Lossy compression

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Quick Questions

Shade in the A or B circle for each question.

1. Which statement describes lossy compression?

A It permanently throws away some detail to make the file smaller

B It shrinks the file and always lets you rebuild the exact original

2. Which file format uses lossy compression?

A ZIP archive of a document

B JPEG photo

3. An MP3 file is smaller than the original song recording. How is this done?

A Sounds that most people can't hear well are removed

B Repeated notes are swapped for a count, so nothing is lost

4. What is the trade-off when you use stronger lossy compression?

A A smaller file and better quality

B A smaller file but lower quality

5. You save a JPEG, open it, edit it, and save it as a JPEG again, over and over. What happens?

A The quality gets worse each time it is saved

B The quality stays exactly the same every time

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1photo.jpg		Asave.jpg -> save.jpg -> save.jpg (quality drops)
2song.mp3		BLossy compression deliberately discards detail that people are unlikely to notice. Keeping every bit is lossless compression.
3Lossy compression lets you trade quality for size: a lower quality setting gives a smaller file		Cquality = 30 -> small file, more detail lost
4Saving a lossy file again and again keeps removing detail, so quality gets worse each time		DA JPEG image that uses lossy compression, so some fine detail has been permanently discarded to make the file smaller
5Unzip the JPEG to get back the original photo exactly		EJPEG is lossy, so the discarded detail is gone for good. Unzipping or reopening cannot rebuild the exact original.
6Lossy compression keeps every bit, so nothing is ever lost		FAn audio file that uses lossy compression, so sounds that are hard to hear are permanently removed to save space

Definitions

lossy compression

Fill in the Blanks

Complete each line of code by filling in the missing parts.

JPEG photos use _____ compression

___ is a lossy format for music files

Lower quality setting = _______ file size

______ 5 TIMES SAVE image as JPEG quality gets lower each time

Lossy: original rebuilt exactly = _____

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A format ← "JPEG"

B compression ← "lossless"

C detail_thrown_away ← TRUE

Question 2

A song ← "track.mp3"

B removed ← "quiet sounds"

C can_rebuild_original ← TRUE

Question 3

A quality ← 90

B size_KB ← 800

C quality ← 30

D size_KB ← 2000

Question 4

A keeps_all_detail ← TRUE

B file_size ← SMALLER

C original_exact ← FALSE

Question 5

A quality_start ← "good"

B REPEAT 5: re-save as JPEG

C quality_end ← "better"

Answer Key

Lossy compression

Section A: Quick Questions

A — It permanently throws away some detail to make the file smaller

B — JPEG photo

A — Sounds that most people can't hear well are removed

B — A smaller file but lower quality

A — The quality gets worse each time it is saved

Section B: Match the Pairs

photo.jpg → A JPEG image that uses lossy compression, so some fine detail has been permanently discarded to make the file smaller

song.mp3 → An audio file that uses lossy compression, so sounds that are hard to hear are permanently removed to save space

Lossy compression lets you trade quality for size: a lower quality setting gives a smaller file → quality = 30 -> small file, more detail lost

Saving a lossy file again and again keeps removing detail, so quality gets worse each time → save.jpg -> save.jpg -> save.jpg (quality drops)

Unzip the JPEG to get back the original photo exactly → JPEG is lossy, so the discarded detail is gone for good. Unzipping or reopening cannot rebuild the exact original.

Lossy compression keeps every bit, so nothing is ever lost → Lossy compression deliberately discards detail that people are unlikely to notice. Keeping every bit is lossless compression.

Section C: Definitions

lossy compression — Shrinks a file by permanently throwing away detail people are unlikely to notice, so the original can't be rebuilt exactly. JPEG and MP3 use it.

Section D: Fill in the Blanks

JPEG photos use lossy compression

MP3 is a lossy format for music files

Lower quality setting = smaller file size

REPEAT 5 TIMES
  SAVE image as JPEG
quality gets lower each time

Lossy: original rebuilt exactly = FALSE

Section E: Spot the Error

B — JPEG uses lossy compression, which permanently throws away detail. It should be compression ← "lossy".

C — An MP3 uses lossy compression, so the removed sound detail is gone for good. The original cannot be rebuilt exactly, so it should be FALSE.

D — Lowering the quality throws away more detail, so the file gets smaller, not bigger. 2000 KB is larger than 800 KB; it should be a smaller number such as 200.

A — Lossy compression does not keep all the detail. It permanently discards some, so this should be FALSE.

C — Each time a JPEG is saved again, a little more detail is thrown away. Repeated saving lowers quality, so the end quality should be "worse".