Method of and apparatus for setting color separation rules

Abstract: The color original image is pre-scanned in order to detect respective color components of sample pixels. Then, it is judged whether or not each sample pixel has color components corresponding to a designated memory color under a predetermined allowable error. When the number of sample pixels having the memory color is larger than a threshold value, a predetermined color separation rule is corrected to obtain a corrected color separation rule which is actually used for separating the color original image into components for image reproduction thereof.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of and an apparatus for setting color separation rules used in separating colors of a color image in a color image processor such as a color process scanner. Description of the Prior Art As well known in the art, a color process is operable to read a color original for each pixel in the order of scanning lines, and color image data thus obtained is subjected to color separation processing, to obtain image data for reproducing a color image. Such color separation is performed in accordance with various color separation rules. Important one out of the rules is that concerning memory colors included in the color original, where the "memory colors" are such colors that humans often obserb in their daily life through visual exprerinece and remember the impression thereof. For example, if the percentage of areas of the memory colors such as a skin color of a human being, a green color of trees and plants, and a blue color of the sky and the sea in the color original is relatively large, the memory colors must be reproduced on a print in a color tone corresponding to the impression on humans in order to make the print a good-quality one. Therefore, the color separation rule for memory colors must be different from standard rules. Furthermore, in some cases gradation curves may be made shifted from normal ones prepared with reference to gray tones. For example, when there is a skin color of a human being in the color original, the intermediate density part of a gradation curve is corrected such that the density of ink on a magenta (M) block is slightly higher than those of an yellow (Y) block and a cyan (C) block. Consequently, a gray portion of the original is reproduced at a slightly reddish tone while a skin-colored portion thereof is a reproduced in a color more similar to the color of the skin remembered by humans, resulting in a desirable print. Such correction processing of memory colors has been conventionally performed through manual work of an operator. However, since most of color originals whose colors are separated by a color scanner include such memory colors, the above-indicated correction for the memory colors is frequently required. Therefore, there is a problem of significantly increasing the burden on the operator. Additionally, considerable experience for a manual operation of a color scanner is required in such setting of the color separation rules for the memory colors. Thus, there are problems of the difficulty in standardizing the manual work and the variations in quality of prints depending on the subjectivity of the operator. In order to deal with the problems, it is desired to automate setting of the color separation rule with respect to memory colors. The publication DE - A - 3 222 488 discloses a color processing apparatus in which, after sampling of an original image, it is determined whether selected colors are present in the image. This determination is carried out by a digital process using specific addresses defining the reference axes of a two-dimensional color space in which the selected colors can be identified. Threshold-comparators are also used for determining if the average exposure of the image is between given limits. SUMMARY OF THE INVENTION The present invention is directed to a method of setting color separation rule in a color image processor, the color separation rule being used to separate a color image data into color components suitable for reproducing a color image represented by the color image data. According to the present invention, the method comprises the steps disclosed in claims 1 to 8. The present inventin is also directed to an apparatus for attaining above-indicated method. As schematically shown in Fig. 1, the apparatus according to the present invention comprises the features disclosed in claims 9 to 15. Since the occupation rate of the memory color in the original color image is used for determining a color separation rule, the process of setting the color separation rule can be attained under objective conditions. Furthermore, when the criterion for judging whether or not each sample pixel has the memory color is determined through a statistical analysis of sample images, the occupation rate can be calculated at a high accuracy. Accordingly, an object of the present invention is to reduce the burden on an operator in setting a color separation rule in a color image processor. Another object is to set a color separation rule suitable for reproducing memory colors. Another object is to shorten the process of setting a color separation rule and to standardize the work for setting a color separation rule. Another object is to provide an apparatus for setting a color separation rule automatically. