1D Convolution Example
Unlike Digital Signal Processing, 1D convolution in deep learning does not flip the kernel — it is simply cross-correlation.
Input $x = [1, 2, 3, 4]$ (length $N=4$), kernel $h = [1, 0, -1]$ (length $K=3$), stride $S=1$.
Output length: $\lfloor (N - K) / S \rfloor + 1 = (4 - 3)/1 + 1 = 2$
Output values:
\[y_1 = 1{\cdot}1 + 2{\cdot}0 + 3{\cdot}(-1) = -2\] \[y_2 = 2{\cdot}1 + 3{\cdot}0 + 4{\cdot}(-1) = -2\] \[y = [-2,\; -2]\]2D Convolution Example
Assume input is 4x4x2
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# Channel 1
[ 1 5 9 13
2 6 10 14
3 7 11 15
4 8 12 16 ]
# Channel 2
[ 1 1 1 1
2 2 2 2
3 3 3 3
4 4 4 4 ]
We want to do convolution with
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conv = nn.Conv2d(
in_channels=2,
out_channels=3,
kernel_size=2,
stride=1,
padding=0,
bias=False
)
internally, the kernel tensor is weight shape = (out_channels = 3, in_channels = 2, kernel_size = 2, kernel_size = 2)
The kernel tensor has shape (out_channels=3, in_channels=2, kH=2, kW=2):
-
Output channel 1
Channel 1 Weights Channel 2 Weights Row 0 [1 0][1 1]Row 1 [0 1][1 1] -
Output channel 2
Channel 1 Weights Channel 2 Weights Row 0 [0 1][0 0]Row 1 [1 0][0 1] -
Output channel 3
Channel 1 Weights Channel 2 Weights Row 0 [ 1 1][-1 -1]Row 1 [ 1 1][-1 -1]
To convolve (2×2 kernel, stride 1, no padding):
- Slide the 2×2 kernel spatially over each input channel.
- For each spatial location:
- Multiply elementwise with the input patch.
- Sum the 4 products → one scalar per input channel.
- Sum the scalars across all input channels → one output value.
Output channel 1
Step 1 — convolve input channel 1 (A) with kernel [[1,0],[0,1]]
Top-left patch example:
\[\begin{bmatrix}1 & 5\\2 & 6\end{bmatrix} \odot \begin{bmatrix}1 & 0\\0 & 1\end{bmatrix} = 1{\cdot}1 + 5{\cdot}0 + 2{\cdot}0 + 6{\cdot}1 = 7\]Full result (stride 1 across the 4×4 input → 3×3 output):
\[A * K_{1}^{(1)} = \begin{bmatrix}7 & 15 & 23\\9 & 17 & 25\\11 & 19 & 27\end{bmatrix}\]Step 2 — convolve input channel 2 (B) with kernel [[1,1],[1,1]] (sum of patch):
Step 3 — sum over input channels:
\[Y_1 = (A * K_{1}^{(1)}) + (B * K_{1}^{(2)}) = \begin{bmatrix}13 & 21 & 29\\19 & 27 & 35\\25 & 33 & 41\end{bmatrix}\]Output channel 2
A with [[0,1],[1,0]] (anti-diagonal):
B with [[0,0],[0,1]] (bottom-right only):
Output channel 3
A with [[1,1],[1,1]] (sum of patch):
B with [[-1,-1],[-1,-1]] (negative sum of patch):
Final output
Stacking $Y_1, Y_2, Y_3$ gives a tensor of shape 3×3×3 (height × width × out_channels).
Shape formula: spatial output = $\lfloor (N - K) / S \rfloor + 1 = (4 - 2)/1 + 1 = 3$, so output is $3 \times 3 \times 3$.
1x1 Convolution Example
A 1×1 convolution acts as a per-pixel learned linear combination across channels — no spatial mixing, just channel mixing at every spatial location independently.
Assume input shape [C_in=2, H=2, W=2]:
Use a 1×1 convolution with C_out=3. Weight tensor shape: (3, 2, 1, 1).
Now we define the kernel, where each per-output-channel weight vector $[w_1,\, w_2]$ is:
| Output channel | $w_1$ (ch 1) | $w_2$ (ch 2) |
|---|---|---|
| #1 | 1 | 0 |
| #2 | 0 | 1 |
| #3 | 1 | −1 |
Each output channel is $Y_k = w_1 \cdot A + w_2 \cdot B$:
\[Y_1 = 1\cdot A + 0\cdot B = \begin{bmatrix}1 & 2\\3 & 4\end{bmatrix}\] \[Y_2 = 0\cdot A + 1\cdot B = \begin{bmatrix}10 & 20\\30 & 40\end{bmatrix}\] \[Y_3 = 1\cdot A + (-1)\cdot B = \begin{bmatrix}1-10 & 2-20\\3-30 & 4-40\end{bmatrix} = \begin{bmatrix}-9 & -18\\-27 & -36\end{bmatrix}\]Final output shape: [C_out=3, H=2, W=2] — the spatial dimensions are unchanged.
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nn.Conv2d(in_channels=2, out_channels=3, kernel_size=1)
