Adobe sought to solve this problem with RT Screening, a screen algorithm devised by Linotype-Hell and licensed by Adobe in PostScript Level 1. A consequence of this is that the cells don't always line up with the recorder grid at all screen angles, specifically the irrational ones. However, it has been done, and several vendors (in particular, Hell Graphics Systems, now Linotype-Hell) have invented and licensed special screening hardware that has the power to effectively handle irrational screen angles.Īs desktop computers didn't have the power to deal with irrational screen angles in the same way as the high-power Linotype-Hell systems, halftones created on them possess cells that are all the same shape, but of different sizes. Needless to say, this requires a very powerful computer.
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This is problematic unless each halftone cell can be described individually, rather than en masse. At irrational screen angles, however, the cells do not fit properly on the recording grid, which results in variably shaped halftone cells and cells comprising different numbers of spots. This becomes important in digital halftone output because at rational screen angles, each halftone cell can fit properly on the grid (or bit map) of the recording device each halftone cell will intersect the grid at the corners of a printer spot, which will allow all the halftone cells to have the same size. At some angles, such as 45º and 0º, that ratio is a rational number (a rational number being one that can be expressed evenly as the ratio of two integers) while for some other angles (such as 15º and 75º), that ratio is an irrational number (an irrational number is one that can'not' be expressed evenly, consisting of decimal places that continue on perhaps forever-like pi, which equals 3.14159. Screen angles are described in terms of their tangents, or the ratio of the opposite and adjacent sides. Hence, rational screen angles and irrational screen angles come into play. Unfortunately, moiré can appear with even the slightest deviation in screen angle (even as little as 0.01º). But the screen angles available for output are dependent upon the device resolution, and it may not be possible to produce a desired screen angle on a particular device. Since each digital halftone dot is made up of smaller printer spots (collectively called a halftone cell), the computer output device (in particular, the device's "]) needs to calculate a screen angle at which to set a particular row of dots. Digital halftoning creates another set of screen angle problems. In the older photographic halftone screening, generating proper screen angles was a simple question of turning the screen to the desired angle before exposing the films. Depending on the application, these angles may be varied, but only by a color separator who knows what s/he is doing. (Even in black-and-white halftone production, a perfectly horizontal screen angle results in more of a visual discernment of the individual dots than does a 45º angle.) Therefore, yellow, which is the lightest color, is best left along one of these axes, while black, the darkest color, is best kept as far from both as possible (or 45º, the midway point between vertical and horizontal). Generally speaking, the further a separation is from either the horizontal or vertical axis, the less intrusive it tends to be. The black separation is 45º from the horizontal, and the cyan separation is 75º from horizontal. The magenta separation is 15º from the horizontal. The screen angle of the yellow separation is 0º, or perfectly horizontal. Experience, though, has resulted in a standard set of default screen angles which work very well in a wide variety of applications. However, since there is only a total of 90º (at least for perfectly round dots) in which to rotate the screens, each screen can't be 30º from each other when printing four colors (30 x 4 = 120º).
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Ideally, moiré is kept minimal when screens are 30º from each other. In order to eliminate undesirable moiré patterns when the four color separation halftones are overprinted in multi-color printing, each screen needs to be placed at a different angle, as the dots of one color interfere with those of another color, creating the distinct moiré patterns. In process color prepress and printing, the angle at which the rows of halftone dots run in relation to the horizontal.