Compression (Cambridge (CIE) A Level Computer Science): Exam Questions

Identify one lossless method of compressing an image.

The colour of each pixel in a bitmapped image is represented by 8 bits.

A bitmap image can be compressed using lossy compression.

Explain the reasons why lossy compression is often suitable for a bitmap image.

The computer can transmit a video made from bitmap images and vector graphic animations to the VR headset.

The bitmap video is not compressed before transmission to the VR headset.

Give two reasons why the video does not need to be compressed.

The photograph is compressed before being uploaded to a web server.

(i) Give three benefits of this photograph being compressed using lossy compression instead of lossless compression.

(3)

(ii) Explain how run-length encoding (RLE) will compress the photograph.

(2)

Music is stored as a sequence of digital samples.

Each digital sample is a denary value in the range 0 to 99999999 (8 digits).

The samples are to be stored in a text file. Each sample is converted to a numeric string and 32 samples are concatenated (joined) to form a single line of the text file.

Each numeric string is 8 characters in length; leading ‘0’ characters are added as required.

Example:

Sample	Denary value	String
1	456	"00000456"
2	48	"00000048"
3	37652	"00037652"
32	673	"00000673"

The example samples will be stored in the text file as a single line:

"000004560000004800037652...00000673"

(i) Identify one drawback of adding leading ‘0’ characters to each numeric string.

(1)

(ii) Suggest an alternative method of storing the samples which does not involve adding leading ‘0’ characters but which would still allow each individual sample to be extracted.

(1)

(iii) State one drawback of the alternative method given in part (b)(ii).

(1)

Explain why a bitmap image is often compressed before it is attached to an email.

A real-time video of a music concert needs to be streamed to subscribers.

Tick (✓) one box to identify the most appropriate type of compression and justify your answer.

The program is distributed by downloading the source code and its library files from a web server.

Explain the reasons for compressing the files.