GCSE · Computer Science · Edexcel · Spec 1CP2

Lossy compression

Two pictures look almost identical. One has had a big chunk of its data thrown away, for good. How is that ever a good idea?

Computer Science · Data

Lossy or lossless: what do they share, and where do they split?

Decide where each statement belongs: lossy only, lossless only, both, or neither. Tick every circle that fits, then check.

A Lossy compression
B Lossless compression

The file ends up smaller than the original
The compressed file must be decompressed before you can access it again
A compression algorithm, which is a sequence of steps, does the work
Some data is permanently removed
Decompressing gives back an exact copy of the original
Decompressing only recreates an approximation of the original
Generally gives the much smaller file when the two are compared
May reduce the quality of the file
The compressed file can be used straight away without decompressing it

Predict, then check

Commit to an answer first. It's the best way to make this one stick.

A photo is compressed with a lossy method, then decompressed. What do you get back?

Lossy compression

Does lossy always ruin the quality?

A friend sends you a photo that was squashed with lossy compression. The file is much smaller than the original, but the picture looks fine to you.

Which of these is closest to what you think right now?

Lossy always gives a poor-quality file. You can always see the damage.Lossy removes data at random, so whatever gets hit gets hit.It looks identical to the original, so nothing can have been removed.It removes data I'm unlikely to notice, so it can look or sound very similar, but the removed data is gone for good.

How sure are you?

I'm sureI think soI'm guessing

Lossy always gives a poor-quality file. You can always see the damage.

This is a common misconception

Easy mistake, because 'lossy' sounds like 'bad'. But lossy removes data that is less noticeable, so it may not affect the perceived quality, especially to the human eye or ear. A picture can look very similar even though the file is much smaller.

Lossy removes data at random, so whatever gets hit gets hit.

This is a common misconception

Not random. The data removed is data considered unnecessary or less noticeable, such as detail the eye or ear is unlikely to notice. That is why the result can still look or sound good.

It looks identical to the original, so nothing can have been removed.

This is a common misconception

Looking very similar is not the same as being identical. Lossy permanently removes some data, so what you have is an approximation of the original, even when you can't tell the difference by eye.

It removes data I'm unlikely to notice, so it can look or sound very similar, but the removed data is gone for good.

This matches the evidence

Exactly. The quality may well be fine, which is the point of lossy. But the file can never be restored to an exact copy, so you've traded away that exactness for a much smaller file.

Lossy removes the less noticeable data, so it doesn't have to look bad. But what it removes is gone for good.

Computer Science · Data

Is lossy a good choice here?

For each file, would lossy compression be suitable? Pick the item, then pick the category.

Still to sort

Lossy is suitable (0)

A slight loss of quality is acceptable for a smaller file.

Where the line is: Ask: does this file have to be exact, or is a slight loss of quality fine in return for a smaller file?

Lossy is not suitable (0)

The file must be exactly preserved, or the loss may be noticeable.

Where the line is: If an exact copy matters, or the loss would show, lossy is the wrong tool.

5 of 5 still to sort.

Make the call for each file, then read the reason.

Your turn

Put the trade-off into words

A photographer saves a large collection of holiday photos as JPEG files. Give one advantage and one disadvantage of using lossy compression for these files. [2 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Advantage: the files are generally much smaller than they would be with lossless compression. (1 mark)
Disadvantage: some data is permanently removed, so the original can't be restored exactly and only an approximation comes back (or: the quality may be reduced). (1 mark)

Model answer

An advantage is that the photos are generally much smaller files than they would be with lossless compression. A disadvantage is that lossy compression permanently removes some data, so the photographer can never get back an exact copy of the original photos.

A full answer names both sides of the trade-off: a much smaller file, at the cost of data that is gone for good.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A much smaller file, with some data gone for good.

