Image Compression

1. Image Compression Russell Taylor Week 3

2. Compression • Two main types – ‘Lossy’ and Lossless • Lossy – A lossy compression method is one where compressing data and then decompressing it retrieves data that is different from the original, but is close enough to be useful. • Lossycompression is most commonly used to compress multimedia data (audio, video, still images), especially in applications such as streaming media and Internet telephony.

3. Lossy Compression • Two main methods of lossy compression • lossy transform codecs, samples of picture or sound are taken, chopped into small segments, transformed into a new basis space, and quantized (reduced colour range or brightness/contrast range). The resulting quantized values are then specially coded • In lossy predictive codecs, previous and/or subsequent decoded data is used to predict the current sound sample or image frame. The error between the predicted data and the real data, together with any extra information needed to reproduce the prediction, is then quantized and coded i.e. Data is taken out of the image and the image saved and when rendered the missing data is emulated from the data before and after the missing / compressed data to show the image.

4. Lossless Compression • Lossless compression is required for text and data files, such as bank records, text articles, etc. • Lossless compression does not alter the quality of the data, just the way it is described. Instead of detailing the attributes of each pixel individually, lossless compression abbreviates the description while still retaining the meaning • E.g. A picture comprising a number of coloured pixels it can be compressed without loss by saying “300 blue dots" instead of “blue dot, blue dot, ...(297 more times)..., blue dot”

5. Compression Codecs • Lossycodecs: Many of the more popular codecs in the software world are lossy, meaning that they reduce quality by some amount in order to achieve compression, but use some algorithm to create the impression of the data being there. Smaller data sets ease the strain on relatively expensive storage sub-systems such as non-volatile memory and hard disk, as well as write-once-read-many formats such as CD-ROM, DVD and Blu-ray Disc. • Lossless codecs: Typically used for archiving data in a compressed form while retaining all of the information present in the original stream. • If preserving the original quality of the stream is more important than eliminating the correspondingly larger data sizes, lossless codecs are preferred. • This is especially true if the data is to undergo further processing (e.g. editing) in which case the repeated application of processing (encoding and decoding) on lossycodecs will degrade the quality of the resulting data such that it is no longer identifiable (visually, audibly or both). Using more than one codec or encoding scheme repeatedly can also degrade quality significantly. • The decreasing cost of storage capacity and network bandwidth has a tendency to reduce the need for lossycodecs for some media.

6. Example of Compression • E.g. 9.9 megabyte 1943 x 1702, 24-bit RGB colour image