1. File Compression
3.3

2. Learning Objectives
3.3.2 Understand the need for data compression and methods of compressing data [lossless, lossy] and that JPEG and MP3 are examples of lossy algorithms
3.3.3 Understand how a lossless, run-length encoding [RLE] algorithm works
3.3.4 Understand that file storage is measured in bytes and that data transmission is measured in bits per seconds, and be able to calculate the time required to transmit a file and storage requirements for files

3. Transfer Rate Why compress data? MBps or Mbps?
If you have 10 high quality images at 4Mb’s each. How much storage will it take up?
What if you were to transfer this data across the internet, how would you calculate the transfer rate, what is it measured in?
Transfer rate (Mbps) = Number of files * file size / time taken * 8
MBps or Mbps?

4. What do we already know? What is colour depth?
How many colours can be represented using 4 bits?
What are the differences between bitmap images and vector images?

5. Answer the following in your books
Use BBC Bitesize to help you:
Google bbc bitesize compression
Why compress an image?
What are 2 compression methods?
What are at least 3 advantages and disadvantages of these 2 compression methods?

6. Understanding Images Music
Images Music
Lossless Lossless
PNG WAV
Lossy Lossy
JPEG MP3
GIF AAC

7. RLE: Run-length encoding 3.3.3
Simple compression method
Lossless as no original data is lost
Converts a stream of data into
Consecutive identical characters separated by
A number count equal to the length of the run
Used a lot on imagery
For example here is 23 pixels in a 1-bit colour image. Think of each pixel as being White or Black B W
1B1W3B4W1B3W3B7W Using a simple code reduces 23 characters down to 16
BWBBWWBWWBBWW along with [3,4,3,3,7] A way of coding the characters and number of counts >1 as a sequence
Across a compete image display, the reduction in code is significant. How is this lossless?

8. Extension 1: Using Data Compression, Why?
Benefits
How are they achieved. Give examples
Reduced storage costs
Reduced transmission costs
e.g. Smaller file size results in lower cost. Streaming costs for a SD movie compared to a HD movie
Increase transmission speed
Increase bandwidth utilisation
Simplify computation using algorithms
Task: Complete this table, research the benefits of Data Compression

9. Activity
1, 3, 1
4, 1

10. Explanation https://www.youtube.com/watch?v=Lto-ajuqW3w 2:30-6:43

11. Plenary Why compress images?
What are the two main ways to compress images?
Name some of the differences
Outline the RLE algorithm
What compression method would you use when uploading an image to Facebook?