Image processing apparatus, and color correction method and lookup table

#The present invention relates to a technique of inputting at least two color difference values, obtaining a saturation value corresponding to these color difference values, and performing color correction and, more particularly, to a technique of efficiently calculating the saturation and hue angle in color information at high speed.

#Various methods and apparatuses have conventionally been proposed for color correction of a digital color image. In many color correction processes, the color is managed and corrected in terms of “three attributes of color”, i.e., brightness Y, hue angle H, and saturation S. A representative example is an L*a*b* color space. In this color space, the color can be processed by means of the three attributes, and the color can be intuitively managed and grasped. Digital color data is generally managed by R, G, and B signals. In practice, it is difficult theoretically to manage the color by means of these R, G, and B signals in color correction. For example, in the L*a*b* color space, the saturation S and hue angle H are obtained by S=(a*2+b*2)1/2 H=tan−1(b*/a*)

#Calculation of the saturation and hue angle using these equations requires calculations using a root function and trigonometric function. When these functions are calculated by, e.g., software running on a personal computer, a standard function such as those in the C language can be utilized, but “floating-point calculation” is generally required. In a system which cannot adopt “floating-point calculation”, the function is realized by an algorithm such as “extraction of square root” which realizes the root calculation by means of arithmetic involving only integer calculation. Alternatively, the trigonometric function is tabled as far as necessary precision can be maintained, and the calculation result of the trigonometric function is obtained by looking it up the table.

#The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

#FIG. 1 is a flow chart showing an example of image processing according to an embodiment of the present invention;

#FIG. 2 depicts a graph for explaining the principle of the embodiment of FIG. 1;

#The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Before the description of each embodiment, the principle of the embodiment will be briefly explained.

#The number of entries of an LUT (LookUp Table) is devised to be substantially halved by utilizing “symmetry” in obtaining the saturation and hue angle. The LUT is specialized in a color space for performing image processing and is so formed as to cover only the color range. In order to obtain necessary results (saturation and hue angle) from the LUT formed in this manner, a main LUT for storing necessary results themselves and a sub-LUT for efficiently accessing an address of the main LUT that corresponds to a combination of necessary elements (in this case, color difference signals) are prepared. A result stored in the main table is obtained by calculating the lookup address of the main LUT from the sum of a value obtained by looking up the sub-LUT for a smaller one of two elements (saturation and hue angle) and the difference (absolute value) between the two elements. To obtain the hue angle, a result obtained from the main LUT is processed in accordance with a combination of the signs of the two elements and their magnitude relationship, thereby covering the color space.

#Image processing to be described below is realized by supplying an image processing program to a computer apparatus such as a personal computer. An assumed image processing program includes driver software which receives an image from an image input device or the like, image editing software which edits an image, and driver software such as a so-called printer driver which outputs an image to an image output device.