Artifex
Imaging Science Technologies
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
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.
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.
--------------------------------------------------------------------------------
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.
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Obtaining
the freely available versions:
An
evaluation version of Even Toned Screening is
available under the Gnu GPL license for free download:
ets-1.1.tar.gz
- To
obtain the latest freely-available version of
Ghostscript PostScript and PDF, please download
from the following URL: http://www.cs.wisc.edu/~ghost/
To
obtain the latest freely-available version of
GhostPCL and Ghostprinter, please download from
the following URL: http://www.artifex.com/downloads/
