Higher Computing Science File Compression

2. File Compression
The purpose of file compression is to reduce the size of files.
Compression makes a file:
•Download / stream more quickly
•Take up less disk space / memory

3. Lossless Compression
• None of the original data is lost
• Methods are found to store the information in a more efficient way

4. Lossless Compression
Example
An image with 16 bit colour has 90 white pixels one after the other
We could store this uncompressed data as 90 x 2 bytes = 180 bytes

5. Lossless Compression
Example
An image with 16 bit colour has 90 white pixels one after the other
We could store this uncompressed data as 90 x 2 bytes = 180 bytes
Instead, we could use 2 bytes to store the number 250 plus a further 2 bytes to store
the colour white = 4 bytes, a saving of 176 bytes

6. Lossy Compression
This method involves sacrificing some of the data that our eyes or ears will
probably not notice
Example
We can reduce the colour bit depth from 24 bits (16,777,216 colours) to 16
bits (65,536 colours)

7. Lossy Compression
This method involves sacrificing some of the data that our eyes or ears will
probably not notice
Example
We can remove sounds that are inaudible to the human ear, very low and
very high frequencies