#11 Compression, Encryption and Hashing

1. Which of these is likely to use lossy compression?

• A. Emailing a PDF
• B. Saving a spreadsheet
• C. Preparing a video for distribution
• D. Uploading a job application

2. Which of the following is not a relevant technical consideration when opting for a compression technique?

• A. CPU processing power/speed
• B. Whether the file will have to be decompressed before it is used
• C. Network connection bandwidth
• D. Volumes of file storage available

3. How can lossy compression be applied to image files?

• A. Reduce the resolution or the colour depth
• B. Reduce the resolution and increase the colour depth at the same time
• C. Increase the resolution
• D. Convert from JPG to BMP

4. What is meant by lossless compression?

• A. No compression is applied
• B. The physical representation of the file is reorganised in such a way that it can be entirely reconstructed when decompressed
• C. Selected parts are removed which were not relevant anyway
• D. Metadata is removed so only the main file contents remain

5. Which of the following file types does not use lossless compression?

• A. ZIP
• B. GIF
• C. PNG
• D. MP3

6. Which of these is not true of the JPEG (JPG) file type?

• A. It remains a popular image file type online due to its significant compression keeping file sizes low
• B. It uses lossless compression
• C. Users manipulating a JPEG can vary the amount of compression applied
• D. Compressing a JPEG is not the same thing as encryption

7. Which of these is a characteristic of dictionary encoding?

• A. The dictionary is used to compress the message but is not used when decompressing the message
• B. It is commonly used to substitute long sequences of the same colour of pixel in an image, so, for example, a sequence of 250 red pixels could be replaced with the code Red250
• C. It is used by BMP, GIF and PNG
• D. The dictionary has to be stored alongside the encoded message

8. Which of these file types uses the LZW (Lempel-Ziv-Welch) algorithm for compression?

• A. BMP
• B. MP3
• C. ZIP
• D. TIFF

9. Why is lossy compression considered desirable?

• A. It achieves significant reduction in file sizes, often beating lossless techniques
• Original files can be reconstructed in their entirety
• B. C. Entire folders of files can be compressed down into one file
• D. It encrypts the data

10. Which one of these statements describes zipping a folder full of files?

• A. It uses run-length encoding
• B. It uses the LZW algorithm, a dictionary encoding technique
• C. It applies lossy compression using dictionary encoding
• D. No dictionary is built up during the compression stage