Why Compress Files?

Digital files — especially images, audio, and video — can be very large. Compression reduces file size so that:

• Files take up less storage space (hard drives, memory cards)
• Files can be transmitted faster over a network (smaller = quicker to download/upload)
• More files can be stored on a device

There are two main types of compression: lossy and lossless.

Lossy Compression

Lossy compression permanently removes data that is considered less important or less perceptible to humans. The original file cannot be perfectly reconstructed from the compressed version.

How it works (JPEG example): removes colour information from areas where the eye is less sensitive to detail — e.g. gradients in skies are simplified. Small details and colour variations humans can barely see are removed, reducing file size greatly.

How it works (MP3 example): removes audio frequencies that are masked by louder sounds or are too high/low for most people to hear clearly.

Lossless Compression

Lossless compression reduces file size without losing any data. The original file can be perfectly reconstructed from the compressed version. It works by finding patterns and redundancy in data and encoding them more efficiently.

Comparing Lossy and Lossless

Run-Length Encoding (RLE)

Run-length encoding is a simple lossless compression algorithm. It replaces consecutive runs of the same value with a (count, value) pair. It works best when there are long sequences of repeated values — common in images with large areas of the same colour.

RLE Example — Pixel colours

RLE Example — Binary image

Limitation: RLE performs poorly on data with very few repeated values (e.g. a photograph with many different colours). In the worst case, it can make the file larger.

Where RLE works well: simple graphics, icons, black-and-white images, fax transmissions.