Data Representation - Compression

What is compression?

Reducing a file's size

(by reducing the number of bits required)

Why do we use compression?

-Compressed files are transmitted quicker

-Reduces storage requirements

-Reduces download times

What are the two main types of compression?

Lossless and Lossy

What are the features of lossless compression?

Compression where

-Storage is made more efficient so that redundant data is not stored

-No reduction in quality

-Original file can be recreated

What are some examples of lossless file types?

-PNG

-GIF

What is the lossless compression type for images where RRRRRRRR becomes 8R?

Run-length encoding

How does run-length encoding work?

Consecutive pixels of the same colour are stored more efficiently. Instead of storing that same colour repeatedly, the colour and the number of consecutive pixels with that colour are stored. This reduces the amount of bits stored.

What is the lossless compression type for text files?

Dictionary-based compression

How does dictionary-based compression work?

Common strings of characters are encoded into a dictionary as unique characters and, instead of needlessly repeating these strings, their smaller dictionary entries are used instead therefore reducing the amount of bits stored.

What are the features of lossy compression?

Compression where

-Unneeded data is permanently removed

-Reduction in quality

-Original file cannot be recreated

What are some examples of lossy file types?

-MP3

-JPEG

Why may lossless compression be more suitable than lossy compression?

-No reduction in quality

-Lossy unsuitable for text files

Why may lossy compression be more suitable than lossless compression?

-Generally reduces size more than lossless

-Lossy compressed images load on webpages faster