JPH04334267A - Color correction system in color picture output device - Google Patents

Abstract

PURPOSE:To correct color tone irregularity due to the color reproduction of a color photograph or color tone irregularity due to the reproduction capacity of a color material to obtain the output color picture of a natural tone as against a color original. CONSTITUTION:At the time of correcting color tone irregularity due to the color reproduction of the color photograph, for example, the color density of red, green and blue from a color picture input device 1, which is obtained by reading the color original, is converted into a hue, lightness and saturation by an HVC conversion circuit 3. A color correction circuit 5 converts lightness and saturation in such a way that a maximum saturation part in the color reproduction area coincides with the maximum saturation part of the color reproduction area of the output color picture for respective hues, and the color reproduction area of the original mapped in the color reproduction area of the output color picture, and the hue, lightness and saturation a after mapping are converted into color material density. A color picture output device 13 outputs the color picture.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color correction method for color image output devices such as color copying machines and color printers. [0002] For example, in a color copying machine, a document image is separated into the three primary colors of red, blue, and green to obtain color signals for each color, and these color signals are divided into yellow, magenta, and cyan. A color copy is obtained by converting the color material signals into color material signals and applying color materials such as color ink or color toner to paper in an amount corresponding to the amount of these color material signals using thermal transfer method, electrophotography method, etc. . When copying a normal color original, it is possible to obtain a color copy with the same tone as the original and no particular problem occurs, but the color image formed on negative paper,
That is, when copying a color photograph, the tone of the color image of the color copy may change significantly compared to the color image of the original. [0004] Furthermore, since there is a limit to the reproduction ability of color materials, it is not possible to accurately reproduce the density according to the value of the color material density signal, and the hue of the copy is shifted compared to the hue of the original. Sometimes. [0005] The reasons for tone deviation due to color reproduction in color photographs and the reasons for tone deviation due to the reproducibility of coloring materials will be explained below. As shown in FIG. 13, the color reproduction range RP (shown by a solid line) of a color photograph is different from the color reproduction range RC (one point) of a color copy made by a thermal transfer method or an electrophotographic method. (shown by the chain line), and this occurs because the chroma is high in the high density area, that is, the low brightness area. Note that FIG. 13 shows the color reproduction range regarding the hue of red, with the horizontal axis showing the saturation C and the vertical axis showing the brightness V. When copying is performed under these conditions, only the shaded area in FIG. 13 can be reproduced, so the colors in the color gamut of the color photo other than the color gamut of the color copy are A color image is formed by being condensed at the boundary of the reproduction area. For this reason, the saturation CP of the highest chroma part of the document, that is, the solid area A, is extremely low chroma CC.
It will be converted to . Furthermore, the gradation of the shadow portion B of the original document is lost, and the image is distorted. For this reason, color reproducibility when copying a color photograph is significantly inferior. [0008] In order to solve this problem, Japanese Patent Laid-Open No. 61-2
It is conceivable to use a color image signal processing method as disclosed in Japanese Patent No. 88662. In the color image signal processing method described in the publication, the saturation and brightness ranges (
If the saturation and brightness reproduction range of the output system (indicated by the diagonal line downward to the left in FIG. A color image signal of the output system is obtained by compressing and mapping the color image signal and brightness to the saturation and brightness of the output system using a predetermined function. In the graph shown in the figure, the horizontal axis indicates saturation C, and the vertical axis indicates brightness L. In the color image signal processing method described in the above-mentioned publication, the lightness is first compressed, as indicated by the vertical arrow in FIG. 14(b). In other words, the maximum and minimum brightness values of the input system for a certain hue are L4 and L1.
