6.3.2 Convolution Basics

Section Overview

Convolution is how CNNs look at images without flattening away spatial structure. This page starts with a picture, then computes a convolution by hand, then verifies the same ideas with nn.Conv2d.

Learning Objectives

• Explain why flattening an image too early is wasteful.
• Compute one convolution output value by hand.
• Understand kernel, stride, padding, channel, and feature map.
• Verify output shapes with PyTorch.
• Explain why stacking convolutions grows the receptive field.

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Look at the Sliding Window First

Read the picture like this:

small window -> multiply by kernel -> sum -> one output value -> slide and repeat

A convolution kernel is a small pattern detector. It does not look at the whole image at once. It scans local regions and writes a score into a feature map.

Why Not Flatten the Image First?

A 32 x 32 grayscale image has 1024 pixels. A fully connected layer with 512 outputs would need:

1024 * 512 = 524288 weights

A 224 x 224 x 3 color image has 150528 input values. A naive fully connected layer explodes in parameters and ignores where pixels are located.

Convolution fixes two problems:

Problem with early flattening	Convolution idea
nearby pixels lose their spatial relationship	look at local windows
every position needs separate weights	reuse the same kernel everywhere
parameter count grows quickly	share parameters across the image

The two core terms are:

• local connection: each output looks at a small area;
• parameter sharing: the same kernel scans many positions.

Lab 1: Compute Convolution by Hand

import numpy as np

image = np.array(
    [
        [1, 2, 0, 0],
        [5, 3, 0, 4],
        [2, 1, 3, 1],
        [0, 2, 1, 2],
    ],
    dtype=np.float32,
)

kernel = np.array(
    [
        [1, 0],
        [0, -1],
    ],
    dtype=np.float32,
)

out = np.zeros((3, 3), dtype=np.float32)
for i in range(3):
    for j in range(3):
        patch = image[i : i + 2, j : j + 2]
        out[i, j] = np.sum(patch * kernel)

print("manual_conv_lab")
print(out)

Expected output:

manual_conv_lab
[[-2.  2. -4.]
 [ 4.  0. -1.]
 [ 0.  0.  1.]]

Top-left output value:

patch = [[1, 2],
         [5, 3]]

kernel = [[ 1,  0],
          [ 0, -1]]

score = 1*1 + 2*0 + 5*0 + 3*(-1) = -2

That is the whole core of convolution.

Lab 2: Use a Kernel as an Edge Detector

This horizontal kernel compares neighboring pixels from left to right.

import numpy as np

image = np.array(
    [
        [0, 0, 0, 0, 0],
        [0, 0, 1, 1, 1],
        [0, 0, 1, 1, 1],
        [0, 0, 1, 1, 1],
        [0, 0, 0, 0, 0],
    ],
    dtype=np.float32,
)

kernel = np.array([[-1, 1]], dtype=np.float32)

out = np.zeros((5, 4), dtype=np.float32)
for i in range(5):
    for j in range(4):
        patch = image[i : i + 1, j : j + 2]
        out[i, j] = np.sum(patch * kernel)

print("edge_lab")
print(out)

Expected output:

edge_lab
[[0. 0. 0. 0.]
 [0. 1. 0. 0.]
 [0. 1. 0. 0.]
 [0. 1. 0. 0.]
 [0. 0. 0. 0.]]

The 1 values appear where the image changes from 0 to 1. That is why early CNN layers often learn edge-like filters.

Stride, Padding, and Output Size

Term	Meaning	Effect
kernel_size	window size	larger kernel sees more local area
stride	how far the kernel moves each step	larger stride makes output smaller
padding	border added around input	preserves edge information and controls size

Output size for one spatial dimension:

output = floor((input + 2*padding - kernel_size) / stride) + 1

Example:

input=6, kernel_size=3, padding=1, stride=2
output = floor((6 + 2*1 - 3) / 2) + 1 = 3

Verify in PyTorch:

import torch
from torch import nn

x = torch.randn(1, 1, 6, 6)
conv = nn.Conv2d(
    in_channels=1,
    out_channels=2,
    kernel_size=3,
    stride=2,
    padding=1,
)
y = conv(x)

print("size_lab")
print("input:", tuple(x.shape))
print("output:", tuple(y.shape))

Expected output:

size_lab
input: (1, 1, 6, 6)
output: (1, 2, 3, 3)

Read the shape as [batch, channels, height, width].

