hierarchical Vision Transformer using Shifted Windows_UQI-LIUWJ的博客-CSDN博客

源码部分：Swin-Transformer/models at main · microsoft/Swin-Transformer (github.com)

输入图片尺寸 Batch_size*H*W
送入SwinTransformer
- PatchEmbedding【Parameter】
  - 每个patch 分别进行Embedding
  - Batch_size*H*W——>Batch_size,Patch_H*Patch_W,emb_dim
- 每个图片加1,Patch_H*Patch*W,emb_dim大小的绝对位置embedding
- 送入BasicLayer
  - 多个SwinTranformer
    - 奇数 shift window attention，偶数 window attention
      - 内部实现
        （如果需要）通过torch.roll进行window shift
        （如果需要）生成Patch_H*Patch_W的mask（针对滑动窗口情况）
        window_partition
        从patch视角转换成window视角
        Batch_size,Patch_H*Patch_W,emb_dim —> Batch_size*num_window,window_size,window_size,emb_dim
        window_attention
        Batch_size*num_window,window_size,window_size,emb_dim ——> Batch_size*num_window,window_size,window_size,emb_dim
        相对位置编码（Parameter）加在QK之上
        (2*window_size-1,2*window_size-1,head)
        window内每个点和另一个点的相对位置索引矩阵
        (head，window_size*window_size,window_size*window_size)
        window_reversion
        从windows视角再回到patch视角
    - Batch_size,Patch_H*Patch_W,emb_dim —> Batch_size,Patch_H*Patch_W,emb_dim
  - （除了末层不需要，其他都需要）Patch_Merging
    - Batch_size,Patch_H*Patch_W,emb_dim —> Batch_size,Patch_H/2*Patch_W/2,emb_dim*2
- 得到Batch_size，L，dim
- 每张图片每个dim的L个patch进行平均池化——>Batch_size,dim
- 全连接进行分类——>Batch_size,num_class.

1 class SwinTransformer

class SwinTransformer(nn.Module):

1.1 init输入部分

1.1.1 主要输入参数

img_size	输入的图片的大小（int，或者int的tuple）默认224
patch_size	patch的大小（int，或者int的tuple）默认4
in_chans	输入图片的channel数默认3
num_classes	图片分类的类别数默认1000
embed_dim	patch embedding的维数默认96
depths	各swin-transformer层的深度（int的tuple）
num_heads	各swin-transformer层的attention的头数（int的tuple）
window_size	窗口大小（窗口内的点进行attention）
mlp_ratio	mlp隐藏层维度：embedding层维度
qkv_bias	QKV是否有bias
drop_rate	dropout rate
attn_drop_rate	attention的drop rate
drop_path_rate	stochastic depth的p值大小
norm_layer	进行何种规范化
ape	是否加绝对位置positional encodding
patch_norm	是否在patch embedding之后进行normalization

1.1.2 代码部分（init）

    def __init__(self, img_size=224, patch_size=4, in_chans=3, num_classes=1000,
                 embed_dim=96, depths=[2, 2, 6, 2], num_heads=[3, 6, 12, 24],
                 window_size=7, mlp_ratio=4., qkv_bias=True, qk_scale=None,
                 drop_rate=0., attn_drop_rate=0., drop_path_rate=0.1,
                 norm_layer=nn.LayerNorm, ape=False, patch_norm=True,
                 use_checkpoint=False, fused_window_process=False, **kwargs):
        super().__init__()

        self.num_classes = num_classes
        self.num_layers = len(depths)
        self.embed_dim = embed_dim
        self.ape = ape
        self.patch_norm = patch_norm
        self.num_features = int(embed_dim * 2 ** (self.num_layers - 1))
        self.mlp_ratio = mlp_ratio


#####################将像素级图片转成patch级图片的类初始类#######################
        # split image into non-overlapping patches
        self.patch_embed = PatchEmbed(
            img_size=img_size, 
            patch_size=patch_size, 
            in_chans=in_chans, 
            embed_dim=embed_dim,
            norm_layer=norm_layer if self.patch_norm else None)
        #用于将图片转换成一个一个patch
        num_patches = self.patch_embed.num_patches
        #一张图片中有几个patch
        patches_resolution = self.patch_embed.patches_resolution
        #一张图片基于patch的分辨率
        self.patches_resolution = patches_resolution
#############################################################################

