1. Field of the Invention
The present invention relates to an apparatus and method for calculating a color temperature, and more particularly to an apparatus and method for calculating a color temperature by using one-dimensional chroma coordinate. The present application is based on Korean Patent Application No. 2001-12431 filed Mar. 10, 2001, which is incorporated herein by reference.
2. Description of the Related Art
Color temperature is the temperature of light from a complete heat radiation, which is measured in Kelvin units (K). When one looks at an object, the overall hue perceived by the viewer varies depending on characteristics of the illumination. For example, under an incandescent lamp, an object appears reddish, while in daylight, the same object appears more bluish than under the incandescent lamp. That means, the higher color temperature results in bluish colors, and the lower color temperature results in reddish color. Since the color is closely related to the color temperature, the color temperature needs to be changed in order to change the color.
Color display systems are widely used in many devices for visually transmitting information to the viewer. Such devices include, for example, TVs, DTVs, thin film transistor (TFT) monitors, color printers, digital cameras, projectors, and mobile phones. Due to visual characteristics of the color display system, a correlated color temperature needs to be calculated accurately. The correlated color temperature of a light source which is measured in Kelvin units (K) is the temperature of a blackbody when the color of the light source is the same. In other words, the correlated color temperature is a wavelength of the light source represented by the Kelvin scale, a standard comparison measuring value.
Color models are used to classify colors, and also qualify colors in accordance with attributes like hue, saturation, chroma, lightness, and brightness. Also, the color models are used for matching, and at the same time the valuable resources for the subjects that have to handle color images on the video or the Web. The color models include RGB model, HSB/HSL model, Munsell color system, and CIE color model. The CIE color model is determined by the International Commission on Illumination, the organization for determining lighting standards. The CIE color model includes CIEXYZ, CIELUV, and CIELAB color models. The CIEXYZ color model uses positive tristimulus values, indicated as XYZ to express colors. The CIEXYZ color model uses a chromaticity diagram.
Conventionally, the color temperature is calculated by a two-dimensional color temperature calculating method that uses X and Y axes of the CIEXYZ color model.
Now, the conventional method for calculating the color temperature will be described below with reference to FIGS. 1 and 2.
The description of the conventional method for calculating color temperature is based on “Color Science: Concepts and methods, quantitative data and formula, 2nd edition, pp.225–229 & p.828, 1982” by Gunter Wyszeck and W. S. Stiles. According to the conventional method for calculating color temperature, the color temperature of the light source is obtained by selecting an isotemperature line most adjacent to two-dimensional chroma coordinates corresponding to the light source. The isotemperature line is the line representing values of color temperature with respect to the selected light source. Accordingly, the two-dimensional chroma coordinates of the CIE 1931 diagram calculated from the RGB chroma coordinates of the input image, in other words, the XY chroma coordinates (Xs, Ys) are inputted (step S110).
Then the XY chroma coordinates (Xs, Ys) are converted into the two-dimensional chroma coordinates (hereinafter called UV chroma coordinates) (Us, Vs) of the CIE 1960 UCS diagram (step S120). The UV chroma coordinates (Us, Vs) are calculated by the following mathematical expression 1:                                          U            s                    =                                    4              ⁢                              X                s                                                                                      -                  2                                ⁢                                  X                  s                                            +                              12                ⁢                                  Y                  s                                            +              3                                      ,                              V            s                    =                                    6              ⁢                              Y                s                                                                                      -                  2                                ⁢                                  X                  s                                            +                              12                ⁢                                  Y                  s                                            +              3                                                          [                  Expression          ⁢                                          ⁢          1                ]            
After the UV chroma coordinates values (Us, Vs) are calculated from the UV chroma coordinates of the CIE 1960 UCS diagram, two isotemperature lines that are most adjacent to the UV chroma coordinates (Us, Vs) are selected (step S130).
Next, distances (di, di+1) between the selected the UV chroma coordinates (Us, Vs) and the two isotemperature lines are obtained (step S140). The distance d1 is calculated by the following mathematical expression 2:                              d          i                =                                            (                                                V                  s                                -                                  V                  i                                            )                        -                                          t                i                            ⁡                              (                                                      U                    s                                    -                                      U                    i                                                  )                                                                        (                              1                +                                  t                  i                  2                                            )                                      1              /              2                                                          [                  Expression          ⁢                                          ⁢          2                ]            
where, the values Us and Vs are UV chroma coordinates with respect to the inputted image.
The values Ui and Vi are UV chroma coordinates on the (i)th isotemperature line that has a slopping degree of ti.
As shown in FIG. 2, if the distance ratio di/dj+1, calculated by the expression 2, is negative, the inputted chroma coordinates (Us, Vs) lie between the two isotemperature lines of i=j and i=j+1.
Next, from the two distance values (dj, dj+1) calculated in S140, a correlated color temperature Tc is calculated (step S150). The correlated color temperature, corresponding to the inputted UV chroma coordinates (Us, Vs) can be obtained by two assumptions. One assumption is that a Planckian Locus (PL) between the two selected color temperatures Tj, Tj+1 can be replaced by a circular arc having a center at an intersection of the two selected isotemperature lines A and B. The other assumption is that the correlated color temperature is a linear function of distance along the circular arc. As a result, the following mathematical expression is obtained:                               T          c                =                              [                                          1                                  T                  j                                            +                                                                    θ                    1                                                                              θ                      1                                        -                                          θ                      2                                                                      ⁢                                  (                                                            1                                              T                                                  j                          +                          1                                                                                      -                                          1                                              T                        j                                                                              )                                                      ]                                -            1                                              [                  Expression          ⁢                                          ⁢          3                ]            
where, θ1 and θ2 are angles between the correlated isotemperature lines meeting with the intersection of the isotemperature lines A and B, after passing through the two isotemperature lines Tj, Tj+1 and the UV coordinates (Us, Vs). With respect to the adjacent isotemperature line having small angles θ1 and θ2, with the expressions θ1/θ2=sin θ1/sin θ2, a fourth mathematical expression is obtained as follows:                               T          c                =                              [                                          1                                  T                  j                                            +                                                                    d                    j                                                                              d                      j                                        -                                          d                                              j                        +                        1                                                                                            ⁢                                  (                                                            1                                              T                                                  j                          +                          1                                                                                      -                                          1                                              T                        j                                                                              )                                                      ]                                -            1                                              [                  Expression          ⁢                                          ⁢          4                ]            
As described above, the conventional method for calculating the two-dimensional color temperature is complicated, and also requires a two-dimensional calculator using the UV chroma coordinates to build hardware. Taking the accuracy of the correlated color temperature Tc into account during the establishment of the hardware, a square root calculator is also required to calculate the distance dj. In addition, a comparator, a multiplier, and a divider, etc. are also required to determine a range of the input value. As a result, establishing the conventional method for detecting the two-dimensional color temperature at a hardware level has considerably low practicality and low utilization in terms of size and costs.