If the maximum and minimum brightness values of the output system are L3 and L2, calculate (L4-L1)/(L3-L2), and according to this ratio, L4 matches L3, and L1 Compress in the lightness direction to match L2. Next, a certain brightness LA
The lengths l1 and l2 in the saturation direction of the area corresponding to the area are determined and compressed in the saturation direction according to the ratio thereof, as shown by the horizontal arrow in FIG. As a result, the same figure (d
), the color reproduction range RIN of the input system is compressed and mapped onto the color reproduction range ROUT of the output system. [0011]However, in the case of tone deviation due to color reproduction of color photographs, in the color image signal processing method described in the above publication, both brightness and saturation are calculated using a simple proportional calculation, that is, linear approximation. Because compression mapping is performed by , there is a problem in that the reproduction of color images becomes unnatural. The reason for this will be explained below. As shown in FIG. 14, when the lightness that provides the maximum saturation in the color reproduction range ROUT of the output system is equal to the lightness that provides the maximum saturation in the color reproduction range RIN of the input system, the length There is no problem if you calculate the ratio of l1 and l2 and compress it in the direction of saturation, but as shown in Figure 13,
The lightness that provides maximum saturation is different between the color reproduction gamut of a color copy and the color reproduction gamut of a color photograph. Therefore, in such a case, the ratio of lengths l1 and l2 cannot be determined. Also, the length l1 at a certain brightness
, l2 is meaningless because it does not correspond to the ratio to be compressed, and the output image obtained by calculating based on this ratio will be significantly different from the original image. turn into. Furthermore, as shown in FIG. 13, the distribution state in the brightness direction in the color reproduction gamut RC of a color copy and the color reproduction gamut RP of a color photograph is as follows: Since the correspondence is not simple, it is possible to simply calculate the brightness difference ratio (L4-L1)/(L3-L2) and perform compression mapping in the brightness direction, as in the color image signal processing method described in the above publication. , it is not possible to obtain a natural image. That is, as shown in FIG. 13, when the color reproduction gamut of a color copy and the brightness that provides the maximum saturation of the color reproduction gamut of a color photograph are different, the conventional color image signal processing method is not effective. [0014] Furthermore, since there is a limit to the reproduction ability of color materials, it is not possible to accurately reproduce the density according to the value of the color material density signal, and the hue of the copy is shifted compared to the hue of the original. Sometimes. The main reasons why this density cannot be accurately reproduced include the amount of coloring material, scattering by coloring material particles within the ink, and the surface smoothness of the paper. [0015] The color tone deviation due to the reproducibility of coloring materials will be explained below. Now, as shown in FIG. 15, among the yellow, magenta, and cyan coloring material density signals, for example, the maximum value SYMAX of the yellow coloring material density signal is the maximum density that can actually be reproduced with yellow ink. The value corresponding to IYM
If higher than AX, IYMAX
Colorant density signals in higher portions are not reproduced. In other words, the yellow density becomes saturated. For this reason, the density ratio of yellow becomes lower than the density ratio of yellow, magenta, and cyan, which should originally be reproduced. Therefore, the hue of the color copy image is shifted in the magenta hue direction compared to the hue of the original image. For example, orange becomes red. Therefore, it is conceivable to compress the level of each coloring material density signal so that the maximum value of each coloring material density signal does not exceed the level corresponding to the maximum density that can be reproduced by ink. However, the saturation of the density described above occurs only for relatively special originals that have extremely high density parts, and for ordinary originals that have an average density range, Color images are reproduced without saturation. Therefore, as mentioned above, if you simply compress the level of each color material density signal, the density will be compressed even for originals that do not originally require compression, and the density of the reproduced color image will become lighter overall. The problem of putting it away arises. The present invention was devised to solve the above-mentioned problems, and in the case of tone deviation due to color reproduction of color photographs, a mapping function for saturation and a mapping function for brightness are used. In addition, in the case of color tone deviation due to the reproducibility of the color material, by switching the compression operation according to the level of the color material density signal, it is possible to obtain an output color image with a natural tone for the color original. With the goal. [Means for Solving the Problems] The first invention converts the color densities of red, green and blue obtained by reading a color original into hue, brightness and saturation, and converts the color density of the original for each hue into hue, brightness and saturation. The lightness and saturation are converted so that the maximum saturation part in the color gamut of the output color image matches the maximum saturation part of the color gamut of the output color image, and the color reproduction gamut of the original is converted into the color reproduction of the output color image. The method is characterized in that the image is mapped to a color area, the hue, brightness, and saturation after mapping are converted into color material density, and a color image is output based on the color material density. Further, the second invention compares the levels of the yellow, magenta, and cyan coloring material density signals with the level corresponding to the maximum density reproducible by the corresponding coloring materials, and selects one of the coloring material density signals. exceeds the level corresponding to the maximum reproducible density, the level of the colorant density signal of the color with the largest amount of excess is compressed to the level corresponding to the maximum reproducible density, and the other It is characterized in that the levels of the color material density signals of the two colors are also compressed at the same ratio. [Operation] First, the operation of the first invention for correcting the tone shift due to color reproduction of a color photograph will be explained. For example, as shown in FIG. 1, when the color reproduction gamut RP (indicated by a solid line) of a color photograph is different from the color reproduction gamut RC (indicated by a dashed-dotted line) of a color copy,
In the color correction method of the present invention, for each hue, the ratio of maximum saturation Cmax to saturation C is set as shown in FIG. 2(a).