########################### 绝对位置编码 #####################################
        # absolute position embedding
        if self.ape:
            self.absolute_pos_embed = nn.Parameter(torch.zeros(1, num_patches, embed_dim))
            trunc_normal_(self.absolute_pos_embed, std=.02)
            #1*num_patches*embed_dim维度，每一个embed_dim都代表了一个patch的绝对位置的向量
#############################################################################


        self.pos_drop = nn.Dropout(p=drop_rate)
        

        # stochastic depth
        dpr = [x.item() for x in torch.linspace(0, drop_path_rate, sum(depths))]  # stochastic depth decay rule
        #层数越深stochastic depth的p（不激活这一层的概率）越大


###############################搭建swin-transformer################################
        # build layers
        self.layers = nn.ModuleList()
        for i_layer in range(self.num_layers):
            layer = BasicLayer(dim=int(embed_dim * 2 ** i_layer),
                               input_resolution=(patches_resolution[0] // (2 ** i_layer),
                                                 patches_resolution[1] // (2 ** i_layer)),
                               #随着层数的推进，维度翻倍，图片分辨率（大小）减半

                               depth=depths[i_layer],
                               #不同层swin-transformer需要不同的block数量

                               num_heads=num_heads[i_layer],
                               #不同层swin-transformer需要不同的注意力头数量

                               window_size=window_size,
                               mlp_ratio=self.mlp_ratio,
                               qkv_bias=qkv_bias, 
                               qk_scale=qk_scale,
                               drop=drop_rate, 
                               attn_drop=attn_drop_rate,
                               drop_path=dpr[sum(depths[:i_layer]):sum(depths[:i_layer + 1])],
                               norm_layer=norm_layer,
                               downsample=PatchMerging if (i_layer < self.num_layers - 1) else None,
                               #除了最后一层，其他的都需要PatchMerge（类似于CNN的池化）

                               use_checkpoint=use_checkpoint,
                               fused_window_process=fused_window_process)
            self.layers.append(layer)
        

        self.norm = norm_layer(self.num_features)
        self.avgpool = nn.AdaptiveAvgPool1d(1)
        self.head = nn.Linear(self.num_features, num_classes) if num_classes > 0 else nn.Identity()
#################################################################################

        self.apply(self._init_weights)
        #对每个子模组进行初始化

1.1.3 _init_weights

对每个子模组分别递归地进行初始化

    def _init_weights(self, m):
        if isinstance(m, nn.Linear):
            trunc_normal_(m.weight, std=.02)
            if isinstance(m, nn.Linear) and m.bias is not None:
                nn.init.constant_(m.bias, 0)
        elif isinstance(m, nn.LayerNorm):
            nn.init.constant_(m.bias, 0)
            nn.init.constant_(m.weight, 1.0)

1.1.4 forward_features

    def forward_features(self, x):
        x = self.patch_embed(x)
        #将图片转成patch级别分辨率，其中每个patch有emb_dim维
        #B,Patch_H*Patch_W,C

        if self.ape:
            x = x + self.absolute_pos_embed
        #绝对位置编码
        #B,Patch_H*Patch_W,C

        x = self.pos_drop(x)
        #Dropout

        for layer in self.layers:
            x = layer(x)
        #依次传入不同的layer层

        x = self.norm(x)  
        #LayerNorm  
        # batch_size, length, dim

        x = self.avgpool(x.transpose(1, 2)) 
        #平均池化  # batch_size,dim,1
        //　每张图片每个dimension取平均池化，就是这个dimension平均的feature

        x = torch.flatten(x, 1)
        #[batch_size,dim]
        #每张图片有dim个特征，每个特征是这张图片各个patch在这一dimension的平均值

        return x

1.1.5 forward

def forward(self, x):
        x = self.forward_features(x)
        #swin transformer 学习特征

        x = self.head(x)
        #全连接层进行分类
        return x

2 PatchEmbed

将像素级图片转化成patch级图片

class PatchEmbed(nn.Module):

