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Original file line number | Diff line number | Diff line change |
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import torch | ||
import torch.nn as nn | ||
import torch.nn.functional as F | ||
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# Adopted from https://gist.github.com/kevinzakka/dd9fa5177cda13593524f4d71eb38ad5 | ||
class SpatialSoftArgmax(nn.Module): | ||
"""Spatial softmax as defined in [1]. | ||
Concretely, the spatial softmax of each feature | ||
map is used to compute a weighted mean of the pixel | ||
locations, effectively performing a soft arg-max | ||
over the feature dimension. | ||
References: | ||
[1]: End-to-End Training of Deep Visuomotor Policies, | ||
https://arxiv.org/abs/1504.00702 | ||
""" | ||
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def __init__(self, normalize=False): | ||
"""Constructor. | ||
Args: | ||
normalize (bool): Whether to use normalized | ||
image coordinates, i.e. coordinates in | ||
the range `[-1, 1]`. | ||
""" | ||
super().__init__() | ||
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self.normalize = normalize | ||
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def _coord_grid(self, h, w, device): | ||
if self.normalize: | ||
return torch.stack( | ||
torch.meshgrid( | ||
torch.linspace(-1, 1, w, device=device), | ||
torch.linspace(-1, 1, h, device=device), | ||
) | ||
) | ||
return torch.stack( | ||
torch.meshgrid( | ||
torch.arange(0, w, device=device), | ||
torch.arange(0, h, device=device), | ||
) | ||
) | ||
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def forward(self, x): | ||
assert x.ndim == 4, "Expecting a tensor of shape (B, C, H, W)." | ||
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# compute a spatial softmax over the input: | ||
# given an input of shape (B, C, H, W), | ||
# reshape it to (B*C, H*W) then apply | ||
# the softmax operator over the last dimension | ||
b, c, h, w = x.shape | ||
softmax = F.softmax(x.reshape(-1, h * w), dim=-1) | ||
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# create a meshgrid of pixel coordinates | ||
# both in the x and y axes | ||
xc, yc = self._coord_grid(h, w, x.device) | ||
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# element-wise multiply the x and y coordinates | ||
# with the softmax, then sum over the h*w dimension | ||
# this effectively computes the weighted mean of x | ||
# and y locations | ||
x_mean = (softmax * xc.flatten()).sum(dim=1, keepdims=True) | ||
y_mean = (softmax * yc.flatten()).sum(dim=1, keepdims=True) | ||
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# concatenate and reshape the result | ||
# to (B, C*2) where for every feature | ||
# we have the expected x and y pixel | ||
# locations | ||
return torch.cat([x_mean, y_mean], dim=1).view(-1, c * 2) | ||
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if __name__ == "__main__": | ||
b, c, h, w = 32, 64, 12, 12 | ||
x = torch.zeros(b, c, h, w) | ||
true_max = torch.randint(0, 10, size=(b, c, 2)) | ||
for i in range(b): | ||
for j in range(c): | ||
x[i, j, true_max[i, j, 0], true_max[i, j, 1]] = 1000 | ||
soft_max = SpatialSoftArgmax()(x).reshape(b, c, 2) | ||
assert torch.allclose(true_max.float(), soft_max) |
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