Convolutions

A convolution slides a small filter over an image, computing a weighted sum at each spot to detect a pattern.

• Early filters find edges; later ones find shapes and objects.
• Output size = (input − filter + 2 × padding) ÷ stride + 1.
• The same filter is reused everywhere, so CNNs need few weights.

size, f, pad, stride = 28, 3, 0, 1
print((size - f + 2 * pad) // stride + 1)

26

Remember: Filter count = how many patterns a layer looks for.

Common mistake: Forgetting the input channels when counting weights: 3×3 on RGB is 27 per filter.