Digital Images as Grids of Pixels

Why it exists

To capture and represent the complex, continuous color and light variations of the real world in a digital format. Computers need a structured, discrete way to store this visual information, and a simple grid of colored points is the most straightforward method.

The mental model

Imagine a digital image as a piece of graph paper or a mosaic. Each small square on the paper is a 'pixel' (picture element), and you fill each square with a single, solid color. When you stand back, the individual colored squares blend together to form a complete picture. The entire image is just a collection of these colored squares arranged in a rectangular grid.

How it works

A raster image is defined by its dimensions (width and height in pixels) and its color depth. For each pixel in its grid, the image file stores a value representing its color. For a simple black and white image, this might be a single bit (0 for black, 1 for white). For a full-color image, it might use 24 bits per pixel to represent millions of different colors, typically by storing values for Red, Green, and Blue channels. The total file size is directly related to the number of pixels and the color depth.

When to use it

Raster graphics are the standard for any image with complex color gradations, textures, and fine details. This makes them the go-to format for digital photographs, scanned artwork, and detailed digital paintings. If you're capturing reality with a camera or a scanner, you are creating a raster image.

When not to use it

The primary weakness is scaling. Because the image is a fixed grid of pixels, making it larger requires software to invent new pixels, which often results in a blurry or 'pixelated' look. For graphics that need to be resized frequently without loss of quality, like logos, fonts, or technical diagrams, vector graphics are a better choice as they use mathematical equations to define shapes.

One canonical example

A JPEG photo from your smartphone is a perfect example. It has fixed dimensions, like 4032x3024 pixels. If you zoom in far enough, you can see the individual square pixels that make up the image. If you try to print it on a large billboard, it will look blurry because the fixed number of pixels are being stretched over a huge area.