Compression

What is Compression?

Within Computer Science, which covers algorithms, programming, data, networks, hardware and safe, responsible use of technology, this topic explains Compression. Computing topics explain how software, hardware and data work, with diagrams and models that show each step in order.

Check that you can explain Compression in your own words and give one example from everyday life. If you can do that, you have understood the core idea.

Understanding Compression helps you design, test and explain programs, websites and systems, from small projects to exam questions.

Compression in detail

Compression reduces file size so that files take less storage space and are quicker to send. Lossy compression permanently removes some data that people are unlikely to notice, such as in JPEG images and MP3 audio, giving smaller files but a loss of quality that cannot be recovered.

Lossless compression reduces size without losing any data, so the original can be rebuilt exactly, as in ZIP and PNG files. Methods include run-length encoding, which replaces repeated data with a count (AAAAA becomes 5A), and dictionary-based methods. Lossless suits text and programs, while lossy suits media where small losses are acceptable.

How the interactive helps you understand Compression

Click a step, or use the Previous and Next buttons, to move through the sequence one stage at a time. Auto-play moves on for you.

Before you change anything, predict the result. Then change one value at a time and check whether your prediction was right. This is a quick way to build a correct mental model of Compression.

Real-life uses and examples

• Trace through a process that uses Compression to check what it does step by step.
• Recognise Compression in an app or device you use every day.
• Apply Compression when you design a small program, website or spreadsheet.

Common mistakes to avoid

• Testing only the normal case and forgetting the edge cases.
• Using the term without being able to say what the computer actually does.
• Learning the definition without testing it. Use the quiz and the interactive to check that you can apply the idea.