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing the concept of the present invention; Fig. 2 is a block diagram showing an image processing apparatus according to a preferred embodiment of the present invention; Fig. 3 is a flow chart showing an overall operation of the image processing apparatus according to the preferred embodiment of the present invention; Fig. 4 is a diagram showing a closed space corresponding to a memory color in three-dimensional color coordinates; Fig. 5 is a flow chart showing correction of color separation rules for images having memory colors: Fig. 6 is a graph showing an example of correction of gradation curves for images having the memory colors; Fig. 7 is a block diagram showing hardware circuits employed in another preferred embodiment of the present invention; and Fig. 8 is a circuit diagram showing an example of the combination of circuits 50 and 51. DESCRIPTION OF THE PREFERRED EMBODIMENTS A. Structure and Overall Operation Fig. 2 is a schematic block diagram showing a color process scanner according to a preferred embodiment of the present invention, and Fig. 3 is a flow chart showing an schematic operation thereof. This color process scanner comprises a pickup unit 1 for photoelectrically reading on image on a color original 100 for each pixel in the order of scanning lines. The pickup unit 1 has a set of light splitters and color filters for separating light from each of pixels constituting the color original 100 into primary light components of red (R), green (G) and blue (B). The primary light components are converted into electric signals through photoelectric cells, respectively. In the step S31 shown in Fig. 3, the color original 100 is first pre-scanned by the pickup unit 1 at high speed as compared with a scan speed for image reproduction, to obtain electric signals representing color densities of each pixel belonging to a 200 x 200 sample pixel array, for example, which is defined on the color original 100. The sample pixel array may consist of sample pixels distributing on the color original 100 at a uniform distribution density, and in this case, the sample pixels can be picked-up while skipping the pixels on the color original 100 at a predetermined skipping ratio. Alternatively, the sample pixel array may be located at a designated small poriton of the color original 100 on which a characteristic part of the original image or a scene including the memory color is found. In addition to control of this sampling, the following various control and arithmetic operations are carried out by a control portion 9. The control portion 9 is constituted by, for example, a microcomputer having a CPU 11 and a memory 10. Image data with respect to the sample pixels thus obtained, which will be referred to as "a sample image data" hereinafter, are stored in an image memory 2. Since the sample image data comprises data indicating color densities with respect to a relatively small number of sample pixels, the image memory 2 may have relatively small capacity. In the next step S32, a density histogram of the color original 100 is generated based on the sample image data in the image memory 2. In the step S33, standard color separation rules and other parameters for image processing are set in a color operation circuit 3. First, a highlight point, standard shadow point, and a gradation curves are determined from the density histogram obtained in the step S32. For this processing, a method can be utilized which is disclosed in, for example, "A Statistical Method for Image Classification and Tone Reproduction Determination", R. Chung, Jr, Appl. Photogr. Sci. Eng., 3,74, 1974. The color operation circuit 3 comprises a highlight and shadow setting circuit 4, a basic masking circuit 5, a color correction circuit 6, a gradation setting circuit 7, an output correction circuit 8 and others. The highlight point, the shadow point and the standard gradation curve obtained by the above described processing are set in the highlight and shadow setting circuit 4, the gradation setting circuit 7, respectively. In addition, a standard color correction rule previously determined and stored in the memory 10 is also temporarily set in the color correction circuit 6 in the step S33. In the step S34, the standard color separation set rule in the step S33 is corrected according to the percentage of memory colors in the image on the color original 100. This processing will be described later in detail. In the step S35, the color original 100 is scanned by the pickup unit 1 at a normal speed, whereby the image data representing the image on the color original 100 for each pixel is obtained. This reading is performed with respect to all pixels in a desired portion of the color original 100 for image repreduction thereof. The image data for each pixel is delivered to the color operation circuit 3 for each of the colors R, G and B. In the color operation circuit 3, the image data representing the color original 100 is subjected to color arithmetic operation processing according to the corrected color separation rule, the image data being outputted to a recording unit 12. The recording unit 12 modulates one or more laser beams based on the image data after this color arithmetic operation processing to expose a photosensitive film 300 to the modulated laser beams, whereby latent images for Y, M, C and K colors are obtained on the photosensitive film 200. B. Principle of