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from __future__ import absolute_import |
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from __future__ import print_function |
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from __future__ import division |
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import warnings |
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import math |
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import torch |
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from torch import nn |
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import torch.nn.functional as F |
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from torch.nn.init import xavier_uniform_, constant_ |
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from ..functions import MSDeformAttnFunction |
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def _is_power_of_2(n): |
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if (not isinstance(n, int)) or (n < 0): |
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raise ValueError("invalid input for _is_power_of_2: {} (type: {})".format(n, type(n))) |
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return (n & (n-1) == 0) and n != 0 |
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class MSDeformAttn(nn.Module): |
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def __init__(self, d_model=256, n_levels=4, n_heads=8, n_points=4): |
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""" |
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Multi-Scale Deformable Attention Module |
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:param d_model hidden dimension |
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:param n_levels number of feature levels |
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:param n_heads number of attention heads |
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:param n_points number of sampling points per attention head per feature level |
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""" |
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super().__init__() |
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if d_model % n_heads != 0: |
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raise ValueError('d_model must be divisible by n_heads, but got {} and {}'.format(d_model, n_heads)) |
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_d_per_head = d_model // n_heads |
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if not _is_power_of_2(_d_per_head): |
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warnings.warn("You'd better set d_model in MSDeformAttn to make the dimension of each attention head a power of 2 " |
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"which is more efficient in our CUDA implementation.") |
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self.im2col_step = 64 |
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self.d_model = d_model |
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self.n_levels = n_levels |
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self.n_heads = n_heads |
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self.n_points = n_points |
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self.sampling_offsets = nn.Linear(d_model, n_heads * n_levels * n_points * 2) |
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self.attention_weights = nn.Linear(d_model, n_heads * n_levels * n_points) |
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self.value_proj = nn.Linear(d_model, d_model) |
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self.output_proj = nn.Linear(d_model, d_model) |
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self._reset_parameters() |
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def _reset_parameters(self): |
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constant_(self.sampling_offsets.weight.data, 0.) |
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thetas = torch.arange(self.n_heads, dtype=torch.float32) * (2.0 * math.pi / self.n_heads) |
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grid_init = torch.stack([thetas.cos(), thetas.sin()], -1) |
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grid_init = (grid_init / grid_init.abs().max(-1, keepdim=True)[0]).view(self.n_heads, 1, 1, 2).repeat(1, self.n_levels, self.n_points, 1) |
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for i in range(self.n_points): |
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grid_init[:, :, i, :] *= i + 1 |
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with torch.no_grad(): |
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self.sampling_offsets.bias = nn.Parameter(grid_init.view(-1)) |
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constant_(self.attention_weights.weight.data, 0.) |
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constant_(self.attention_weights.bias.data, 0.) |
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xavier_uniform_(self.value_proj.weight.data) |
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constant_(self.value_proj.bias.data, 0.) |
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xavier_uniform_(self.output_proj.weight.data) |
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constant_(self.output_proj.bias.data, 0.) |
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def forward(self, query, reference_points, input_flatten, input_spatial_shapes, input_level_start_index, input_padding_mask=None): |
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""" |
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:param query (N, Length_{query}, C) |
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:param reference_points (N, Length_{query}, n_levels, 2), range in [0, 1], top-left (0,0), bottom-right (1, 1), including padding area |
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or (N, Length_{query}, n_levels, 4), add additional (w, h) to form reference boxes |
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:param input_flatten (N, \sum_{l=0}^{L-1} H_l \cdot W_l, C) |
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:param input_spatial_shapes (n_levels, 2), [(H_0, W_0), (H_1, W_1), ..., (H_{L-1}, W_{L-1})] |
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:param input_level_start_index (n_levels, ), [0, H_0*W_0, H_0*W_0+H_1*W_1, H_0*W_0+H_1*W_1+H_2*W_2, ..., H_0*W_0+H_1*W_1+...+H_{L-1}*W_{L-1}] |
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:param input_padding_mask (N, \sum_{l=0}^{L-1} H_l \cdot W_l), True for padding elements, False for non-padding elements |
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:return output (N, Length_{query}, C) |
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""" |
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N, Len_q, _ = query.shape |
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N, Len_in, _ = input_flatten.shape |
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assert (input_spatial_shapes[:, 0] * input_spatial_shapes[:, 1]).sum() == Len_in |
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value = self.value_proj(input_flatten) |
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if input_padding_mask is not None: |
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value = value.masked_fill(input_padding_mask[..., None], float(0)) |
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value = value.view(N, Len_in, self.n_heads, self.d_model // self.n_heads) |
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sampling_offsets = self.sampling_offsets(query).view(N, Len_q, self.n_heads, self.n_levels, self.n_points, 2) |
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attention_weights = self.attention_weights(query).view(N, Len_q, self.n_heads, self.n_levels * self.n_points) |
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attention_weights = F.softmax(attention_weights, -1).view(N, Len_q, self.n_heads, self.n_levels, self.n_points) |
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if reference_points.shape[-1] == 2: |
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offset_normalizer = torch.stack([input_spatial_shapes[..., 1], input_spatial_shapes[..., 0]], -1) |
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sampling_locations = reference_points[:, :, None, :, None, :] \ |
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+ sampling_offsets / offset_normalizer[None, None, None, :, None, :] |
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elif reference_points.shape[-1] == 4: |
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sampling_locations = reference_points[:, :, None, :, None, :2] \ |
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+ sampling_offsets / self.n_points * reference_points[:, :, None, :, None, 2:] * 0.5 |
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else: |
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raise ValueError( |
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'Last dim of reference_points must be 2 or 4, but get {} instead.'.format(reference_points.shape[-1])) |
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output = MSDeformAttnFunction.apply( |
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value, input_spatial_shapes, input_level_start_index, sampling_locations, attention_weights, self.im2col_step) |
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output = self.output_proj(output) |
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return output |
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