MaxUnpool2d takes in as input the output of MaxPool2d including the indices of the maximal values and computes a partial inverse in which all non-maximal values are set to zero.
Note
MaxPool2d can map several input sizes to the same output sizes. Hence, the inversion process can get ambiguous. To accommodate this, you can provide the needed output size as an additional argument output_size in the forward call. See the Inputs and Example below.
>>> pool = nn.MaxPool2d(2, stride=2, return_indices=True)
>>> unpool = nn.MaxUnpool2d(2, stride=2)
>>> input = torch.tensor([[[[ 1., 2., 3., 4.],
[ 5., 6., 7., 8.],
[ 9., 10., 11., 12.],
[13., 14., 15., 16.]]]])
>>> output, indices = pool(input)
>>> unpool(output, indices)
tensor([[[[ 0., 0., 0., 0.],
[ 0., 6., 0., 8.],
[ 0., 0., 0., 0.],
[ 0., 14., 0., 16.]]]])
>>> # Now using output_size to resolve an ambiguous size for the inverse
>>> input = torch.tensor([[[[ 1., 2., 3., 4., 5.],
[ 6., 7., 8., 9., 10.],
[11., 12., 13., 14., 15.],
[16., 17., 18., 19., 20.]]]])
>>> output, indices = pool(input)
>>> # This call will not work without specifying output_size
>>> unpool(output, indices, output_size=input.size())
tensor([[[[ 0., 0., 0., 0., 0.],
[ 0., 7., 0., 9., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 17., 0., 19., 0.]]]])
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