Finding Colors Regarded as Memory Color Described in this section is a principle for determining whether or not the color of a sample pixel corresponds to a memory color. Fig. 4 is a diagram showing a closed region LC defined in three-dimensional color coordinates, which is introduced for discriminating memory colors from other colors. First, a plurality of (preferable, a lot of) sample originals on which images having a designated memory color, e.g., a skin color, are prepared, and the color density on the sample originals are photoelectrically read for each pixel to be separated into components of B, G and R colors. Supposing that M is the total number of the pixels on the sample originals and i is a serial number of the pixels (i.e., 1 ≦ i ≦ M), the color components of the i-th pixel can be represented as B i , G i and R i . The color components B i , G i and R i are averaged in the range of 1 ≦ i ≦ M, whereby averaged color components BO, GO and RO are obtained. Preferably, originals including the designated memory color at a preferred hue are selected as the sample originals, since the designated memory color in the sample original gives standards in discriminating the memory color from other colors in reproduction of the original 100. In the three-dimensional color coordinates or color coordinate space, a point P₀ having the coordinate values of the averaged components (BO, GO, RO) is defined as shown in Fig. 4. The point P₀ is a characteristic or reference point indicating the designated memory color. However, in almost all originals, a memory color included therein does not necessarily coincide with the point P₀ and colors corresponding to the memory color are statistically distributed around the point P₀. Therefore, in the preferred embodiment, it is determined in a predetermined allowable error range whether or not the color of a sample pixel corresponds to the designated memory color. If the region having this allowable error range in the three-dimensional coordinates is referred to as "memory color region", this memory color region can be represented by the three-dimensional closed region LC including the point P₀ in the three-dimensional color coordinates. This memory color region LC may be determined so as to have an arbitrary shape. However, in view of the situation in which memory colors in an actual color original are statistically distributed, it is desirable to determine the shape, the size and the position of the memory color region in such a form as to reflect the character of distribution. As is well known, a statistical distribution in most of optical quantities is a normal distribution (Gaussian distribution). Accordingly, it is assumed that colors corresponding to the designated memory color in the actual color original are distributed according to the normal distribution, and the color components B, G and R of the sample pixels extracted from the population consisting of all pixels in the original are also distributed according to the normal distribution. The equiprobability surface in the distribution is an ellipsoid or the surface of a three-dimensional ellipse centered at the point P₀ (BO, GO, RO). Thus, assuming that a predetermined threshold value as described below is L², points which exist inside of the closed region or ellipse LC as represented by the following inequality (1) can be statistically considered as the designated memory color: L² ≧ C₁₁ (B - BO)² + C₂₂ (G - GO)² + C₃₃ (R - RO)² + 2C₁₂ (B - BO) (G - GO) + 2C₂₃ (G - GO) (R - RO) + 2C₃₁ (R - RO) (B - BO) The constants or coefficients C₁₁ to C₃₁ in the equation (1) represent respective weights of the colors B, G and R characterizing the designated memory color. According to statistics, the coefficient C₁₁ to C₃₁ can be determined as respective elements in an inverse matrix of a variance-covariance matrix A represented by the following equation (2): where, V b = {1 / (M - 1)} Σ (B i - BO)² = {1 / (M - 1)} {Σ B i ² - (Σ B i )² / M} V g = {1 / (M - 1)} {Σ G i ² - (Σ G i )² / M} V r = {1 / (M - 1)} {Σ R i ² - (Σ R i )² / M} C bg = {1 / (M - 1)} x Σ (B i - BO) (G i - GO) = {1 / (M - 1)} {Σ B i G i - (Σ B i ) (Σ G i ) / M} C gr = {1 / (M - 1)} {Σ G i R i - (Σ G i ) (Σ R i ) / M} C rb = {1 / (M - 1)} {Σ R i B i - (Σ R i ) (Σ B i ) / M} This mathematical principle itself is disclosed in, for example, "Multivariate Analysis Method" issued by Nikka Giren Publishing Company, 1981, pp. 261-267. Following is an example of numerical values taken when the designated memory color is a skin color. According to this principle, if a sample pixel having the density values(B i , G i , R i ) satisfies the equation (1), the color thereof is regarded as a substantially same color with the memory color. If a relatively small value is employed as the threshold value L² in the inequality (1), the allowable error is decreased and the criteria for discriminating then memory color from other colors becomes severe, so that only colors quite resembling the designated memory color are regarded as a substantially same color with the designated memory color. Therefore, it is desirable to select the threshold value L² while considering a balance between rigorousness in detection of the memory color and the allowable error. According to the preferred embodiment, the threshold value L² is statistically determined from the standpoint of what percent of colors normally distributed is to be considered