1. Field of the Invention
The present invention relates to: an image processing apparatus for performing color correction processing on input values of input image signals in a color coordinate system so that a predetermined color region (e.g., skin color region) is shifted to an ideal color region; a camera apparatus using the image processing apparatus; an image output apparatus (e.g., a liquid crystal display (LCD) or a printer) using the image processing apparatus; an image processing method using the image processing apparatus; a color correction processing program for making a computer execute the image processing method; and a computer readable recording medium on which the color correction processing program is recorded. The present invention is useful when images captured by an image capturing apparatus (e.g., a video camera or a digital still camera) are output from an image output apparatus (e.g., a printer or a plotter) or a display apparatus (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)).
2. Description of the Related Art
In recent years, apparatuses for capturing, printing or displaying images (e.g., a video camera, a digital still camera, a mobile phone camera, a liquid crystal display (LCD), a printer, etc.) have been widely used. Thus, it is desired that the quality of the captured image and the quality of output image be improved. Also, rather than faithfully reenacting an original image actually captured so as to display an original image in its original form, it is desired that a color correction be performed to obtain an image preferable to a user so as to display and print a vivid image, which is referred herein to as memory color.
The conventional image processing apparatuses use a color correction method for increasing the image quality of images, such as a color correction method for better representing a shade of skin color and a shade of blue which would be observed in the sky as well as a shade of green which would be observed in trees.
Herein, a color correction method with a general conventional linear conversion will be described with reference to FIG. 16.
FIG. 16 is a diagram showing a position of a skin color in a color space.
An image is represented by a one-dimensional space representing a light intensity (which is referred to as luminance) and a two-dimensional space, shown in FIG. 16, representing color. It is common to perform signal processing (color correction) on both of the above mentioned. Herein, as a method used in general, an image signal is separated into a luminance and a color space. The method is described with reference to the (L*a*b) space (herein, L, *a and *b denote luminance, a red color component in the color space and a blue color component in the color space, respectively).
The L, *a, *b are respectively calculated by using the following equations. The (L*a*b) space is a color space and is widely used in general in an image processing apparatus which handles images. The (L*a*b) space is defined by CIFA.L*=116(Y/Yn)^⅓−16a*=500[(X/Xn)^⅓−(Y/Yn)^⅓]b*=200[(Y/Yn)^⅓−(Z/Zn)^⅓]
where, in X, Y, Z, Xn, Yn and Zn, stimulus values of actual light sources are the stimulus values of light.
As shown in FIG. 16, a region A1 represents a region which is the maximum amount of color space viewed by a user. As for a camera, it is desired that an input light signal be processed appropriately so that the size of the preferred region is closer to the size of this region. Also, it is desired that a display (such as an LCD) represent the size of this region.
However, with the conventional techniques, it is difficult to represent the entire region of the maximum amount of region A1 viewed by a user. For example, a region A2 is the maximum amount of color space represented by an apparatus. Thus, the processing is performed within a range of the maximum amount of color space, depending on the performance of an image processing apparatus as indicated by the region A2. A region A3 is a local skin color region in which a skin color in an image captured or an image displayed is represented.
In order to briefly explain the conventional technique, herein, a representation of a skin color will be described as an example. Also, regarding a region of the color space where image processing is performed, description will be mainly centered on a two-dimensional color space which represent a color of the (*a*b) space shown in FIG. 16.
FIG. 17 shows a color space after the skin color region A3 is moved by adding certain values to the current values. FIG. 18 is a diagram showing details of moving the skin color region A3 of FIG. 17 by adding certain values to the current values. FIG. 19 shows a color space after the skin color region A3 is moved by multiplying the current values by an appropriate factor. FIG. 20 is a diagram showing details of moving, by integration, the skin color region A3 of FIG. 19.
