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In computing, compression is when the size of a file is reduced but the original message is retained. Think of a photograph. To make the file size of the photo smaller, compression doesn't crop or cut the photo in any way. Instead it reduces the amount of data used to store the file.

Compression is used for different types of data, eg documents, sound, video. It is either applied to transfer a file, whether on a disk or over a network, to ensure that the file can fit on the storage device, or both. The aim of compression is reduce the quantity of the file size but to keep the quality of the original data.

When we talk about compressing graphics for use in multimedia, we mean bitmap graphics not vectors. There are two reasons for this:

  1. Vectors generate smaller file sizes
  1. They aren't supported on the web, therefore don't need to be compressed

Bitmap graphics can produce some of the largest file sizes compared to other media elements, such as text files, vectors, Flash animations. These large file sizes result in slow loading times, particularly on the web and a heavy burden on the system resources (memory and storage space).

You may wonder why this is an issue now that we have faster Internet connections and computers with more memory. Despite this, the file sizes that bitmap graphics can create can still take too long to download, or take up too much space. The more advanced technology becomes the more users want from it.

"The sizes of bitmaps and vectors are affected by the content of images in different ways. The memory requirements for any 45mm square, 24-bit colour bitmapped image of 72 pixels per inch is 48 kilobytes, no matter how complicated the image may be. ... In a vector representation, we store a description of all the objects making up the image; the more complex the picture, the more objects there will be, and the larger the description that will be required, but since we do not actually store the pixels, the size is independent of any resolution." (Chapman & Chapman 2004)

Next: Compression Techniques