as "memory colors". For example, when a risk percentage is set at 3% and there is a probability of 97 % at which an arbitrary sample having a color in the vicinity of the memory color satisfies the equation (1), it is assumed that L² = 9.12 from a table of numerical values representing the χ²-distribution with three degrees of freedom, i.e., χ²(3). In general, it is desirable to determine the threshold value L² such that approximately 95 to 98 % of the samples used for finding the average density value (BO, GO, RO) of the designated memory color satisfies the equation (1). Alternatively, the following criteria can be used. First, chromaticities b, g and r are defined from the densities of colors B, G and R through the following equations (4): b = B / (B + G + R) g = G / (B + G + R) (4) r = R / (B + G + R) The chromaticities b, g and r always satisfy the following condition for normalization: b + g + r = 1 The combination of b, g and r provides a point (b, g, r) on a plane defined by the equation (5) in the three-dimensional b, g and r coordinates. A memory color region in the two-dimensional space or plane is a two-dimensional region defined by arbitrary two of the chromaticities b, g and r. For example, an ellipse on the two-dimensional b and g coordinates may be defined by the following equation (6) to assume that colors of points inside thereof are regarded as the memory color. All the chromaticities b, g and r include a normalization factor (B + G + R) in their dominators, and therefore, the respective magnitudes of the color components B, G and R are taken into consideration by arbitrary two chromaticities. Since the probability that the chromaticities b and g of an arbitrary sample pixel satisfy the inequality (6) is expressed by the χ²-distribution with two degrees of freedom, a threshold value l² can be set in the same manner as the threshold value L² in the above described inequality (1). In addition, a manner for determining averages b0 and g0 and coefficients c₁₁, c₂₂ and c₁₂ is also similar to that for the inequality (1). l² ≧ c₁₁ (b - b0)² + c₂₂(g - g0)² + 2c₁₂ (b - b0) (g - g0) Data representing the above described inequality (1) or (6) has been previously stored in the memory 10 as the criteria for discriminating memory colors from other colors. When two or more colors are designated as memory colors, the inequality (1) or (6) is prepared and stored for each memory color. Although either of the inequalities (1) and (6) may be employed, the apparatus according to the preferred embodiment employs the inequality (1). C. Actual Process of Finding Occupation Rates of Memory Colors and Correction of Color Separation Rule Fig. 5 is a flow chart showing the details of processing for finding occupation rates of memory colors in the original image and correcting the standard color separation rule, which corresponds to the details of the step S34 shown in Fig. 3. First, in the step S41, the sample image data representing the color density of one sample pixel is fetched from the image memory 2. In the step S42, it is determined by performing an arithmetic operation based on the inequality (1) whether or not a point whose coordinate values (B, G, R) are defined by the color components of this sample pixel is located in the internal range of the memory color region or ellipse LC in the three-dimensional color coordinate space. More particularly, the right hand side of the inequality (1) is calculated with respect to the extracted sample pixel, and the calculated vlaue is compared with the threshold value L². In the step S43, if it is concluded that the sample pixel is in the memory color region LC, the process proceeds to the step S43, to increment a count value n by "1". This count value n is used for counting the number of sample pixels having colors in the memory color region LC. In the initial stage, the count value has been cleared to "0". If the result of judgement in the step S43 is that the sample pixel is outside of the memory color region LC, the count value n remains unchanged. Thus, the count value n will indicate the number of sample pixels in the memory color region LC among all sample pixels, when all sample pixels are subjected to the process. By the presence of the step S45, the foregoing processing is performed with respect to all the sample pixels in the image memory 2. When processing is terminated with respect to all the sample pixels, a value n / N is found which is the ratio of the number n of sample pixels in the memory color range to the number N of all the sample pixels. In other words, the value n / N represents an occupation ratio of the pixels having the memory color in the original image. After the ratio n / N is obtained, the ratio n / N is compared with a predetermined threshold value P, in the step S46. The threshold value P is a constant defining a minimum occupation rate of the memory color in the original image above which the standard color separation rule should be corrected for the memory color. This threshold value P is previously found experientially or experimentally for each memory color because the reproduction quality of memory colors affects the total quality of a print at different degrees for respective memory colors. For example, the quality of the print is substantially affected if the percentage of the green color of the trees and plants in the area of the entire scene is 30 % to 50 % or more