Bitmap images (Edexcel GCSE Computer Science): Revision Notes

Bitmap images

What are bitmap images?

Digital images stored on computers are called bitmap images. These images are created using binary code - sequences of 1s and 0s that computers can understand and process. Think of bitmap images as a digital way of recreating the pictures we see in real life.

Pixels - the building blocks

Every bitmap image is made up of thousands or millions of tiny coloured dots called pixels (short for picture elements). Each individual pixel contains its own specific colour information. The more pixels an image contains, the more detailed and sharp it will appear.

Image size and dimensions

The image size tells us how many pixels wide and how many pixels tall an image is. This is usually written as width × height in pixels.

For example:

• A small web image might be 800 × 600 pixels
• A high-quality photo might be 3000 × 2000 pixels
• A smartphone photo could be 4000 × 3000 pixels

The total number of pixels in an image equals the width multiplied by the height. So a 3000 × 1000 pixel image contains 3,000,000 individual pixels!

Colour depth and bit depth

Colour depth (also called bit depth) determines how many different colours each pixel can display. It's measured in bits - the basic units of computer memory.

The relationship works like this:

• 1 bit per pixel = 2 possible colours (usually black and white)
• 8 bits per pixel = 256 possible colours
• 24 bits per pixel = 16,777,216 possible colours

The magic formula is: Number of colours = $2^{\text{number of bits}}$

Resolution and image quality

Resolution describes how many pixels fit into each inch when an image is displayed on a screen or printed on paper. Higher resolution means more pixels packed into the same space, creating sharper, clearer images.

When you enlarge a small image to cover a larger area, it becomes less sharp because there are fewer pixels per unit area. Each pixel has to be stretched bigger to fill the space. This creates a pixelated effect where you can actually see the individual square pixels - the image looks blocky and blurred.

Image file size calculation

The size of an image file (measured in bits or bytes) depends on three factors:

• Width (W) - how many pixels wide
• Height (H) - how many pixels tall
• Colour depth (D) - how many bits per pixel

File size formula: $W \times H \times D$

This tells us the file size in bits. To convert to more practical units:

• Divide by 8 to get bytes
• Divide by 1024 to get kilobytes (KB)
• Divide by 1024 again to get megabytes (MB)

Worked example

Remember!