New display concept for realistic reproduction of high-luminance colors [see more details: links]

Abstract

A new display concept for reproduction of high-luminance colors based on a liquid crystal display has been developed using a brighter backlight unit and color mapping algorithms. The new concept is able to display brighter colors close to a peak luminance of a display white than conventional displays so that realistic scene of brighter colors is better reproduced. It may also be one of the future display solutions needed to extend the color gamut in the direction of brighter colors, which is a principal limitation in conventional displays even in high-dynamic range display systems. With the new concept, an xvYCC- (extended-video YCbCr) compatible display can be easily realized.

Problem in conventional display system:

The limitation of high-luminance colors in relation to the peak display white of the conventional display degrades image qualities during high-luminance color reproductions. This often occurs during display on a high dynamic range (HDR) imaging system; for example, to capture and to display the colored illumination on a stage during live music performances (see Fig.1) or colors of brighter LED-lighting of Christmas trees.

Display Concept & Standard Color Reproduction

The above-mentioned low-luminance color problem in conventional display can easily be solved by using high-luminance primary colors; this is realized, for example, by using a brighter backlight in an LCD with an appropriate color-mapping algorithm. As the following step, a mapping of an input video signal (source gamut) to the enlarged display gamut (target gamut) will be required. For example, as shown in Fig. 2 right, the relative lower luminance of “w1” of the target gamut (Gamut2) can be selected as the matched white point of the source gamut. The point of “w1” is freely selectable within the target gamut according to the desired mapping strategy. Therefore, the source gamut can be mapped inside the target gamut. If the source gamut is the Rec. 709 or sRGB gamut, then the source gamut can be mapped into the target gamut without any change of colors, as is done for the conventional sRGB display.

In contrast, if a video signal is encoded by the wide gamut of the xvYCC, nearly the entire region of the xvYCC source gamut, excepting only some low-luminance colors (marked as “Out of Gamut Colors” in Fig. 2-R; marked in green color but outside the target gamut), can be successfully matched into the target gamut, marked in green. This means that the high-luminance colors (marked as “High-luminance Colors”; marked in green color but outside the upper sRGB gamut) of xvYCC can be reproduced more than by the conventional display system.

[Fig.2. Gamut enlargement from “Gamut1” of the conventional display to the enlarged “Gamut2” by replacing a brighter backlight and the conceptual mapping diagram of the standard color reproduction in the proposesd display]

The new display can reproduce more bright colors with minimal change of hue and lower desaturation of colors than the conventional methods of “sRGB camera/sRGB display” and “xvYCC camera/sRGB display.” Consequently, the proposed display preserves more image details than the conventional display.

sRGB source gamut expansion:

To improve a pleasantness of the popular sRGB images, the sRGB gamut can be mapped to the freely definable “target-expanding gamut” as a part of the target gamut of a display with a brighter backlight. To do this, we used the simple mapping algorithm using a concept of constant saturation as shown in Fig. 4. Any color inside the sRGB gamut will be linearly increased to the target-expanding gamut along the line of constant saturation. A linearly increasing gain can be determined by comparing the two gamut boundaries (source and target-expanding gamut) for a given constant saturation. As a mapping result (see fig.5), the luminance of the sRGB chromatic colors will be increased while maintaining their hue so that the image may look more brilliant and colorful than the sRGB original.