Chapter Overview
- Compression is reducing file size for storage or transmission
- Lossy compression loses data but greatly reduces size (images, video, audio)
- Lossless compression preserves all data but reduces size less (documents, data)
- Lossy is irreversible; lossless is reversible
- Each has different use cases based on data type and quality requirements
The Need for Compression
What is Compression?
Compression is the process of reducing the size of a file so that it can be transmitted across a network (like the internet) or stored more efficiently on secondary storage devices (hard drives, SSDs, mobile phones).
Think of compression like packing a suitcase—you're using the same space more cleverly to fit more in, or making what you have fit into a smaller bag.
Why Do We Need Compression?
There are two main scenarios where compression becomes essential:
- Maximise storage capacity: Mobile phones, tablets, and personal computers have limited storage space. By compressing files (especially photos, videos, and audio), you can store significantly more on your device without running out of space.
- Minimise transfer time: Uploading or downloading files across the internet takes time. Smaller files transfer faster. For streaming (like watching Netflix), compression is critical—it allows high-quality video to stream without buffering.
How Do We Reduce File Size?
For images specifically, file size can be reduced by:
- Reducing colour depth: Using fewer colours in an image (e.g., 8-bit colour instead of 24-bit) reduces the amount of data needed.
- Reducing resolution: A 1000×1000 pixel image requires more data than a 500×500 pixel image. Smaller images = smaller files.
But there's a trade-off: smaller files might look worse. That's where the two main compression methods come in.
Big Picture: Compression isn't magic—it's about being clever with data. Either you delete information you don't need (lossy) or you reorganise information more efficiently (lossless).
Lossy Compression
What is Lossy Compression?
Lossy compression is when data is deliberately removed from a file in order to reduce its size. Once the data is gone, it's gone for good—you can't get it back. This is called irreversible compression.
Definition
Lossy Compression: Data is lost to reduce file size. Irreversible. Cannot be undone.
How Does Lossy Work?
Lossy compression uses algorithms that identify information humans are unlikely to notice missing. For example:
In photographs: Lossy compression tries to group similar colours together, reducing the total number of colours in the image without compromising the overall visual quality too much. Your eye doesn't perceive every tiny colour difference—it sees the big picture.
Analogy: Imagine describing your friend's face. You might say "brown eyes" instead of listing every single shade of brown—you're keeping the important information but dropping details nobody really needs.
Trade-off: Size vs Quality
Lossy compression can greatly reduce file size (often to 10% of the original or smaller), but at the expense of losing quality. The more you compress, the worse it looks.
Lossy is only suitable for data where reducing quality is acceptable:
- 📸 Photographs and images (quality loss often unnoticed)
- 🎬 Video (human eye is forgiving)