7 Oct 2005: Version 1.37 of ETS is now available. This release contains a number of refinements over previous versions, including a much refined static threshold modulation pattern (previous versions used white noise), and improvements to the vector-optimized versions to make them more consistent with the C version.
9 Feb 2004: Version 1.30 of ETS is now available. This release contains a number of dramatic improvements over previous releases. For one, it includes full support for variable ink droplet sizes. It also screens multiple ink planes at the same time to optimize interactions between the planes. Last but not least, this version also contains assembly language speedups for Altivec and Pentium 4 architectures.
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.
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.
With Even Toned Screening With Floyd-Steinberg
"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.
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.
An evaluation version of Even Toned Screening is available under the GNU GPL license for free download:
Levien, R. "Practical Issues in Color Inkjet Halftoning, Electronic Imaging, San Jose, 2003.
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.
For more information about licensing, contact:Miles Jones
V.P. of Sales
101 Lucas Valley Road #110
San Rafael, CA 94903
tel +1 415 492 9861
fax +1 415 492 9862