CVPR CVPR #**** #**** CVPR 2010 Submission #****. CONFIDENTIAL REVIEW COPY. DO NOT DISTRIBUTE. 000 054 001 055 002 056 003 Display Gamut Reshaping for Color Emulation and Balancing 057 004 058 005 059 006 060 007 061 008 062 009 Abstract 063 010 064 011 Emerging next generation digital light projectors are us- 065 012 ing multiple LED/laser sources instead of one white lamp. 066 013 This results in a color gamut much larger than any exist- 067 014 ing display or capture device. Though advantageous in the- 068 015 ory, when used to display contents captured/processed at a 069 016 smaller gamut, a large gamut expansion results in hue-shift 070 017 artifacts. 071 018 We present a hardware-assisted 3D gamut reshaping 072 019 method that handles the gamut expansion in LED based 073 NTSC Traditional Single Source DLP 020 DLP displays by hierarchical temporal multiplexing of the Projectors (-2.9, -12.41) 074 HDTV (+7.7, +10.3) Multiple LED Source DLP 021 multiple primaries. This, in turn, results in a color emula- Projectors (+71, +66.6) 075 022 LCD Panels/Traditional Multiple Laser Source DLP 076 tion technique by which projectors with such large gamuts Singl le S ource LCD Pro ject ors PjtProjectors (+ 151, +96 .9) 023 can also achieve a standard color gamut and white point – (+11.5, +3.8) 077 024 the two most important color properties in terms of display Figure 2. Comparison of the different standard 2D color gamuts 078 025 quality, with an additional advantage of increased bright- with the gamuts provided by the LED or laser based projectors, 079 026 ness and dynamic range. The same method can also be used both in CIE xy and u’v’ space. The bold numbers indicate the 080 027 for color balancing across multiple projectors that are often percentage deviation in the area of the 2D gamut when compared 081 028 used to create large-scale high resolution displays. to the NTSC gamut, both in CIE xy and u’v’ space respectively 082 029 083 ple light sources, one for each primary, created from one 030 1. Introduction 084 031 or more light emitting diodes (LEDs) [8, 29, 10]. The pri- 085 maries are then switched ON and OFF or multiplexed tem- 032 The traditional digital light projection (DLP) technology 086 porally independent of each other (Figure1). 033 includes a white light bulb and a color wheel with differ- 087 034 ently colored filters. The filters are temporally multiplexed The LEDs in these projectors provide more saturated col- 088 035 at a high speed to selectively pass any one of the multiple ors than the color wheel resulting in a much larger color 089 036 primaries at any instant of time on to the digital micromir- gamut than any traditional projector and standard color 090 037 ror device (DMD) array [34]. The number of filters on the gamuts like NTSC, PAL, and even the most recent HDTV 091 038 color wheel can be three (R,G and B), four (R,G, B and (56% larger in the CIE u’v’ chromaticity chart). In fact, 092 039 W) or more [30, 13, 24,1, 25,6, 15](Figure1). These are the emerging laser projectors, due to monochromatic pri- 093 040 wide band filters creating a gamut smaller than the stan- maries, promise to provide an even larger color gamut, cov- 094 041 dard industry-specified gamuts like NTSC, PAL and HDTV ering almost all the colors visible to the human eye (almost 095 042 (Figure2). Hence, media in one of these standard gamuts is double than that of the NTSC gamut in the CIE u’v’ space) 096 043 mapped to the smaller gamut of the display. [20]. The percentage increase/decrease of different display 097 044 gamuts when compared to the NTSC gamut both in the CIE 098 045 xy and u’v’ chromaticity charts is quantified in Figure2. 099 046 Though larger color gamut assures reproducibility of 100 047 a larger range of chrominance, this causes gamut expan- 101 048 sion creating several problems (e.g. hue-shifts, white- 102 049 point shift and non-optimal utilization of color resources) 103 050 Figure 1. Left to right: Light Path of a traditional DLP projector when displaying existing media generated in devices with 104 051 and a DLP projector with multiple LED sources (Blue filter is ON). a much smaller gamut(Section3). As a result, these up- 105 052 The projection industry has recently introduced projec- coming projectors cannot be used in applications using 106 053 tors where the color wheel is eliminated by using multi- projectors and cameras in a tightly coupled feedback loop 107 1 CVPR CVPR #**** #**** CVPR 2010 Submission #****. CONFIDENTIAL REVIEW COPY. DO NOT DISTRIBUTE. 