Image compression can be lossy or lossless. Lossless compression is preferred for artificial images such as technical drawings, icons or comics. This is because lossy compression methods usually introduce highly visual artifacts near sharp edges. Lossy methods are suitable for natural images such as photos or medical imagery.
Methods for lossless image compression are:
Scalability generally refers to a quality reduction achieved by manipulation of the bistream or file (without decompression and re-compression). Other names for scalability are progressive coding or embedded bistreams. Despite its contrary nature, scalability can also be found in lossless codecs, usually in form of coarse-to-fine pixel scans. Scalability is especially useful for pre-viewing images while downloading them (e.g. in a web browser) or for providing variable quality access to e.g. databases. There are several types of scalability:
Meta information. Compressed data can contain information about the image which can be used to categorize, search or browse images. Such information can include color and texture statistics, small preview images and author/copyright information.