Raster and Vector Images

Digital images are stored in one of two ways: as a raster (also called bitmap) image or as a vector image. The difference becomes obvious when you make an image bigger.

Raster images

A raster image is made from a grid of pixels — tiny squares, each holding one colour. The image is just that grid, so its quality depends on how many pixels are in it. If you zoom in far enough, you see the individual squares. If you enlarge the image, the software stretches the pixels it already has and guesses the gaps, so edges turn jagged and blocky. This is pixelation.

For example, a photo of a school netball team taken at a small size looks fine on a phone. Blown up to cover a sports hall banner, the players' faces become a patchwork of coloured squares. Raster images suit photographs, where every pixel may be a slightly different shade.

Vector images

A vector image is not a grid of pixels. It is made from lines, curves and fills, stored as instructions: draw a curve from here to there, fill this shape with blue. When the image is enlarged, the instructions are used to redraw it at the new size, so the edges stay crisp. A vector logo on a small membership card can be scaled up for a large shop sign with no loss of quality.

Vector images suit logos, icons and diagrams built from clean shapes. A vector image is not automatically better quality — a simple drawing is still simple — but scaling it does not cause pixelation.

Comparing raster and vector