OCR AS and A level Computer Science Exercises

Q1.(a) Explain why compression is considered necessary for images on the web. [2] Transmission of data is faster (web pages will load more quickly) and uses less data allowance with smaller image files. Q1.(b) Explain why lossy compression techniques would not be suitable for use with files containing large bodies of text. [1] Since lossy methods do not retain 100% of the original data, a text file would lose some characters and become more difficult to read. Q1.(c) Suggest a suitable lossless method for compressing text. [1] Dictionary-based compression. Q2.(a) Explain lossy data compression Lossy compression permanently removes unnecessary data giving a close approximation of the original file after decompression. This may look the same, but a byte-by-byte comparison will show differences. Q2.(a) Explain lossless data compression Lossless compression uses an algorithm to compress the file with instructions to replicate it again as an exact copy. Q2.(c) RLE encoding is a lossless compression method. Give one disadvantage of lossless compression over lossy methods for the compression of images. [1] Although compressed compared to the original, lossless methods result inlarger files than with lossy compression.