Hybrid Screening Algorithms.

A computer program that creates halftone screens using both amplitude modulation (AM) and frequency modulation (FM) techniques was developed. The objective was to eliminate the characteristic graininess in flat areas found in FM screens, while still maintaining the detail rendering capabilities of frequency modulation. So as to decide where to apply each screening technique, a detail detecting program was also developed, which scans the image prior to screening. The detailed areas are screened using FM, graininess not being an issue due to the amount of detail. However, the flat or low detail areas, which are subject to graininess or objectionable patterns in FM screens, are screened using conventional AM dots. A steep transition between AM and FM screened areas is not desirable, so algorithms that blend these techniques and create Hybrid dots were developed and are disclosed here. The addition of noise in FM screens is evaluated and optimized, allowing for minimum perturbation, thus increasing the detail rendering capabilities of FM screens. Images generated using this technique proved to have outstanding quality, with reduced graininess and excellent detail. An alternative application for the detail detecting algorithms was found to be in determining the required amount of noise to be added to pure frequency modulation algorithms.