Halftones, or periodic cluster dithers, are point-to-point, contone pixel to halftone dot, conversion processes. This paper
describes an approach to achieving accurate halftone reproduction by using the errors caused during the scan conversion
process without comparison to the source pixels and maintaining the point-to-point scan conversion paradigm.
Two types of density errors encountered in the halftoning process are considered. 1. An error describing the difference
between the input pixel value and the halftone threshold value. 2. An error describing the difference between the density
level of the output data and the reflection density of the ink or toner on the page.
Many images are received for distribution and reproduction without the possibility of a comparison to the source image. Image quality is difficult to measure and quantify without such a source image. However, test photographs consisting of memory colors can be compared to a remembered image. Test forms consisting of photographs of the real world are the only source of memory colors. They are used to test a graphic-arts color workflow. Such tests: (1) Display the color quality of various output devices. (2) Describe color differences by comparing the difference between reproduction of a test photograph and the remembered colors. (3) Help achieve a preferred state of color reproduction. (4) Characterize reproduction gamut problems of an output device. (5) Remotely trouble-shoot a color workflow. The human visual system cannot provide a precise measurement of quality but does provide direction and goals for a
correction process. The criteria for the selection of image content and the usefulness of test photographs are described.
KEYWORDS: RGB color model, CMYK color model, Halftones, Color management, Inkjet technology, Photography, Visualization, Printing, Particles, Systems modeling
Duotone refers to an image with various shades of a hue mapped in an vector or wedge through a color space. The colorant, the gradient curve, and the number of colorants used define the slice through the color space. The image is printed with two or more analogue colorants. The colorants may be custom formulated or selected from a named color system. Typically two colorants are placed on a substrate by a halftone procedure, and the visual result, the mixture of the two colorants, is a third color. A gamut map of the colorants requires an accurate model of the thrid color that results from halftoning and printing the two inks. Color management procedures convert this gamut model to a vector through a monitor RGB color space and then to CMYK for proofing. This paper describes such a color management procedure.
Space-filling curves are regularly proposed as methods of traversing an image plane. Such methods, though more complicated than point movement by rows, have the advantage of moving the point within an area before moving to another area. Point movement is inherently within tiles and within areas of similar frequencies. Isis Imaging's Standard threshold-modulation screen, first commercially released in March 1994, is a device-independent screening method available in software residing on the host computer. It is widely used for print and photography applications. The algorithm is described, its properties examined and its output evaluated.
Accurate, automatic color reproduction is the goal of much of color technology. However, there is a need to improve reproduction in only the luminous or gray axis. Quadtone reproduction takes advantage of the four device CMYK color planes to provide greater gray-scale depth within the limitations of 8-bit per channel band-width. 'Quadtone' refers to photos reproduced using four tones of the same colorant. It is the printed imposition of four carefully selected shades of ink that result in a greater number of densities. Guerilla printing is a collection of algorithms using the CMYK channels to simulate traditional photography on an inkjet printer. Guerilla printing increases density values, defines detail and produces near continuous-tone screens.
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