Compression

Why compress?

Smaller files use less storage and transfer faster over a network (less bandwidth).

Lossless compression

• No data is lost — the original is restored exactly.
• Works by storing patterns efficiently:
• Run-Length Encoding (RLE) stores "5 white pixels" instead of listing each.
• Dictionary coding replaces repeated data with shorter codes.
• Used for text, code, spreadsheets (ZIP, PNG, FLAC).

Lossy compression

• Permanently removes data the eye/ear barely notices.
• Much smaller files, but quality is reduced and cannot be recovered.
• Used for photos (JPEG), music (MP3), video (MP4).

Comparison

	Lossless	Lossy
Quality	Perfect	Reduced
File size	Larger	Smaller
Reversible?	Yes	No
Use	Text, code	Photos, music, video

Exam tip

If a question mentions text or program files, the answer is lossless — losing data there would corrupt the file.