108 [18, 17, 21, 37, 19, 35, 28]. When the projected images are The XYZ coordinates of a color can be derived easily from 162 109 captured by the lower gamut cameras severe gamut clipping (I; x; y) using 163 110 artifacts occur. 164 111 In this paper, we present an algorithm to address this (X; Y; Z) = (xI; yI; I(1 − x − y)): (2) 165 112 gamut expansion. Unlike traditional single source projector 166 113 architecture where at any particular instance of time only Further, matching two colors, (I1; x1; y1) = (I2; x2; y2) 167 114 one of the primaries can be turned on, in projectors with assures that they also match in their XYZ coordinates i.e. 168 115 multiple LED sources more than one primary can be turned (X1;Y1;Z1) = (X2;Y2;Z2). Finally, the most important 169 116 on at the same time. Our method takes advantage of this point to note is that, for colors of similar chrominance, I 170 117 key property of simultaneous ON times to design a hard- scales proportionally to the luminance Y . Hence, in dis- 171 118 ware assisted scheme of hierarchical temporal multiplex- plays, for considering each primary or the grays, I and Y 172 119 ing of the LED primaries that can emulate a standard color are both scaled equally when the inputs are scaled. 173 120 gamut and white point without compromising other proper- It can be shown that in the CIE XYZ space, a ray through 174 121 ties like brightness, contrast and light efficacy, and enables the origin is the locus of colors with the same chromaticity 175 122 the following. coordinate (x; y) but different TTVs I. The chromaticity 176 123 1. Dynamic Color Emulation: Operability at standard color coordinates (x; y) is a 2D projection of these rays on the 177 124 gamut (like HDTV, NTSC and PAL) and white point (like X + Y + Z = d plane. The set of all chrominance visi- 178 125 D85 and D65) is very important for any display. Our ble to the human eye creates a horse-shoe shaped plot in the 179 126 method emulates many different color gamut and white xy space that represents different chrominance values phas- 180 127 point standards from the same set of LED primaries dy- ing out the I. This is called the chromaticity chart (Figure 181 128 namically as demanded by the application, just by chang- 2). The point (0:33; 0:33) in this chart indicates a perfect 182 129 ing the parameters of the temporal multiplexing (Section achromatic color with X = Y = Z. As the colors move 183 130 4). These parameters can be precomputed automatically away radially from this point towards the periphery of the 184 131 and then stored in the projector itself. horse-shoe shape, they change in saturation, while the hue 185 132 2. Robustness to Manufacturing Imprecision: The only way remains constant. 186 133 to achieve a desired color specification in a traditional sin- Finally, it can be shown with simple algebra, that adding 187 134 188 gle source projector is to control the color properties of their two colors, (I1; x1; y1) and (I2; x2; y2), result in a color 135 189 color filters via precision manufacturing. Since our method (I3; x3; y3) where I3 is the sum of the TTVs of the super- 136 can achieve the same standard color properties from LEDs imposing colors and chrominance is the weighted convex 190 137 that have a large variation in color, such strict control in combination of the chrominance of the superimposing col- 191 138 manufacturing can be avoided. This can make the technol- ors in the xy chromaticity chart, where the weights are given 192 139 ogy more flexible and cost effective. by the proportion of their TTVs. In other words, 193 140 194 3. Color Balancing in Multi-Projector Displays: Finally, 141 x1I1 + x2I2 y1I1 + y2I2 195 the same hierarchical scheme can also be used to achieve (I3; x3; y3) = I1 + I2; ; : 142 color balancing across multiple projectors, common for I1 + I2 I1 + I2 196 143 building large-area high-resolution displays (Section 4.2). (3) 197 144 2. Notation This result can be generalized to n colors, where the 198 145 chrominance of the new color lies within the convex hull 199 146 We first briefly describe our color notation. Our algo- of the chrominance of the constituting n colors. Thus, in 200 147 rithm involves only color matching and does not deal with a projector with three primaries, the reproducible chromi- 201 148 color distances. Hence, all computations in our algorithm nance lies within the triangle spanned by the chrominance 202 149 are carried on in CIE XYZ space. However, when evaluat- of the three primaries (Figure2). This is called the 2D color 203 150 ing the display quality, we use a perceptually uniform color gamut or simply the color gamut.