Application Driven Design of Multi-Primary Displays

Implementations of multi-primary displays for home cinema, mobile screens and soft proofing are described and analyzed. The design flexibility gained by using extra primaries is discussed for each, and the different considerations are highlighted. We show that in all cases the multi-primary displays suit the requirements of the application better than RGB displays. Introduction Our imaging needs are varied and differ in their required "look and feel"; we want to make a movie look on our TV just like it looks at the cinema, use digital monitors for soft-proofing and achieve perfect color-match to printed paper. Translating these needs to a display's technical spec, one would find significant differences in key parameters such as color gamut, dynamic range, brightness and contrast ratio required for each application, to such degree implying that not one display can serve all. Nevertheless, up until recently the supply of electronic color displays was limited to only one type of device, CRT, whose color reproduction characteristics had almost no variance. Practices of gamut mapping and color management have been developed to meet the different imaging needs mentioned above with the CRT gamut. However, even the most sophisticated ones have their shortcomings that derive primarily from the physical differences between the source and the target as can be seen from figure 1, which depicts the different color gamut of film, offset print on paper and the HDTV standard (Rec. 709 [1]). In recent years, certain technological developments have facilitated the emergence of new displays. For RGB displays, these developments mostly mean that the gamut area boundary of Rec 709 may be surpassed [2]. However, one of the most significant breakthroughs is the ability to create Multi-Primary (MP) displays, which have gained a lot of interest lately [3-6]. Whereas RGB displays use red green and blue primaries to reproduce colors, in an MP display more than three primary colors are combined to create the colors. Due to the use of more than three primaries, the three dimensional color gamut structure of an MP display is significantly different than that of an RGB display. Furthermore, the increase in the degrees of freedom gained by the additional primaries allows a design of the gamut structure according to the application the display is intended for. The display’s gamut would ideally be identical or at least very similar to the target gamut, thus enabling inherently good color match and relatively simple image processing. In the following we examine three different applications: home cinema displays, mobile displays and specialized displays for soft proofing of offset printing. We discuss the drawbacks of RGB displays and show how the use of multiprimary displays solves them. Figure 1. Film, Rec. 709 and print color gamut Home Cinema Home cinema is the high end niche of the TV market, and its intention is to create the cinema experience at home, both in terms of sound and image. While this has been achieved in audio, the video part is still slogging behind. Home cinema displays are usually large (front projectors and large TVs based on rear projection, plasma and recently also LCD panels), but lack the necessary color gamut and appearance of film. The film gamut is wider than Rec. 709 gamut (figure 1), in particular in the yellow and cyan color areas. The new RGB display technologies, which are not limited to Rec. 709 color gamut, still have triangular gamut shape. Thus the choice of the green point dictates the nature of the display. A green point near the edge of the chromaticity diagram, as in the case of the Sony Cineza home cinema front projector [7], provides good coverage of yellow colors, but limits the coverage of cyan (figure 2). A green inclined towards the cyan region, as in the Sony Bravia LCD [7] has the opposite effect. In both cases there is no coverage of the film gamut. The difference in color gamut between these two displays implies that their color gamut design is driven by technology rather than by the application which is common to both. A triangle that covers film gamut more closely than Rec 709 may be selected, such as the gamut defined by the DCI for digital cinema projectors [8]. Yet, a projector with such gamut will have very low brightness efficiency. Simple analysis shows that the white point luminance drops by >25%, when the color gamut of a projector with a given optical engine is extended from Rec. 709 to DCI gamut. This inefficiency is due to the fact that in order to increase the gamut in RGB displays, the spectral pass band of the RGB filters used in projectors and LCD displays to filter a white light source must be narrowed. Since a large screen with a reasonable luminance is a must for home Pre-print, September 2006 cinema application, this implies that more light power has to be put into the engine. Although this may look like a simple solution, one must remember that more light power means more heat, implying tougher cooling requirements, resulting in more noise that should be masked so it would not disturb sound performance. As a result one ends up with a larger, more complicated system, with shorter lifetime and higher cost. Figure 2. Comparison between the gamut of film (solid gray), two RGB displaysCineza (dotted) and Bravia (dashed), and an MP display (solid line) Multi-primary displays can overcome both problems mentioned above. The use of more than three primaries allows a non-triangular gamut shape, thus providing coverage of both yellow and cyan regions. As an example the MP display configuration shown in figure 2 demonstrates a gamut optimized for film compatibility, larger than the DCI gamut by ~20%, yet with 30% increase in the luminance of the white point. This is achieved by adding yellow and cyan primaries and narrowing the band pass of the R, G and B primaries. For four-primary configuration, addition of yellow enhances the luminance and allows flexibility in the chromaticity of the green primary, thus again enabling better coverage of the film gamut. The use of more primaries allows better utilization of the white light source, since the yellow region of the white light source may be included. Light sources used in the industry for projection engines, e.g. UHP and Xenon lamps, have intense yellow emission, which is not used in RGB displays, but may be fully included in a multi-primary display. Even more important factor is the spectral overlap of filters for MP displays, which results in better effective transparency than RGB filter set. To understand the guideline for choosing the color filters and the issue of spectral overlap let us examine the MacAdam limits of the D65 optimal color stimuli depicted in figure 3 [9]. It is clear from the curve that adding a yellow primary is beneficial, since yellows may be rather saturated (i.e. very near the edge of the chromaticity curve) at relatively high luminance. Furthermore, the black line drawn on the curve indicates the chromaticity obtained by various long wavelength transmission spectra with different cutoffs. We note that the line follows the chromaticity edge quite closely until a knee at a cutoff of 520 nm. Choosing a yellow filter corresponding to the knee point would yield a highly saturated yellow, which transmits as much as possible from the white source spectrum. Furthermore, a red filter with a cutoff at 600 nm (marked by a diamond on figure 3) is completely overlapped by the transmission of the yellow filter, thus the red part of the spectrum is transmitted twice through the system (regardless of the cutoff frequency of the red filter). Similar reasoning applies for the cyan and the blue. Figure 3. The MacAdam limits for optimal color stimuli under D65 Moreover, parts of the green transmission spectrum are overlapped by the cyan and the yellow filters. As a result, although each of the filters in an n-primary display (where n>3) occupies 1/n of the pixel area (in an LC display) or on average 1/n of the time [10], the average transparency is higher than that of an RGB system since most of the white light spectrum transmits through two of the filters. Finally, although the analysis here is done in terms of D65 illumination it may be also done for other light sources (such as CCFL, UHP Xenon lamp or white LED) with similar conclusions. Important considerations determining the choice of primaries are the required white point color temperature and the relative luminance of the primaries with respect to the white. Cinema look and feel implies "warm" (low) color temperature, typically 5000 – 6500K. In the CRT age, TV color temperature was pushed to significantly higher levels typically 9000 – 12000K, thus rendering a much "cooler" appearance. Here again, additional primaries mean additional degrees of freedom. The introduction of a yellow primary means a much warmer device white point, between 5000 – 6500K. Thus, a very important aspect affecting cinema look and feel is achieved at the physical level of the display, rather than at the image processing level. As an example, the white point color temperature of the MP display shown in figure 2 is 5400K, typical for film. In figure 4, luminance – excitation purity [11] curves representing cross sections of the color gamut of film and MP display along different hue lines (straight lines connecting two saturated colors with opposite hues through the white point on an x-y chromaticity diagram) are shown. It can be seen that a good Pre-print, September 2006 match is achieved between MP performance and cinema envelope, in contrast to Rec 709 which falls short in saturated colors and throughout the whole range in yellow. Figure 4. Luminance vs. excitation purity for the principle hues, for film (solid lines), Rec. 709 display (dotted) and MP display (dashed) Nevertheless, as home cinema products are used for viewing not only feature films but also other types of content (typically Rec 709), other color spaces and their 3 dimensional volumes have to be considered. This consideration pertains mainly to the relative luminance of the primaries with respect to the white point, which may be essential to the compatibility with the color space represented by the data. The chromaticity of a primary usually trades-off with its luminance; therefore a suitable choice of a primary should represent an optimum between both aspects. Currently, color data is given in terms of device dependent RGB data. The Rec. 709 standard for example, maps this device dependent RGB to absolute color space within the Rec. 709 triangle. To make use of the benefits of the MP display a suitable color rendering and gamut mapping method has been developed, as well as color conversion from a three dimensional color space into MP signals. The conversion is performed by transferring the incoming device dependent data into an absolute color space where color rendering and mapping are performed, and at the last stage the resulting XYZ data is converted to multi-primary signals. The use of absolute color provides a reference to which the primary signals may be aligned by calibration. The conversion from three to n>3 channels is a problem with a multitude of solutions and for video applications the solution must be chosen at real time speed, sufficient for processing 1080P data, implying about 6 ns total processing time per pixel. Gamut mapping is required because the color gamut of the MP display is different from that of Rec. 709, and most available color content is prepared to comply with Rec 709 standard. Therefore, the extended envelope of performance will not be utilized without color re-mapping. Color is hence re-mapped to the full gamut of the display, while appearance heuristics constrained by image quality aspects are applied. The integrity of neutral, natural and other memory colors is protected, and out of device gamut colors are re-mapped inside, preserving color differentiation and smooth transitions.