A question about convlstm_pytorch

[DavideA]

Sure!

It's a common trick when dealing with the LSTM family.

1. As convolution is a linear operation, you can merge the two convolutions for each gate (horizontally, by reading the equations) by concatenating x_t and h_{t-1} and provide the concatenated tensor as input to a single convolution. This leaves us with 4 convolutions. Now,
2. Since each output channel is computed independently (and each convolution acts on the same input tensor, i.e. the concatenated tensor of point 1), you can actually do a single convolution and split channels afterward. This mechanic becomes clearer by looking at the output channels of the conv (they are 4 * self.hidden_dim) and the line in which they are split:

cc_i, cc_f, cc_o, cc_g = torch.split(combined_conv, self.hidden_dim, dim=1) 

Does this make sense?

D

[Melika-Ayoughi]

No it's not. I don't have problem with the order.
what u are doing is:
i_t = sigmoid(W *(x_t, h_{t-1}, c_{t-1}))
f_t = sigmoid(W *(x_t, h_{t-1}, c_{t-1}))

So first of all W is the same for all lines, so instead of having W_xi, W_xf, W_hi, W_hf, ... u have one W.
second of all the products are not modelled (for instance W_{ci} .* c_{t-1})

[DavideA]

Hi @Melika-Ayoughi,

and thank you for your interest.

I'm not sure I understood your concern.
Is it possibly a duplicate of this issue?

Please forgive me if it's not.

D

[DavideA]

Ok, I think I see your point now.

This repo implements the following dynamic:

Which is significantly different from the one you reported. Thank you for pointing it out, we should mention it in the README.

D

[Melika-Ayoughi]

great! I still don't understand how your implementation has all these weights and not just one weight. can u explain that? because u only convolve x_t and w once. so I assumed there would only be one w and not 8!

[NickLucche]

I have a question regarding your implementation:
As I understood the original convolutional lstm formulation is as follows:

But in your implementation, u used only one convolution layer. I don't understand how these 2 correspond with each other. because in the formulation, c is only used in the Hadamard product and not in convolutions, but here c and h are both used in convolutions.
in fact, all weights are shared for all 4 formulas, although there are 11 weights in the original formula.

The reported formulation of the ConvLSTM was first introduced in this paper (I believe): https://arxiv.org/pdf/1506.04214.pdf.
The real difference between this implementation and the reported formulation lies in the fact that the paper 'extends' a variant of the LSTM, where each gates gets a contribution from the past cell state.

[Weiming-Hu]

Hi, I'm curious are there any particular reasons for implementing this variant of the LSTM model? Rather than the original one from the paper https://arxiv.org/pdf/1506.04214.pdf? I'm specifically asking about using $c_{t-1}$ for $o_t$ rather than in the paper using $c_t$.

Thank you!