← IGCSE Computer Science Paper 1

Lossy compression permanently removes data

Data removed by lossy compression is gone for good, so the original quality cannot be restored. Keep the original file, as re-saving or compressing again only lowers quality further.

Part of: IGCSE Computer Science 0478 — 1.3 Data Storage and Compression — Module 34, ACARA Digital Technologies Years 9–10 — Data Compression — Module 27

Understanding Lossy compression permanently removes data in IGCSE Computer Science Paper 1

What it does: Changing the format or saving again cannot recreate the lost data, so the detail stays missing. Each lossy compression removes more data, so repeating it makes quality worse every time. The original holds all of the data. The compressed copy has permanently lost some, so only the original keeps full quality.

Common mistakes: Lossy compression permanently removes data, so the original file must be kept. Deleting song.wav leaves only the lossy copy, which cannot be restored to full quality. The line should save the original: SAVE original AS "song.wav". Each lossy re-save removes more data, so quality goes down, not up. The line should be quality ← quality - 10.

Class: _____________________  ·  Name: _____________________  ·  Date: _________

Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 86–109) and word search (Modules 86–120).

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Learning
Duo
A student compresses a photo using lossy compression. Can the original quality be restored from the compressed file?
No, the removed data is gone for goodYes, the removed data is stored and can be put back
Matches
Open the lossy file, save it at high quality, and the lost detail returns
↓
Saving at a higher quality setting does not restore removed data. The lost detail is gone for good and only the original file has it.
Applying
Meanings
Term
permanent loss
Meaning
Data removed by lossy compression is gone for good, so the original quality cannot be restored. Keep the original file, as re-saving or compressing again only lowers quality further.
Practice
original ← "photo.bmp"
Testing
Gaps
Lossy compression removes data
Check Answer
Spots
1DELETE "song.wav"
2copy ← LossyCompress(original)

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Quick Questions

1. A student compresses a photo using lossy compression. Can the original quality be restored from the compressed file?
A. No, the removed data is gone for good ✓
B. Yes, the removed data is stored and can be put back
Lossy compression permanently removes data. The compressed file does not contain the lost detail, so it cannot be restored.
2. A JPEG photo has been compressed and looks blurry. The student opens it and saves it as a new file at the highest quality setting. What happens to the detail?
A. The lost detail is restored
B. The lost detail does not come back ✓
Saving again does not bring detail back. The data was already removed, so a higher quality setting cannot recreate it.
3. Which is the best way to protect your photos before applying lossy compression?
A. Keep a copy of the original file ✓
B. Delete the original to save space straight away
Since lossy compression permanently removes data, the original file is the only way to get the full quality back.
4. A song is compressed to MP3 three times in a row, each time with lossy compression. What happens to the sound quality?
A. It stays the same after the first compression
B. It gets lower each time it is compressed ✓
Repeated lossy compression removes more data each time, so quality drops further with every round.
5. Which statement about lossy compression is TRUE?
A. Data removed is permanently lost ✓
B. Data removed can be rebuilt when the file is opened
Lossy compression makes the file smaller by permanently discarding some data, so the file cannot be rebuilt exactly as it was.

Match the Pairs

permanent loss → Data removed by lossy compression is gone for good and cannot be restored
keep the original file → Store an uncompressed copy so the full quality can be used again later
Saving a lossy file again does not bring back the detail that was removed → photo.jpg (compressed) -> save as photo_new.png -> detail still missing
Compressing a file with lossy compression again lowers the quality further → photo.jpg -> compress -> compress -> compress = lower quality each time
Open the lossy file, save it at high quality, and the lost detail returns → Saving at a higher quality setting does not restore removed data. The lost detail is gone for good and only the original file has it.
Delete the original file after compressing it because it can be rebuilt from the lossy copy → The original cannot be rebuilt from the lossy copy because data was permanently removed. You should keep the original file.

Key Term

permanent loss
Data removed by lossy compression is gone for good, so the original quality cannot be restored. Keep the original file, as re-saving or compressing again only lowers quality further.

Fill in the Blanks

1. Lossy compression removes data permanently
Hint: Type the word meaning the data is gone for good.
2. Saving again will not restore detail
Hint: Type the word for the fine information that was removed and cannot come back.
3. Always keep the original file
Hint: Type the verb for what you should do with the uncompressed original.
4. Compressing again lowers quality further
Hint: Type the verb for what repeated lossy compression does to quality.
5. original ← "photo.bmp" backup ← original small ← LOSSY(backup)
Hint: Type the variable name that holds the untouched file being copied as a backup.

Spot the Error

Question 1
A DELETE "song.wav"
B copy ← LossyCompress(original)
C SAVE copy AS "song.mp3"
Line A — Lossy compression permanently removes data, so the original file must be kept. Deleting song.wav leaves only the lossy copy, which cannot be restored to full quality. The line should save the original: SAVE original AS "song.wav".
Question 2
A quality ← 100
B FOR i ← 1 TO 3
C ReSaveJPEG(photo)
D quality ← quality + 10
E NEXT i
Line D — Each lossy re-save removes more data, so quality goes down, not up. The line should be quality ← quality - 10.
Question 3
A photo ← LoadImage("a.jpg")
B detail_restored ← TRUE
C SAVE photo AS "a.png"
Line B — Saving a lossy JPEG in another format does not bring back the detail that was already removed. It is gone for good, so detail_restored must be FALSE.
Question 4
A song ← LoadSound("track.wav")
B mp3 ← LossyCompress(song)
C original_data_lost ← FALSE
D OUTPUT original_data_lost
Line C — Lossy compression removes data permanently, so the original data is lost. The line should set original_data_lost to TRUE.
Question 5
A photo ← LoadImage("pic.png")
B COPY photo TO small
C LossyCompress(photo)
D SAVE small AS "small.jpg"
Line C — This compresses the original, which permanently removes its data. Compress the copy so the original is kept: LossyCompress(small).

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