IS&T©s 2000 PICS Conference IS&T©s 2000 PICS Conference Copyright 2000, IS&T sRGB Color Calibration of Standard Color Displays Henry D’Souza, Alp Bayramoglu and William Nott Compaq Computer Corporation Houston, Texas USA Abstract the next. We mesaured the primary color coordinates of A comprehensive and practical method is presented for seven monitors of different manufacturers in our lab. The ensuring that standard color displays deliver optimal per- results are shown in Figure 1. Visually, there were some ceptual color matching capability within sRGB standards. perceivable differences between these monitors. These dif- Display specific data is measured on the assembly line and ferences become much more startling when comparing be- stored in EDID data space within each unit. When the dis- tween display technologies such as LCD (Liquid Crystal play is connected to a computer, a software utility retrieves Display) vs CRT (Cathode Ray Tube) monitors. Figure 2 the data and designs an optimizing filter to correct R, G, shows the difference between a Compaq Flat Panel Moni- B and white points at all intensity levels for best percep- tor and a Compaq CRT monitor. tual match to sRGB standards. The basic philosophy and Computer Graphics Systems render image bitmaps to method is presented, empirical data is analyzed and feasi- the monitor screen. These bitmaps consist of RGB triplets bility is evaluated. that directly relate to specific colors. This is because the image bitmap is generated under the assumption that the display device will conform to the standard sRGB color 1. Introduction space [2]. Since we have evidence that all monitors do not conform exactly to the sRGB specification, we are inter- In computer systems, the digital representation of color is ested in a method for processing the original bitmap given in terms of variable mixes of three primary additive colors: valid data regarding the actual primary color coordinates of Red, Green and Blue (R, G, B). The human visual system the specific monitor that we are working with. A method predictably perceives the close juxtaposition of the three is developed below to generate a filter that will correct for basic colors as one resultant color. This illusion is the ba- the deviations from the sRGB specification and essentially sis for color image processing. Using a-priori knowledge calibrate the monitor to display the best approximation of of the human visual system, it is possible to manipulate the the color that an sRGB monitor would display, given the intensity mix of the three basic colors to cause various de- original unfiltered bitmap. sired color shades to be perceived [1]. In fact, a wide range of colors may be perceived in this manner. For a high fidelity color system, it is of fundamental 2. Altering the Gamut of a display bitmap importance that the actual shade of color produced by any mix of the three primary colors be predictable. For this to The x,y CIE (Commission International d’Eclairage) chro- occur, it is necessary to have precise proportional control maticity coordinates of the primary colors of a display rep- of the intermediate intensities of each component primary resent the apices of a triangle that bounds the range of col- color. It is also necessary to know, with reasonable ac- ors that may be rendered by the display [3]. Any partic- curacy, the actual shade of the basic colors. Currently in ular color rendered corresponds to one and only one or- computer systems, this fidelity is lacking due to imprecise dered triplet of primary color intensity values (R,G,B). Im- proportional brightness control of basic colors as well as ages consisting of an array of these ordered triplets (image no control/knowledge of the actual shading of the primary bitmaps) are developed based upon the assumption that the colors. primary monitor R,G,B colors have certain CIE color co- This paper addresses the second issue viz. variations ordinates. This assumption is (or should be) related to a and deviations in the color coordinates of the primary mon- standard color space. The sRGB standard, among other itor colors. The primary color coordinates can vary from things, specifies the coordinates for the primary colors