Multi-Channel Convolution

Color images have three input channels: red, green, and blue. In PyTorch, a batch of RGB images usually has shape:

[batch, 3, height, width]

A 3 x 3 convolution over an RGB image actually has kernel shape:

[out_channels, in_channels, kernel_height, kernel_width]

Run it:

import torch
from torch import nn

x = torch.randn(2, 3, 32, 32)
conv = nn.Conv2d(in_channels=3, out_channels=8, kernel_size=3, padding=1)
y = conv(x)

print("channel_lab")
print("input:", tuple(x.shape))
print("output:", tuple(y.shape))
print("weight:", tuple(conv.weight.shape))
print("bias:", tuple(conv.bias.shape))

Expected output:

channel_lab
input: (2, 3, 32, 32)
output: (2, 8, 32, 32)
weight: (8, 3, 3, 3)
bias: (8,)

Interpretation:

• 2: two images in the batch;
• 3: RGB input channels;
• 8: eight learned output feature maps;
• (8, 3, 3, 3): eight kernels, each looking across three input channels.

Receptive Field: How CNNs See More Over Depth

One 3 x 3 convolution sees a small local region. If you stack layers, later features indirectly depend on larger regions of the original image.

Intuition:

Layer depth	What it often learns
shallow	edges, color changes, textures
middle	corners, simple shapes, parts
deep	larger object parts and semantic patterns

This hierarchy is why CNNs work well for images: small local clues can be composed into larger visual ideas.

Basic Conv2d Checklist

import torch
from torch import nn

x = torch.randn(1, 1, 8, 8)
conv = nn.Conv2d(
    in_channels=1,
    out_channels=4,
    kernel_size=3,
    stride=1,
    padding=1,
)
y = conv(x)

print("conv2d_lab")
print("input:", tuple(x.shape))
print("output:", tuple(y.shape))
print("weight:", tuple(conv.weight.shape))
print("bias:", tuple(conv.bias.shape))

Expected output:

conv2d_lab
input: (1, 1, 8, 8)
output: (1, 4, 8, 8)
weight: (4, 1, 3, 3)
bias: (4,)

When you read any Conv2d, ask:

1. What is the input shape [N, C, H, W]?
2. Does in_channels equal the input C?
3. How many feature maps does out_channels create?
4. How do kernel_size, stride, and padding change H and W?

Common Mistakes

Mistake	Why it hurts	Fix
using image shape [H, W, C] in PyTorch	PyTorch expects [N, C, H, W]	use permute when converting from image libraries
wrong in_channels	Conv2d cannot match the input	print x.shape before the layer
forgetting padding	feature maps shrink unexpectedly	calculate output size or print shapes
treating convolution as magic	hard to debug features	remember patch * kernel -> sum
flattening too early	spatial structure is lost	use conv blocks before classifier head

Exercises

1. Change the hand-written 2 x 2 kernel and observe how the output changes.
2. Manually compute out[1, 0] in Lab 1 and compare with the printed output.
3. Change stride=1 in the size lab. What output shape do you get?
4. Change out_channels=16 in the channel lab. Which shapes change?
5. Convert an image-like tensor from [N, H, W, C] to [N, C, H, W] with permute.

Key Takeaways

• Convolution preserves local spatial structure better than early flattening.
• A kernel is a small pattern detector shared across positions.
• stride and padding control how the kernel moves and how output size changes.
• Multi-channel convolution combines information across input channels.
• Stacked convolution layers grow receptive field and build visual hierarchy.