The problem of inconsistent effects between torchsparse's conv3d downsampling and minkowski's

[Jovendish]

I'm using torchsparse's conv3d to do a downsampling operation with stride 2, but found that this operation not only reduces the size of the feature tensor, but also the coordinates, which is inconsistent with minkowski's performance. I was hoping to find a way to make torchsparse's conv3d's downsampling operation consistent with minkowski.

I checked the documentation of torchsparse but didn't find a relevant solution.

Is there any other parameter setting or custom operation method that can make torchsparse's conv3d downsampling operation consistent with minkowski? If you have any suggestions or guidance I would be very grateful.

Details

![WechatIMG11](https://github.com/mit-han-lab/torchsparse/assets/25397930/d9d041e8-1903-
4308-847c-b3987b67c739)

[zhijian-liu]

When applying a downsampling operation with a stride of 2, the coordinates are effectively halved. If you wish to maintain the original coordinate scale, you can easily achieve this by multiplying the coordinates by 2.

[Jovendish]

thanks for your reply.

In my code, I have three layers of downsampling operations. I have tried to scale the coordinates back to their original scale by multiplying them by two after each downsampling layer. However, I have noticed that this operation only works for the first downsampling layer, as the subsequent downsampling layers yield the same results. I'm unsure if I made a mistake in my implementation or if I have encountered some specific mechanism in torchsparse.

Partial code

`

F.set_conv_mode(2)
F.set_kmap_mode('hashmap')
F.set_downsample_mode('minkowski')

class Encoder(torch.nn.Module):
    def __init__(self, channels=[1, 16, 32, 64, 32, 8]):
        super().__init__()

        self.stack_0 = nn.Sequential(
            spnn.Conv3d(channels[0], channels[1], 3, 1, bias=True),
            spnn.ReLU(inplace=True),
            spnn.Conv3d(channels[1], channels[2], 2, 2, bias=True),  # DownScale
            spnn.ReLU(inplace=True),
        )

        self.stack_1 = nn.Sequential(
            spnn.Conv3d(channels[2], channels[2], 3, 1, bias=True),
            spnn.ReLU(inplace=True),
            spnn.Conv3d(channels[2], channels[3], 2, 2, bias=True),  # DownScale
            spnn.ReLU(inplace=True),
        )

        self.stack_2 = nn.Sequential(
            spnn.Conv3d(channels[3], channels[3], 3, 1, bias=True),
            spnn.ReLU(inplace=True),
            spnn.Conv3d(channels[3], channels[4], 2, 2, bias=True), # DownScale
            spnn.ReLU(inplace=True),
        )

    def forward(self, x):
        out_0 = self.stack_0(x)
        out_0.C[:, 1:] *= 2

        out_1 = self.stack_1(out_0)
        out_1.C[:, 1:] *= 2

        out_2 = self.stack_2(out_1)
        out_2.C[:, 1:] *= 2

        return [out_2, out_1, out_0]

`

Result

[ys-2020]

Hi @Jovendish , in the 2nd and 3rd layers, you are downsampling by stride=2 with the coordinates that has been multiplied by 2. Thus, the number of points will remain the same as the previous layers.

A potential solution might be:

def forward(self, x):
        out_0 = self.stack_0(x)
        out_1 = self.stack_1(out_0)
        out_2 = self.stack_2(out_1)

        out_0.C[:, 1:] *= 2
        out_1.C[:, 1:] *= 4
        out_2.C[:, 1:] *= 8

        return [out_2, out_1, out_0]

[Jovendish]

Thank you very much for your patience. But actually, I want to be able to scale back in the middle of each layer, because I need to do some extra work in the middle of each layer, and I am wondering why torchsparsev2.1 changed the behavior of the downsampling layer，are there any considerations ?

[zhijian-liu]

You can follow @ys-2020 's approach to clone the coordinate tensor and do the scaling in the middle. We change this behavior to follow SpConv.