---
title: "Lossless versus lossy compression"
subject: "Multimedia System"
chapter: 5
key: "semester_8/multimedia_system:lossy-vs-lossless"
source: https://www.bctengineer.com/q/semester_8/multimedia_system/lossy-vs-lossless
site: BCT Engineering
---

# Lossless versus lossy compression

Multimedia System · Chapter 5: Data Compression

- Asked in: 2073 Bhadra, 6 marks
- Video: [Lossy and Lossless Compression | Differences | CG | Computer Graphics | Lec-66 | Bhanu Priya](https://www.youtube.com/watch?v=e9hG_zjKkvI) — Education 4u
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/multimedia_system/lossy-vs-lossless
- Whole chapter: https://www.bctengineer.com/md/semester_8/multimedia_system/chapter-5

## Question

Differentiate between lossless and lossy data compression.

## Short answer

Lossless compression preserves the original data exactly, with low to moderate ratios (2:1 to 3:1), using statistical redundancy. Lossy compression permanently discards some information, achieving high ratios (10:1 to 100:1) by exploiting human perception limits. Lossless is reversible and used for text/programs; lossy is irreversible and used for photos, music, video.

## Answer

Lossless compression reduces size so that the decompressed data is identical to the original. Lossy compression discards some information permanently, so the decompressed data is only an approximation of the original.

| Point | Lossless | Lossy |
|---|---|---|
| Data recovery | Exact original is recovered | Only an approximation is recovered |
| Information loss | None | Some detail is lost for good |
| Compression ratio | Low to moderate (about 2:1 to 3:1) | High (10:1 to 100:1 or more) |
| Exploits | Statistical redundancy | Redundancy plus human perception limits |
| Typical media | Text, program files, medical and legal images | Photos, music, video, streaming |
| Techniques | RLE, Huffman, arithmetic, LZW | DCT, quantization, DPCM, vector quantization |
| Reversibility | Fully reversible | Not reversible |
| Examples/formats | ZIP, PNG, GIF, FLAC, TIFF | JPEG, MP3, MPEG, AAC |
| Quality on repeated saving | Unchanged | Degrades each time it is re-compressed |
| Output size | Larger | Much smaller |

**Choice:** use lossless when every bit matters (a document, an executable), and lossy when small size matters more than perfect fidelity (a streamed video).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the key difference in recovery of data?
   A. Both give an approximation
   B. Lossless recovers the exact original; lossy gives only an approximation
   C. Lossy recovers the exact original; lossless approximates
   D. Neither can be decompressed
   Answer: B. Lossy permanently discards information, so it is not reversible. (Key point: Data recovery)

2. Which is typical of lossy compared with lossless?
   A. Lower ratio, 2:1 to 3:1
   B. Same ratio as lossless
   C. Ratio of exactly 1:1
   D. Higher ratio, 10:1 to 100:1 or more
   Answer: D. Lossy exploits human perception limits as well as redundancy, so output is much smaller. (Key point: Compression ratio)

3. Which choice is appropriate?
   A. Lossless for an executable, lossy for a streamed video
   B. Lossy for an executable, lossless for a streamed video
   C. Lossy for legal documents
   D. Lossless for all music streaming
   Answer: A. Use lossless when every bit matters and lossy when small size matters more than fidelity. (Key point: Choosing the type)

4. What happens to quality when a file is repeatedly re-saved?
   A. Lossless degrades each time
   B. Lossy degrades each time; lossless stays unchanged
   C. Neither changes
   D. Both improve
   Answer: B. Each lossy re-compression discards more detail, while lossless is exactly reversible. (Key point: Repeated saving)

## Related in Multimedia System

- [Why lossy compression is preferred](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Awhy-lossy-preferred) — Multimedia System, chapter 5
- [Meaning of lossy compression](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Alossy-compression-meaning) — Multimedia System, chapter 5
- [Data compression techniques](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Adata-compression-techniques) — Multimedia System, chapter 5, asked 2×
- [Need for data compression](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Aneed-for-data-compression) — Multimedia System, chapter 5

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