Representing images

Last time

Before we start: last time, in Number systems, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A photo is a grid

Zoom right into any photo on your phone and you will see it is made of tiny squares. Each square is called a pixel. Each pixel is one single colour, and that colour is stored as a binary number.

Pixel

A pixel is the smallest part of an image. It is a single point of one colour. The word comes from picture element.

A black and white image

The simplest image is black and white. Each pixel needs just one bit: one for black, zero for white. Here is a smiley face, eight pixels by eight. Each row of the picture is one row of bits.

Its size

The smiley has eight rows of eight pixels: sixty-four pixels. Each pixel needs one bit, so the image is sixty-four bits. That is eight bytes.

Colour depth

To have more colours, each pixel needs more bits. The number of bits used for each colour is called the colour depth. With n bits, you can have two to the power n colours.

More bits, more colours

One bit gives two colours. Four bits give sixteen. Eight bits give two hundred and fifty-six. And twenty-four bits give over sixteen million colours: true colour, used for photographs.