Editing Halftone (section)

===Multiple screens and color halftoning===
[[File:Halftoningcolor.svg|frame|Three examples of modern color halftoning with CMYK separations. From left to right: The cyan separation, the magenta separation, the yellow separation, the black separation, the combined halftone pattern, and finally how the human eye would observe the combined halftone pattern from a sufficient distance.]]
[[File:CMYK raster print.jpg|thumb|250px|This close-up of a halftone print shows that magenta on top of yellow appears as orange/red, and cyan on top of yellow appears as green.]]
[[File:CMYK screen angles.svg|thumb|Examples of typical [[CMYK color model|CMYK]] halftone [[screen angle]]s]]
[[File:Screens. Halftone process in printing. img 01.jpg|right|thumb|300 px|Purple screens used in [[offset printing]]: Angles 90°, 105°, 165°]]

When different screens are combined, a number of distracting visual effects can occur, including the edges being overly emphasized, as well as a [[moiré pattern]]. This problem can be reduced by rotating the screens in relation to each other. This ''screen angle'' is another common measurement used in printing, measured in degrees clockwise from a line running to the left (9 o'clock is zero degrees). These angles are optimized to avoid patterns and reduce overlap, which can cause colors to look dimmer<!--and drop expansion?-->.{{citation needed|date=January 2014}}

Halftoning is also commonly used for printing color pictures. The general idea is the same, by varying the density of the four secondary printing colors, cyan, magenta, yellow, and black (abbreviation [[CMYK color model|CMYK]]), any particular shade can be reproduced.<ref>{{usurped|1=[https://web.archive.org/web/20120222202831/http://www.prepressx.com/Images/color-theory.html Halftone Line Screens in Printing]}} "Use of halftone line screens for printing digital images on press". (last checked on 2009-04-20)</ref>

In this case there is an additional problem that can occur. In the simple case, one could create a halftone using the same techniques used for printing shades of grey, but in this case the different printing colors have to remain physically close to each other to fool the eye into thinking they are a single color. To do this the industry has standardized on a set of known angles, which result in the dots forming into small circles or rosettes.