Representing Text, Sound, Images and Storage: Question 5
Syllabus 1.3
A scanned copy of a black-and-white document contains large blocks of a single colour, such as wide plain margins. The document needs to be compressed before it is emailed, without permanently losing any of the original scanned data.
Which compression method would achieve this?
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Worked solution
Why the other options are wrong
- (A) is a lossy method that permanently removes some recorded sound detail, and it is not even relevant to compressing the scanned image itself.
- (B) permanently reduces the number of colours each pixel can be given, so some of the original colour data is lost. This is a lossy method.
- (D) permanently reduces the number of pixels used to store the image, so some of the original image data is lost. This is also a lossy method.
Why C is correct
The document described, large blocks of a single repeated colour, is exactly the situation where run-length encoding (RLE) works well. RLE looks for runs of the same value repeated one after another and stores the value once together with a count of how many times it repeats, instead of storing the value separately for every repeat. Because the original run can be rebuilt exactly from the stored value and count, no data is permanently lost: RLE is a lossless compression method, which matches the requirement in the question.
Final answer
C, applying run-length encoding (RLE) compresses the large blocks of repeated colour without permanently losing any of the original scanned data.