Compression (Cambridge (CIE) A Level Computer Science) : Revision Note

The need for compression

What is compression?

• 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

• Compression can be achieved using two methods, lossy and lossless

Lossy vs lossless

What is lossy compression?

• 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

• 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

• In the images above, lossy compression is applied to a photograph and dramatically reduces the file size

• Data has been removed and the overall quality has been reduced, however it is acceptable as it is difficult to visually see a difference

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

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

• In a document, lossless compression uses algorithms to analyse the contents looking for patterns and repetition

• In the image above, lossless compression is automatically applied to document formats such as DOCX and PDF with a different rate of success

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

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