Colour correction device

Abstract: It is a purpose of the invention to provide a colour correction device which minimises colour differences throughout an area to be subjected to colour reproduction. There is provided a configuration constituted by a dynamic range conversion means (1) for cyan, magenta, and yellow, a separation means (2) for separating them into high order bits and low order bits, a CMY table reference means (3) for obtaining CMY table values corresponding to the high order bits, an interpolation means (4) for performing an interpolation process based on the low order bits and the CMY table values, a K-table reference means (5) for obtaining four K-table values corresponding to black, and a CMY correction means (6) for correcting the values of cyan, magenta, and yellow based on three interpolated data and three data obtained by the K-table reference means. This provides an effect that a colour difference between an original image and a reproduction image can be reduced.

Description: The present invention relates to a colour correction device for improving colour reproducibility of a colour image in digital colour copiers, colour facsimile machines, colour scanners, colour printers, and the like. A colour correction process is performed in conventional digital colour copiers, colour facsimile machines, colour scanners, colour printers and the like. The purpose is to improve colour reproducibility of a recorded colour image, and this is a process required to prevent turbidity in spectral characteristics of printed ink and deterioration of colour purity caused by non-linear distortion during colour mixing in colour printing. It is known that a colour scanner or the like uses first-order masking while a colour printer or the like uses second-order or third order masking which is masking of a higher order for correcting non-linear distortion in colour reproduction. A masking coefficient used for masking can be obtained using the least square method. The method disclosed in Japanese unexamined patent publication (KOKAI) No. S60-220660 is known wherein an area to be subjected to colour reproduction is divided into a plurality of areas, and a plurality of masking coefficients are prepared for respective areas. In the case of masking of higher orders wherein an enormous amount of calculation is required, a look-up table (LUT) method is known which is suitable for real-time processing. However, conventional methods have had a problem in that colour differences cannot be reduced when an entire area to be subjected to colour reproduction is corrected using a single masking coefficient. Further, in the case of the method wherein an area to be subjected to colour reproduction is divided into a plurality of areas and a plurality of masking coefficients are prepared for respective areas, there has been a problem in that distortion of correction occurs at boundaries between such areas. Masking of higher orders has had a problem in that an enormous amount of calculation is required. The look-up table method suitable for real-time processing also has had a problem in that a table of a large capacity is required and in that sufficient interpolation accuracy cannot be achieved. It is therefore an object of the present invention to provide a colour correction device which minimises colour differences throughout an area to be subjected to colour reproduction in order to solve the problems as described above in the prior art. In order to solve the above-described problems, the present invention provides a colour correction device to which colour image data of cyan, magenta, yellow, and black are input and which outputs colour image data of cyan, magenta, yellow, and black which have been subjected to colour correction, characterised by comprising: a dynamic range conversion means for converting a dynamic range of each of the cyan, magenta and yellow input colour image data; a separation means for separating the data converted by said conversion means into high order bits and low order bits; a CMY table reference means having CMY tables to which said high order bits of said cyan, magenta, and yellow output by said separation means are input for outputting the data related to each of cyan, magenta, and yellow in accordance with the input to said CMY tables; an interpolation means for performing an interpolation process based on said low order bits of said cyan, magenta, and yellow output by said separation means and said data related to each of cyan, magenta, and yellow output by said CMY table reference means and for outputting interpolation data related to cyan, magenta, and yellow, respectively; a K-table reference means having K-tables to which said black colour image data is input for outputting data related to cyan, magenta, yellow, and black, respectively in accordance with said black colour data; and a CMY correction means for outputting cyan, magenta, and yellow colour data which have been subjected to colour correction using the three interpolation data interpolated by said interpolation means and the three data relating to cyan, magenta and yellow output by said K-table reference means. Advantageously, a dynamic range conversion means for converting a dynamic range of the black input colour image data can be provided. In a colour correction device having the above-described configuration, CMY tables for colour correction on cyan, magenta, and yellow are stored in the CMY table reference means, and K-tables for colour correction on black are stored in the K-table reference means. When image data of cyan, magenta, yellow, and black are input, the dynamic range conversion means converts the dynamic ranges of the input cyan, magenta, and yellow; the separation means separates said converted data into high order bits and low order bits; the CMY table reference means obtains CMY table values corresponding to the high order bits from the CMY tables; and the interpolation means performs an interpolation process based on the low order bits separated by said separation means and the CMY table values obtained by said CMY table reference means. Further, the K-table reference means obtains four K-table values corresponding to the input black from the K-tables. In addition, the CMY correction table corrects the values of cyan, magenta, and yellow based on three data interpolated by said interpolation means and three data obtained by said K-table reference means to output colour image data of cyan, magenta, yellow, and black which have been subjected to colour correction. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic figures, in which; Fig. 1 illustrates a colour correction device according to the present invention. Fig. 2 is a conceptual view illustrating a CMY table reference means of a colour correction device according to the present invention. Fig. 3 is a conceptual view illustrating an interpolation means of a colour correction device according to the present invention. Fig. 4 illustrates the flow of a process in a CMY correction means of a colour correction device according to the present invention. Fig. 5 illustrates an embodiment of a process in an interpolation means of a colour correction device according to the present invention. Fig. 6 illustrates an embodiment of a process in an interpolation means of a colour correction device according to the present invention. Fig. 7 illustrates an embodiment of a process in an interpolation means of a colour correction device according to the present invention. Fig. 8 illustrates an embodiment of a process in an interpolation means of a colour correction device according to the present invention. An embodiment of the present invention will now be described with reference to the drawings. Fig. 1 is a block diagram illustrating a configuration of a colour correction device according to the present invention. Fig. 2 is a conceptual view illustrating a part of a CMY table which is referred to by a CMY table reference means according to the present invention. Fig. 3 is a view showing the concept of an interpolation means according to the present invention in the form of a plan view wherein yellow has an arbitrary value. Here, input colour data is represented by Cin (cyan), Min (magenta), Yin (yellow), and Kin (black), and output image data after colour correction is represented by Cout (cyan), Mout (magenta), Yout (yellow), and Kout (black). Each of the input colour image data is constituted by 8 bits and has a value in the range from 0 to 255. Each of the output colour image data is also constituted by 8 bits and has a value in the range from 0 to 255. As described later, the CMY table referred to by the CMY table reference means includes an LUT of 41 3 bytes for each of cyan, magenta, and yellow, and the K-table referred to by the K-table reference means is a table of 256 x 4 bytes. For clarity, it is assumed in the following description that the interpolation means performs linear interpolation using three interior division points as shown in Fig. 3 and that the CMY correction means performs a compression process based on proportional distribution at the time of overflow. Referring to Fig. 1, CMY tables for colour correction on cyan, magenta, and yellow are stored in a CMY table reference means 3 in advance, and K-tables for colour correction on black are stored in a K-table reference means 5 in advance. When input colour image data (Cin, Min, Yin, and Kin) each constituted by 8 bits are input, a dynamic range conversion means 1 converts the dynamic ranges of the input image data Cin, Min, and Yin using dynamic range conversion means 1a, 1b, and 1c, respectively. According to the dynamic range conversion of the present embodiment, 8-bit input colour image data represented by a value in the range from 0 to 255 is converted into 8-bit colour image data represented by a value in the range from 0 to 160. The reason for the conversion of the data into a value in 161 steps from 0 to 160 is as follows. It is desired that a CMY table referred to by the CMY table reference means 3 has a capacity which is as small as possible and is free from any problem in actual use for reasons including the cost involved in the fabrication of the colour correction device and size and accuracy of the same. Taking such a condition into consideration, the CMY tables are configured so that cyan, magenta, and yellow data each having a value in the range from 0 to 40 are input and so that they provide output data of 8 bits. Such a table is provided for each of cyan, magenta and yellow to output cyan data, magenta data, and yellow data, respectively. As apparent from the above description, one table is an LUT having a capacity of 41 3 bytes. Since the interpolation process performed by an interpolation means 4 in the present embodiment uses three interior division points as shown in Fig. 3, the dynamic range conversion means converts input colour image data having a value in 256 steps from 0 to 255 into data in 161 steps [41 + (40 x 3)] from 0 to 160 so that three interior division points are included in 41 steps. This dynamic range conversion may be either linear conversion or non-linear conversion, and the type of conversion employed is determined by the desired specifications. As for the specific configuration of the dynamic range conversion means, a look-up table may be used or actual calculation for conversion may be performed on a software or hardware basis. In the present embodiment, the dynamic range conversion is linear conversion, and the following calculation is carried out on a