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# How do I calculate the partial derivative with respect to \$x\$?

I am trying to implement CNN using python Numpy.

I searched so much, but all I found was for one filter with one channel for Convolution.

Suppose we have an X as Image with this shape: `(N_Height, N_Width, N_Channel) = (5,5,3)`

And Let’s say I have `16` filters with this shape: `(F_Height, F_Width, N_Channel) = (3,3,3)` , `stride=1` and `padding=0`

Forward:

Output shape after conv2d will be

`(`
`math.floor((N_Height - F_Height + 2*padding)/stride + 1 )),`
`math.floor((N_Width- F_Width + 2*padding)/stride + 1 )),`
`filter_count`
`)`

So the output of this layer will be an array with this shape: `(Height, Width, Channel) = (3, 3, 16)`

BackPropagation:

Suppose $$dL/dh$$ is the input for my layer in backpropagation with this shape: `(3,3,16)`

Now I must find $$dL/dw$$ and $$dL/dx$$: $$dL/dw$$ to update my filters params and $$dL/dx$$ to pass it as input to the previous layer as Loss respect to the input X.

From this answer Error respect to filters weights I found how to calculate $$dL/dw$$.

The problem I have in BackPropagation is I don’t know how to calculate $$dL/dx$$ having this shape:`(5,5,3)` and pass it to the prev layer.

I read lots of articles in Medium and other sites but I don’t get how to calculate it:

Thanks in advance 🙂