while being affected even if the percentage of the skin color of a human being therein is several %. If the ratio n / N is larger than the threshold value P, the percentage of the memory color in the color original 100 is estimated to be high, and the standard color separation rule is corrected in the next step S47. The correction affects color correction rules and gradation curves, and the modified color correction rule and the corrected gradation curves are set in the color correction circuit 6 and the gradation setting circuit 7, respectively. in the other hand, if the ratio n / N is equal to or smaller than the threshold value P, the standard color separation rule is not corrected, the standard color separation rule having been set in the color correction circuit 6 and the gradation setting circuit 7 is used for image reproduction without any modificaion. The correction or modification of the color separation rule in the step S47 may be conducted in the following manner according to a predetermined modification rule stored in the memory 10. For example, when the color separation rule is corrected for a skin color, the color of the actual skin of the yellow race such as the Japanese is generally more yellowish than the skin color remembered by humans. Thus, it is preferable to correct the color separation rule so as to somewhat suppress the yellowness in printing. Accordingly, the color correction rules are modified such that the density of a yellow color in reproduction is somewhat suppressed. Consequently, obtained is a reproduced picture having the yellowness samewhat suppressed, in which the color of a skin-colored portion becomes a healthy skin color remembered by humans. Furthermore, the standard gradation curves are corrected in order to attain gradation correction suitable for the memory color. That is, for the skin color, the halftone dot percentage in an intermediate density portion of a standard gradation curve G M1 for a M block of negative reproduction shown in Fig. 6A is decreased to obtain a corrected gradation curve G M2 . On the other hand, the halftone dot percentages in intermediate density portions of standard gradation curves G Y1 and G C1 for Y and C blocks of negative reproduction shown in Fig. 6B are increased to obtain corrected gradation curves G Y2 and G C2 . Consequently, the entire image is slightly reddish so that the color of the skin is favorably reproduced. Although it is preferred to modify both of the color correction rules and the gradation curves, correction of the gradation curves may be omitted. Additionally, if there are a plurality of memory colors requiring correction of the color separation rule in the same color original, it is preferred to previously determine the priority order in correction of the color separation rule among the plurality of memory colors and to attain the correction for one or more memory colors having high priority or importance. In general, the skin color is the most important, followed by the blue color of the sky and the sea and the green color of the trees and plants. This order of priority may be varied depending on the character of the original image and the use of a print. Since the color separation rule is corrected and set through the foregoing steps, the designated memory color is reproduced based on memory of a humans, whereby a favorable print is obtained through the color blacks thus obtained. D. Other Embodiments A plurality of (for example, two or three) threshold values in modification of the standard color separation rule may be prepared with respect to one memory color. In this case, the ratio n / N is compared with the plurlity of threshold values P₁, P₂, ... and a minimum threshold value is found among the threshold values being larger than the ratio n / N. Then, the degree in modification of the standard color separtion rule is determined or selected in accordance with the found minimum threshold value. Preferably, the degree in modification is increased when the found minimum threshold value is relatively large. Through the selection of the degree in the modification or correction, the quality of the reproduced image is further improved. In addition, although it is desirable to define the closed region having the statistical allowable error range as a memory color region, the memory color region may be determined based on another criterion. It should be noted that the present invention can be applied to image processing apparatus other than a process scanner. In order to construct a color image processing apparatus according to the present invention, hardware circuits may be employed. Fig. 7 is a block diagram showing a process scanner in which an electronic construction for determining whether or not the color of a sample pixel can be regarded as a memory color and that for counting the number of sample pixels whose colors are regarded as memory colors are embodied by hardware circuits. In this case, the control portion 9 now be also constituted by hardware circuits, that is, a comparator, a latch circuit and other logic circuit, for example. However, high universality and high convenience can be attained when the control portion 9 is constituted by a memory 10, a CPU 11 and the others and to use a software program to achieve the object function. The operation of the process scanner of Fig. 7 is the same as that of the the above described color process