How to assign a image like tensor with indexes

The target tensor is X (B, H, W, C). The index tensor is I (B, N, 2). And the source tensor is S (B, N, C).
So how can I implement X[i, I[i,j,0], I[i,j,1]] = S[i, j] efficiently and parallelly?

I tried to use for loop to assign X. But it took too much time in loss.backward().

This code might work:

B, H, W, C = 2, 3, 3, 4
N = 5
x = torch.randn(B, H, W, C)
I = torch.randint(0, H, (B, N, 2))
S = torch.ones(B, N, C)

x[torch.arange(B).unsqueeze(1),
  I[torch.arange(B), :, 0],
  I[torch.arange(B), :, 1]] = S

Could you please double check it using your manual approach with the for loop?

PS: Please don’t create double posts, as other users might answer the same question :wink:

1 Like

It’s great.

I have tried to change the index matrix I to a new matrix with size (B*N,3) including the batch index.
And this code works:

x[I[:,0], I[:,1], I[:2]] = S

However, your implementation is faster than mine.

And I have another question. If some indexes are repeating in I, which value in S would replace the value in x? Is the latter one in I?

PS: This is the first time I post the problem on the forum. I’m sorry for my misoperation. Could you please delete another question? Thanks.

On the CPU it might be the last value. I think this behavior is undefined on the GPU.

Sure, we’ll delete the other post. :wink: