Color Reproduction Considering the Average Picture Level and Flare Effect in PDP Displays

This paper proposes a method of color reproduction after considering the viewing conditions of PDP displays. Owing to a change of peripheral environments by a temporal and spatial location of observers, the ambient lightings should be considered in the process of color reproduction of displays. A conventional method enhances the contrast and saturation of images by controlling voltage gains in each channel, so that there is a limit to implementing the phenomenon of human adaptation. This method also faces difficulties in reproducing the perceived colors under a variety of viewing conditions. Accordingly, in order to solve this problem, we first characterize the device by considering the average picture level (APL) of PDP displays since the maximum luminance value of a PDP display varies according to the average picture level in order to keep the maximum electric power. In the next step, information about an ambient illuminant is obtained by a sensor and this includes the luminance and correlated color temperature values of ambient lighting. Then, a flare effect is taken into account to exclude the reflected lights in the surface of PDP displays. The chromatic adaptation is finally applied to reproduce colors in PDP displays using the information from the viewing conditions. In the experiments, the images reproduced by the proposed method are visually more similar than those reproduced by a conventional method. Introduction Color reproduction in displays is generally carried out in dark rooms in order to maintain any reference conditions. Yet, the conditions by which common observers watch displays vary continuously. At this time, an observer’s peripheral environments are affected by inner illumination, daylight, or sunset. Therefore, algorithms that can reproduce colors for each type of ambient illumination in a display are needed. This is because the human visual system changes according to viewing conditions which is called a chromatic adaptation [1, 2]. The curves of a human’s LMS cone responses change independently when viewing conditions vary. For example, under an incandescent lamp, the range of the L cone sensitivity is large. In other words, the L cone response for the same stimulus decreases under an incandescent lamp compared to under daylight. Therefore, images have to change in order to consider the variation of the human visual system. Recently, several researchers have studied the goals set by the CIE Technical Committee TC8-04: “To investigate the state of adaptation of the visual system when comparing soft-copy images on self-luminous displays and hard copy images viewed under various ambient lighting conditions [3]. ” Kato [4] proposed an adaptation model using the S-LMS space in order to match images between a softcopy versus a softcopy. His method hypothesized that the human visual system is partially adapted to the display’s white point and partially to an ambient light. Our proposed method is also similar to Kato’s method with respect to using the white point of a display. However, our method considers viewing conditions more adaptively by using a sensor. The proposed method consists of four steps. First, the PDP display is characterized to convert device-dependent RGB values to a device independence space. At this time, in order to consider the characteristics of a PDP display, sample patches are arranged according to common viewing conditions. Second, a sensor is used to extract the information about a particular viewing condition. Among several sensors, the sensor that can extract the information about color temperature and the luminance of an ambient light is used. Third, a flare effect is calculated to compensate for contrast and saturation by the viewing illuminant. Finally, the image in the PDP TV is reproduced by the chromatic adaptation model using the coefficient determined between the white point of PDP displays and the ambient light obtained by a sensor. Characterization of PDP Displays Considering Average Picture Level The process of device characterization is to obtain deviceindependent values from digital RGB signals or its inverse process. Therefore, we can determine the relation between reference tristimulus XYZ values and digital RGB values through device characterization. Potential models, includes GOG, S-curve, and Measurement-based. Among these, the GOG model represents the relation between electric values and light via an exponential curve. This model is accurate for PDP displays and it only needs a few sample patches. Therefore, we characterized PDP devices by the GOG model in this paper. Via the GOG model, input-to-output is represented by the coefficients of a gain, an offset, and a gamma. Luminance for each channel is modeled by red, green, and blue channels as Eq. (1) where d r , d g , and d b are the normalized values for an input. N is the number of bits for a display device. R, G, and B are the normalized values between 0 and 1 for a luminance value. We can obtain a device independent of the XYZ tri-stimulus from the digital value of a display through a matrix operation, after the coefficients of gain, offset, and gamma are determined using training patches for each channel. To characterize the display, some patches are measured in advance. Equipment for measuring is generally located at a distance of 4 times the height of the display. As well, the size of 15th Color Imaging Conference Final Program and Proceedings 163