Understanding Convolutional Neural Networks (CNNs): A Beginner's Guide

May 16, 2024 by
Understanding Convolutional Neural Networks (CNNs): A Beginner's Guide
DxTalks, Ibrahim Kazeem

Have you ever scrolled through social media and seen a friend tagged in a photo by facial recognition software? Or marveled at the image recognition abilities of your smartphone camera. These feats are powered by Convolutional Neural Networks (CNNs). 

CNNs are a type of artificial intelligence that excels at analyzing visual data, such as images and videos. They are like sophisticated image scanners that can learn to identify patterns and features within them. 

In this guide, we explain how CNNs work step-by-step without getting frustrated with complex math and algorithms.   By the end, you'll clearly understand how CNNs power amazing technologies like facial recognition and self-driving cars. Let's go!

What are Convolutional Neural Networks?

 A convolutional neural network (CNN) is a type of artificial neural network specifically designed to process and analyze visual data, such as images and videos. CNNs are inspired by the human visual cortex and have proven remarkably effective in recognizing image patterns and objects.

At the core of a CNN are convolutional layers, which act like filters that scan the input image and detect specific features, such as edges, curves, or textures. These layers are followed by pooling layers, which downsample the feature maps and make the network more robust to small changes in the input image.

The real power of CNNs lies in their ability to automatically learn and extract relevant features from raw pixel data, without the need for manual feature engineering. This makes CNNs incredibly versatile and capable of tackling various computer vision tasks.

Imagine showing thousands of pictures of cats and dogs to a computer program. That's essentially what CNNs do, but instead of getting overwhelmed, they learn to identify key features—pointy ears for cats, floppy ears for dogs. This lets them become super good at recognizing these patterns in new images.

In the real world, CNNs are being used in various applications, such as:

  • Image recognition: Facebook and Google use CNNs to automatically recognize and tag people, objects, and scenes in uploaded images.
  • Self-driving cars: Companies like Tesla and Waymo rely on CNNs to detect and classify objects, lane markings, and traffic signs in real-time for autonomous driving.
  • Medical imaging: CNNs analyze medical scans, such as X-rays and MRI images, to assist in disease diagnosis and treatment planning.
  • Facial recognition: Many smartphones and security systems employ CNNs to recognize faces, unlock devices, and grant access to authorized individuals.

Basic Architecture: How Convolutional Neural Networks Work

CNNs are built using a series of specialized layers, much like a layered cake. Each layer performs a specific task, ultimately leading to image recognition. Here's a breakdown of the key players:

Image source

1. Convolutional Layers:

Imagine a detective meticulously examining a crime scene photo, searching for clues. Convolutional layers act similarly. 

They are the core of a CNN, responsible for extracting features from the image. Think of these features as the building blocks that make up an object. For example, edges, lines, and curves can be features used to identify shapes.

These layers work by applying small filters that slide across the image, analyzing tiny patches at a time. 

As the filter moves, it calculates a score indicating how well the patch features match the filter's pattern. This process creates a new image, called a feature map, highlighting areas with specific features.

CNNs can have multiple convolutional layers, each using different filters to identify various features. The first layers might detect basic edges and lines, while later layers can combine these features to identify more complex shapes, like eyes or noses in a face.

2. Pooling Layers:

Just like a detective might summarize their findings, pooling layers act as a condensing unit in a CNN. They take the feature maps generated by the convolutional layers and reduce their size. This helps control the network's complexity and prevents overfitting, where the network memorizes specific training data rather than learning generalizable patterns.

There are different pooling methods, but a common one is max pooling. Here, the pooling layer picks the highest value from a small grid of pixels in the feature map, essentially summarizing the most prominent feature within that area. This downsampling process reduces the image size while preserving crucial information for further analysis.

3. Fully-Connected Layers:

The fully-connected layers take over after the convolutional and pooling layers have extracted and summarized the features. These layers work similarly to regular neural networks, where neurons are interconnected and learn by adjusting their weights based on the data they receive.

In CNNs, the fully connected layers receive the flattened output from the pooling layers (imagine stretching the feature map into a long list) and analyze the overall relationships between the extracted features. 

This allows the network to make the final decision, such as entirely classifying the image as a cat, dog, or something else.

The Power of Combining Layers to Form a Convolution

Imagine a child learning to identify animals. At first, they might focus on basic features—floppy ears for dogs and pointed ears for cats. These early steps are similar to the work of the initial convolutional layers in a CNN. 

They extract fundamental visual building blocks like edges, lines, and color variations.

As the child progresses, they start to combine these features. Floppy ears paired with a wagging tail solidify the concept of "dog." This is where later convolutional layers take over. 

They learn to combine the outputs of previous layers, building more complex representations. Here, the network might identify a combination of curved lines and specific color patterns to recognize a cat's face.