Jaws
gDocEarly stochastic screening (also known as frequency modulated or FM screening) was developed in an attempt to solve quality issues such as moiré associated with conventional screens. In stochastic screening, the screened image is created by dots of a fixed size with the number of dots in an area, rather than their size, controlling the perceived intensity of the printed image.
FM Screening with a Difference
Harlequin Dispersed Screening (HDS) is Global Graphics’ patented second generation stochastic screening system that addresses many of the deficiencies in other stochastic screening techniques.
It is part of the Harlequin Screening Library and creates reproductions with increased detail rendition in comparison with conventional screening methods.
It also offers complete freedom from both cross screen and object moirés.
Conventional screening fixes the location and number of the dots but varies the size; stochastic techniques use dots of a constant size but vary their number and location.
A pseudo-random element to the screen controls the scattering and clustering of dots over a small area of the image. Different numbers of dots create different output shades based upon the grey level in a particular area of the original continuous tone image.
The very minute dots used in stochastic screening allow for better rendition of detail and significantly better quality images than conventional screening.
Since stochastic dots are not placed in a rigid grid and do not require screens to be angularly displaced, the moiré caused by interference between screens for different colors, or between the screen and objects in the image is entirely eliminated.
Also, the use of smaller dispersed dots virtually eliminates the problems of banding and mid-tone jump. With no screen angles to align, the need for extremely accurate press registration is less important and the make-ready time to prepare a press to produce accurate color can be shortened.
Despite these advantages, early stochastic screens had a number of characteristics that made them difficult to use in practice. Some of the issues were due to the characteristics of the output devices and media in use at the time, others were due to factors such as press process control; however the majority of issues were deficiencies in the early screening algorithms.
While eliminating many of the problems associated with conventional screening methods, the distribution of stochastic screening dots caused a host of other quality issues due mainly to the random manner in which dots were clustered together. Some of the issues encountered in practice include: grainy effect in mid-tones areas, loss of highlight dots and plugging in shadow areas, leading to loss of highlight and shadow detail, dot gain or limitations in usage because some FM methods demand specialized equipment.
