| """ Selective Kernel Convolution/Attention |
| |
| Paper: Selective Kernel Networks (https://arxiv.org/abs/1903.06586) |
| |
| Hacked together by / Copyright 2020 Ross Wightman |
| """ |
| import torch |
| from torch import nn as nn |
|
|
| from .conv_bn_act import ConvBnAct |
| from .helpers import make_divisible |
|
|
|
|
| def _kernel_valid(k): |
| if isinstance(k, (list, tuple)): |
| for ki in k: |
| return _kernel_valid(ki) |
| assert k >= 3 and k % 2 |
|
|
|
|
| class SelectiveKernelAttn(nn.Module): |
| def __init__(self, channels, num_paths=2, attn_channels=32, |
| act_layer=nn.ReLU, norm_layer=nn.BatchNorm2d): |
| """ Selective Kernel Attention Module |
| |
| Selective Kernel attention mechanism factored out into its own module. |
| |
| """ |
| super(SelectiveKernelAttn, self).__init__() |
| self.num_paths = num_paths |
| self.fc_reduce = nn.Conv2d(channels, attn_channels, kernel_size=1, bias=False) |
| self.bn = norm_layer(attn_channels) |
| self.act = act_layer(inplace=True) |
| self.fc_select = nn.Conv2d(attn_channels, channels * num_paths, kernel_size=1, bias=False) |
|
|
| def forward(self, x): |
| assert x.shape[1] == self.num_paths |
| x = x.sum(1).mean((2, 3), keepdim=True) |
| x = self.fc_reduce(x) |
| x = self.bn(x) |
| x = self.act(x) |
| x = self.fc_select(x) |
| B, C, H, W = x.shape |
| x = x.view(B, self.num_paths, C // self.num_paths, H, W) |
| x = torch.softmax(x, dim=1) |
| return x |
|
|
|
|
| class SelectiveKernel(nn.Module): |
|
|
| def __init__(self, in_channels, out_channels=None, kernel_size=None, stride=1, dilation=1, groups=1, |
| rd_ratio=1./16, rd_channels=None, rd_divisor=8, keep_3x3=True, split_input=True, |
| drop_block=None, act_layer=nn.ReLU, norm_layer=nn.BatchNorm2d, aa_layer=None): |
| """ Selective Kernel Convolution Module |
| |
| As described in Selective Kernel Networks (https://arxiv.org/abs/1903.06586) with some modifications. |
| |
| Largest change is the input split, which divides the input channels across each convolution path, this can |
| be viewed as a grouping of sorts, but the output channel counts expand to the module level value. This keeps |
| the parameter count from ballooning when the convolutions themselves don't have groups, but still provides |
| a noteworthy increase in performance over similar param count models without this attention layer. -Ross W |
| |
| Args: |
| in_channels (int): module input (feature) channel count |
| out_channels (int): module output (feature) channel count |
| kernel_size (int, list): kernel size for each convolution branch |
| stride (int): stride for convolutions |
| dilation (int): dilation for module as a whole, impacts dilation of each branch |
| groups (int): number of groups for each branch |
| rd_ratio (int, float): reduction factor for attention features |
| keep_3x3 (bool): keep all branch convolution kernels as 3x3, changing larger kernels for dilations |
| split_input (bool): split input channels evenly across each convolution branch, keeps param count lower, |
| can be viewed as grouping by path, output expands to module out_channels count |
| drop_block (nn.Module): drop block module |
| act_layer (nn.Module): activation layer to use |
| norm_layer (nn.Module): batchnorm/norm layer to use |
| """ |
| super(SelectiveKernel, self).__init__() |
| out_channels = out_channels or in_channels |
| kernel_size = kernel_size or [3, 5] |
| _kernel_valid(kernel_size) |
| if not isinstance(kernel_size, list): |
| kernel_size = [kernel_size] * 2 |
| if keep_3x3: |
| dilation = [dilation * (k - 1) // 2 for k in kernel_size] |
| kernel_size = [3] * len(kernel_size) |
| else: |
| dilation = [dilation] * len(kernel_size) |
| self.num_paths = len(kernel_size) |
| self.in_channels = in_channels |
| self.out_channels = out_channels |
| self.split_input = split_input |
| if self.split_input: |
| assert in_channels % self.num_paths == 0 |
| in_channels = in_channels // self.num_paths |
| groups = min(out_channels, groups) |
|
|
| conv_kwargs = dict( |
| stride=stride, groups=groups, drop_block=drop_block, act_layer=act_layer, norm_layer=norm_layer, |
| aa_layer=aa_layer) |
| self.paths = nn.ModuleList([ |
| ConvBnAct(in_channels, out_channels, kernel_size=k, dilation=d, **conv_kwargs) |
| for k, d in zip(kernel_size, dilation)]) |
|
|
| attn_channels = rd_channels or make_divisible(out_channels * rd_ratio, divisor=rd_divisor) |
| self.attn = SelectiveKernelAttn(out_channels, self.num_paths, attn_channels) |
| self.drop_block = drop_block |
|
|
| def forward(self, x): |
| if self.split_input: |
| x_split = torch.split(x, self.in_channels // self.num_paths, 1) |
| x_paths = [op(x_split[i]) for i, op in enumerate(self.paths)] |
| else: |
| x_paths = [op(x) for op in self.paths] |
| x = torch.stack(x_paths, dim=1) |
| x_attn = self.attn(x) |
| x = x * x_attn |
| x = torch.sum(x, dim=1) |
| return x |
|
|
