TWO DIMENSIONAL COLOR CALIBRATION FOR FOUR PRIMARY DISPLAYS Carlos Eduardo Rodr´ ıguez-Pardo, Gaurav Sharma ECE Dept. University of Rochester Rochester, NY, USA Xiao-Fan Feng Sharp Laboratories of America Camas, WA, USA ABSTRACT The process to ensure a fixed and desired response from a color dis- play generally consists of a per-channel calibration transform com- bined with a multi-dimensional characterization transformation. In this paper We focus on the former, i.e., the channel calibration. Con- ventional one-dimensional channel calibration strategies are inad- equate for high resolution LCD displays because of inter-channel crosstalk. We address this problem by developing a color calibration strategy for a four primary LCD display, based on a two dimensional structure for channel calibration, which allows for simultaneously meeting the dual objectives of perceptual linearization of individ- ual channels and gray balance along the device gray axes, despite inter-channel crosstalk. The two-dimensional nature of the trans- form represents a good balance between the dual objectives of low complexity and accurate control of key attributes of the displayed colors via channel calibration and experimental results demonstrate that the proposed scheme accomplishes its objectives offering a sig- nificant improvement over the per channel calibration for our four primary display system. Index Terms— Color calibration, multi-primary display, LCD display 1. INTRODUCTION The diversity of technologies for display systems, and the spread and interconnection of different digital imaging devices make necessary the use color management systems that allow the exchange of im- ages in encodings that are device independent. Color management for display and print devices is commonly partitioned into channel color calibration and multi-dimensional characterization that maps device independent color encodings of images and videos to the cal- ibrated device control space provided by the calibration [1, 2]. Dis- play color calibration is the focus of this paper. For color displays, the (channel) calibration aims to remap the device control space into a calibrated device control space which, apart from gamut limita- tions, ideally transforms linearly to a tristimulus color space, say the CIEXYZ color space [3]. If the calibration transformation ac- complishes this objective with good accuracy, the subsequent multi- dimensional color characterization can be accomplished via simple linear (affine) transformations from device independent tristimulus space, specifically, a 3 × 3 matrix for three primary systems. Traditional approaches for calibration 1 are one-dimensional based on the assumption of channel independence and color con- stancy that characterized the behavior of Cathode Ray Tubes (CRT). Contact: pardo@ece.rochester.edu 1 Throughout this paper, we use the term calibration to refer to channel color calibration, even though the term is often used elsewhere to refer to both channel calibration and the characterization. However, for Liquid-Crystal Displays (LCD), the channel indepen- dence assumption is challenged by the continuous reduction of the pixel size for the development of higher resolution displays. This reduction comes with the increase in the level of the interference, or crosstalk, between the electronic units, an effect that becomes strongest for spatial adjacent sub-pixels [4], affecting the channel independence and thus, the performance of traditional calibration functions [5, 6]. Other challenges arise due to the introduction of multi-primary displays. The use of four or more primaries in display systems has emerged as a novel approach to widen the gamut of color devices [7–9]. Moreover, power saving [10–12], view angle im- provement [7], and more recently, high resolution image represen- tation [8], are some of the other advantages obtained by the use of multi-primary technology, which makes it an active area of research with promising results. We develop a two dimensional scheme for calibration of a four primary display. Conceptually, the method is similar to the tech- nique presented in [13] although the differences in the application setting and the objectives also force significant differences in our de- velopment here. Specifically, whereas the work in [13] addressed calibration for color printers and was based on a three dimensional calibrated device control space, here we address color displays and specifically four primary displays. The specifics of our calibration methodology are motivated by LCD displays and the need to ad- dress the crosstalk between the primaries, which cannot be handled by traditional one-dimensional calibration approaches. Experimen- tal evaluation of the proposed approach on a four primary display demonstrates that it successfully accomplishes its goal of simultane- ously achieving perceptual linearization of the individual channels and gray balance along the device gray axes, offering a significant improvement over the traditional one-dimensional method. The manuscript is organized as follows. Section 2 introduces the calibration problem formulation and motivation for the two di- mensional scheme proposed in Section 3. Experimental validation of the strategy for a four primary display is presented in Section 4. Section 5 concludes the paper. 2. PROBLEM FORMULATION For our discussion, consider a display with N primaries, and a device control vector α =[α1, ..., αN ], an N ×1 vector whose entries lie in [0, 1] and represent the control signals to drive each of the primaries. Figure 1 shows the diagram for a general display system, where the display response,d(α), to the control vector α, is modeled in the CIEXYZ color space as, d(α)= Pf (α), (1) where P is a 3 × N matrix whose columns contain the color co- ordinates of the display primaries in the CIEXYZ color space. The 2820 978-1-4799-0356-6/13/$31.00 ©2013 IEEE ICASSP 2013