software basis for the data of each colour. (converted data) = (data before conversion) x 160/255 Each of the colour image data of cyan, magenta, and yellow which have been subjected to the dynamic range conversion is separated by a separation means 2 into high order six bits which is a value in 41 steps from 0 to 40 and low order two bits which is a value in four steps from 0 to 3. The high order bits are transferred to the CMY table reference means 3 while the low order bits are transferred to the interpolation means 4. The CMY table reference means 3 refers to an independent CMY table for each of cyan, magenta, and yellow based on the data of cyan, magenta, and yellow separated into a value in 41 steps from 0 to 40 to obtain three sets of eight lattice point data d000, d001, d010, d011, d100, d101, d110, and d111 for cyan, magenta, and yellow. A more specific description will be made with reference to Fig. 2. Let us assume that Cpnt, Mpnt, and Ypnt respectively represent the values of the data of cyan, magenta, and yellow separated by the separation means 2. Then, eight lattice point data are obtained which are specifically data d000 of a coordinate (Ypnt, Mpnt, cpnt), data d001 of a coordinate (Ypnt, Mpnt, Cpnt+1), data d010 of a coordinate (Ypnt, Mpnt+1, Cpnt), data d011 of a coordinate (Ypnt, Mpnt+1, Cpnt+ 1), data d100 of a coordinate (Ypnt+ 1, Mpnt, Cpnt), data d101 of a coordinate (Ypnt+1, Mpnt, Cpnt+1), data d110 of a coordinate (Ypnt+1, Mpnt+1, Cpnt), and data d111 of a coordinate (Ypnt+1, Mpnt+1, Cpnt+1). The interpolation means performs interpolation processes on cyan, magenta, and yellow independently of each other based on the three sets of eight lattice point data obtained by the CMY table reference means 3 and three sets of data having a value in four steps from 0 to 3 separated by the separation means 2 and outputs cyan data dc, magenta data dm, and yellow data dy. The above-described interpolation process will be more specifically described with reference to Fig. 3 and Figs. 5 though 8. Let us assume that Ca, Ma, and Ya respectively represent the values of the low order bits of cyan, magenta, and yellow separated by the separation means 2 and a set of eight lattice point data obtained by the CMY table reference means 3 are represented by d000, d001, d010, d011, d100, d101, d110, and d111. Then, the calculations as shown in Figs. 5 through 8 are carried out based on the values Ca, Ma, and Ya using the set of eight lattice point data d000, d001, d010, d011, d100, d111, d111, and d111 to obtain interpolation data. An interpolation process on each colour data is then performed by the interpolation means 4 to obtain the interpolated data dc, dm, and dy of cyan, magenta, and yellow, respectively. Although the present embodiment employs linear interpolation with sufficient results, it goes without saying that non-linear interpolation may be employed. Kin among the input colour image data is input to the K-table reference means 5 which then refers to independent K-tables for cyan, magenta, yellow, and black and outputs four 8-bit data, i.e., cyan data kc, magenta data km, yellow data ky, and black data Kout. The contents of each of the independent K-tables for cyan, magenta, yellow, and black is empirically determined. For example, a reproduction image output by a recording device which generates a reproduction image from the data (Cin, Min, Yin, Kin) of an original image is compared with the original image, and the data on the K-tables are determined to minimise the difference therebetween. A CMY correction means 6 performs a correction process using the cyan data dc, magenta data dm, yellow data dy calculated by the interpolation means 4 and the cyan data kc, magenta data km, yellow data ky output by the K-table reference means 5 to calculate cyan data Cout, magenta data Mout, and yellow data Yout. A more specific description will be made with reference to Fig. 4 wherein the cyan data, magenta data, and yellow data calculated by the interpolation means 4 are represented by dc, dm, and dy, respectively, and the cyan data, magenta data, and yellow data output by the K-table reference means 5 are represented by kc, km, and ky, respectively (S5a); next, the data dc, dm, dy output by the interpolation means 4 are respectively added to the data kc, km, and ky output by the K-table reference means 5 for each of the cyan, magenta, and yellow data (S5b); if cyan data cc, magenta data mm, and yellow data yy obtained by the addition are all less than 256, the cyan data cc, magenta data mm, and yellow data yy as a result of the addition become the output data Cout, Mout, and Yout (S5c, S5d). If any one of the cyan data cc, magenta data mm, and yellow data yy as a result of the addition is equal to or greater than 256, a maximum value maxCMY is obtained from the cyan data cc, magenta data mm, and yellow data yy (S5c, S5e), and a compression process is performed based on the obtained maximum value maxCMY and the cyan data cc, magenta data mm, and yellow data yy to calculate the output data Cout, Mout, and Yout (S5f). Although the compression process in the present embodiment is on a linear basis as shown in Fig. 4, the present invention is not limited thereto. The process as described above provides output colour image data (Cout, Mout, Yout, Kout) which have been subjected to colour correction. According to the present embodiment, the cyan data cc, magenta data mm, and yellow data yy are obtained by adding the data dc, dm, and dy output by the interpolation means 4 and the data kc, km, and ky output by the K-table reference means 5, respectively, for each of the cyan, magenta, and yellow data. However, the addition may be carried out after multiplying the data by some coefficient, or a non-linear process