A halftone image shows colors running into each other in every possible tone gradation, i.e. from the deepest black to the lightest gray, from the lightest hue to the darkest. Photographs are usually halftone images.
Due to the nature of the process, letterpress and offset printing cannot produce true halftones like gravure printing . If a halftone image is reproduced with this printing process , the original must be broken down (rasterized) into raster dots during image reproduction by means of a raster . H. a screened image is basically a line image that simulates a halftone image. In English usage, the screened halftone image is often referred to as halftone .
In contrast to the line image or full tone image, a halftone image has brightness gradations of the motif in a tone scale from white to black (grayscale image). The ( logarithmic ) measure of the difference in brightness between the lightest and the darkest (blackest) tone value (light to shadow) is called gradation , measured in gamma (γ). If the gradation is low, the image is called soft ; in the extreme case, dull , if it is high, the image appears hard (i.e. very high-contrast ).
If the tonal values are reduced to a few, the poster effect (US English posterized image ; posterization ) occurs. If there are only two tone or color values left, it is a line image .
The printing technique of the halftone image produces a special effect when the usual black is replaced by other dark printing colors, for example a brown sepia tone .