2.1 init

    def __init__(self, img_size=224, patch_size=4, in_chans=3, embed_dim=96, norm_layer=None):
        super().__init__()
        img_size = to_2tuple(img_size)
        #224——>（224，224）
        patch_size = to_2tuple(patch_size)
        #4——>（4，4）

        patches_resolution = [img_size[0] // patch_size[0], img_size[1] // patch_size[1]]
        #一张图片基于patch的分辨率

        self.img_size = img_size
        self.patch_size = patch_size
        self.patches_resolution = patches_resolution

        self.num_patches = patches_resolution[0] * patches_resolution[1]
        #一张图片有几个patch

        self.in_chans = in_chans
        self.embed_dim = embed_dim

        self.proj = nn.Conv2d(in_chans, embed_dim, kernel_size=patch_size, stride=patch_size)
        '''
        卷积核是patch_size*patch_size,stride是patch_size
        ——>每个patch*patch*in_chans的部分，通过proj，变成1*1*embed_dim
        
        '''


        if norm_layer is not None:
            self.norm = norm_layer(embed_dim)
        else:
            self.norm = None

2.2 forward

 def forward(self, x):
        B, C, H, W = x.shape
        //batch_size,channel_num,height,width

        # FIXME look at relaxing size constraints
        assert H == self.img_size[0] and W == self.img_size[1], \\
            f"Input image size (H*W) doesn't match model (self.img_size[0]*self.img_size[1])."


        x = self.proj(x).flatten(2).transpose(1, 2)  
        '''
        每个patch里面的内容进行卷积
        将每个patch_size*patch_size的内容变成1*1的内容

        proj——> B,emb_dim,Patch_H,Patch_W
        flatten(2)——>B,emb_dim,Patch_H*Patch_W
        transpose(1,2)——>B,Patch_H*Patch_W,emb_dim
        '''


        if self.norm is not None:
            x = self.norm(x)
        return x

3 BasicLayer

一个stage的swin transformer层

class BasicLayer(nn.Module):
    """ A basic Swin Transformer layer for one stage.

3.1 主要参数

dim	输入channel的数量
input_resolution	输入的分辨率
depth	block的数量
num_heads	attention头的数量
window_size	window的大小，window_size*window_sizw的内容进行attention
mlp_ratio	mlp隐藏层维度：embedding层维度
qkv_bias	QKV是否有bias
drop	dropout rate
attn_drop	attention的drop rate
drop_path	stochastic depth的p

3.2 init

def __init__(self, dim, input_resolution, depth, num_heads, window_size,
                 mlp_ratio=4., qkv_bias=True, qk_scale=None, drop=0., attn_drop=0.,
                 drop_path=0., norm_layer=nn.LayerNorm, downsample=None, use_checkpoint=False,
                 fused_window_process=False):

        super().__init__()
        self.dim = dim
        self.input_resolution = input_resolution
        self.depth = depth
        self.use_checkpoint = use_checkpoint

        # build blocks
        self.blocks = nn.ModuleList([
            SwinTransformerBlock(dim=dim, 
                                 input_resolution=input_resolution,
                                 num_heads=num_heads, 
                                 window_size=window_size,
                                 shift_size=0 if (i % 2 == 0) else window_size // 2,
                                 #由于一个window attention加一个shift window attention是一个swin-transformer块
                                 #所以这里需要根据奇偶判断shift_size是windos_size的一半还是0
                                 mlp_ratio=mlp_ratio,
                                 qkv_bias=qkv_bias, 
                                 qk_scale=qk_scale,
                                 drop=drop, 
                                 attn_drop=attn_drop,
                                 drop_path=drop_path[i] if isinstance(drop_path, list) else drop_path,
                                 norm_layer=norm_layer,
                                 fused_window_process=fused_window_process)
            for i in range(depth)])
            

        # patch merging layer
        if downsample is not None:
            self.downsample = downsample(input_resolution, dim=dim, norm_layer=norm_layer)
        else:
            self.downsample = None
        #除了最后一层，其他的都需要PatchMerge（类似于CNN的池化）

3.3 forward

 def forward(self, x):
        ///x:B,Patch_H*Patch_W,C
        for blk in self.blocks:
            if self.use_checkpoint:
                x = checkpoint.checkpoint(blk, x)
            else:
                x = blk(x)
        #依次送入这个basic block 里面的每个swin-transformer block

        if self.downsample is not None:
            x = self.downsample(x)
        #除非最后一层，否则都进行PatchEmerging
        return x

