Gabriel G. Marcu

Error diffusion algorithm with output position constraints for homogeneous highlight and shadow dot distribution

In most images processed using error diffusion techniques, a nonhomogeneous arrangement of dots (‘‘worm’’ artifacts) may occur for highlight and shadow regions. In this article we propose a method to reduce the worm artifacts using spatial constraints for the dot arrangement. For a pixel in the shadow or highlight region, a dot is placed only if a minimum distance constraint between the current pixel and the placed dots is satisfied. If the distance constraint is not satisfied, the placement of the current dot is postponed and the quantizer error is diffused to the unprocessed pixels. The proposed method verifies the distance constraint without computing geometric distances. Instead, a roadmap enlarging from the current pixel location is used to check for placed dots. A dot is placed only if no other dot along the scanned roadmap is found. The scanned length of the roadmap depends on the input gray level and is accessed from a look-up table. The paper includes an example of halftone image processed with the proposed method.

Three-dimensional histogram visualization in different color spaces and applications

A visualization procedure for the 3-D histogram of color images is presented. The procedure assumes that the histogram is available as a table that associates to a pixel color the number of its appearances in the image. The procedure runs for the RGB, YMC, HSV, HSL, L*a*b*, and L*u*v* color spaces and it is easily extendable to other color spaces if the analytical form of color transformations is available. Each histogram value is represented in the color space as a colored ball, in a position corresponding to the place of the color in the space. A simple drawing procedure is used instead of more complicated 3-D rendering techniques. The 3-D histogram visualization offers a clear and intuitive representation of the color distribution of an image. The procedure is applied to derive a clusterization technique for color classification and visualize its resuIts, to display comparatively the gamut of different color devices, and to detect the misalignment of the Rc3B planes of a color image. Diagrams illustrating the visualization procedure are presented for each application.

Gray tracking correction for TFT-LCDs

The consistency of the white point with the input gray level is referred in the world of display technology as gray tracking. Gray tracking is an issue for Twisted Nematic (TN) TFT-LCD screens which typically show a bluish color shift of the white point when the gray level decreases. This paper discusses some causes of this color shift in TN displays and proposes a method for correcting it by using the existing video card look-up tables (VLUT) in the graphic controller driving the panel. The correction process performs the gamma correction and the gray tracking correction using a single set of three 1D look-up tables. The gray tracking correction uses the luminance and the chrominance information of the R, G, B channels. For a target gamma and white point, each VLUT entry that corresponds to a certain luminance and chrominance (target color), the method computes the output RGB values of the VLUT such that, the resulted gray has the minimum color difference to the target color. The method proves to be effective in removing the colorcast on the TN TFT-LCD screens. The method is different from a previous published paper from the same author by minimizing the loss of the luminance due to the tuning of the chrominance.

Computer simulation of printed colors on textile materials

The printing process of textile materials uses an ink set dependent of the image to be printed and referred as the primary color palette. The colors of the printed textile material depend on the printing sequence of the ink masks and are referred as the secondary color palette. A single primary color palette may conduct to different secondary color palettes, as a function of printing sequence. This paper provides an analysis of the mechanism of color appearance on the printed textile materials. The analysis conducts to a model to simulate on the computer display the appearance of the textile printed colors as a function of a number of parameters. The simulation includes a generalized Neugebauer model. A hierarchical structure is introduced for the colors of the secondary palette in order to provide the coefficients of the Neugebauer model. For a certain textile material, the color hierarchy is dependent on the ink set and the printing sequence. The color hierarchy is established as result of color calibration process. Printed samples are used for calibration procedure.

17.3: Color Correction in TFTLCD Displays for Compensation of Color Dependency with the Viewing Angle

Most todays twisted nematic TFTLCD displays show a color dependency with the viewing angle. This paper describes a method for compensation of the pixel color variations in these displays relative to a single viewer position. The color correction can be recomputed automatically as the viewer position changes, as long as the position is known. The method determines the color correction required for each pixel of the screen, such that a single viewer for a given position can see the color unaffected by the viewing angle differences to the screen. Color is measured in few points over the screen relative to a viewer position, and then an interpolation procedure is used to compute the color correction for each intermediate pixel, between the measured positions. The measurements for different positions of the viewer are used in a second interpolation procedure for computing color correction for intermediate position of the viewer between the measured positions. The method was experimented for portable and desktop displays. It was found that, for relative small viewing angle variations, with acceptable approximation, the color variation with the viewing angle can be modeled as variations of the transfer function. It was found that the proposed method can also be effectively used to compensate for panel uniformity.

