Retinex Theory-based RGB to RGBW Conversion for RGBW Displays

Abstract

RGBW displays adding a white sub-pixel to conventional RGB displays are widely used for solving the low transmittance problem of high definition displays. In spite of better transmittance characteristics, RGB to RGBW signal conversion process is a major obstacle to replacing RGB with RGBW displays because the latter feature asymmetric color space between achromatic and chromatic colors. This study proposes a retinex theory-based approach to RGB to RGBW conversion, which preserves human color perception within a pre-determined level of color distortion for RGBW displays. The proposed method primarily consists of two procedures. In the first, it searches for the maximum intensity level that induces no color distortion for a given image by extracting white spectra from the common components of the RGB primary colors and adjusting all the pixels’ gains uniformly. In the second, the proposed method applies an additional gain to each pixel based on its chromaticness and controls the color distortions arising from the individual gains by using the color perception estimated by retinex theory and a feedback mechanism. Experimental results showed that the proposed method was more effective than conventional methods in terms of intensity increment and color preservation. For Kodak test images, the proposed method increased the average intensity by 1.4987 times with a color-distortion level of 0.0094 compared to reference RGB displays, whereas the conventional methods increased the average intensity by 0.8624-1.3429 times with color-distortion levels of 0.0325-0.0682. The surplus intensity yielded by the proposed method can be used to dynamically reduce the power consumption of a liquid crystal display (LCD) backlight or to provide brighter images on LCDs. Furthermore, applications of RGBW displays to premium TVs, especially with a wide color gamut (WCG) and high dynamic range (HDR), are also studied in this work. For WCG applications, the perceived brightness effect of the WCG is estimated by using the Helmholtz–Kohlrausch (H–K) effect on RGB and RGBW LCDs. The simulation results showed that the RGBW LCD was more suitable for a WCG than the RGB LCD in terms of the brightness balance of achromatic and chromatic colors and the results were also confirmed by subjective tests. Secondly, an effective method to implement HDR display based on the RGBW LCD is proposed. The data stretch, considering a local adaptation characteristic of the human visual system (HVS), greatly enhanced the details of dark regions and local peak dimming using white channel analysis and the white channel data itself increased the expressiveness of peak luminance in irradiative or specular regions up to 1,500 nits.