Representing Images

Computers store images as collections of tiny dots called pixels. This lesson explains how bitmap images are represented in binary, and the factors that affect image quality and file size.

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What Is a Bitmap Image?

A bitmap image is made up of a grid of pixels (picture elements). Each pixel is a single coloured dot. When you zoom into a bitmap image, you can see the individual square pixels.

The computer stores a binary value for each pixel that represents its colour. The entire image is stored as a long sequence of binary values — one for each pixel.

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Key Concepts

Resolution

Resolution refers to the number of pixels in an image, usually expressed as width × height (e.g. 1920 × 1080 pixels).

• A higher resolution means more pixels, which produces a sharper, more detailed image.
• A higher resolution also means a larger file size because there are more pixels to store.

Colour Depth

Colour depth is the number of bits used to represent the colour of each pixel.

Colour Depth	Number of Colours	Example
1 bit	2	Black and white
2 bits	4	4-colour palette
4 bits	16	16-colour palette
8 bits	256	256-colour palette
16 bits	65,536	High colour
24 bits	16,777,216	True colour (RGB)

The formula for the number of available colours is:

Number of colours = 2^(colour depth)

• A greater colour depth means more colours can be represented, producing a more realistic image.
• A greater colour depth also means a larger file size because each pixel requires more bits.

True Colour (24-bit)

In 24-bit colour (also called true colour), each pixel uses 24 bits — 8 bits for red, 8 bits for green, and 8 bits for blue (the RGB colour model).

• Red: 8 bits = 256 shades (0–255)
• Green: 8 bits = 256 shades (0–255)
• Blue: 8 bits = 256 shades (0–255)
• Total combinations: 256 × 256 × 256 = 16,777,216 colours

Example Colours in 24-bit RGB

Colour	Red	Green	Blue	Hex Code
White	255	255	255	#FFFFFF
Black	0	0	0	#000000
Pure Red	255	0	0	#FF0000
Pure Green	0	255	0	#00FF00
Pure Blue	0	0	255	#0000FF
Yellow	255	255	0	#FFFF00

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Metadata

A bitmap image file also contains metadata — additional data stored alongside the pixel data. Metadata typically includes:

• The width and height of the image (in pixels)
• The colour depth (bits per pixel)
• The file format and compression information

This metadata is stored in the file header and is needed so that the computer knows how to reconstruct the image from the binary data.

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Image Representation Pipeline

flowchart LR
    A[Image: grid of pixels] --> B[Each pixel: colour value]
    B --> C[Colour depth: bits per pixel]
    A --> D[Resolution: width x height]
    C --> E[File size in bits]
    D --> E
    E --> F[Bytes / 8, KB / 1024, MB / 1024]

Calculating Image File Size

The file size of a bitmap image can be estimated using:

File size (bits) = Width (pixels) × Height (pixels) × Colour depth (bits per pixel)

Worked Example 1

An image is 800 × 600 pixels with a colour depth of 24 bits.

• File size = 800 × 600 × 24 = 11,520,000 bits
• 11,520,000 ÷ 8 = 1,440,000 bytes
• 1,440,000 ÷ 1024 = 1406.25 KB
• 1406.25 ÷ 1024 ≈ 1.37 MB

Worked Example 2

An image is 200 × 200 pixels with a colour depth of 8 bits.

• File size = 200 × 200 × 8 = 320,000 bits
• 320,000 ÷ 8 = 40,000 bytes
• 40,000 ÷ 1024 ≈ 39.06 KB

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Effect of Resolution and Colour Depth on Quality and File Size

Change	Effect on Quality	Effect on File Size
Increase resolution	Image is sharper and more detailed	File size increases
Decrease resolution	Image is blockier and less detailed	File size decreases
Increase colour depth	More colours, more realistic image	File size increases
Decrease colour depth	Fewer colours, less realistic image	File size decreases

Exam Tip: File size calculations are very common in the exam. Make sure you can convert between bits, bytes, kilobytes, and megabytes. Remember: 8 bits = 1 byte, 1024 bytes = 1 KB, 1024 KB = 1 MB. Always show your working clearly.

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Summary

• Bitmap images are stored as a grid of pixels, with each pixel represented by a binary colour value.
• Resolution is the number of pixels (width × height) — higher resolution means more detail and larger file size.
• Colour depth is the number of bits per pixel — higher colour depth means more colours and larger file size.
• 24-bit true colour uses 8 bits each for red, green, and blue, giving over 16 million colours.
• File size = width × height × colour depth (in bits).
• Image files also contain metadata in the file header.

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Extended Worked Examples for Image File Size

Worked Example 3: 1-bit black-and-white logo

A logo is 256 × 256 pixels with a colour depth of 1 bit (each pixel is either black or white).

• Total bits = 256 × 256 × 1 = 65,536 bits
• Bytes = 65,536 ÷ 8 = 8,192 bytes
• KB = 8,192 ÷ 1024 = 8 KB

This shows how cheap monochrome images are to store. A 1-bit colour depth gives 2^1 = 2 colours.

Worked Example 4: Smartphone photograph