1.3 Compression

Question 1

Why is compression needed?

Answer 1

• Data files can be very large.
• It would take too long to send an uncompressed file.
• The compressed file will download faster.

Question 2

Lossy compression

Answer 2

Lossy compression removes data from the file, resulting in loss of detail.

Question 3

Lossless compression

Answer 3

Lossless compression reduces file size without losing any original detail. Allowing the original file to be perfectly reconstructed.

Question 4

A program written in a high-level language.

Use Lossy or Lossless?

Answer 4

Lossless

1) All the data is needed
2) If data is lost, the program will not run.

Question 5

A photograph emailed to a friend?

Lossy or Lossless?

Answer 5

Lossy

1) Not all the data is required.
2) Number of colours can be reduced without the user noticing.
3) Email requires a smaller file size.

Question 6

Uploading a video to a website?

Lossy or Lossless?

Answer 6

Lossy

1) Some loss of quality will not be noticed.
2) A more significant reduction may be needed.
3) Takes a shorter time to upload.

Question 7

How can a text file be compressed?

Answer 7

Huffman compression, repeated sequences can be represented with a variable to lower file sizes.

Question 8

How can a Bitmap image be compressed?

Answer 8

Run-length encoding, storing the number of times each color occurs.

Question 9

How can a sound file be compressed?

Answer 9

Lossy compression, sample resolution can be reduced so fewer bits will be used to to each sample so less data stored.

Question 10

What is meant by Run-length encoding?

Answer 10

• lossless form of compression.
• reduces the physical size of a string of adjacent pixels
• the repeating string is encoded into the frequency: notation form.

Question 11

Explain how temporal redundancy compresses

a video.

Answer 11

• Identifies pixels that do not change between frames.
• Records only the differences between the frames.
• Makes file size smaller.