4 SwinTransformerBlock

4.1 主要输入参数、

dim	输入channel的数量
input_resolution	输入的分辨率
num_heads	attention头的数量
window_size	window的大小，window_size*window_sizw的内容进行attention
shitf_size	是否需要滑动窗口，偶数层不用奇数层用
mlp_ratio	mlp隐藏层维度：embedding层维度
qkv_bias	QKV是否有bias
drop	dropout rate
attn_drop	attention的drop rate
drop_path	stochastic depth的p

4.2 init

def __init__(self, dim, input_resolution, num_heads, window_size=7, shift_size=0,
                 mlp_ratio=4., qkv_bias=True, qk_scale=None, drop=0., attn_drop=0., drop_path=0.,
                 act_layer=nn.GELU, norm_layer=nn.LayerNorm,
                 fused_window_process=False):
        super().__init__()
        self.dim = dim
        self.input_resolution = input_resolution
        self.num_heads = num_heads
        self.window_size = window_size
        self.shift_size = shift_size
        self.mlp_ratio = mlp_ratio

        if min(self.input_resolution) <= self.window_size:
            # if window size is larger than input resolution, we don't partition windows
            self.shift_size = 0
            self.window_size = min(self.input_resolution)
        #如果当前图片的分辨率大小比window size小，那么将window size设置成图片的分辨率大小。同时不进行shift window


        assert 0 <= self.shift_size < self.window_size, "shift_size must in 0-window_size"
        #默认shift window比windows size 小

        self.norm1 = norm_layer(dim)
        self.attn = WindowAttention(
            dim, 
            window_size=to_2tuple(self.window_size), 
            num_heads=num_heads,
            qkv_bias=qkv_bias, 
            qk_scale=qk_scale, 
            attn_drop=attn_drop, 
            proj_drop=drop)

        self.drop_path = DropPath(drop_path) if drop_path > 0. else nn.Identity()
        self.norm2 = norm_layer(dim)
        mlp_hidden_dim = int(dim * mlp_ratio)
        self.mlp = Mlp(in_features=dim, hidden_features=mlp_hidden_dim, act_layer=act_layer, drop=drop)


##################################滑动窗口WSA###########################
        if self.shift_size > 0:
            # calculate attention mask for SW-MSA
            H, W = self.input_resolution
            img_mask = torch.zeros((1, H, W, 1))  # 1 H W 1
            h_slices = (slice(0, -self.window_size),
                        slice(-self.window_size, -self.shift_size),
                        slice(-self.shift_size, None))
            w_slices = (slice(0, -self.window_size),
                        slice(-self.window_size, -self.shift_size),
                        slice(-self.shift_size, None))
            cnt = 0
            for h in h_slices:
                for w in w_slices:
                    img_mask[:, h, w, :] = cnt
                    cnt += 1

            mask_windows = window_partition(img_mask, self.window_size) 
            #B,H,W,C——>B*num_W, window_size, window_size, C  
            #num_W表示可以划分成几个窗口


            mask_windows = mask_windows.view(-1, self.window_size * self.window_size)
            #B*num_W*C，window_size*window_size

            attn_mask = mask_windows.unsqueeze(1) - mask_windows.unsqueeze(2)
            #B*num_W*C，window_size*window_size，window_size*window_size
            attn_mask = attn_mask.masked_fill(attn_mask != 0, float(-100.0)).masked_fill(attn_mask == 0, float(0.0))
        else:
            attn_mask = None
##############################################################################

        self.register_buffer("attn_mask", attn_mask)
        self.fused_window_process = fused_window_process

不同数字的cnt对应的是下面这九块

对于滑动窗口这一部分，我们举个例子：H=W=6，window_size=2，shift_size=1

img_mask
'''
tensor([[[[0.],[0.],[0.],[0.],[1.],[2.]],

         [[0.],[0.],[0.],[0.],[1.],[2.]],

         [[0.],[0.],[0.],[0.],[1.],[2.]],

         [[0.],[0.],[0.],[0.],[1.],[2.]],

         [[3.],[3.],[3.],[3.],[4.],[5.]],

         [[6.],[6.],[6.],[6.],[7.],[8.]]]])
'''

mask_windows
'''
tensor([[0., 0., 0., 0.],
        [0., 0., 0., 0.],
        [1., 2., 1., 2.],
        [0., 0., 0., 0.],
        [0., 0., 0., 0.],
        [1., 2., 1., 2.],
        [3., 3., 6., 6.],
        [3., 3., 6., 6.],
        [4., 5., 7., 8.]])
'''

