Compression

What is compression and why is it needed

• Compression is reducing the the size of a file so that it takes up less space on secondary storage

• There are scenarios where compression may be needed, such as:

  • Maximise the amount of data you can store on a digital device such as a mobile phone or tablet

  • Minimise the transfer time of data being uploaded, downloaded or streamed across a network such as the Internet

Two types

• lossy

• Lossless

Lossy

• Lossy compression is when data is lost in order to reduce the size on secondary storage

• Lossy compression is irreversible

• Lossy can greatly reduce the size of a file but at the expense of losing quality

When is lossy compression acceptable

• Lossy is only suitable for data where reducing quality is acceptable, for example images, video and sound

• In photographs, lossy compression will try to group similar colours together, reducing the amount of colours in the image without compromising the overall quality of the image

Benefit of lossy compression

• Lossy compressed photographs take up less storage space which means you can store more and they are quicker to share across a network

lossless compression

• Lossless compression is when data is encoded in order to reduce the size on secondary storage

• Lossless compression is reversible, the file can be returned to its original state

• Lossless can reduce the size of a file but not as dramatically as lossy

• Lossless compressed documents take up less storage space which means you can store more and they are quicker to share across a network

When is lossless compression used

Lossless can be used on all data but is more suitable for data where a loss in quality is unacceptable, for example documents

How does lossless compression compress files

In a document, lossless compression uses algorithms to analyse the contents looking for patterns and repetition. For example, repeating characters are replaced with a single character and the number of occurrences in the document (“EEEEE” becomes “E5”)

decompression algorithms

When you open a lossless compressed document the decompression process reverses the algorithms and returns the data back to its original state