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

The Need For Compression

What is compression?

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

• The impact of compression is:

  • Less storage space required

  • Less bandwidth required

  • Shorter transmission time

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

Lossy Compression

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

Lossless Compression

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 algorithms such as run length encoding (RLE) can be used to analyse the contents looking for patterns and repetition.

What is run length encoding?

• Run length encoding (RLE) is a form of lossless data compression that condenses identical elements into a single value with a count

• For a text file, "AAAABBBCCDAA" is compressed to "4A3B2C1D2A"

• The string has four 'A's, followed by three 'B's, two 'C's, one 'D', and two 'A's

• RLE is used in bitmap images to compress sequences of the same colour

• For example, a line in an image with 5 red pixels followed by 3 blue pixels could be represented as "5R3B"

Lossless file formats

• 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