 mask_windows.unsqueeze(1) - mask_windows.unsqueeze(2)
'''
我们记：
A=[[ 0.,  0.,  0.,  0.],
   [ 0.,  0.,  0.,  0.],
   [ 0.,  0.,  0.,  0.],
   [ 0.,  0.,  0.,  0.]]
B=[[ 0.,  1.,  0.,  1.],
   [-1.,  0., -1.,  0.],
   [ 0.,  1.,  0.,  1.],
   [-1.,  0., -1.,  0.]]
C=[[ 0.,  0.,  3.,  3.],
   [ 0.,  0.,  3.,  3.],
   [-3., -3.,  0.,  0.],
   [-3., -3.,  0.,  0.]]
D=[[ 0.,  1.,  3.,  4.],
   [-1.,  0.,  2.,  3.],
   [-3., -2.,  0.,  1.],
   [-4., -3., -1.,  0.]]
结果是[A,A,B,A,A,B,C,C,D]
'''


atten_mask
'''
我们记：
A[[   0.,    0.,    0.,    0.],
  [   0.,    0.,    0.,    0.],
  [   0.,    0.,    0.,    0.],
  [   0.,    0.,    0.,    0.]]
B=[[   0., -100.,    0., -100.],
   [-100.,    0., -100.,    0.],
   [   0., -100.,    0., -100.],
   [-100.,    0., -100.,    0.]]
C=[[   0., -100.,    0., -100.],
   [-100.,    0., -100.,    0.],
   [   0., -100.,    0., -100.],
   [-100.,    0., -100.,    0.]]
D=[[   0., -100., -100., -100.],
   [-100.,    0., -100., -100.],
   [-100., -100.,    0., -100.],
   [-100., -100., -100.,    0.]]
结果是[A,A,B,A,A,B,C,C,D]
'''

A,B,C,D分别对应Window 0，1，2，3

4.3 forward

def forward(self, x):
        #x:B,Patch_H*Patch_W,C
        H, W = self.input_resolution
        B, L, C = x.shape
        assert L == H * W, "input feature has wrong size"

        shortcut = x
        x = self.norm1(x)
        x = x.view(B, H, W, C)
        #x:B,Patch_H,Patch_W,C

#############################（如果需要的话）滑动窗口############################
        # cyclic shift
        if self.shift_size > 0:
            if not self.fused_window_process:
                shifted_x = torch.roll(x, shifts=(-self.shift_size, -self.shift_size), dims=(1, 2))
                #向右下方横纵各平移shift_size 
                #（最左边和最上面的翻折下来）   

                # partition windows
                x_windows = window_partition(shifted_x, self.window_size)  
                # num_Window*B, window_size, window_size, C
            else:
                x_windows = WindowProcess.apply(x, B, H, W, C, -self.shift_size, self.window_size)


        else:
            shifted_x = x
            # partition windows
            x_windows = window_partition(shifted_x, self.window_size)  
            # num_Window*B, window_size, window_size, C

        x_windows = x_windows.view(-1, self.window_size * self.window_size, C)  
        # num_Window*B, window_size*window_size, C        
############################################################################


############################（滑动）窗口attention#################
        # W-MSA/SW-MSA
        attn_windows = self.attn(x_windows, mask=self.attn_mask) 
        #根据是否是滑动窗口attention，来进行窗口attention/滑动窗口attention
        # num_Window*B, window_size*window_size, C

        # merge windows
        attn_windows = attn_windows.view(-1, self.window_size, self.window_size, C)
        # num_Window*B, window_size，window_size, C

        # reverse cyclic shift
        if self.shift_size > 0:
            if not self.fused_window_process:
                shifted_x = window_reverse(attn_windows, self.window_size, H, W)  
                # B H W C
                #从window级别视角转换回patch级别视角

                x = torch.roll(shifted_x, shifts=(self.shift_size, self.shift_size), dims=(1, 2))
                #将向右下方平移后的矩阵平移回去
            else:
                x = WindowProcessReverse.apply(attn_windows, B, H, W, C, self.shift_size, self.window_size)
        else:
            shifted_x = window_reverse(attn_windows, self.window_size, H, W)  
            #如果没有滑动窗口，只要从window级别视角转换回patch级别视角即可
            x = shifted_x
######################################################################


        x = x.view(B, H * W, C)
        x = shortcut + self.drop_path(x)
        #每一个SwinTransformerBlock做完后（window att/shift window att），都进行一次stochastic depth

