Myong-Young Lee

Color signal decomposition method using 3-D Gamut boundary of multi-primary display

Color signal decomposition method is to decompose the conventional three-primary colors (RGB) into the multi-primary control values of multi-primary display (MPD) under the constraints of tri-stimulus matching. For the satisfaction of tri-stimulus matching between input and output display systems, the MPD color signals have to be estimated from a device-independent color space such as CIEXYZ or CIELAB. However, as the target of MPD is to display moving-picture data, it is necessary to simplify the color space conversion between input and output systems and decomposition of the multi-primary control values. In this paper, color signal decomposition method for MPD is proposed using 3-dimensional look-up-table (3D-LUT) in linearized LAB space. The linearized LAB space satisfies the linearity and additivity in color space conversion and easily constructs the 3-D LUT considering lightness, chroma, and hue. For the reproduction of moving-picture data in MPD, the proposed method is based on 3-D LUT structure to reduce the complexity of hardware and processing time. First, 3-D LUT containing gamut boundary data of MPD is created to derive the multi-primary control values. The input images are transformed to linearized LAB values. Then, the corresponding color signals on the gamut boundary points, which have the same lightness and hue as the input point, are calculated. Also, the color signal for a point on the gray axis is calculated. Based on the gamut boundary points and a point on the gray axis, the MPD color signals for the input values can be obtained by interpolating between the gamut boundary points and a point on the gray axis. In particular, the neighboring gamut boundary points are used in the region of occurring abrupt signal change for a smooth change of hue. As a result, the proposed method guarantees computational efficiency and color signal continuity and requires less memory rather than conventional color decomposition methods.

RGB look-up table design for color matching between monitor and mobile display

This paper proposes a color-matching 3D look-up table that simplifies the complex color-matching procedure between a monitor and a mobile display device, where the image colors are processed in a device-independent color space, such as CIEXYZ or CIELAB, and gamut mapping performed to compensate the gamut difference. When compared with a monitor, mobile displays are unable to display images with good color fidelity due to their smaller gamut, dimmer luminance, and worse color reproduction ability related to their low power consumption. As such, the image colors displayed on a monitor and mobile display can be significantly different for the same input digital values. Thus, to solve this problem, a color matching process between a monitor and a mobile display is needed that includes both color management in a device-independent color space1 and gamut mapping to compensate for the significant gamut difference. Yet, since these procedures involve many complex arithmetic functions, simplification is required for realization in mobile devices. Accordingly, this paper proposes a color-matching look-up to simplify the complex color-matching procedures for use in a mobile display. Moreover, the performance of the proposed color-matching look-up table is evaluated with different sizes of look-up table to determine the minimum size. Color-matching experiments between a monitor and a mobile display show that the images on the mobile display reflect the monitor images better after color matching than without color matching.

Modeling for hue shift effect of human visual system on high luminance display

This paper proposes a color correction method based on modeling the hue shift phenomenon of human visual system (HVS). Observers tend to perceive same color stimuli, but of different intensity, as different hues, what is referred to as the hue shift effect. Although the effect can be explained with the Bezold-Brücke (B-B) effect, it is not enough to apply the B-B model on high luminance displays because most displays have a broad-band spectrum distribution and results vary according to type of display. In the proposed method, the quantities of hue shift between a high luminance display and a normal luminance display were first modeled by a color matching experiment with color samples along the hue angle of the LCH color space. Based on the results, the hue shift was then modeled piecewise and was finally applied to the inverse characterization of display to compensate the original input image. From evaluating the proposed method using the psychophysical experiment with some test images, we confirmed that the proposed modeling method is effective for color correction on high luminance displays.

Fast multilevel vector error diffusion based on adaptive primary color selection

This paper proposes multi-level vector error diffusion based on adaptive primary color selection for fast and accurate color reproduction. Conventional bi-level vector error diffusion uses eight primary colors(R, G, B, C, M, Y, W, K). However, multi-level vector error diffusion uses more primary colors (this paper uses 64 primary colors) depending on the printing device, thereby significantly increasing the time complexity due to the additional increment of computation. Moreover, the output image can also include color artifacts that have a noticeable primary color under the influence of large quantization error and increased primary color. Accordingly, to reduce these problems, we proposed the quantization process to decide a candidate primary among the 64 primary colors using lightness difference. First, we classified the 64 primary colors into 60 chromatic colors and 4 achromatic colors and then we exclude primary colors with the large lightness difference against the input color from a set of 60 chromatic primary colors. Using both 4 achromatic primary colors and a candidate primary colors, we calculated the vector norm to select output color. Also this paper determine optimal threshold experimentally to remove smear artifacts resulting from the diffusion of large quantization error. As a result, this paper archives fast multi-level vector error diffusion by avoiding additional computation and produces visually pleasing halftone pattern by excluding noticeable primary colors.

Physical‐based photon mapping for an LED backlight lighting simulator

— Liquid-crystal displays (LCDs) have become increasingly popular due to their lower price and larger sizes. In particular, backlights having an RGB LED source have recently attracted attention, because they have a wider color gamut, higher luminance, and lower power consumption. However, even when the backlight area is uniformly covered with light modules based on arrays of individual LEDs, this does not ensure a uniform chromaticity and luminance over the backlight panel, thereby stressing the need for lighting simulation of the backlight. Accordingly, this paper proposes an effective lighting simulator to predict the chromaticity and luminance distribution of an LED backlight panel for an LCD. First, the spectrum-based photons are all initially generated using a random function with a constraint satisfying the spectral power distribution of the actual LED light sources, while their emitting directions are determined based on a pre-calculated probability using a random variable angle. The optical characteristics of the inner sheets in the LCD backlight structure are then modeled using the wavelength and incident angle to predict the next direction of each photon based on the reflection and transmittance at an intersection. All the photons that reach the unit area of the outward panel are gathered to shape their spectral power distribution, then converted to CIEXYZ values and multiplied with a color-matching function. Finally, a realistic image visualization of these CIEXYZ values is achieved through standardized device characterization using the sRGB mode. Experiments confirm that the proposed spectrum-based photon mapping can effectively predict the chromaticity and luminance distribution of an LED backlight panel, providing a good lighting simulation of an LED backlight before manufacturing the LCD.

A Lamp-Lit Image Rendering of Automotive Rear Lamp Using Spectral-Based Ray Tracing Method

This study proposes a new algorithm that can render a realistic image of an automotive rear lamp using the backward ray tracing method. To produce a realistic image similar to that perceived by the human eye, the physical transformation procedures of light energy are traced by a spectral interaction definition. The incident light energy at the eye point estimated by a ray tracing algorithm is represented by XYZ tri-stimulus values, which are then converted into RGB values considering the particular display device. A more effective form of light source modeling than the Monte-Carlo integration method is also applied for accurate simulation. Finally, comparisons of traced results and real measurements are performed in terms of the spectral distribution, image detail, and visual experiments.