Youngshin Kwak

Luminance contrast and chromaticity contrast preference on the colour display for young and elderly users

Abstract The human visual system changes with aging and one of the most important changes is the decrease of spatial contrast sensitivity. We investigated this change both for luminance contrast and chromaticity contrast, and both for threshold contrast and preferred contrast, (preferred by users to carry out a visual recognition task), in a series of psycho-physical experiments with achromatic and chromatic sinusoid gratings of different values of spatial frequency, hue, and luminance level, and with two observer groups: young and elderly observers. We investigated the spatial frequency range of 0.1–10 cycles per degrees. Our results indicate that, beyond the expected luminance contrast sensitivity decline of the elderly observers, the difference between the preferred luminance contrast of the elderly and the preferred luminance contrast of the young is even more significant than the threshold difference. The small preference differences between the age groups for chromaticity contrast compared to luminance contrast suggests that while with increasing age both the chromatic and the achromatic contrast sensitivity drops, preferred contrast stays more stable for chromaticity contrast than for luminance contrast.

Preferred skin color reproduction on the display

We develop a methodology to find the optimal memory color (colors of familiar objects) boundary in YCbCr color space and a local image adjustment technique called preferred color reproduction (PCR) to improve image quality. The optimal memory color boundary covers most familiar object colors taken under various viewing conditions. The PCR algorithm is developed based on the idea that colors of familiar objects (memory colors) are key factors in judging the naturalness of an image. The PCR algorithm is applied to pixels detected as having a memory color. Memory color detection is conducted using color information by checking if an input color is inside the predetermined memory color boundary. The PCR algorithm transforms colors inside the memory color boundary to be shifted toward the boundary of constant interval in the center. The PCR algorithm is applied to skin colors, and psychophysical experiments using real images were conducted to determine the best parameters for the algorithm resulting in the most preferred image.

Generating vivid colors on red-green-blue-white electonic-paper display

Color characteristics of an RGBW (red, green, blue, white) electrophoretic display (EPD) prototype developed by Samsung Electronics are analyzed. EPD shows strong crosstalk between subpixels because of both the fringe field between subpixels and the scattering phenomena at the display surface. An RGB-to-RGBW color-decomposition algorithm optimized to EPD characteristics is developed that compensates for color deterioration due to the fringe field and scattering phenomena. For the four-color-decomposition algorithm, white is added to the primary colors to enhance the reflectance of the vivid colors while minimizing chroma loss. The psychophysical experimental result shows that images rendered with the algorithms developed in this study are preferred more than 90% of the time over those rendered with algorithms from previous studies. This research proves that, in spite of the limited physical property of EPD, the color quality can be improved dramatically through the use of well-designed color-rendering algorithms.

Optimal color primaries for three- and multiprimary wide gamut displays

We present a collection of principles to compare two sets of color primaries for wide gamut displays. A new, algorithmic threedimensional method to find optimal color primaries both for threeprimary and multiprimary displays is described. The method was implemented in a computer program. The resulting optimal color primary sets are discussed. We show that two-dimensional methods to find optimal color primaries by using a chromaticity diagram are inferior to three-dimensional optimization techniques that include luminance information.

Image-color-quality modeling under various surround conditions for a 2-in. mobile transmissive LCD

— This study aims to develop an image-color-quality (ICQ) model for a 2-in. mobile transmissive liquid-crystal display (LCD). A hypothetical framework for ICQ judgment was made to visually assess ICQ based the cognitive processes of the human visual system (HVS), and then an illumination adaptive ICQ model applicable for various surround conditions was developed. The memory color reproduction ratio (MCRR) of a locally adapted region of interest in a complex image reproduced on a mobile display was first computed. The colorfulness index and luminance contrast for all of the pixels in the image were then calculated by a global adaptation process. Finally, an ICQ model including all of the three attributes was developed under dark conditions using an assessed set of psychophysical data. The model gave more accurate performance than the mean accuracy for all of the observers. It was also visually tested under three different outdoor conditions, including overcast, bright, and very bright conditions, and the illuminance level range was from 7000 to 35,000 to 70,000 lx. The effect of outdoor illumination could be quantified as an exponential decay function and the ICQ model could be extended to cover a wide variety of outdoor illuminations conditions.

