Making data smaller

Compression stores data in fewer numbers, so it takes less space and less time to send. Run-length encoding stores how many of each colour there are in turn (3, 4, 1) instead of every pixel. It gives back exactly the same picture (lossless), and works best on data with long runs.

1. A shorter row
2. Try: paint a run
3. Worked example: encode a picture › Four rows
4. Worked example: encode a picture › Find the mistake
5. Worked example: encode a picture › Why compress?
6. Vary: one change at a time › A whole run
7. Vary: one change at a time › No runs
8. Vary: one change at a time › Fewest numbers
9. Vary: one change at a time › Compression ratio
10. Vary: one change at a time › When RLE works
11. Practise: codes › Encode
12. Practise: codes › Decode the length
13. Practise: codes › Text
14. Practise: codes › Bigger?
15. Draw from the codes

Lossless compression gives back exactly the original data. Lossy compression throws away detail people are unlikely to notice, such as tiny differences in shade, so files get much smaller, but that detail is gone for good. Music and photos online are usually lossy; text, medical scans and evidence must be lossless.

1. Does it come back?
2. Try: raise the tolerance
3. Worked example: lossless versus lossy › The trade-off
4. Worked example: lossless versus lossy › Find the mistake
5. Worked example: lossless versus lossy › Which must be lossless?
6. Vary: one setting at a time › Back to lossless
7. Vary: one setting at a time › All the way
8. Vary: one setting at a time › Worth it?
9. Vary: one setting at a time › Streaming
10. Practise: choose a format › Formats
11. Practise: choose a format › Evidence
12. Practise: choose a format › Save again
13. Practise: choose a format › Review: run-length codes
14. The edge of lossless