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• Screening

Imaging Science

Artifex offers outstanding imaging science technologies in the areas of screening and color management. Artifex is pleased to announce a major revision of Ghostscript's color handling architecture. Please see our white paper on this new color capability: Ghostscript Color Architecture

Please download our datasheet for more information:Imaging Science datasheet

Screening

Artifex offers two high-performance screening technologies, Even Toned Screening (ETS) and threshold array screening.

Even Toned Screening

Even Toned Screening controls undesirable halftone patterns and textures, a form of error diffusion screening optimized for inkjet printers, using a technique known as Output-Dependent Feedback Screening.

Error diffusion is a technique for breaking a continuous tone image into a dot structure suitable for ink-on-paper printing. Error diffusion produces a very finely dispersed dot structure, which is particularly well suited to inkjet printers. Most other printing technologies, including laser and offset printing have difficulty reproducing such fine structure.

The earliest error diffusion algorithm was invented by Floyd and Steinberg in 1975, and is still considered one of the best. However, many error diffusion algorithms produce visually noticeable artifacts as a result of the dot placement choices. Techniques to reduce these artifacts are an active area of research.

Exceptional features of error diffusion screening include total freedom from moire, as well as unparalleled fidelity in reproducing fine detail. These features have helped ensure error diffusion's dominant role in halftoning for inkjet printers.

Overall quality

Producing high quality output with any screening technique requires custom-tuned color profiling targeted for that screen. The smooth response of ETS makes it more convenient to make a superior color profile, and therefore an overall superior quality output.

This high quality screening technique adds output-dependent feedback to the conventional error diffusion halftone, improving these halftones significantly.

''Wormy'' Textures in Highlights and Shadows

Error diffusion halftones are plagued by a ``wormy'' texture in highlights and shadows. This undesirable effect is caused by limitations in the logic used by error diffusion to place dots. With Even Toned Screening, the visual texture of highlight and shadow tones are made more uniform, eliminating the wormy textures that can occur with conventional error diffusion halftones. This screening technique is effectively identical to placing each halftone dot in the center of a circle and packing the circles tightly together. The resulting texture appears very homogeneous, and is visually pleasing to the eye. While Even Toned Screening can be applied to both highlight and shadow dots, on inkjet printers only the highlights cause visually noticeable patterns, because of the ink spread. Thus, applying the output dependent feedback to highlight textures only can speed processing.

''Tearing'' Adjacent to High-Contrast Areas

With the smooth highlight areas created with Even Toned Screening, ``tearing'' is also avoided. Tearing refers to a screening artifact where the halftone dots drop out next to high-contrast areas, creating a hole or "tear" in the halftone image. This ``tearing'' effect is particularly severe for mixed pages containing images and line art or colored text. The use of Even Toned Screening ensures excellent results for all types of source images.

Availability

Even Toned Screening is available in two forms. First, it is a complete error diffusion algorithm using a tuned version of Floyd-Steinberg to achieve excellent smoothness for all tone values. Second, it is available as a modification to existing error diffusion algorithms, placing the highlight (and optionally shadow) dots more smoothly while leaving the midtones unchanged. The second option is desirable when work has already been done tuning an error diffusion algorithm to a particular device. The final result is a halftone with exceptionally smooth highlight regions, free of "worming" and "tearing," with modest computational cost. The speed of Even Toned Screening is within a factor of two of simple error diffusion techniques such as basic Floyd-Steinberg, the earliest and most well-known error diffusion technique.

Threshold Array Screening

Artifex offer two Threshold Array Screening options:

Ordered dither, using traditional parameters for lines per inch (screen resolution), angle, and spot shape.

Dispersed dot stochastic dither screen.

Both technologies allow for independent horizontal and vertical device resolution. Both technologies generate screens with at least 256 levels. In the ordered dither (conventional dot) screen, a supercell is used consisting of an array of multiple dots within the screen to provide enough gray levels, using stochastic dot growth to prevent moire patterns. The threshold array dimensions will be those of the supercell.

The dispersed dot stochastic dither threshold array can be generated for any specified width and height, and the number of levels is 1+(width*height). This technology also allows for a 'minimum dot' setting when generating dispersed dots to allow the resultant screens to be practical on printing engines that cannot reliably image true 'dispersed dot' screens. This is sometimes referred to as a 'green screen' in that the peak spatial frequency at a particular level is lower than a blue noise distribution or that of error diffusuion, but the perceived visual quality may be better than an ordered dither screen. The dispersed dot approach relies on a seed or simple spatical rotation, rather than the screen angle in order to prevent correlation among individual screens for each color channel (e.g., CMYK). This prevents color shifts if the print engine registration among the color channels changes within the page, or from page to page.

Awards

In partnership with RIPit, Even Toned Screening won the 2003 "PDF Shootout" at Seybold San Francisco for best quality PDF output, besting far more expensive solutions.

Publications

Levien, R. ``Output Dependent Feedback in Error Diffusion Halftoning'', IS&T 46th Annual Conference, Cambridge, Massachusetts, May 1993. (also reprinted in ``Recent progress in digital halftoning'', Reiner Eschbach, ed., IS&T, 1995) Levien, R. ``Method and apparatus for error diffusion screening of images with improved smoothness in highlight and shadow regions'', US Patent 5,917,614, June 29, 1999.