CNN Geometry: What Shape Reaches the Next Layer?

Fri, 28 Aug 2026 10:15:00 +0100

Here is a batch of 32 RGB images and an ordinary first convolution.

x = torch.randn(32, 224, 224, 3)
conv = nn.Conv2d(in_channels=3, out_channels=16, kernel_size=3)
conv(x)

RuntimeError: Given groups=1, weight of size [16, 3, 3, 3],
expected input[32, 224, 224, 3] to have 3 channels, but got 224 channels instead

The images have three channels. PyTorch says there are 224. The message names a number that appears nowhere in the model definition, so the obvious reading is that the layer was declared wrong, and the obvious repair is to declare it right:

Adaptive Pooling on Programmer.ie

CNN Geometry: What Shape Reaches the Next Layer?