Pixelization (British English, pixelisation) or Mosaic is any technique used in editing images or video, whereby an image is blurred by displaying part or all of it at a markedly lower resolution. It is primarily used for censorship. The effect is a standard graphics filter, available in all but the most basic bitmap graphics editors.
A familiar example of pixelization can be found in television news and documentary productions, in which vehicle license plates and faces of suspects at crime scenes are routinely obscured to maintain the presumption of innocence, as in the television series COPS. Bystanders and others who do not sign release forms are also customarily pixelized. Footage of nudity (including male and female genitals, buttocks, nipples, pubic hair, or areolae) is likewise obscured in some media: before the watershed in many countries, in newspapers or general magazines, or in places in which the public cannot avoid seeing the image (such as on billboards). Drug references, as well as gestures considered obscene (such as the finger) may also be censored in this manner. Pixelization is not usually used for this purpose in films, DVDs, subscription television services, or pornography (except for countries in which the law requires it). When obscene language is censored by an audible bleep, the mouth of the speaker may be pixelized to prevent lip reading, often as in COPS. Graphic injuries and excess blood can also be pixelized.
Pixelization may also be used to avoid unintentional product placement, or to hide elements that would date a broadcast, such as date and time stamps on home video submissions. Censorship for such purposes is most common on reality television series.
Pixelization has also been used for artistic effect, notably in the art print The Wave of the Future, a reinterpretation of Katsushika Hokusai's The Great Wave at Kanagawa. In this updated print, the image of the large ocean wave shifts from the traditional style of the Japanese woodcut print to a pixelized image and finally to a wireframe model computer graphics image. Westworld (1973) was the first feature film to use digital image processing to pixelize photography to simulate an android's point of view.
A black rectangular or square box (known as censor bars) may be simply be used to occlude parts of images completely (for example, a black bar covering the eyes instead of the entire face being pixelized). Censor bars were extensively used as a graphic device in the January 2012 Protests against SOPA and PIPA.
A drawback of pixelization is that any differences between the large pixels can be exploited in moving images to reconstruct the original, unpixelized image; squinting at a pixelized, moving image can sometimes achieve a similar result. In both cases, integration of the large pixels over time allows smaller, more accurate pixels to be constructed in a still image result. Completely obscuring the censored area with pixels of a constant color or pixels of random colors escapes this drawback but can be more aesthetically jarring.
International legal standards
Nudity is obscured on television networks in the United States. Japanese pornography laws require that genitals in films (including animated works) and other forms of adult media (such as eroge, drawings, etc) be obscured. In Thailand, restrictions are placed on television broadcast depiction of cigarettes being smoked, alcohol being consumed, or guns being pointed at people. Pixelization is one method of censoring this content; otherwise, censor bars are used. In the Philippines, pixelization is also used if there are scenes of naked people or cadavers, bloody depiction of death by any means (e.g. gunshot wounds) and exposure of innards (sometimes rendered in black and white and blurred), and the finger gesture, among objectionable content. However, nudity and some bloody scenes are cut entirely and pointing of guns or blades to oneself or others are cropped.
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