        # FFN
        x = x + self.drop_path(self.mlp(self.norm2(x)))
        #这论文的模型图里没有说明，但应该也是一个stochastic depth的操作

        return x

5 WindowAttention

逐window的attention

5.1 输入参数

dim	输入channel的数量
num_heads	attention头的数量
window_size	window的大小，window_size*window_sizw的内容进行attention
qkv_bias	QKV是否有bias
attn_drop	attention的drop rate
proj_drop	输出层的droprate

5.2 init

 def __init__(self, dim, window_size, num_heads, qkv_bias=True, qk_scale=None, attn_drop=0., proj_drop=0.):

        super().__init__()
        self.dim = dim
        self.window_size = window_size  
        # Wh, Ww
        self.num_heads = num_heads
        head_dim = dim // num_heads
        #由于需要保持维度，所以每个window attention输入输出的维度都是dim
        #由于window attention有num_heads个头，所以每个头的dim就是dim//num_heads

        self.scale = qk_scale or head_dim ** -0.5


#############################相对位置编码#################################
        # define a parameter table of relative position bias
        self.relative_position_bias_table = nn.Parameter(
            torch.zeros((2 * window_size[0] - 1) * (2 * window_size[1] - 1), num_heads))  

        # 2*Wh-1 * 2*Ww-1, n_heads

        # get pair-wise relative position index for each token inside the window
        coords_h = torch.arange(self.window_size[0])
        coords_w = torch.arange(self.window_size[1])
        coords = torch.stack(torch.meshgrid([coords_h, coords_w]))  
        # 2, Wh, Ww
        coords_flatten = torch.flatten(coords, 1)  
        # 2, Wh*Ww
        relative_coords = coords_flatten[:, :, None] - coords_flatten[:, None, :]  
        # 2, Wh*Ww, Wh*Ww
        relative_coords = relative_coords.permute(1, 2, 0).contiguous()  # Wh*Ww, Wh*Ww, 2
        relative_coords[:, :, 0] += self.window_size[0] - 1  # shift to start from 0
        relative_coords[:, :, 1] += self.window_size[1] - 1
        relative_coords[:, :, 0] *= 2 * self.window_size[1] - 1
        relative_position_index = relative_coords.sum(-1)  # Wh*Ww, Wh*Ww
        self.register_buffer("relative_position_index", relative_position_index)
############################################################################


        self.qkv = nn.Linear(dim, dim * 3, bias=qkv_bias)
        #这里和分别写三个dim——>dim的q，k，v Linear function是异曲同工的
        self.attn_drop = nn.Dropout(attn_drop)
        self.proj = nn.Linear(dim, dim)
        self.proj_drop = nn.Dropout(proj_drop)

        trunc_normal_(self.relative_position_bias_table, std=.02)
        self.softmax = nn.Softmax(dim=-1)

5.2.1 相对位置编码详述

relative_position_bias_table是一个可更新的Parameter
我们现在假定窗口大小是2*2
- 那么从相对位置来说(0,0)——>(0,1);(1,0)——>(1,1)，他们的相对位置是一样的
  (0,0) (0,1)
  (1,0) (1,1)
换言之，需要这样的相对位置索引（值相同的表示相对位置是一样的）

4 (0,0)—>(0,0)	2 (0,0)—>(0,1)	1 (0,0)—>(1,0)	0 (0,0)—>(1,1)
5 (0,1)——(0,0)	4 (0,1)—>(0,1)	3 (0,1)—>(1,0)	1 (1,0)—>(1,1)
7 (1,0)——>(0,0)	6 (1,0)—>(0.1）	4 (2,2)—>(2,2)	2 (1,0)—>(1,1)
8 (1,1)—>(0,0)	7 (1,1)—>(0,1)	5 (1,1)—>(1,0)	4 (3,3)—>(3,3)

coords_h = torch.arange(window_size[0])
coords_w = torch.arange(window_size[1])
coords_h,coords_w
'''
(tensor([0, 1]), tensor([0, 1]))
'''

coords = torch.stack(torch.meshgrid([coords_h, coords_w]))
coords
'''
(tensor([[0, 0],
         [1, 1]]),
 tensor([[0, 1],
         [0, 1]]))
'''

coords_flatten = torch.flatten(coords, 1)
coords_flatten
'''
tensor([[0, 0, 1, 1],
        [0, 1, 0, 1]])
'''

relative_coords = coords_flatten[:, :, None] - coords_flatten[:, None, :] 
relative_coords
'''
每个window和另一个window之间相对位置，一共4*4个，所以这里是4*4的矩阵
第一个4*4矩阵是相对位置的纵轴；第二个4*4是相对位置的横轴
tensor([[[ 0,  0, -1, -1],
         [ 0,  0, -1, -1],
         [ 1,  1,  0,  0],
         [ 1,  1,  0,  0]],

