Compression (Cambridge (CIE) O Level Computer Science)

Revision Note

Robert Hampton

Written by: Robert Hampton

Reviewed by: James Woodhouse

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

  • 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

3 images of a husky dog, 1 with no compression, 1 at 50% and 1 at 80%
  • 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

Diagram to show loss in file size from DOCX to PDF
  • 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

Worked Example

An email is sent containing a sound file.

Lossy compression is used to compress the sound file.

Explain two reasons why using lossy compression is beneficial. [4]

How to answer this question

  • What are the differences between lossy and lossless?

  • Can you state two differences? [2 marks]

  • Can you say why each point is a benefit? [2 marks]

Answer

  • Lossy will decrease the file size [1]

  • ...so it can sent via email quicker [1]

  • Lossy means data is lost [1]

  • ...the difference is unlikely to be noticed by humans [1]

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Robert Hampton

Author: Robert Hampton

Expertise: Computer Science Content Creator

Rob has over 16 years' experience teaching Computer Science and ICT at KS3 & GCSE levels. Rob has demonstrated strong leadership as Head of Department since 2012 and previously supported teacher development as a Specialist Leader of Education, empowering departments to excel in Computer Science. Beyond his tech expertise, Robert embraces the virtual world as an avid gamer, conquering digital battlefields when he's not coding.

James Woodhouse

Author: James Woodhouse

Expertise: Computer Science

James graduated from the University of Sunderland with a degree in ICT and Computing education. He has over 14 years of experience both teaching and leading in Computer Science, specialising in teaching GCSE and A-level. James has held various leadership roles, including Head of Computer Science and coordinator positions for Key Stage 3 and Key Stage 4. James has a keen interest in networking security and technologies aimed at preventing security breaches.