of one monitor to the next from the same manufacturer. This the monitor [2]. If the monitor rendering the image bitmap variation may also be expected from one manufacturer to does not conform to the sRGB standard for these colors, 1031 IS&T©s 2000 PICS Conference IS&T©s 2000 PICS Conference Copyright 2000, IS&T then any and all colors rendered will likely be perceived by computing the Y weighted average for u and v and incorrectly. by summing the Y values to get the resultant color. If [Y ; Ù; Ú ]Ê Y Ù Ú Ö Ö The goal here is to develop a method of ”filtering” the is the intensity Ö with coordinates and of Y ; Ù; Ú ]B image bitmap such that the output of the filter is a new the red component of the display and likewise [ Y ; Ù; Ú ]G image bitmap with modified (R,G,B) triplets that take into and [ correspond to the blue and green compo- consideration the actual color coordinates of the particular nents respectively, then mixing these three will yield an monitor in comparison with the color coordinates assumed analytically predictable resultant color with intensity given in the creation of the image bitmap. In this particular treat- by ment, we shall assume that the original color coordinates = Y · Y · Y used in the creation of the original image bitmap lie within Y g b Ö (7) the color gamut of the monitor. Below, we will discuss how the issue may be optimally resolved if the original color and coordinates given by coordinates extend beyond the color gamut of the monitor. Y Ù · Y Ù · Y Ù Ö Ö g g b b = In order to develop a color correcting filter, it is nec- Ù (8) Y · Y · Y g b essary to have a deterministic method of mapping from Ö R,G,B bitmap data directly to the perceived color coor- Y Ú · Y Ú · Y Ú Ö Ö g g b b = dinates. An experiment was conducted to determine if it Ú (9) Y · Y · Y g b is possible to directly compute the perceived color coor- Ö dinates resulting from any mix of known primaries. The So, to shift the display gamut from the triangle defined coordinates of each primary were mesaured. Then 25%, by 50% and 75% of each of the other primaries were added Y; Ù; Ú]Ê ;[Y ; Ù; Ú ]G;[Y ; Ù; Ú ]B to the first primary, together and separately, and the coor- [ (10) dinates measured each time. The idea was to determine if the coordinates of the colors resulting from the mixed pri- to the triangle defined by maries would align graphically so as to suggest a simple ~ ~ ~ Y;Ù;~ Ú~]Ê ;[Y;Ù;~ Ú~]G;[Y;Ù;~ Ú~]B deterministic method of color prediction. See Figure 3 and [ (11) Figure 4 for the results. It was found that it is not possi- (with the requirement that the ~ triangle is contained ble to work linearly within the CIE 1931 x,y chromaticity within the original triangle) we simply select an attenua- color space. That is to say that, given tion and mixing matrix A given by: ¼ ½ Y ; Ü; Ý ]Ê ;[Y ; Ü; Ý ]G;[Y ; Ü; Ý ]B [ (1) a a a ÖÖ Ög Öb @ A a a a A = gg gb it is not possible to predict by linear combination the gÖ (12) a a a bÖ bg bb Y ; Ü; Ý ]Ê · G · B [ (2) such that the following holds true that results from adding the three given intensities of Y a Ù · Y a Ù · Y a Ù ÖÖ Ö g Ög g b Öb b colors together. Ö Ù~ = Ö (13) Y a · Y a · Y a Ö ÖÖ g Ög b Öb Y ; Ü; Ý ] [Y; Ù; Ú] It was determined that mapping [ to us- ing the mapping [4] Y a Ù · Y a Ù · Y a Ù Ö gÖ Ö g gg g b gb b Ù~ = g (14) 4Ü Y a · Y a · Y a Ö gÖ g gg b gb = Ù (3) ¾Ü ·½¾Ý ·½ Y a Ù · Y a Ù · Y a Ù Ö bÖ Ö g bg g b bb b Ù~ = and b (15) Y a · Y a · Y a Ö bÖ g bg b bb 9Ý = Ú (4) and similarly ¾Ü ·½¾Ý ·½ Y a Ú · Y a Ú · Y a Ú ÖÖ Ö g Ög g b Öb b yields a color space that is linear in the sense that given Ö Ú~ = Ö (16) Y a · Y a · Y a Ö ÖÖ g Ög b Öb Y ; Ù; Ú ]Ê ;[Y ; Ù; Ú ]G;[Y; Ù; Ú]B [ (5) Y a Ú · Y a Ú · Y a Ú Ö gÖ Ö g gg g b gb b Ú~ = It is entirely possible to predict the resulting color co- g (17) Y a · Y a · Y a gÖ g gg b gb ordinates Ö Y a Ú · Y a Ú · Y a Ú Ö bÖ Ö g bg g b bb b Ú~ = b (18) [Y ; Ù; Ú ]Ê · G · B a · Y a · Y a (6) Y Ö bÖ g bg b bb 1042 IS&T©s 2000 PICS Conference IS&T©s 2000 PICS Conference Copyright 2000, IS&T An iterative error minimizing computer program was Y ÌÚ · Y ÌÚ · Y Ú Ö g g b b used to determine the elements of the attenuation and mix- Ö Ú~ = b (25) Y Ì · Y Ì · Y g b ing matrix A.