Pytorch maxpooling over channels dimension

Question

I was trying to build a cnn to with Pytorch, and had difficulty in maxpooling. I have taken the cs231n held by Stanford. As I recalled, maxpooling can be used as a dimensional deduction step, for example, I have this (1, 20, height, width) input ot max_pool2d (assuming my batch_size is 1). And if I use (1, 1) kernel, I want to get output like this: (1, 1, height, width), which means the kernel should be slide over the channel dimension. However, after checking the pytorch docs, it says the kernel slides over height and width. And thanks to @ImgPrcSng on Pytorch forum who told me to use max_pool3d, and it turned out worked well. But there is still a reshape operation between the output of the conv2d layer and the input of the max_pool3d layer. So it is hard to be aggregated into a nn.Sequential, so I wonder is there another way to do this?

Answer 1

Would something like this work?

from torch.nn import MaxPool1d
import torch.nn.functional as F


class ChannelPool(MaxPool1d):
    def forward(self, input):
        n, c, w, h = input.size()
        input = input.view(n, c, w * h).permute(0, 2, 1)
        pooled = F.max_pool1d(
            input,
            self.kernel_size,
            self.stride,
            self.padding,
            self.dilation,
            self.ceil_mode,
            self.return_indices,
        )
        _, _, c = pooled.size()
        pooled = pooled.permute(0, 2, 1)
        return pooled.view(n, c, w, h)

Or, using einops

from torch.nn import MaxPool1d
import torch.nn.functional as F
from einops import rearrange


class ChannelPool(MaxPool1d):
    def forward(self, input):
        n, c, w, h = input.size()
        pool = lambda x: F.max_pool1d(
            x,
            self.kernel_size,
            self.stride,
            self.padding,
            self.dilation,
            self.ceil_mode,
            self.return_indices,
        )
        return rearrange(
            pool(rearrange(input, "n c w h -> n (w h) c")),
            "n (w h) c -> n c w h",
            n=n,
            w=w,
            h=h,
        )

Answer 2

To max-pool in each coordinate over all channels, simply use layer from einops

from einops.layers.torch import Reduce

max_pooling_layer = Reduce('b c h w -> b 1 h w', 'max')

Layer can be used in your model as any other torch module

Answer 3

I'm not sure why the other answers are so complicated. Max pooling over the whole channels dimension to get an output with only 1 channel sounds equivalent to just taking the maximum value over that dimension:

torch.amax(left_images, dim=1, keepdim=True)

Note: If your goal is just to perform dimensional reduction, generally people use 1x1 conv layers with large to small number of channels rather than maxpooling over the channels dimension.