# Stuck in creating custom Pooling layer in Pytorch

Hello everyone,
I am studying about quaternion convolutional neural network following this repo on github: https://github.com/Orkis-Research/Pytorch-Quaternion-Neural-Networks

The repo’s work is great but i want to implement a max amplitude pooling layer to utilize the quaternion network.

The basic idea follows this paper: https://ieeexplore.ieee.org/document/8632910
The pooling will take 4 input layer, compute the amplitude (length) then apply a max pooling. The torch.max function return pooled result and indices for max values.

My question is how to apply these indices to the input layer to get pooled results.

In my custom pooling layer, indices can be a list or a tensor with shape (n, c, h, w); input has shape (n, 4c, h, w) and amplitude has shape (n, c, h, w)

This is my forward function:

def forward(self, x):

``````    # x = F.pad(x, self._padding(x), mode='constant')

amplitude = get_modulus(x, vector_form=True)

input_size = x.size()

amp_size = amplitude.size()

x_r = get_r(x)

x_i = get_i(x)

x_j = get_j(x)

x_k = get_k(x)

amp = amplitude.unfold(2, self.k, self.stride).unfold(3, self.k, self.stride)

print("amp unfold size: ", amp.size())

x_r = x_r.unfold(2, self.k, self.stride).unfold(3, self.k, self.stride)

x_i = x_i.unfold(2, self.k, self.stride).unfold(3, self.k, self.stride)

x_j = x_j.unfold(2, self.k, self.stride).unfold(3, self.k, self.stride)

x_k = x_k.unfold(2, self.k, self.stride).unfold(3, self.k, self.stride)

"""

tensor.unfold(axis, size, step)

Returns a view of the original tensor which contains all slices of size size from self tensor in the dimension dimension.

Step between two slices is given by step.

"""

amp = amp.contiguous().view(amp.size()[:4] + (-1,))

print("amp size after view: ",amp.size())

x_r = x_r.contiguous().view(x_r.size()[:4] + (-1,))

x_i = x_i.contiguous().view(x_i.size()[:4] + (-1,))

x_j = x_j.contiguous().view(x_j.size()[:4] + (-1,))

x_k = x_k.contiguous().view(x_k.size()[:4] + (-1,))

"""

tensor.contiguous() => create a copy of tensor

"""

amp_max, indice = torch.max(amp, dim = -1)

indice = torch.squeeze(indice).tolist()

# print(indice)

"""

initialize output channel

"""

# x_r_out = torch.zeros_like(x_r.mean(4))

# x_i_out = torch.zeros_like(x_r.mean(4))

# x_j_out = torch.zeros_like(x_r.mean(4))

# x_k_out = torch.zeros_like(x_r.mean(4))

"""

apply max amplitude position => max amplitude pooling

"""

# x_r_out

# x_i_out

# x_j_out

# x_k_out

print('size of input: ', input_size)

print('size of amplitude: ', amp_size)

print('size of pooling: ', amp_max.size())

I’d imagine that it is something along the lines of

``````qs = torch.randn(1, 4, 6, 6) # batch 1, 4 channels (1 quaternion) and 6x6 matrix, bs = 1
bs, _, h, w = qs.shape
amp = torch.norm(qs, dim=1, keepdim=True)
amp_uf = amp.unfold(3, 2, 2).unfold(2, 2, 2).reshape(bs, 1, h // 2, w // 2, 2 * 2)
qs_uf = qs.unfold(3, 2, 2).unfold(2, 2, 2).reshape(bs, 4, h // 2, w // 2, 2 * 2)
maxamp = amp_uf.argmax(-1, keepdim=True).expand(-1, 4, -1, -1, -1)
qs_pooled = qs_uf.gather(-1, maxamp).view(bs, 4, h // 2, w // 2)
``````

(The gather gets goes over the dimensions except the last and then picks the value at the maxamp index in the last dimension.)

Best regards

Thomas

2 Likes

That’s amazing! Thank you so much for a clever solution!

Thanks much, Thomas.
It is really amazing. I wonder if you can help to come back from qs_pooled to qs (I mean upsampling or MaxUnpool2d PyTorch equivalent)
Regards
Lyes