Week 8 Compression

Question 1

What is compression?

Answer 1

The process of reducing the number of bits required to represent data

Question 2

Why might compression be used? [3]

Answer 2

• Reduce the storage requirements;
• Ensure faster transmission;
• Comply with file size restrictions (e.g. on email)

Question 3

Lossless Compression

Answer 3

When decompressing the image, the original uncompressed image is restored exactly as it was.

Question 4

Lossy Compression

Answer 4

When irrelevant data is discarded such that when decompressed, the original file is not recovered.

Question 5

State the advantages of lossless compression [2]

Answer 5

The file can be reproduced exactly as it was originally;
A. The quality of an image/sound/video would not be reduced.

The original data can be fully recovered if lossless compression has been used // lossless data compression can be reversed;
NE. no data is lost NE. no loss of quality

The original data cannot be recovered if lossy compression has been used // lossy compression cannot be reversed // the data is degraded by lossy compression;
A. redundant / less important data removed
NE. data is lost
NE. quality is reduced

Question 6

State one disadvantage of lossless compression [2]

Answer 6

Not as big a file size reduction as lossy compression

Question 7

State one advantage of lossy compression [1]

Answer 7

Greater file size reduction than lossless compression

Question 8

State two disadvantages of lossy compression [2]

Answer 8

May result in lower quality image/sound; Unable to recover discarded data

Question 9

When can run-length encoding (RLE) be used with bitmaps?

Answer 9

If three or more consecutive pixels contain the same bit pattern, then a run of cells can be compressed.

Question 10

How does RLE reduce file size?

Answer 10

The run of identical pixels can be stored in two bytes: number in the run; colour of the pixels.

Question 11

Describe how dictionary-based compression works

Answer 11

• (Variable) length strings of symbols/substrings of original data are represented by single tokens;
• A table/dictionary is formed using the tokens as the keys/index;
• The strings of symbols are used as the entries;