        [[ 0, -1,  0, -1],
         [ 1,  0,  1,  0],
         [ 0, -1,  0, -1],
         [ 1,  0,  1,  0]]])
'''

relative_coords = relative_coords.permute(1, 2, 0).contiguous() 
relative_coords
'''
每一行是一个相对位置索引
tensor([[[ 0,  0],
         [ 0, -1],
         [-1,  0],
         [-1, -1]],

        [[ 0,  1],
         [ 0,  0],
         [-1,  1],
         [-1,  0]],

        [[ 1,  0],
         [ 1, -1],
         [ 0,  0],
         [ 0, -1]],

        [[ 1,  1],
         [ 1,  0],
         [ 0,  1],
         [ 0,  0]]])
'''

relative_coords[:, :, 0] += window_size[0] - 1
relative_coords[:, :, 1] += window_size[1] - 1
relative_coords[:, :, 0] *= 2 * window_size[1] - 1
'''
tensor([[[3, 1],
         [3, 0],
         [0, 1],
         [0, 0]],

        [[3, 2],
         [3, 1],
         [0, 2],
         [0, 1]],

        [[6, 1],
         [6, 0],
         [3, 1],
         [3, 0]],

        [[6, 2],
         [6, 1],
         [3, 2],
         [3, 1]]])
'''

relative_position_index = relative_coords.sum(-1)
relative_position_index
'''
tensor([[4, 3, 1, 0],
        [5, 4, 2, 1],
        [7, 6, 4, 3],
        [8, 7, 5, 4]])
'''

5.3 forward

    def forward(self, x, mask=None):
        """
        Args:
            x: input features with shape of (num_windows*B, N, C)
            mask: (0/-inf) mask with shape of (num_windows, Wh*Ww, Wh*Ww) or None
        """
        B_, N, C = x.shape
        #num_Window*B, window_size*window_size, C

        qkv = self.qkv(x).reshape(B_, N, 3, self.num_heads, C // self.num_heads).permute(2, 0, 3, 1, 4)
        #3，B_,self.num_heads,N,C // self.num_heads
        #3，num_Window*B,self.num_heads,window_size*window_size,C // self.num_heads

        q, k, v = qkv[0], qkv[1], qkv[2]  
        # make torchscript happy (cannot use tensor as tuple)
        #num_Window*B,self.num_heads,window_size*window_size,C // self.num_heads

        q = q * self.scale
        attn = (q @ k.transpose(-2, -1))
        #Q,K内积
        #B_,self.num_heads,N,N
        #num_Window*B,self.num_heads,window_size*window_size,window_size*window_size

        relative_position_bias = self.relative_position_bias_table[self.relative_position_index.view(-1)].view(
            self.window_size[0] * self.window_size[1], 
            self.window_size[0] * self.window_size[1], 
            -1)  
        # window_size*window_size,window_size*window_size,self.num_heads


        relative_position_bias = relative_position_bias.permute(2, 0, 1).contiguous()  
        # self.num_heads,window_size*window_size,window_size*window_size

        attn = attn + relative_position_bias.unsqueeze(0)
        #Batch张图片中每个window都加上这个relative position

        if mask is not None:
            nW = mask.shape[0]
            attn = attn.view(B_ // nW, nW, self.num_heads, N, N) + mask.unsqueeze(1).unsqueeze(0)
            #B_ // nW,num_Window,self.num_heads,window_size*window_size,window_size*window_size

            #mask：#nW，window_size*window_size，window_size*window_size

            attn = attn.view(-1, self.num_heads, N, N)
            attn = self.softmax(attn)
        else:
            attn = self.softmax(attn)

        attn = self.attn_drop(attn)

        x = (attn @ v).transpose(1, 2).reshape(B_, N, C)
        x = self.proj(x)
        x = self.proj_drop(x)
        return x

