Imaging Science

EvenArtifex is pleased to offer outstanding imaging science technologies in the areas of screening and color management.

Please download our datasheet for more information: Imaging Science datasheet

Screening

Artifex offers two high-performance screening technologies, Well Tempered Screening (WTS) and Even Toned Screening (ETS).

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.

with Even Toned Screening - - - - - - - - - - - - - - -with Floyd-Steinberg

 

``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.

ETS "Green Screen"

Green Screen is a second-order FM screening that uses variable dot placement and noise. This not only eliminates moirй, but also improves the reproduction of flesh tones. In addition Green Screen gives excellent rendition of fine detail and produces smooth tints and vignettes with no noise.

• Exceptional reproduction of fine detail
• Smooth multi-color vignettes with no noise
• No visible dot structure in skin tones
• Increased color gamut gives greater visual impact and wider range of process colors
• Screened type is sharp and legible
• Flat tints are smooth

First-order Stochastic Screening

First-order stochastic screening uses fixed spot sizes, and adjusts their frequency and proximity as needed for midtones, highlights, and shadows. Also known as FM (frequency modulated) screening, it prevents moire and rosette patterns and allows textiles, metal products, flesh-tones, and complex images to render with amazing detail and clarity.

Second-order Stochastic Screening

A less desirable trait of first-order FM screening is graininess. Green Screen corrects this deficiency by using a fixed spot size only in the highlights and shadows while allowing the midtones to vary slightly. Green Screen produces visual density by increasing both the size and frequency of spots, enabling better tonal control and smoother, more natural reproduction. Unlike other stochastic technologies, Green Screen does not use tiles for dot placement, which can cause repetition artifacts. Instead, Green Screen uses error diffusion placement. Additionally, advanced algorithms virtually eliminate the worm-like patterns inherent to diffusion screening. The implementation of Green Screen is in standard C, with additional SSE2 (Pentium 4) optimizations to achieve blazing performance on modern platforms.

Argyll Color Management SystemTM

Argyll is an ICC compatible color management system. It supports accurate ICC profile creation for scanners, RGB or CMYK printers, film recorders and display monitors. Spectral sample data is supported, allowing a selection of illuminants observer types, and paper fluorescent whitener additive compensation. Profiles can also incorporate source specific gamut mappings for perceptual and saturation intents. Gamut mapping and profile linking uses the CIECAM02 appearance model, a unique gamut mapping algorithm, and a wide selection of rendering intents. It also includes code for the fastest portable 8 bit raster color conversion engine available anywhere, as well as support for fast, fully accurate 16 bit conversion. Device color gamuts can also be viewed and compared using a VRML viewer. Argyll also includes a general purpose ICC profile format access library, icclib, and a general purpose CGATS file format I/O library. One of the most compelling aspects of the Argyll Color Management System is that we provide full source code. This allows users in mission-critical projects to tune and customize color handling to meet their specific needs. This is one of the reasons that Argyll was chosen as the color management system for the DreamWorks animated movie "Shrek 2".

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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.