Compression
Lossy & Lossless
What You'll Learn
- Why compression is needed and when to use it
- The difference between lossy and lossless compression
- Advantages and disadvantages of each type
- How each type affects the file
- Which compression type to use in different scenarios
Predict
Test your intuition about compression
Worked Examples
Real-world compression scenarios
Trace
Analyse compression decisions
Parsons Problems
Put concepts in order
Quiz
Test your knowledge
1. The Need for Compression
Why Do We Need Compression?
Digital files can be very large. Without compression, many everyday tasks would be impractical or impossible:
Email Attachments
Most email services limit attachments to 25MB. An uncompressed photo could be 20MB+!
Music Streaming
An uncompressed 3-minute song is ~30MB. With compression (MP3), it's only ~3MB.
Limited Storage
Phones have limited space. Compression lets you store thousands of photos instead of hundreds.
Web Pages
Compressed images load faster, saving bandwidth and improving user experience.
Video Streaming
A 2-hour 4K movie uncompressed would be ~1TB! Compression makes streaming possible.
Backups
Compressing backups saves storage space and reduces backup time.
What is Compression?
Compression is the process of reducing the file size by encoding data more efficiently.
Original File
100 MB
Compressed File
25 MB
Compression reduces file size, making storage and transmission more efficient
Common Scenarios Requiring Compression
| Scenario | Why Compression Helps |
|---|---|
| Sending files via email | Stay within attachment size limits |
| Uploading to cloud storage | Faster uploads, use less storage quota |
| Streaming music/video | Reduce bandwidth, enable smooth playback |
| Website images | Faster page load times |
| Archiving old files | Save storage space |
| Sending multiple files | Bundle into one smaller file (ZIP) |
| Mobile data usage | Reduce data consumption |
2. Types of Compression
Two Main Types
Lossless Compression
"Shrink it, but keep everything"
The original data can be perfectly reconstructed when decompressed.
No data is lost.
Lossy Compression
"Shrink it more, lose some detail"
Some data is permanently removed to achieve smaller file sizes.
Some data is lost.
3. Lossless Compression
How Lossless Works (Simplified)
Lossless compression finds patterns and redundancy in data and encodes them more efficiently.
Simple Example: Run-Length Encoding
Original: AAAAABBBCCCCCCCC (16 characters)
Compressed: 5A3B8C (6 characters)
The pattern "5 A's, 3 B's, 8 C's" uses fewer characters but contains the same information!
Common Lossless Formats
General file compression
Images (graphics, screenshots)
Audio (music archival)
Simple images, animations
File archives
High-ratio file compression
When to Use Lossless
Text Documents
Every character matters - you can't lose any text!
Program Files / Software
A single bit change could break the program
Spreadsheets / Databases
Financial data, records - accuracy is critical
Graphics with Sharp Edges
Logos, diagrams, screenshots - PNG is ideal
Medical Images
Doctors need every detail for accurate diagnosis
Advantages & Disadvantages of Lossless
Advantages
- No quality loss - original perfectly preserved
- Can compress and decompress repeatedly
- Essential for text, code, and critical data
- Reversible - always get original back
Disadvantages
- Lower compression ratio (files still larger)
- Not as effective for photos/video
- Files may still be too large for some uses
4. Lossy Compression
How Lossy Works (Simplified)
Lossy compression achieves smaller files by permanently removing data that humans are less likely to notice.
Examples of What Gets Removed
- Images: Subtle colour variations, fine details
- Audio: Frequencies humans can't hear well (very high/low)
- Video: Details between similar frames
Common Lossy Formats
Photos, web images
Music, podcasts
Video files
Audio (iTunes, streaming)
Web images
Video codec
Quality Levels
Lossy compression usually lets you choose a quality level - a trade-off between file size and quality.
When to Use Lossy
Photographs
JPEG compression removes details humans don't notice
Music for Everyday Listening
MP3 at 320kbps sounds nearly identical to CD
Video Streaming
Essential - uncompressed video would be impossibly large
Web Images
Fast loading is more important than perfect quality
Mobile Apps
Smaller assets = faster downloads, less storage
Advantages & Disadvantages of Lossy
Advantages
- Much smaller file sizes (often 90%+ reduction)
- Makes streaming media practical
- Faster uploads/downloads
- Saves significant storage space
- Quality loss often imperceptible
Disadvantages
- Quality loss is permanent and irreversible
- Re-compressing degrades quality further
- Not suitable for text, code, or critical data
- Visible/audible artifacts at high compression
Compression Artifacts
When lossy compression is too aggressive, you may notice artifacts - visible or audible flaws.
JPEG Artifacts (Images)
- Blockiness: Visible squares, especially in gradients
- Colour banding: Smooth gradients become stepped
- Blurring: Loss of fine detail
- Ringing: Halos around sharp edges
MP3 Artifacts (Audio)
- Pre-echo: Ghost sounds before loud notes
- Swirling: Metallic sound on cymbals
- Muffled highs: High frequencies sound dull
5. Lossless vs Lossy: Side by Side
Complete Comparison
| Feature | Lossless | Lossy |
|---|---|---|
| Data lost? | No - original preserved perfectly | Yes - some data permanently removed |
| File size reduction | Moderate (typically 20-50%) | High (typically 70-95%) |
| Quality | Identical to original | Reduced (may not be noticeable) |
| Reversible? | Yes - fully reversible | No - changes are permanent |
| Re-compression | No quality loss | Further quality loss each time |
| Best for | Text, code, archives, graphics | Photos, music, video, web |
| Common formats | ZIP, PNG, FLAC, GIF | JPEG, MP3, MP4, AAC |
Decision Flowchart
What type of file?
Every bit matters!
Quick Reference Card
Why Compress?
- Save storage space
- Faster file transfers
- Stay within size limits
- Enable streaming
- Reduce bandwidth costs
Lossless
- No data lost
- Fully reversible
- Smaller reduction
- ZIP, PNG, FLAC
- Use for: text, code, data
Lossy
- Some data lost forever
- Not reversible
- Much smaller files
- JPEG, MP3, MP4
- Use for: photos, music, video
The Fundamental Trade-Off
Lossless: Keep everything, but files are still relatively large
Lossy: Much smaller files, but some quality is permanently lost