6 window partition

将patch级别的图片划分成窗口级别

def window_partition(x, window_size):
    """
    Args:
        x: (B, H, W, C)
        window_size (int): window size
    Returns:
        windows: (num_windows*B, window_size, window_size, C)

        比如原来是(1,56,56,3),窗口大小为7，可分成8*8个窗口
        那么返回维度是(64,7,7,3)
    """
    B, H, W, C = x.shape
    x = x.view(B, H // window_size, window_size, W // window_size, window_size, C)
    windows = x.permute(0, 1, 3, 2, 4, 5).contiguous().view(-1, window_size, window_size, C)
    return windows

举例

a=torch.arange(16).reshape(1,4,4,1)
print(a)
'''
tensor([[[[ 0],[ 1],[ 2],[ 3]],
         [[ 4],[ 5],[ 6],[ 7]],
         [[ 8],[ 9],[10],[11]],
         [[12],[13],[14],[15]]]])
'''
w=window_partition(a,2)
w
'''
tensor([[[[ 0],[ 1]],
         [[ 4],[ 5]]],


        [[[ 2], [ 3]],
         [[ 6],[ 7]]],


        [[[ 8],[ 9]],
         [[12],[13]]],


        [[[10],[11]],
         [[14],[15]]]])
'''

7 window reversion

把窗口级别的还原成patch级别

def window_reverse(windows, window_size, H, W):
    """
    Args:
        windows: (num_windows*B, window_size, window_size, C)
        window_size (int): Window size
        H (int): Height of image
        W (int): Width of image
    Returns:
        x: (B, H, W, C)
    """

    B = int(windows.shape[0] / (H * W / window_size / window_size))
    #(H * W / window_size / window_size)就是num_windows

    x = windows.view(B, H // window_size, W // window_size, window_size, window_size, -1)
    # B, n_patch_H,n_patch_W,window_size,window_size,C
    x = x.permute(0, 1, 3, 2, 4, 5).contiguous().view(B, H, W, -1)
    #(B,H,W,C)
    return x

举例

w
'''
tensor([[[[ 0],[ 1]],
         [[ 4],[ 5]]],


        [[[ 2], [ 3]],
         [[ 6],[ 7]]],


        [[[ 8],[ 9]],
         [[12],[13]]],


        [[[10],[11]],
         [[14],[15]]]])
'''

window_reverse(w,2,4,4)
'''
tensor([[[[ 0],[ 1],[ 2],[ 3]],
         [[ 4],[ 5],[ 6],[ 7]],
         [[ 8],[ 9],[10],[11]],
         [[12],[13],[14],[15]]]])
'''

8 PatchMerging

class PatchMerging(nn.Module):
    r""" Patch Merging Layer.
    Args:
        input_resolution (tuple[int]): Resolution of input feature.
        dim (int): Number of input channels.
        norm_layer (nn.Module, optional): Normalization layer.  Default: nn.LayerNorm
    B,H,W,C——>
    """

    def __init__(self, input_resolution, dim, norm_layer=nn.LayerNorm):
        super().__init__()
        self.input_resolution = input_resolution
        self.dim = dim
        self.reduction = nn.Linear(4 * dim, 2 * dim, bias=False)
        self.norm = norm_layer(4 * dim)

    def forward(self, x):
        """
        x: B, H*W, C
        """
        H, W = self.input_resolution
        B, L, C = x.shape
        assert L == H * W, "input feature has wrong size"
        assert H % 2 == 0 and W % 2 == 0, f"x size (H*W) are not even."

        x = x.view(B, H, W, C)

        x0 = x[:, 0::2, 0::2, :]  # B H/2 W/2 C
        x1 = x[:, 1::2, 0::2, :]  # B H/2 W/2 C
        x2 = x[:, 0::2, 1::2, :]  # B H/2 W/2 C
        x3 = x[:, 1::2, 1::2, :]  # B H/2 W/2 C
        x = torch.cat([x0, x1, x2, x3], -1)  # B H/2 W/2 4*C
        x = x.view(B, -1, 4 * C)  # B H/2*W/2 4*C

        x = self.norm(x)
        x = self.reduction(x)

        return x

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(0,0)	(0,1)
(1,0)	(1,1)