Image compression may belossy orlossless. Lossless compression is preferred for archival purposes and often for medical imaging, technical drawings,clip art, or comics. Lossy compression methods, especially when used at lowbit rates, introducecompression artifacts. Lossy methods are especially suitable for natural images such as photographs in applications where minor (sometimes imperceptible) loss of fidelity is acceptable to achieve a substantial reduction in bit rate. Lossy compression that produces negligible differences may be called visually lossless.
Discrete Cosine Transform (DCT) – The most widely used form of lossy compression. It is a type ofFourier-related transform, and was originally developed byNasir Ahmed, T. Natarajan andK. R. Rao in 1974.[2] The DCT is sometimes referred to as "DCT-II" in the context of a family of discrete cosine transforms (seediscrete cosine transform). It is generally the most efficient form of image compression.
DCT is used inJPEG, the most popular lossy format, and the more recentHEIF.
Color quantization - Reducing thecolor space to a few "representative" colors in the image. The selected colors are specified in the colorpalette in the header of the compressed image. Each pixel just references the index of a color in the color palette. This method can be combined withdithering to avoidposterization.
Whole-image palette, typically 256 colors, used in GIF and PNG file formats.
block palette, typically 2 or 4 colors for each block of 4x4 pixels, used inBTC,CCC,S2TC, andS3TC.
Chroma subsampling. This takes advantage of the fact that the human eye perceives spatial changes of brightness more sharply than those of color, by averaging or dropping some of the chrominance information in the image.
The best image quality at a given compression rate (orbit rate) is the main goal of image compression, however, there are other important properties of image compression schemes:
Scalability generally refers to a quality reduction achieved by manipulation of the bitstream or file (without decompression and re-compression). Other names for scalability areprogressive coding orembedded bitstreams. Despite its contrary nature, scalability also may be found in lossless codecs, usually in form of coarse-to-fine pixel scans. Scalability is especially useful for previewing 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:
Quality progressive or layer progressive: The bitstream successively refines the reconstructed image.
Resolution progressive: First encode a lower image resolution; then encode the difference to higher resolutions.[6][7]
Component progressive: First encode grey-scale version; then adding full color.
Region of interest coding. Certain parts of the image are encoded with higher quality than others. This may be combined with scalability (encode these parts first, others later).
Meta information. Compressed data may contain information about the image which may be used to categorize, search, or browse images. Such information may include color and texture statistics, smallpreview images, and author or copyright information.
Processing power. Compression algorithms require different amounts ofprocessing power to encode and decode. Some high compression algorithms require high processing power.
The quality of a compression method often is measured by thepeak signal-to-noise ratio. It measures the amount of noise introduced through a lossy compression of the image, however, the subjective judgment of the viewer also is regarded as an important measure, perhaps, being the most important measure.
TheJPEG 2000 standard was developed from 1997 to 2000 by a JPEG committee chaired by Touradj Ebrahimi (later the JPEG president).[18] In contrast to the DCT algorithm used by the original JPEG format, JPEG 2000 instead usesdiscrete wavelet transform (DWT) algorithms. It uses theCDF 9/7 wavelet transform (developed byIngrid Daubechies in 1992) for its lossy compression algorithm,[19] and the Le Gall–Tabatabai (LGT) 5/3 wavelet transform[20][21] (developed by Didier Le Gall and Ali J. Tabatabai in 1988)[22] for its lossless compression algorithm.[19]JPEG 2000 technology, which includes theMotion JPEG 2000 extension, was selected as thevideo coding standard fordigital cinema in 2004.[23]
^Shao, Dan; Kropatsch, Walter G. (February 3–5, 2010). Špaček, Libor; Franc, Vojtěch (eds.)."Irregular Laplacian Graph Pyramid"(PDF).Computer Vision Winter Workshop 2010. Nové Hrady, Czech Republic: Czech Pattern Recognition Society.Archived(PDF) from the original on 2013-05-27.
^Le Gall, Didier; Tabatabai, Ali J. (1988). "Sub-band coding of digital images using symmetric short kernel filters and arithmetic coding techniques".ICASSP-88., International Conference on Acoustics, Speech, and Signal Processing. pp. 761–764 vol.2.doi:10.1109/ICASSP.1988.196696.S2CID109186495.