scanner shown in Fig. 2. Sample image data obtained from a pickup unit 1 during high-speed pre-scanning are sequentially stored in an image memory 2 while being inputted to a discriminating circuit 50 for discriminating a designated memory color from other colors. The number of sample pixels whose colors are judged to be the designated memory color by the designated circuit 50 is counted by a counting circuit 51. After all the sample pixels are subjected to this processing, a count value of the counting circuit 51 is inputted to the CPU 11 in the control circuit 9, to perform processing similar to that in the above described step S46 and the subsequent steps shown in Fig. 5. The discriminating circuit 50 may be constituted using a look-up table or LUT which is supplied with the color components B, G and R and outputs a value or pulse "1" only when the color components satisfy the inequality (1). Furthermore, the counting circuit 51 may be constituted by a combination of standard logic circuits such as 74LS161 in SN7400 series supplied from Texas Instruments, USA. Fig. 8 is a circuit diagram showing structures of the discriminating circuit 50 and the counting circuit 51. In this example, each of components of colors B, G and R of the sample image data is inputted to the discriminating circuit 5 in eight bits. Since the color space shown in Fig. 4, is a wide space, a memory having a large capacity (2²⁴ x 1 bit, for example) is required when an LUT is constructed such that the inequality (1) for entire space is provided in the LUT. Therefore, in this embodiment, the color space is divided into a region in the vicinity of point P₀ shown in Fig. 4 and the other region. The LUT is prepared only in the region in the vicinity of the point P₀ to reduce the required memory capacity of the LUT to practical one (256k x 1 bit, for example). Subtracters 58 to 60 perform arithmetic operations or subtractions using equation µ B = B - BO, µ G = G - GO, and µ R = R - RO, respectively. Each of the inputs B, G and R and BO, GO and RO has eight bits, while each of outputs µ B , µ G and µ R has nine bits because it has one sign bit added thereto. Signals µ′ B , µ′ G and µ′ R which are the more significant four bits (one bit thereof is a sign bit) of the 9-bit outputs µ B , µ G and µ R are inputted to comparators 64 to 66, respectively. Each of the comparators 64 to 66 is operable to compare the input thereto with a value "0", so that a value or pulse "1" is outputted therefrom when the input is equal to "0" while a value "0" is outputted when the input is not equal to "0". Since the signals µ′ B , µ′ G and µ′ R are obtained by extracting the more significant four bits from the signals µ B , µ G and µ R , the condition µ′ B = 0 (µ′ G = 0, µ′ R = 0) indicates that the absolute value of the signal µ B (µ G , µ R ) is smaller than a value 2⁴ representing a predetermined distance in the color space. Thus, with respect to points spaced apart from the point P₀ by the predetermined value or more, at least one of outputs of the comparators 64 to 66 becomes "0" and an output of an AND circuit 68 becomes "0". When the color of a sample pixel is close to the point P₀ which is the reference coordinate point of a designated memory color, the signals µ′ B , µ′ G and µ′ R are all "0", so that the outputs of the comparators 64 to 66 are all "1". On the other hand, six-bits signals µ″ B , µ″ G and µ″ R obtained from the eight-bits signals µ B , µ G and µ R are inputted to an LUT 67. Each of the inputs µ″ B , µ″ G and µ″ R comprises the less significant five bits of the corresponding signal µ B , µ G or µ R and the sign bit being a plus or minus sign, and can represent values in the range from "-32" to "+31". The following inequality (1′) obtained by substituting µ″ B , µ″ G and µ″ R for (B - BO), (G - GO) and (R - RO) in the inequality (1) is expressed in a table form to be stored in the LUT 67: L² ≧ C₁₁µ″ B ² + C₂₂µ″ G ² + C₃₃µ″ R ² + 2C₁₂µ″ B µ″ G + 2C₂₃µ″ G µ″ R + 2C₃₁µ″ R µ″ B When the values µ″ B , µ″ G and µ″ R which are respective parts of sample pixel data satisfy the inequality (1′), an output of the LUT 67 becomes "1". More particularly, the LUT 67 is constructed such that a value "1" is outputted therefrom only when the value obtained by subtracting L² from the right-hand side of the inequality (1′) is zero or a negative value, or equivalently, only when the value obtained by subtracting the righ-hand side from the value L² is a positive value. Then, when the output of the LUT 67 is "1" under the condition where the outputs of the comparators 64 to 66 and the LUT 67 are all "1", the output of the AND circuit 68 is "1" so that the count value of the counting circuit 51 is incremented by "1". Meanwhile, the counting circuit 51 has supplied with a clear pulse CP to clear the same before the pickup unit 1 is enable for the pre-scan. In addition, sampling clock pulses SCK for specifying the sampling pixels are inputted to a sampling clock terminal of the counting circuit 51. In this example, if the densities of the colors B, G and R of each of the sample pixels represent values in the density range 0 to 4.0, the least significant bit represents the density of approximately 0.016 obtained by dividing 4.0 by 2⁸. Thus, the data range of the LUT 67 in which each of the inputs µ″ B , µ″ G and µ″ R is acceptable by the LUT 67 is approximately ±0.5 in density. Thus, the memory color region