Posterization (from English posterization ; posterized images ) describes the photographic effect of tone separation (or tone value separation) with simultaneous tone value reduction. The effect is used as a means of artistic creation or as a first step in vectorization . Another name for the effect is the Greek Isohelie , which refers to the resulting zones of equal brightness .
First, three or more density separations are created from normal halftone negatives by repeated copying onto hard- working lith material . The result is an image that converts the entire tonal range of the original recording into a few tonal values (e.g. white, gray, black). In conventional photography, hard-working lith film is preferably used when copying , which is exposed or developed for different lengths of time. The films representing the individual tonal values are precisely mounted on top of each other and enlarged. From Agfa there was a special film for this purpose, the AGFA Contour Professional , as the sheet film was offered and significantly simplified the procedure.
As with most photographic effects, the effort involved in electronic image processing is also simplified here . In the image on the right (see below), manipulated in a standard image processing program, the tone values were reduced to five, at the same time the remaining ones were partially colored after the contrast had also been reduced for some of them . This creates an image that is reminiscent of Pop Art .
The effect can also arise as an undesirable artifact of the technical - mostly digital - process. It is then called banding . Banding is the fact that a brightness profile is not uniform, but in visible delimited strips (engl. Tape is shown).
Some of the causes of banding in digital cameras are:
- Unfavorable tone value curves with the camera sensor .
- Too low a brightness resolution in the processing chain - eight bits per color channel ( JPEG ) are common, i.e. 256 brightness values. In high-quality cameras, ten, twelve or fourteen bits are sometimes used in order not to limit the dynamic range of the sensor, which reaches 9.0, for example.
- Subsequent image processing such as brightening or changing the contrast.