Setting custom kernel for CNN in pytorch

Is there a way to specify our own custom kernel values for a convolution neural network in pytorch? Something like kernel_initialiser in tensorflow? Eg. I want a 3x3 kernel in nn.Conv2d with initialization so that it acts as a identity kernel -

0 0 0
0 1 0
0 0 0

(this will effectively return the same output as my input in the very first iteration)

My non-exhaustive research on the subject -

I could use nn.init but it only has some pre-defined kernel initialisaition values.

I tried to follow the discussion on this thread but it doesn’t suit my needs.

I might have missed something in my research please feel free to point out.

I have asked the same on SO here but couldn’t find any answer.

4 Likes

I think the easiest way would be to use the functional API.
You would have to define the weights and use F.conv2d to apply the convolution.
Here is a small example:

nb_channels = 1
h, w = 5, 5
x = torch.randn(1, nb_channels, h, w)
weights = torch.tensor([[0., 0., 0.],
                        [0., 1., 0.],
                        [0., 0., 0.]])
weights = weights.view(1, 1, 3, 3).repeat(1, nb_channels, 1, 1)

output = F.conv2d(x, weights)
4 Likes

Thank you for your response.

I already know about F.conv2d but I wanted to use kernels not just for convolution but for CNN (nn.Conv2d) where learning of weights take place.

I don’t think F.conv2d will help. :frowning:

You would need to set requires_grad=True for the weights and it would also work as nn.Conv2d internally just calls the functional API, see here. :wink:

However, if you prefer to use the module, you could try the following code:

weights = ...
conv = nn.Conv2d(nb_channels, 1, 3, bias=False)
with torch.no_grad():
    conv.weight = nn.Parameter(weights)

output = conv(x)
output.mean().backward()
print(conv.weight.grad)
7 Likes

That makes sense, thank you. Let me try it out. :slight_smile:

Hi,
I want to cast the data once they completed dot product before addition in f.conv2d. Can you please give me any suggestion ?

As far as I understand you would like to split the convolution operation and add a custom op before the summation.
If so, I think you would need to implement the conv op manually using e.g. unfold.

How can I see the algorithmic implementation of convolution using pytorch?

aten/src/ATen/native/Convolution.cpp might be a good starting point to see which algorithms are being dispatched.

So, how does it work when I have multiple channels?
For example, the below code will output tensors of shape (B, 2, W, H). Instead, I would like the two kernels to be each applied to each channel with output image (B, 2C, W, H)

def get_grad_kernel(channels):
  Iy = [[ 1, 2, 1],
        [ 0, 0, 0],
        [-1,-2,-1]]
  Ix = [[-1, 0, 1],
        [-2, 0, 2],
        [-1, 0, 1]]
  return torch.Tensor([ [Ix]*channels, [Iy]*channels ])/4

grad_kernel = get_grad_kernel(channels=3)
compute_gradient = lambda image: F.conv2d(input=image, weight=grad_kernel, padding=1)

EDIT: From ptrblck’s reply, making the kernel as the following

  return torch.Tensor([ [Ix], [Iy] ]*channels)/4

and setting groups=image.size(1) works nicely

Setting groups=in_channels might work for your use case.

1 Like

Thanks for your informative answers.
I am struggling to add more than one weight (let’s say 2 kernels 3x3) in conv2d() to get more than one output at the same time (nn.Conv2d(nb_channels, 2, 3, bias=False).
Could you please help me out with this

The number of kernels is defined in dim0 in the weight matrix and you could use this modified code for it:

nb_channels = 1
h, w = 5, 5
x = torch.randn(1, nb_channels, h, w)
weights = torch.tensor([[[0., 0., 0.],
                         [0., 1., 0.],
                         [0., 0., 0.]],
                        [[0., 0., 0.],
                         [0., 1., 0.],
                         [0., 0., 0.]]])
weights = weights.view(2, 1, 3, 3)

output = F.conv2d(x, weights)

Let me know, if this would work for you or if you get stuck somewhere. :slight_smile:

2 Likes

Thanks a lot for your answer.
This works out for me but I want to implement the discussed Conv2D weights into this simple network:

class CNN(nn.Module):

def __init__(self):
    super(CNN, self).__init__()
    self.features =nn.Conv2d(1, 2, kernel_size=3, stride=1, padding=0)
                           nn.ReLU(),
                           nn.MaxPool2d(kernel_size=2, stride=1)
                                 )
    self.drop_out = torch.nn.Dropout(0.6)

def forward(self, x):
    x=self.features(x)
    x=self.drop_out(x)
    return x

cnn=CNN()

I would be appreciated if give me a hint on this as well
Thanks

You can assign a new weight parameter to self.features via:

self.features = nn.Conv2d(...)
with torch.no_grad():
    weights = torch.tensor(...)
    self.features.weight = nn.Parameter(weights)
2 Likes

Thank you for your answer

Is there a way to adapt this approach so that rather than performing convolutions using different kernels per channel, it performs convolutions on all channels for a tensor in a batch but the kernel for each tensor in the batch changes? So each batch item undergoes a convolution with a unique prespecified kernel.

e.g. something like this:

import torch 

batch_size = 8
channels = 10
img_size = 30
kernel_size = 3

batch = torch.rand((batch_size,channels,img_size,img_size))

# Make a unique kernel for each batch member but the kernel is convolved 
# with every channel 
weights = torch.rand((batch_size,1,kernel_size,kernel_size)).repeat(1,channels,1,1)
print(weights.shape)

conv = torch.nn.Conv2d(channels,channels,kernel_size,padding=4,bias=False)

with torch.no_grad():
    conv.weight = torch.nn.Parameter(weights,requires_grad=False)

output = conv(batch)
print(output.shape)

Edit: This has been solved using for loops or groups here

Hello @ptrblck I try this code but I got error "Illegal instruction (core dumped)
"

I’m not sure which code you were trying. Could you post more information about your use case, setup (installed PyTorch version, CUDA etc.) and could you also try to get a backtrace from gdb?