大气集团企业网站模板,长春网站设计制作培训,wordpress商城主题模板,设置网站建设Swin Transformer
简介 下采样的层级设计#xff0c;能够逐渐增大感受野。采用window进行注意力计算#xff0c;极大降低了内存消耗#xff0c;避免了整张图像尺寸大小的qkv矩阵滑窗操作包括不重叠的 local window#xff0c;和重叠的 cross-window。不重叠的local window…Swin Transformer
简介 下采样的层级设计能够逐渐增大感受野。采用window进行注意力计算极大降低了内存消耗避免了整张图像尺寸大小的qkv矩阵滑窗操作包括不重叠的 local window和重叠的 cross-window。不重叠的local windows将注意力计算限制在一个窗口window size固定而cross-windows则让不同窗口之间信息可以进行关联实现了信息的交互。
整体架构 Patch Partition结构将图像切分重排并进行embeddingPatch Merging结构下采样方法实现层次化结构Swin Transformer Block一个W-MSA ,一个SW-MSA,也即是一个window-多头注意力机制和一个shift-windows多头注意力机制实现将自注意力机制限制在一个windows中进行计算同时通过shift-window解决限制在一个windows中后不同windows之间无信息共享的问题。
Patch Embedding
在图像切分重排中采用的是使用patch size大小的conv2d进行实现
class PatchEmbed(nn.Module):r Image to Patch Embedding图像切分重排Args:img_size (int): Image size. Default: 224.patch_size (int): Patch token size. Default: 4.in_chans (int): Number of input image channels. Default: 3.embed_dim (int): Number of linear projection output channels. Default: 96.norm_layer (nn.Module, optional): Normalization layer. Default: Nonedef __init__(self, img_size224, patch_size4, in_chans3, embed_dim96, norm_layerNone):super().__init__()img_size to_2tuple(img_size)patch_size to_2tuple(patch_size)patches_resolution [img_size[0] // patch_size[0], img_size[1] // patch_size[1]]self.img_size img_sizeself.patch_size patch_sizeself.patches_resolution patches_resolutionself.num_patches patches_resolution[0] * patches_resolution[1]self.in_chans in_chansself.embed_dim embed_dimself.proj nn.Conv2d(in_chans, embed_dim, kernel_sizepatch_size, stridepatch_size)if norm_layer is not None:self.norm norm_layer(embed_dim)else:self.norm Nonedef forward(self, x):B, C, H, W x.shape# FIXME look at relaxing size constraintsassert H self.img_size[0] and W self.img_size[1], \fInput image size ({H}*{W}) doesnt match model ({self.img_size[0]}*{self.img_size[1]}).x self.proj(x).flatten(2).transpose(1, 2) # B Ph*Pw Cif self.norm is not None:x self.norm(x)return x
Patch Merging
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.LayerNormdef __init__(self, input_resolution, dim, norm_layernn.LayerNorm):super().__init__()self.input_resolution input_resolutionself.dim dimself.reduction nn.Linear(4 * dim, 2 * dim, biasFalse)self.norm norm_layer(4 * dim)def forward(self, x):x: B, H*W, CH, W self.input_resolutionB, L, C x.shapeassert L H * W, input feature has wrong sizeassert H % 2 0 and W % 2 0, fx size ({H}*{W}) are not even.x x.view(B, H, W, C)x0 x[:, 0::2, 0::2, :] # B H/2 W/2 Cx1 x[:, 1::2, 0::2, :] # B H/2 W/2 Cx2 x[:, 0::2, 1::2, :] # B H/2 W/2 Cx3 x[:, 1::2, 1::2, :] # B H/2 W/2 Cx torch.cat([x0, x1, x2, x3], -1) # B H/2 W/2 4*Cx x.view(B, -1, 4 * C) # B H/2*W/2 4*Cx self.norm(x)x self.reduction(x)return x SW-MSA设计
如下所示w-msa mask避免窗口5和窗口3进行相似度计算通过mask只在窗口内部进行计算。
通过对特征图移位并给Attention设置mask来间接实现的。能在保持原有的window个数下最后的计算结果等价 Window Attention A t t e n t i o n ( Q , K , V ) S o f t m a x ( Q K T d B ) V Attention(Q,K,V)Softmax(\frac{QK^T}{\sqrt{d}}B)V Attention(Q,K,V)Softmax(d QKTB)V
相对位置编码
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)对于相对位置编码在2维坐标系中当偏移从0开始时21和12相对00的位置编码是不同的而转为1维坐标后却是相同数值为了解决这个问题采用对x坐标2 * self.window_size[1] - 1操作从而进行区分。而该相对位置编码需要2 * self.window_size[1] - 1编码数值。
A Survey of Transformers
图解Swin Transformer - 知乎 (zhihu.com)
