Pixels & images

Every photo on your phone, every emoji, and every video game is made of tiny dots. If you get really close to a screen, you can sometimes even see them. Here you'll find out how a computer builds a whole picture out of such dots and why some images need more storage than others.

Images are grids of dots

A digital image is a grid of rows and columns, like squared paper. Each little square in it is called a pixel, a picture dot with exactly one colour. In the simplest image there is only black and white. Then a single bit per pixel is enough: 1 means black, for example, and 0 means white. From a long row of zeros and ones the computer can rebuild the image at any time.

Resolution: more pixels, finer image

How many pixels an image has is called its resolution. It is given as width and height, a Full HD screen for example has 1920 by 1080 pixels. An image with 8 by 8 pixels looks rough and blocky, with 1000 by 1000 pixels it looks smooth and sharp. You work out the total number of pixels like the area of a rectangle: width times height. More pixels mean a finer image, but also more storage, because every pixel has to be stored individually.

Colour needs more bits

With one bit per pixel you can only tell two colours apart. For colourful images, each pixel needs more bits. Screens mix every colour from red, green, and blue, RGB for short. If you store 8 bits for each of the three base colours, a pixel has 24 bits and can take on 16777216 shades, that is over 16 million. That's why a colour photo is much bigger than the same image in black and white.