Image Representation: Pixels and Colour Depth

A pixel (picture element) is a single point in a digital image. Every digital image stored on a computer is made up of a rectangular grid of pixels — this type of image is called a bitmap.

Each pixel stores a single colour value as a binary number. When the pixels are small enough and packed tightly enough, the human eye perceives a continuous image rather than a grid of discrete dots.

Property	What it means
Pixel	One point in the image, storing one colour value
Bitmap	An image stored as a grid of pixel colour values
Image dimensions	The width and height of the grid, measured in pixels
Colour depth	The number of bits used to represent each pixel's colour

A black-and-white image uses 1 bit per pixel (0 = white, 1 = black). A full-colour image typically uses 24 bits per pixel — 8 bits each for red, green, and blue channels.

A pixel has no fixed physical size. The same 800 × 600 pixel image printed on a postcard looks sharp; stretched across a billboard it looks blocky. The number of pixels does not change — only the physical size of each one does.

An image's dimensions are expressed as width × height in pixels. Multiplying width by height gives the total pixel count — the number of individual colour values that must be stored.

Examples:

Image	Width (px)	Height (px)	Total pixels
Thumbnail	100	100	10,000
Standard HD	1280	720	921,600
Full HD	1920	1080	2,073,600
4K UHD	3840	2160	8,294,400

A higher total pixel count captures more spatial detail. A 1920 × 1080 image contains roughly 200 times more pixels than a 100 × 100 thumbnail. However, resolution alone does not determine file size — colour depth also plays a critical role.

Colour depth is the number of bits used to represent each pixel. More bits per pixel means more possible colour values, producing a more detailed and realistic image.

The number of possible colours is $2^{\text{colour depth}}$:

Colour depth (bits)	Possible colours	Example use
1	2	Black and white
4	16	Early computer graphics
8	256	Simple web graphics
16	65,536	High colour
24	16,777,216	True colour (typical photographs)

Worked example — how many colours can a 4-bit image represent?

$2^4=16\text{ possible colours}$

A 4-bit image can only assign one of 16 colours to each pixel. Switching to 8-bit doubles the number of bits but squares the number of available colours from 16 to 256.