What you need to know

Compression reduces the number of bits a file needs, using an algorithm, which is just a sequence of steps.
A compressed file has to be decompressed before you can access it again.
Lossy compression permanently removes some of the data to make the file smaller.
The data it removes is unnecessary or less noticeable, like detail your eye or ear is unlikely to spot.
Have a goLossy compression is a tidy-up with one rule: bin whatever nobody will miss. Which gets binned first: detail you'd barely notice, or detail that jumps out at you?
The detail you'd barely notice.
Lossy removes data that is unnecessary or less noticeable, such as detail the eye or ear is unlikely to spot.
Because that data is gone, decompressing only recreates an approximation. You never get an exact copy back.
Compared with lossless, lossy generally gives a much smaller file, but it may reduce the quality.
Have a goYou need the smallest file you can get, and a slight loss of quality is fine. Lossy or lossless: which one is generally the better fit?
Lossy.
Lossy generally gives a much smaller file than lossless, with the cost that it may reduce the quality.
Lossy doesn't always mean poor quality. The removed data is the less noticeable stuff, so a picture can look very similar.
Lossy formats include JPEG for images, MP3 (and AAC) for audio and MP4 for video.
Have a goA classmate announces, very confidently: 'JPEG is the lossy format for songs.' Fix their sentence.
JPEG is for images. Songs use MP3 (or AAC).
Each lossy format goes with a type of file: JPEG for images, MP3 and AAC for audio, MP4 for video.
Choosing a method depends on how accurate the data must be, how much quality loss is acceptable and how much a small file matters.
Lossy suits cases where a slight loss is fine for a smaller file. It doesn't suit text files or an image enlarged on a very large print.

The big picture

Lossy compression makes a file much smaller by permanently removing data that is less noticeable. You can't get that data back, so decompressing only gives you an approximation. It's a trade-off, and your job is to decide when it's worth it.

Key points

1Compression reduces the number of bits needed to represent a file, and a compressed file must be decompressed before it can be accessed again.

2Lossy compression permanently removes some data, usually data that is less noticeable, such as detail the human eye or ear is unlikely to notice.

3A lossy file can never be restored to an exact copy of the original. Decompression recreates only an approximation.

4Lossy generally gives a much smaller file than lossless but may reduce quality. It does not always give a poor-quality file.

5Choose lossy when a slight loss of quality is acceptable for a smaller file, and not when the file must be exactly preserved.

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Worked example

Problem

A bank wants to shrink its file of customers' account balances. A podcast team wants a smaller file for each episode. For each one, decide whether lossy compression is suitable.

⚠ Watch out

Thinking lossy compression always ruins the quality, or that you can decompress a lossy file back to the original. It removes less noticeable data, so the result can look or sound very similar, but it can never be restored exactly.

🧠

Memory hook

Lossy = lost for good. Both words start with L, so if data goes missing in a lossy file, it's lost, and it stays lost.

✓

Check yourself

Without looking back: a friend decompresses a lossy file and says they now have the exact original. What would you tell them, and why?

Flashcards

(11)

What does compression do to a file?

It reduces the number of bits needed to represent the file, using a compression algorithm (a sequence of steps).

What has to happen to a compressed file before you can access it again?

It must be decompressed.

What does lossy compression do to the data?

It permanently removes some of it, to reduce the file size.

What sort of data does lossy compression remove?

Data considered unnecessary or less noticeable, such as detail the human eye or ear is unlikely to notice.

After lossy compression, what does decompression give you?

Only an approximation of the original, never an exact copy. The compression is not fully reversible.

Lossy compared with lossless: what happens to file size and quality?

Lossy generally gives a much smaller file, but it may reduce the quality.

Does lossy compression always give a poor-quality file?

No. It removes less noticeable data, which may not affect the perceived quality, especially to the human eye or ear.

Name a lossy format for images, one for audio and one for video.

Images: JPEG. Audio: MP3 (and AAC). Video: MP4.

What three things decide between lossy and lossless?

How accurate the data must be, how much loss of quality is acceptable and how important a small file is.

When is lossy compression a good choice?

When a slight loss of quality is acceptable in return for a smaller file.

Give two cases where lossy compression is not suitable.

Files that must be exactly preserved, such as text files, and an image to be enlarged on a very large print, where the loss may be noticeable.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Lossy or lossless: what do they share, and where do they split?

Does lossy always ruin the quality?

Is lossy a good choice here?

Put the trade-off into words

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