GCSE Computer Science • Data Representation

Bitmap Images

Master how images are stored in binary, calculate file sizes, and understand the trade-offs between quality and storage.

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Key Concepts

Click on each card to review the essential definitions

Pixel
A picture element – the smallest addressable unit of a bitmap image. Each pixel represents a single point of colour.
Bitmap
A method for storing images by 'mapping' binary data to pixels. Each pixel's colour is stored as binary values in sequence.
Resolution
The density of pixels in an image, typically measured in pixels per inch (PPI) or dots per inch (DPI). Higher resolution = more detail.
Colour Depth
The number of bits used to encode the colour of each pixel. More bits = more possible colours. Common values: 1-bit, 8-bit, 24-bit.
Metadata
'Data about data' – information stored with the image such as width, height, colour depth, resolution, and creation date.
Colour Table
A lookup table that maps binary codes to specific colours. Used in indexed colour images to reduce file size.

📐 File Size Formula

File Size (bits) = Width × Height × Colour Depth

To convert: ÷8 for bytes, ÷1024 for KiB, ÷1024 again for MiB

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Colour Depth Quick Reference
Colour Depth Max Colours (2n) Typical Use
1-bit 2 colours Black & white
8-bit 256 colours Greyscale, GIF
16-bit 65,536 colours High colour
24-bit 16.7 million colours True colour (RGB photos)

Trade-offs: Quality vs Storage

Understand how changing image properties affects file size and quality

📐 Doubling width AND height
📊 Resolution: No change to PPI
💾 Storage: 4× larger (quadrupled)
🌐 Network: Takes 4× longer to transmit
🔍 Tripling pixels per inch
📊 Resolution: 3× higher
💾 Storage: 9× larger
🌐 Network: Takes 9× longer to transmit
🎨 Reducing colour depth 24-bit → 8-bit
📊 Resolution: No change
💾 Storage: 3× smaller
🌐 Network: Transmits 3× faster

Memory Match

Match the terms with their definitions

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Find the Pairs

Click on cards to flip them. Match each term with its correct definition.

Parsons Puzzle

Arrange the steps in the correct order to calculate file size

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Calculate file size in KiB for a 640×480 image with 24-bit colour

📦 Code Blocks

📝 Your Solution

Categorize

Drag items into the correct categories

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Sort These Image Concepts

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Pixel Art Builder

Create pixel art and see the binary representation

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Draw Your Image

Your Canvas (8×8)

📊 Image Metadata

Width
8 pixels
Height
8 pixels
Colour Depth
3 bits
File Size
192 bits

🔢 Binary Data (Colour Table)

📄 Binary Data (File Contents)

File Size Calculator

Calculate image file sizes and practice conversions

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Calculate File Size
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Practice Problems

Knowledge Check

Test your understanding with these exam-style questions