Pseudo-Random Halftone Screening for Colour and Black&White Printing

The idea of using pseudo-random spatial structures in graphic arts (color and black & white rendering algorithms) was suggested by psychologists and biologists about ten years ago. They observed that some natural pseudo-random structures such as the spatial distribution of receptors in the human retina play an important part in the perceptual process. Our modelization of pseudo-random spatial structures resembles the retinal mosaic. It starts by obtaining the quasi-random distribution of tile centers according to some well-defined spectral characteristics. We then obtain the desired tesselation of the output device space by applying the Voronoi polygonization process. Two slightly different approaches to the output image computation are being explored. In the first approach, an analytic black-dot curve is calculated according to the resampled input signal level and the area of each given tile. This analytic curve is scan-converted to obtain the blackened pixels. In the second approach, we associate threshold values to all pixels inside every tile according to some specially tailored analytic spot function. Then, the standard threshold comparison process is applied

An interface for the interactive design of artistic screens

The article presents the concepts and the tools involved in the interactive design of artistic screens. The screen elements are derived from a small set of analytical contours provided by the screen designer. We present the requirements that these contours must satisfy in order to generate consistent screens. Software tools have been developed which provide automatic means for verifying and enforcing these constraints. They include a way of specifying the periodicity of the screen dot and a graphical interface offering a convenient way of specifying and tuning the growth of the screen do

Dithering algorithms for variable dot size printers

Dither-based methods for the halftoning of images on multi-level printing devices such as multi-level inkjet printers are presented. Due to the relatively large size of single droplets, halftoning algorithms are still needed. However, since halftoning occurs between the basic levels attainable by printing one, two or several droplets at the same position, artefacts are less visible than in equal resolution bilevel printers. When dithering algorithms are used for the halftoning task, the dither threshold tiles should have oblique orientations so as to make the halftoning artifacts less visible. They should be designed so as to break up the inherent artifacts of variable dot size printers, such as for example continuous lines made up of elongated elliptic dots. The resulting visual effects are shown by simulating the printed dots of a multilevel inkjet printe

Two approaches in scanner-printer calibration: colorimetric space-based vs. "closed-loop"

Studies two different table-based approaches for the calibration of electronic imaging systems. The first approach, which is the classical one, uses the device-independent CIE-XYZ colorimetric space as an intermediate standard space. Input and output devices such as scanners, displays and printers are calibrated separately with respect to the objective CIE-XYZ space. The calibration process requires establishing a 3D mapping between a scanner's device-dependent RGB space and a device-independent colorimetric space such as CIE-XYZ. Measured samples belonging to the calibration set are used for splitting the colorimetric space into Delaunay tetrahedrons. The second approach, the so-called “closed-loop” approach, directly calibrates scanner-printer pairs, without any reference to an objective colorimetric space. It enables a 3D mapping to be built between the scanner's RGB space and the printer's CMY space without requiring any colorimetric measurement. It offers very accurate calibrated output for input samples having the same characteristics (halftone dot, ink spectral reflectance) as the printed samples used for the calibration process. When the desktop scanners' RGB sensibilities are not a linear transform of the CIE x¯,y¯,z¯ matching curves, an accurate calibration can only be made if input color patches are based on the same primary inks as the patches used for device calibratio

Structure Artifact Free Multi-Level Error Diffusion Algorithm

Error-diffusion is widely used to generate intensity levels between the primary levels of multi-level colour printing devices (ink-jet printers, electrophotographic printers). Standard error-diffusion algorithms produce structure artifacts at rational intensity levels such as 1/3, 1/2 and 2/3. The boundary between structure artifacts breaks the visual continuity in regions of low intensity gradients and generates undesirable false contours. These undesirable structure artifacts are also visible when error-diffusion is used to generate intermediate intensity levels between primary levels. In this contribution, we propose to remove these structure artifacts by introducing small discontinuities in the tone correction curve, thereby avoiding reproducing the intensity levels responsible for the generation of structure artifacts. The method can not be applied to bilevel printing, since the forbidden intensity regions responsible for the structure artifacts would be too large. In multi-level colour printing however, the forbidden intensity regions are small enough and do not produce any visible intensity breaks in varying intensity wedges

Method and Apparatus for Generating Halftone Images by Discrete One-to-one Dither Tile Rotation

The invention relates to a method and apparatus for automatic high-speed generation of digital angled halftone screens, specially suited for obtaining screens approximating the irrational angles which are generally required by high-quality colour reproduction. The method enables colour separations to be generated which minimize Moire effects, interferences and artifacts by applying discrete one-to-one rotations to digital halftone screens of the required period in order to reach the final screen angle. Dither tiles incorporating assemblies of the basic screen element are rotated by one-to-one discrete rotation and transformed into a new type of dither array, the scanning dither array. The scanning dither array is composed both of dither thresholds and of displacement vectors, providing the means to scan the dither array at image generation time. Several different discrete one-to-one rotation variants are proposed: a small angle rotation technique valid for a subset of rational rotation angles, a rigid band technique and an improved band technique valid for all rational rotation angles and a technique based on discrete shearing transformations. The high- quality of the so rotated dither tile is due to the fact that discrete one-to-one rotation preserves the exact number of elementary cells per screen element and their exact dither threshold values. Since discrete one-to-one rotation enables screen tiles generated by any existing or new method to be rotated, it provides a new range of solutions for obtaining high-quality digital angled halftone screens. In this range of solutions, high-quality solutions can be found for generating three digital angled halftone screens, each 30 deg. apart from each other, as known from traditional photographic colour screening techniques. Further solutions minimizing Moire effects may be obtained by halftone screens whose first order frequency component vectors sum up to zero. Since most of the proposed discrete one-to-one rotation variants can be accomplished by simple and incremental operations such as additions, subtractions, shifts, replications and table accesses, discrete one-to-one rotation is capable of generating angled halftone screens at high-speed. The invented process has turned out to be particularly effective when printing with color ink jet printers at resolutions between 150 and 800 dpi as well as with xerographic printers at resolutions between 300 and 1200 dpi

Method and apparatus for generating halftone images by evolutionary screen dot contours

The present invention is related to the synthesis, display and printing of halftone images. The invention comprises a method capable of generating screen elements with sophisticated screen dot shapes such as artistic shapes, microletters and ideograms. The method can be used to generate screen elements whose screen dots are made of evolving artistic shapes at increasing intensity level. For generating screen elements at consecutive intensity levels, intermediate contours which bound the white and black parts of each screen element are obtained by interpolating between fixed predefined contours. Such interpolated contours defining screen dots may be transformed from a screen dot definition space to a screen dot rendition space before being converted into discrete screen elements by a scan conversion and filling operation. Since the method allows producing large screen elements made up of many distinct subscreen dot shapes, the process can be used to generate discrete subscreen elements whose geometry may vary from one subscreen dot to another for the same intensity level. Some subscreen dot shape variations may be used to avoid counterfeiting by photocopying machines and digital scanners. The contours which make up the screen elements' dot shapes may be of arbitrary complexity. This characteristic makes the invented process useful in fields where halftoning with sophisticated screen dot shapes is desired, i.e. for printing banknotes, postage stamps or valuable papers. Screen dot shapes made of latin, greek cyrillic or arabic typographic characters as well as screen dot shapes made of ideograms offer new design freedom for creating posters and graphic designs