← IGCSE Computer Science Paper 1

Comparing lossy and lossless compression

Lossy makes smaller files but permanently removes data; lossless keeps all data so the original can be restored, but saves less space.

Part of: IGCSE Computer Science 0478 — 1.3 Data Storage and Compression — Module 46, ACARA Digital Technologies Years 9–10 — Data Compression — Module 36, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 57

Understanding Comparing lossy and lossless compression in IGCSE Computer Science Paper 1

What it does: Run-length encoding is lossless because decoding 4W2B gives back exactly WWWWBB, with no data lost. Reducing the resolution removes pixels permanently. The file size drops, but the original detail cannot be restored. Lossy throws data away, so it usually saves more space. Lossless keeps everything, so it saves less. RLE stores repeats as counts, so it can be decoded back to the exact original. That makes it lossless. Losing even one character could corrupt the file, so lossless is needed. The removed data cannot be put back, so the result is only an approximation of the original. The mistake reverses the size comparison. Lossless trades a bigger file for keeping all the data.

Common mistakes: Data is removed and cannot be restored, which describes lossy compression. Line 1 should be method = 'lossy'. Lossless compression keeps all the data, so no data is removed. Line 2 should be data_removed = False.

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Learning

Duo

Which type of compression lets you restore the original file exactly?

Lossy compressionLossless compression

Matches

Lossy compression: all data is kept, so the original can be restored

↓

This describes lossless compression. Lossy compression permanently removes data, so the original cannot be restored exactly.

Applying

Meanings

Term

lossy vs lossless

Meaning

Lossy makes smaller files but permanently removes data; lossless keeps all data so the original can be restored, but saves less space.

Practice

// Lossless: RLE keeps all the data

Testing

Gaps

type ← ""

Check Answer

Spots

1method = 'lossless'

2data_removed = True

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Quick Questions

1. Which type of compression lets you restore the original file exactly?

A. Lossy compression

B. Lossless compression ✓

Lossless keeps all the data, so the original can be rebuilt exactly. Lossy removes data permanently.

2. Which type of compression usually gives the smaller file?

A. Lossy compression ✓

B. Lossless compression

Lossy throws data away, so it usually saves more space. Lossless keeps everything, so it saves less.

3. A student reduces an image's resolution to shrink the file. Which type of compression is this?

A. Lossy, because pixels are removed for good ✓

B. Lossless, because the image can be restored

Reducing resolution removes pixel data permanently. The original detail cannot be recovered.

4. Run-length encoding (RLE) is an example of which type of compression?

A. Lossless, because no data is lost ✓

B. Lossy, because the file gets shorter

RLE stores repeats as counts, so it can be decoded back to the exact original. That makes it lossless.

5. Which statement about data is true?

A. Lossless removes some data permanently; lossy keeps all of it

B. Lossy removes some data permanently; lossless keeps all of it ✓

Lossy = data removed. Lossless = all data kept.

Match the Pairs

Lossy compression → Makes the file smaller by permanently removing some data, so the original cannot be restored exactly

Lossless compression → Shrinks the file but keeps all the data, so the original can be restored exactly

A lossless method: repeated values are stored as a count and a value, and every pixel can be restored → RLE: WWWWBB -> 4W2B

A lossy method: the image has fewer pixels, so the file is smaller but the lost detail cannot return → Resolution: 1920x1080 -> 960x540

Lossy compression: all data is kept, so the original can be restored → This describes lossless compression. Lossy compression permanently removes data, so the original cannot be restored exactly.

Lossless compression always gives a smaller file than lossy compression → Lossy compression usually gives smaller files because it throws data away. Lossless saves less space because it must keep everything.

Key Term

lossy vs lossless

Lossy makes smaller files but permanently removes data; lossless keeps all data so the original can be restored, but saves less space.

Fill in the Blanks

1. method ← "RLE" type ← "lossless"

Hint: Type the kind of compression that keeps all the data so the original can be restored.

2. method ← "reduce resolution" type ← "lossy"

Hint: Type the kind of compression that permanently removes data to make the file smaller.

3. type ← "lossy" restorable ← FALSE

Hint: Type the Boolean value showing whether the original can be restored after lossy compression.

4. lossySize ← 2 losslessSize ← 5 OUTPUT lossySize < losslessSize

Hint: Type the comparison operator that checks the lossy file is smaller than the lossless file.

5. original ← "AAAABB" encoded ← "4A2B" decoded ← "AAAABB"

Hint: Type the run length (count) for the repeated A characters in this lossless RLE example.

Spot the Error

Question 1

A method = 'lossless'

B data_removed = True

C can_restore = False

Line A — Data is removed and cannot be restored, which describes lossy compression. Line 1 should be method = 'lossy'.

Question 2

A method = 'lossless'

B data_removed = True

C can_restore = True

Line B — Lossless compression keeps all the data, so no data is removed. Line 2 should be data_removed = False.

Question 3

A IF method = 'lossy' THEN

B OUTPUT 'data removed'

C OUTPUT 'can restore'

D ENDIF

Line C — Lossy compression permanently removes data, so the original cannot be restored. Line 3 should output 'cannot restore'.

Question 4

A data = 'AAAABBB'

B rle = '4A3B'

C type = 'lossy'

Line C — Run-length encoding keeps all the data, so the original can be rebuilt exactly. It is lossless, not lossy.

Question 5

A image = 'photo.bmp'

B type = 'lossless'

C method = 'lower resolution'

Line B — Reducing the resolution throws away pixel detail permanently, so this is lossy compression. Line 2 should be type = 'lossy'.

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