Color characterization issues for TFTLCD displays

This paper describes few issues related to TFTLCD display color rendition and characterization. The paper points out specific aspects of color rendition on TFTLCD devices that differentiate the traditional CRT self luminous devices from the TFTLCD transmissive devices. Two TFTLCD technologies are discussed, TN (Twisted Nematic) and IPS (In Plane Switching) with and without dual domain improvement. The paper discusses specific aspects of color rendering for TN and IPS technologies, the display primaries, color gamut and brightness, the transfer function, the white point, some viewing angle issues and the color model. The paper explains why the TFTLCD displays offer perceptually a larger gamut than the CRT displays, even if chromatically their gamut triangle is smaller in the CIE chromaticity diagram than the one of a typical CRT. The paper also explains the hue shift effect of the primaries with the input voltage and the effect of luminance variation with the viewing angle, both encountered in some TN TFTLCD displays. The paper confirms that despite their advantages due to high brightness, high contrast, high sharpness, virtually no geometric image distortion, in terms of color capability, with few exceptions, TFTLCD devices have not yet surpassed the CRT devices with larger chromatic gamut and no color variation with the viewing angle.

Gamut mapping for color simulation on CRT devices

The gamut mapping procedures transform the colors of the original image into the colors that can be reproduced by another device, according with different reproduction criteria. This paper discusses three gamut mapping procedures: gamut clipping, white shifting technique recently introduced by Konica, and an example of UltraColor technique used by Kodak. A new proposed method that maps the out of gamut colors to the gamut colors based on the morphing technique along the constant angle direction or constant device hue in CIELAB or CIELUV is investigated. The mapping procedures dependent on the content of the image are compared with the procedures based only on the device to device gamut mapping. The image dependent mapping procedures exploit more efficiently the potential of adaptation of the observer eye to the contrast of the image than the device mapping methods and conduct to better results. A 3D rendering procedure is used to illustrate different gamut devices and to compare the effect of mapping procedures. The CIELAB and CIELUV color spaces are compared for optimum performance of mapping of the outside gamut colors from one device to another. The mapping methods dependent on the content of the image are evaluated using the 3D histogram visualization technique simultaneously with the gamut visualization. The techniques are applied for simulation on the CRT of the different reproduction media.

Color separation for printing with non-standard inks

The printing process that uses an arbitrary set of inks, deposited on a substrate in an arbitrary order through variable ink masks is referred to as non-standard printing. The color separation consists of deriving the ink mask values that print with the specific set of inks to the desired color. The color separation method proposed in this paper introduces a special partition of the color space in tetrahedrons, that enables baricentric interpolation of any arbitrary input color as function of tetrahedron vertices. The vertices are selected in the points corresponding to representation of the color combinations resulted by overlapping of the input colors set in the color space.<<ETX>>

Color designing and simulation in nonconventional printing process

The conventional printing technologies use the standard four ink (yellow, magenta, cyan, and black) process. In some special cases, an unconventional ink set printing process can be selected. The unconventional inks set printing process uses a particular set of inks, selected according to the image to be reproduced. The inks are deposited on the substrate material in a certain order, through individual ink masks. Designing the unconventional printing process requires us to find the ink color set, named primary color palette, the sequence of ink mask printing, and the individual ink masks (color separation). In addition to the primary color palette, the secondary color palette is defined as the set of all the color combinations resulting by overlapping the ink masks after the complete printing process. A single primary color palette may conduct after printing process to a number of different secondary color palettes. This paper provides a color separation method which converts the input color components to the ink mask values that determine the amount of ink deposited on the substrate. The separation process depends essentially on the color calibration process that determines the colors of the secondary palette. The color calibration is accomplished using the printed samples. The calibration process conducts to a hierarchical structure of color combinations of the secondary color palette. The simulation of the printed colors is described, based on the generalized Neugebauer model for an arbitrary number of inks and an arbitrary set of inks. The color hierarchy established during the calibration process is used to derive the coefficients of the Neugebauer model.

A color classification algorithm

The color classification process requires that a color image be partitioned into a set of uniform colored regions. A method of automatic classification of a color picture, based on the advantages of a high-speed binary tree splitting algorithm and completed with a stopping criterion derived from color cluster detection methods, is presented. The histogram of distance of the current color point to the center of the investigated cluster is analyzed to stop the binary splitting algorithm. The color space was divided and labeled accordingly to the closest color cluster or region corresponding to a color of the color set. A dynamic dividing method of the color space was used. A post-processing algorithm makes it possible to filter out the unwanted transition colors by absorption and erosion.<<ETX>>