4.2.4
File compression
Compression reduces the data needed to store or send a file. One question decides everything: can the original information be rebuilt exactly, or has some of it been thrown away for good?
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What you need to know
- Define lossy and lossless compression accurately.
- Explain why media products get compressed.
- Weigh file size against the need to preserve the original data.
- Apply the choice to images, audio and moving images.
Start here
Why bother
Media files are big. Compression cuts the amount of data needed to represent them, which means less storage, faster uploads and downloads, and less mobile data burnt through. That matters most exactly when it is hardest — distributing to a lot of people, or to devices with little space and a poor connection.
In an answer, follow the chain through. “It makes the file smaller” is the immediate effect. “So visitors can download the audio guide quickly on mobile data at the trail head” is why it matters here. What compression does not do is make anyone’s internet connection faster — avoid writing that.
How it works
Lossless: smaller, and fully reversible
Lossless compression stores the same data more efficiently — recording repeated patterns compactly rather than spelling everything out. Decompress it and the original data comes back exactly. That is what you want when losing anything is unacceptable: graphic assets, original audio, a folder of project files being sent to a colleague.
How much smaller depends on the data. Something full of repeated patterns compresses well; something already efficiently compressed may barely shrink at all. And note that lossless is not the same as uncompressed — both preserve the data, but only one of them used a compression method to do it in less space.
The idea in one line
Writing “ten white pixels” instead of listing white ten times saves space and still lets you rebuild every pixel. Real formats are more sophisticated, but that is the principle: nothing had to be thrown away.
How it works
Lossy: smaller, and one-way
Lossy compression throws information away to get the file smaller. Good methods aim at information people are least likely to notice, but it is gone — the original cannot be rebuilt from the compressed file. Push it hard and you can see and hear where it went: blocks around edges, smeared movement, thin distorted audio.
That is often the right trade for a web image, a listening copy or streamed video, where a smaller file means more people can actually use it. Check the export at the size and on the device it is meant for. And never let a small delivery copy become your only master — decoding and re-encoding lossy files repeatedly stacks the losses up.
| Lossless | Lossy |
|---|---|
| The original data can be rebuilt exactly | Some original information is discarded for good |
| For when exact preservation matters | For when compact delivery is worth some loss |
| PNG images, FLAC audio, ZIP archives | JPEG images, MP3 audio, most delivery video |
| How much smaller depends on the patterns in the data | Usually much smaller, because it is allowed to lose things |
Try it
Lossy, lossless or uncompressed?
Eight files. The third option exists because compressed and lower quality are not the same thing, and PNG is the proof.
In the exam
Sending a design, then publishing it
One caution on the ZIP: if the folder is already full of compressed images and video, do not promise a big size reduction. Already-compressed data has few patterns left to exploit.
Worked answer
4 marks
A designer needs to send a folder of editable project files to a colleague, and to export a photographic banner for the website. Explain the compression choice in each case.
Tempting, but this does not score
They should compress both of them to make the files smaller so they are quicker to send and quicker for the website to load.
It never says which kind of compression, which is the entire distinction being tested. The two cases need opposite answers, and this treats them as the same job.
Worth writing
For the project files, lossless compression such as a ZIP archive — the colleague has to recover those files exactly to carry on editing, and lossy compression would corrupt work in progress. It also bundles them into one transfer. For the banner, a JPEG at a sensible quality setting, because lossy compression cuts the download size for visitors and a photograph tolerates that loss well. The designer keeps the original project separately as the master, since converting that JPEG to PNG later would preserve its current pixels but recover nothing JPEG had already discarded.
Where the marks come from
- Names the type of compression in both cases, not just “compress it”.
- Justifies lossless by what the colleague needs — files back unchanged.
- Justifies lossy by the constraint that applies — visitor download size.
- Adds the master, and explains why converting back does not undo a lossy save.
In the exam
Building a comparison
- Name the compression type, not just a file extension.
- Say whether the original data is preserved or discarded.
- Explain the effect on the product — exact editing, crisp text, clear speech, reliable playback.
- Connect file size to storage, transfer time or mobile data.
- If the command word is evaluate, weigh those up and recommend one for this client.
Not everything that shrinks a file is compression
Cropping an image, shortening a clip or reducing its pixel dimensions all make the file smaller, and none of them is lossy or lossless compression. When the question asks about compression, answer about encoding and what was preserved.
Key takeaways
- Lossless is reversible. Lossy discards information permanently.
- Compressed does not mean lower quality — that is only true of lossy.
- Uncompressed and lossless both preserve the data; only one used a compression method.
- Keep a master, export a delivery copy suited to the platform.
- Cropping or shrinking an image is not compression, even though it reduces file size.
Glossary
- Compression
- Reducing the data needed to represent a file.
- Lossless
- Compression that lets the original data be rebuilt exactly.
- Lossy
- Compression that permanently discards some original information.
- Uncompressed
- Data that was never reduced by a compression method at all.
- Decompression
- Decoding a compressed file so its contents can be used.
- Compression artefact
- Visible or audible damage left by compression — blocks, smearing, distortion.
Test yourself
6 questions
Can a file be smaller without losing any original data?
Yes — that is lossless compression. It stores the data more efficiently and rebuilds it exactly.
Why is lossy compression acceptable for a web photograph?
It cuts the download size a lot, and a photograph tolerates the loss well enough for the size it is displayed at.
Which method preserves editable files exactly for a colleague?
Lossless — a ZIP archive, for example, because the files have to come back unchanged.
Does converting a lossy file to a lossless format restore what was discarded?
No. It preserves the current decoded data. What was already lost stays lost.
Is PNG compressed?
Yes — losslessly. It is compressed and preserves the image data exactly, which is why “compressed” and “worse” are not the same word.
Is cropping an image a form of compression?
No. It makes the file smaller by removing part of the picture, which is a different thing from encoding the same picture in less data.
Common questions
Does compression always make a big difference?
No. It depends on the data and how it is already stored. Recompressing something already compressed may save almost nothing.
Is lossy compression always the wrong choice?
No. It is often the right delivery choice, because a smaller file means more people can access the product. Exact preservation is what calls for lossless or uncompressed.
What is the difference between uncompressed and lossless?
Both preserve every bit of the original data. Uncompressed never used a compression method; lossless used one and can undo it.