Squash It

A photo can be shrunk to a fraction of its size. Learn why files are compressed, tell lossless from lossy, and squash a run of pixels yourself with run-length encoding.

⏱️ 15 min 🎯 14 activities Teachers Not yet rated Students Not yet rated

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What you'll cover

Making files smaller 🗜️

A single photo can be millions of bytes. **Compression** squashes a file down so it takes less space and travels faster across the internet. There are two kinds: **lossless**, which keeps every last bit, and **lossy**, which throws some data away to shrink the file further. This module tells them apart.

The language of compression 🔑

Four ideas run through this module:

Lossless and lossy ⚖️

The two methods trade quality against file size in opposite ways.

Which can be undone? ♻️

Which type of compression lets you restore the original file exactly, with nothing lost?

Lossless compression
Lossy compression
Neither type
Both types equally well

Match each term to its meaning 🔗

Lossless
Lossy
Run-length encoding
The need for compression

The original file is restored perfectly
Some detail is permanently thrown away
Storing a count next to each repeated value
Smaller files transfer and store more easily

Why compress? 📉

Select the TWO reasons a business might compress its files.

To transfer them faster over the internet
To use less storage space
To make the files impossible to open
To add extra detail to the files

Run-length encoding 🏷️

Run-length encoding, or RLE, is a **lossless** method. It replaces a run of the same value with a **count** followed by that value. For example, WWWWWBBB becomes 5W3B. It works brilliantly on images with large blocks of one colour, but adds little where values rarely repeat.

How run-length encoding works 🪜

An interactive activity.

Squash the run 🔢

Using run-length encoding, how is the string AAAABB compressed?

4A2B
A4B2
AAAABB with no change
6AB

Compression in words 🧩

Compression makes a file _____. _____ compression loses no data, so the original is restored perfectly, while _____ compression removes some data for good. Run-length encoding is a lossless method that stores a _____ next to each repeated value. Smaller files also transfer more _____.

smaller Lossless lossy count quickly larger colour name slowly binary

True of lossy ✂️

Select the TWO statements that are true about lossy compression.

It makes smaller files than lossless compression
Some data is removed permanently
The original file can be restored exactly
It is the best choice for a legal document

Spot the true statements 🖍️

An interactive activity.

Choose the method 🧭

An interactive activity.

Lossy or lossless? ✍️

An interactive activity.