can be defined on the LUT 67 with a required range and a resolution sufficient for discriminating the memory color from other colors at high accuracy. The hardware circuits having the structures shown in Figs. 7 and 8 can respectively determine whether or not the color of each of the sample pixels is included in the memory color region and count the number of sample pixels whose colors are included in the memory color region during high-speed pre-scanning, so that the color separation rule can be corrected immediately after the high-speed pre-scanning. Although the preferred embodiments have been described in the case where the standard color separation rule has been previously set in the process scanner to be corrected through the predetermined correction rule or modification rule if the percentage of the memory colors in the original is a predetermined percent of more, two or more of standard color separation rules being different from each other may be prepared so that one of them is selected to be used in accordance with the percentage of the memory colors. Furthermore, correction of the color separation rule may be conducted while taking variation of memory colors into consideration. For example, when the skin color is a flesh color of a white race or that of black race (e.g., brown or the like) rather than a yellow race, the correction rule is modified according to the character of the designated flesh color. If it is desired to reproduce sky-blue more faithfully to the human's memory than sea-blue, the point P₀ may be set so as to represent a typical sky-blue so that the correction of the standard color separation rule is conducted mainly for sky-blue among various blue colors, e.g., sky-blue, sea-blue, cyan-blue and others. According to the present invention, since a predetermined color separation is corrected or selected if the rate of an area having memory colors in a color original is a threshold value or more. Accordingly, a criterion for determining whether the correction of the color separation rule is required or not is objectified, so that the burden on an operator is reduced, the process can be shortened, and the work is standardized, thereby to obtain a reproduced image whose finish does not vary even if the operator has no considerable experience. Therefore, this method is suitable for automation. Additionally, the present invention provides an apparatus in which setting of a color separation rule is automated and an allowable error in discrimination of memory colors can be statistically determined.

Claims: A method of setting a color separation rule in a color separation means provided in a color image processor which is used for separating color image data representing a color original image (100) into color components (R,G,B) in order to reproduce said color original image for each color component, said method comprising the steps of: (a) designating a memory color, (b) designating sample pixels among pixels of a color original image prepared for image reproduction, (c) determining respective color components for each sample pixel, characterized by (d) defining an ellipse (LC) in a color coordinate space so that a center point (Po) of said ellipse is located at a point representing said memory color, (e) counting the number of sample pixels whose respective color components are included in said ellipse in said color coordinate space, (f) calculating a ratio of said number to a total number of said sample pixels to compare said ratio with a threshold ratio, (g) storing a first color separation rule in said color separation means when said ratio does not exceed said threshold ratio, and storing a second color separation rule in said color separation means when said ratio exceed said threshold ratio. A method according to claim 1 wherein the step (d) includes the steps of: (d-1) separating each of a plurality of sample images into respective color components, each of said plurality of sample images having said memory color, (d-2) obtaining a variance-covariance matrix representing a color distribution of said color components of said plurality of sample images around said memory color, and (d-3) obtaining respective matrix components of an inverse matrix of said variance-covariance matrix, and (d-4) determining coefficients of said ellipse by said respective matrix components. A method according to claim 2 wherein said color coordinate space is a three-dimensional space having three variables corresponding to three primary colors, and said ellipse is a three-dimensional ellipse having said three variables. A method according to claim 2 wherein said color coordinate space is a three-dimensional space having thee variables corresponding to three primary colors, and said ellipse is a two-dimensional ellipse having two variables which are obtained by dividing two of said three variables by a sum of said three variables, respectively. A method according to claim 1 wherein the step (g) includes the steps of: (g-1) designating a plurality of color separation rules, (g-2) selecting one of said plurality of color separation rules in accordance with the result of comparison in the step (f) to define said second color separation rule by a selected one among said plurality of color separation rules. A method according to claim 1 wherein the step (a) includes the steps of: (a-1) designating a plurality of memory colors, (a-2) determining a