When a vivid skin color region B3 which is actually preferably obtained is calculated from the skin color region A3 represented in FIGS. 17 and 18, it is required to alter the color reproduction for the skin color. In order to alter the color reproduction for the skin color, a method in which the entire (*a*b) space is altered as shown by a dashed line in FIG. 17 is generally used. For example, when the skin color space is moved by adding a movement amount Δd as shown in FIG. 18, the skin color region is made to approach the ideal and vivid skin color region B3 by using a skin color correction method. This skin color correction method moves the entire (*a*b) space by using the following equation.
Δd(x) is a movement amount of Δd in the x space.*a=*a+Δd(a)*b=*b+Δd(b)
Next, another skin color correction method will be described with reference to FIGS. 19 and 20.
As another skin color correction method, a method is used in which the skin color region is made to approach the ideal skin color region by integrating the entire (*a*b) space to cover the skin color to be moved as shown in FIGS. 19 and 20. For example, the skin color region is moved by integrating the (*a*b) space with Δdi according to the following equation.
Δdi(x) is an integration amount of Δdi in the x space.*a=*a×Δdi(a)*b=*b×Δdi(b)
Also, as a conventional technique, Reference 1 and Reference 2 describe locally (partially) performing a color correction on a skin color.    [Reference 1] Japanese Laid-Open Publication No. 2002-185973    [Reference 2] Japanese Laid-Open Publication No. 2002-223366
When a color/luminance correction is locally (partially) performed, for example, on a skin color or a green color, the reproduction of the color and the luminance of the entire image rather than local (partial) image is altered with the conventional linear conversion, and as a result, the reproduction of images other than the local (partial) image would not be obtained in the aforementioned conventional color correction method. Also, when a simple non-linear conversion is used, there is a possibility that some portions are not represented due to a loss of continuity. In contrast, when a complex non-linear conversion is used, a processing amount (calculating amount) is excessive. For example, the aforementioned References 1 and 2 do not sufficiently consider reducing the burden of the processing amount.
Specifically, reference is made to the maximum amount of color space B2 after a correcting movement in FIG. 17. A problem occurs in that the color balance of other regions is affected along with the change of the entire color space. Also, depending on a display apparatus using a conventional image processing apparatus, it is considered that the region B2 which has been moved from the region A2 would not be represented as the entire color space.
Also, in other skin color correction methods, there is a problem of affecting the color balance of other regions as shown in FIG. 20. This problem occurs along with the change of the entire color space from the maximum amount of color space region A2 represented by an image processing apparatus to the maximum amount of color space region C2 after a correcting movement. Also, when a region A3 is moved to a region B3, the region A3 becomes a region C3, which is slightly larger than the region A3 due to the integration of the region A3. As a result, as in the case of FIG. 17, depending on the particular display apparatus which uses an image processing apparatus, it is considered that the region C2, which has been enlarged from the region A2 after the correcting movement, becomes a region which is not represented by the entire color space due to its protrusion from the region represented by the apparatus.
In this manner, the conventional techniques perform a color correction on skin color by using respective skin color correction methods described above. However, in the conventional techniques as described above, not only the change of skin color but also the change of other colors and the balance with respect to the maximum amount of space represented by the display apparatus must be considered and adjusted in order to perform a color correction on the skin color.
References 1 and 2 describe color correction methods in which color corrections are performed in a non-linear manner. However, Reference 1 does not take the amount of processing into consideration and thus, the amount of processing is excessively large, thereby resulting in an increase in the size of a circuit and software. Reference 2 does not specifically describe the methodology for the color correction.
The present invention is made to solve the aforementioned conventional problems and the objective thereof is to provide: an image processing apparatus capable of not changing a surrounding color region outside a selected color region to be corrected, of not clipping the moved region due to its protrusion from the region represented by an apparatus and of suppressing an increase of the processing amount (calculation amount) by locally performing a color correction by using a continuous non-linear conversion; a camera apparatus using the same; an image output apparatus (e.g., a liquid crystal display (LCD) or a printer) using the same; an image processing method using the image processing apparatus; a color correction processing program for instructing a computer to perform the method; and a computer readable recording medium for recording the same.