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The RTI Harlequin RIP offers you complete control of screen angles for both process and spot colors. A long-established reason for choosing the screening angles (and frequencies) of color separations has been to pick a set of values that minimize inaccuracies in the screens and consequent moiré patterning when they are superimposed. This reason is becoming less important as the accuracy of screen generation increases.
The RIP includes Harlequin Precision Screening (HPS), a technology that provides more accurate screening and reduces moiré without unduly limiting the choice of screen angle and frequency. Harlequin Precision Screening allows users to choose how rosettes are formed in the image and gives users the ability to generate extra gray levels which allow the use of higher screen frequencies than the resolution would normally allow. With 'Generate Extra Gray Levels' enabled you can also limit the number of distinct gray levels at 128, 256, 512, 1024, 2048 or 4,096.
HPS allows you to select any screen frequency and to use the usual CMYK screen angles of 0°, 15°, 45°, and 75° (plus multiples of 90°). To reduce moiré patterning, HPS uses an adaptive screening technique that can adjust each halftone dot so that it is placed within one half pixel of its ideal location. HPS also allows you to choose how rosettes are formed in the image. When color separations are combined, the dots form one of two rosette patterns (clear-centered rosettes or spot-centered rosettes). Clear-centered rosettes are less likely than spot-centered rosettes to show a significant color shift if separations are printed slightly out of register, but the output produced is normally less saturated, lighter, and has a more noticeable rosette structure. In general, output at high frequencies, or where the output may be run on presses with poor registration, should use clear-centered rosettes, while output at low screen frequencies or on well registered devices (including most color printers) should use spot-centered rosettes.
Clear-Centered and Spot-Centered Rosette Sample
You may wish to override a set of angles requested in a job. This is especially useful if the job has requested a set of angles that optimize the output quality for a particular output device, such as a laser printer, but that may diminish the quality on other devices, such as a platesetter. You may want consistency. For example, on all pages of a single publication when the jobs come from different sources. Enforcing settings in the RIP is the simplest way of getting such consistency.
A good choice of angles for general use with color separations in offset litho work is a set in which the colors are separated by 30°, for example: 15°, 75°, 0°, and 45° respectively for CMYK (and related sets using these angles plus or minus multiples of 90°). For use with elliptical dots, a separation of 60° is recommended, leading to angles of 15°, 75°, 0°, and 135° for CMYK.
Using a Euclidean dot shape produces better saturated grays at gray values above 50%, especially at finer (higher) screen frequencies. Euclidean strategies increase the fill of halftone cells from the corners, instead of the centers, when the gray value exceeds 50%. That is, when the gray value is less than 50%, the dots are black, the background is white, and the dot size increases as the gray value increases; when the gray value reaches 50%, the dots become white, the background becomes black, and dot size decreases as the gray value increases.
Manipulating the dot shape generated can greatly influence the amount of dot gain in an image. The Harlequin RIP offers a variety of dot shapes, and your choice between them might depend on the output device, resolution as well as media being used. Standard dot shapes included with the RIP are: Euclidean, Euclidean PDF, Round, Cosine, Diamond, Diamond 0, Diamond 1, Diamond 2, Diamond 3, Diamond 4, Diamond Round, Double Dot, Ellipse, Ellipse A, Ellipse B, Ellipse B2, Ellipse C, Ellipse C PDF, Elliptical1, Elliptical2, EllipticalP, EllipticalQ1, EllipticalQ2, Inverted Double, Inverted DoubleDot, Inverted Ellipse A, Inverted Ellipse B, Inverted Ellipse C, Inverted Simple Dot, Line, Line 90, Rhomboid, Square1, Square2, Square Concave and Cross Dot.