can be performed. In addition, although the CMY correction means 6 may be configured on a software basis as in the present embodiment, it may alternatively be configured on a hardware basis. As described above, the present invention employs a configuration constituted by a dynamic range conversion means 1 for cyan, magenta, and yellow, a separation means 2 for separating them into high order bits and low order bits, a CMY table reference means 3 for obtaining CMY table values corresponding to the high order bits, an interpolation means 4 for performing an interpolation process based on the low order bits and the CMY table values, a K-table reference means 5 for obtaining four K-table values corresponding to black, and a CMY correction means 6 for correcting the values of cyan, magenta, and yellow based on three interpolated data and three data obtained by the K-table reference means. As a result, there is an advantage in that a colour difference between an original image and a reproduction image can be reduced. Further, the above-described configuration wherein separate tables are configured for black image data and image data of other colours provides an advantage in that the capacity of each table can be small. The invention provides a colour correction device to which colour image data consisting of cyan, magenta, yellow, and black are input and which outputs colour image data of cyan, magenta, yellow, and black which have been subjected to colour correction by a colour correction means or the like, colour reproduction is optimised by providing a dynamic range conversion means which converts the dynamic ranges of cyan, magenta, and yellow, a separation means to which said converted data is input and which separates it into high order bits and low order bits, a CMY reference means which obtains CMY table values corresponding to said high order bits from CMY tables, and an interpolation means which performs an interpolation process based on the low order bits separated by said separation means and the CMY table values obtained by said CMY table reference means, a K-table reference means which obtains four K-table values corresponding to the black from K-tables, and a CMY correction means which corrects the values of cyan, magenta, and yellow based on three data interpolated by said interpolation means and three data obtained by said K-table reference means. The aforegoing description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention.

Claims: A colour correction device to which colour image data of cyan, magenta, yellow, and black (Cin, Min, Yin, Kin) are input and which outputs colour image data of cyan, magenta, yellow, and black (Cout, Mout, Yout, Kout) which have been subjected to colour correction, characterised by comprising: a dynamic range conversion means (1a, 1b, 1c) for converting a dynamic range of each of the cyan, magenta and yellow input colour image data; a separation means (2a, 2b, 2c) for separating the data converted by said conversion means into high order bits and low order bits (Ya, Ma, Ca); a CMY table reference means (3) having CMY tables to which said high order bits of said cyan, magenta, and yellow output by said separation means are input for outputting the data related to each of cyan, magenta, and yellow in accordance with the input to said CMY tables; an interpolation means (4) for performing an interpolation process based on said low order bits of said cyan, magenta, and yellow output by said separation means and said data related to each of cyan, magenta, and yellow output by said CMY table reference means and for outputting interpolation data (dc, dm, dy) related to cyan, magenta, and yellow, respectively; a K-table reference means (5) having K-tables to which said black colour image data is input for outputting data (kc, km, ky, Kout) related to cyan, magenta, yellow, and black, respectively in accordance with said black colour data; and a CMY correction means (6) for outputting cyan, magenta, and yellow colour data which have been subjected to colour correction using the three interpolation data interpolated by said interpolation means and the three data relating to cyan, magenta and yellow (kc, km, ky) output by said K-table reference means. The colour correction device according to Claim 1, wherein said data relating to black (Kout) output by said K-table reference means is used as black image data after colour correction. The colour correction device according to Claim 1, wherein said CMY correction means includes an addition means for adding said interpolation data and said data relating to cyan, magenta and yellow output by said K-table reference means, respectively, and a determination means for determining the results of said addition as colour-corrected cyan, magenta, and yellow image data, respectively, when all of the results of said addition are less than predetermined values. The colour correction device according to Claim 3, wherein said CMY correction means further includes a compression means for compressing said respective results of said addition when at least one of said results of said addition is equal to or greater than said predetermined value. The colour correction device according to Claim 4, wherein said compression means performs a compression process based on linear proportional distribution. The colour correction device according to any preceding claim and further comprising a dynamic range conversion means for converting a dynamic range of the black input colour image data.

Inventors: YAMADA YASUMASA, YAMADA, YASUMASA, Yamada, Yasumasa, c/o Seiko Inst. Inf. Dev. Inc.
Applicants: SEIKO INSTR INFORMATION DEVICE, SEIKO INSTRUMENTS INFORMATION DEVICES INC.
Application year: 1996
Main classification: H04N1
IPC codes: H04N1, G06T1, G06T5