priority order of said plurality of memory colors, and (a-3) selecting the one of said plurality of said memory colors which is the top of said priority order to thereby designate said memory color. A method according to claim 1 wherein the step (c) includes the step of: (c-1) designating said sample pixels so that said sample pixels are uniformly distributed on said color original image. A method according to claim 1 wherein the step (c) includes the step of: (c-1) designating a part of said color original image, said sample pixels being pixels arrayed on said part of said color original image. An apparatus for setting a color separation rule in a color image processor, said color separation rule being used to separate color image data into color components (R,G,B) suitable for reproducing an original color image (100) represented by said color image data, said apparatus comprising: (a) sampling means (1) for extracting sample pixels from a color original image to determine respective color components for each sample pixel, characterized by (b) first memory means for storing information representing an ellipse (LC) which is defined in a color coordinate space such that a center point (Po) of said ellipse is located at a point representing a designated memory color in said color coordinate space and a size of said ellipse is determined by distribution of respective color components of a plurality of sample images having said memory color, (c) means for judging whether or not a point having coordinate values represented by said color components of each sample pixel is located in said ellipse defined in said color coordinate space, (d) counter means for couting the number of sample pixels whose colors are located in said ellipse to generate a count value proportional to said number, (e) comparator means for comparing said count value with a threshold value, and (f) means for selectively storing one of first and second color separation rules in said color image processor in accordance with the result of comparison made in said comparator means, where said first and second color separation rules are different from each other. An apparatus according to claim 9 wherein said means (f) includes: (f-1) second memory means for storing a correction rule for correcting color-balance of said memory color, (f-2) means for storing said first color separation rule in color separation means provided in said color image processor, and (f-3) means for correcting said first color separation rule according to said correction rule when said count value is larger than said threshold value to thereby convert said first color separation rule into said second color separation rule in said color separation means provided in said color image processor. An apparatus according to claim 9 wherein said means (f) includes: (f-1) second memory means for storing said first and second color separation rules, and (f-2) means for selecting one of said first and second color separation rules in accordance with the result of comparison made in said comparator means to set a selected color sepatation rule in said color image processor. An apparatus according to claim 9 wherein said means (c) includes: (c-1) means for calculating respective differences between said color components of each sample pixel and coordinate values of said point representing said memory color, where each of said respective differences is represented by a combination of N-bits digital data and a sign bit, the number N being an integer larger than one, (c-2) means for separating said N-bits digital data into more significant N₁-bits and less significant N₂-bits, where the numbers N₁ and N₂ are positive integers satisfying N₁ + N₂ = N , (c-3) means for judging whether or not an object point whose coordinate values are defined by a combination of said less significant N₂-bits and a said sign bit corresponding to respective color components of each sample pixel is located in said ellipse, (c-3) means for comparing said more significant N₁-bits with zeros, and (c-4) means for generating a detection signal when said object point is located in said ellipse and said more significant N₁-bits are zeros, and said counter means includes_ (d-1) means for counting said detection signal in order to count the number of sample pixels whose colors are located in said ellipse. An apparatus according to claim 9 wherein said sampling means includes: (b-1) means for photoelectrically reading said color original image while skipping pixels on said color original image at a predetermined skipping ratio to determine said repective color components for each sample pixel. An apparatus according to claim 13 wherein said means (b-1) includes: (b-11) means for scanning said color original image at a scanning speed faster than a scanning speed at which said color original image is scanned for image reproduction in order to determine said repective color components of said sample pixels. An apparatus according to claim 9 wherein said sampling means includes: (b-1) means for scanning a designated portion of said color original image in order to determine said respective color components of said sample pixels.

Inventors: ASADA SHINJI DAINIPPON SCREEN, ASADA, SHINJI DAINIPPON SCREEN MFG. CO., LTD.
Applicants: DAINIPPON SCREEN MFG, DAINIPPON SCREEN MFG. CO., LTD.
Application year: 1989
Main classification: H04N1
IPC codes: H04N1, B41J2, G06F17, G03F3