Subpixel arrangements and color image rendering methods for multiprimary displays

Design principles are formulated to develop visually optimal multiprimary subpixel architectures. Two new hexagonal subpixel architectures optimized for multiprimary color displays are shown, each designed according to these principles. These new multiprimary pixel architectures are considered to be useful to eliminate the color fringe artifact. They are expected to yield images of better visual quality than previous three primary color architectures including the RGB stripe architecture. A new image rendering method is also formulated for multiprimary subpixel architectures. This method can be used with and without subpixel rendering. An error function is defined enabling proper chromaticity reproduction and enhanced luminance resolution.

Optimal chromaticities of the primaries for wide gamut 3-channel display

The theoretical approach is introduced to design the optimal chromaticities for primaries with a given size of triangular color gamut in xy-plane. Optimal primaries are defined as a set of chromaticities of red, green and blue primaries with fixed white point that most optimally satisfying four criteria, i.e. gamut size, gamut shape, coverage of object colors and hue of the primaries, in the visually uniform color space, CIECAM02. It is assumed that the optimal gamut should cover that of sRGB and have similar maximum chroma for each hue. The number of SOCS data located outside the gamut is used as a criterion to judge the coverage of object colors. Also it is set the hues of primaries to be close to those of sRGB. The simulation results showed that the optimal primaries for 85% of NTSC area have similar points with sRGB for red and blue, and green primary is located in between sRGB and NTSC. For 100% of NTSC area, the optimal chromaticities are located near those of NTSC for red and green and that of sRGB for blue.

11.3: Depth-Image-Based Rendering (DIBR) Using Disocclusion Area Restoration

The embodiment of 3D display has been evolving from stereoscopic 3D system into multi-view 3D system, which provides images corresponding to the different visual points. Currently because of input source format such as two dimensional image, multi-view display system needs technology to generate virtual view images of multi visual points. Due to the changes in the visual points, occlusion region of the original image becomes disoccluded area, resulting in problem to restore the output image information which is exclusive of input image. This paper proposes the method for generation of multi-view images through two steps which are depth map refinement and disocclusion area estimation and restoration. The first step ‘depth map refinement’ removes depth map noise and compensates mismatches between RGB and depth, preserving boundaries and object shape. The second step ‘the disocclusion area estimation and restoration’ predicts disocclusion area using disparity and restore information of the area using neighbor frames information which is the most similar with occlusion area.

Skin color reproduction algorithm for portrait images shown on the mobile display

The preferred skin color reproduction algorithm is developed for the mobile display especially for a portrait image with one person as a main object occupying most of the screen. According to the developed technique, the skin area in an image is detected using color value of each pixel in YCbCr color space. The skin color boundary is defined as a quadrangle in Cb-Cr plane. The colors of pixels belonging to skin area are shifted toward the preferred colors while there is no color change for the other pixels. The psychophysical experiments are conducted to investigate the optimal model parameters providing the most pleasant image to the users. Then, the performance of developed algorithm is tested using the optimal parameters. The result shows that for more than 95% cases, the observers prefer the images treated with the developed algorithm compared to the original image. It is believed that the developed algorithm can be applied to the mobile application to improve the image quality regardless the input sources.

The Colour Preference Control based on Two-Colour Combinations

This paper proposes a framework of colour preference control to satisfy the consumers colour related emotion. A colour harmony algorithm based on two-colour combinations is developed for displaying the images with several complementary colour pairs as the relationship of two-colour combination. The colours of pixels belonging to complementary colour areas in HSV colour space are shifted toward the target hue colours and there is no colour change for the other pixels. According to the developed technique, dynamic emotions by the proposed hue conversion can be improved and the controlled output image shows improved colour emotions in the preference of the human viewer. The psychophysical experiments are conducted to investigate the optimal model parameters to produce the most pleasant image to the users in the respect of colour emotions.