Mapping is performed using a linear conversion characteristic based on c/Cmaxp, and non-linear mapping is performed for brightness using a polygonal conversion characteristic such that the solid parts A and a of both reproduction areas match, as shown in the same figure (b). I do. Note that in FIGS. 2(a) and 2(b), the horizontal axes indicate input saturation CIN and input brightness VIN, respectively, and the vertical axes indicate output saturation COUT and output brightness VOUT, respectively. Furthermore, the brightness levels P and Q shown in FIG. 1 and FIG. 2(b) correspond to each other. This conversion is performed for each hue. As a result, even if the lightness that gives the maximum saturation of the color gamut of a color photo is different from the brightness that gives the maximum saturation of the color gamut of a color copy, the color gamut of the color photo is determined by the shape of the color gamut. is mapped to the color gamut of a color copy without changing significantly. Therefore, the solid area A of a color photograph is reproduced as a high-density solid area A even in a color copy. In addition, since the shadow portion B of the document is also reproduced with the gradation maintained, although the brightness changes, the shadow portion B is not crushed. Furthermore, the mapping conversion characteristic in the brightness direction between each hue can be obtained by interpolation from the brightness that gives the maximum saturation in the six hues of red, yellow, green, cyan, blue, and magenta. Now, let Vi be the lightness that gives the maximum saturation in the six hues, and let VH be the brightness that gives the maximum saturation in any hue H among the hues. As can be seen from FIG. 1] [0027] Note that Hi and Hi+1 are 6 hues in the order of red, yellow, green, cyan, blue, and magenta.
It means the i-th and (i+1)-th hue when the colors are arranged. Using this equation (1), at any hue H, the brightness VHP and VHC that give the maximum saturation of color photographs and color copies can be determined. Next, at a certain hue H, mapping is performed from the color gamut of the color photograph to the color gamut of the color copy using the following mapping function fH (v0). However, v is the brightness before conversion. Here, as shown in FIG. 4, it is assumed that the maximum brightness and minimum brightness of the color gamut of the color photograph and the color copy are the same, and are 9.25 and 1, respectively. ##EQU00002## Next, mapping in the direction of chroma will be considered. Now, let Ci be the maximum saturation in the six hues, and let CH be the maximum saturation in any hue H among the hues, then the following equation is obtained. [0035] As a result, the maximum saturation CHP for color photographs and the maximum saturation C for color copies at any hue H
HC is determined respectively. At a certain hue H, the maximum saturation CHP for color photography is changed to the maximum saturation CHC for color copying.
Find the following linear mapping function gH(C) that converts into . ##EQU00004## Through the above processing, the converted lightness value fH (v0) for the input lightness at any hue H is obtained.
, and the converted saturation value gH (C
) is required. FIG. 5 is an explanatory diagram showing the conversion from the maximum saturation point D of a color photograph in a hue H other than six hues to the maximum saturation point d for color copying. In addition, in the figure, RCR
, RPR are the color reproduction ranges of color copies and color photographs for red, and RCY and RPY are the color reproduction ranges of color copies and color photographs for yellow. Next, the operation of the second invention for correcting the color tone shift due to the reproducibility of the coloring material will be explained. In the color correction method in the color image output device of the second invention, the levels of the yellow, magenta and cyan colorant density signals are compared with the levels corresponding to the densities reproducible by the corresponding colorants, Only when the level of any colorant density signal is higher than the level corresponding to reproducible density, the level of the colorant density signal of the color with the greatest excess is compressed to the level corresponding to reproducible density. At the same time, the levels of the color material density signals of the other two colors are also compressed at the same ratio. [0041] As a result, density saturation does not occur even in high-density originals, and since the density ratio of each coloring material is kept constant, no change in hue occurs. Furthermore, since the level of the color material density signal is not compressed for a normal original, sufficient reproduction density is ensured. [Embodiments] Hereinafter, the features of the present invention will be specifically explained based on embodiments with reference to the drawings. FIG. 6 shows an overall block diagram of a color copying machine to which the color correction method of the first invention is applied. The output of the color image input device 1 for reading a color original is supplied to the RGB color separation circuit 2, where it is converted into red, green, and blue density signals DR, DG, and DB, and further processed by the HVC conversion circuit 3. , hue signal H, brightness signal V
, is converted into a chroma signal C. These signals H, V, and C are supplied to the HVC adjustment circuit 4. An operation panel unit 20 is connected to the HVC adjustment circuit 4, and as described later,
The hue, brightness, and saturation signals H, V, and C are adjusted according to instructions from the operation panel section 20. [0045] Hue, brightness, and saturation signals H, V, and C after adjustment
is supplied to the color correction circuit 5, and the brightness and saturation signals V and C are corrected according to the hue of the original. The output after color correction is supplied to the HVC inverse conversion circuit 6, and red, green, blue density signals DR, DG,
After being converted into DB, the color correction circuit 7 converts the yellow, magenta, and cyan coloring material density signals DY, DM, and D into
Converted to C. The details of the above-mentioned HVC conversion circuit 3, HVC adjustment circuit 4, color correction circuit 5, and HVC inverse conversion circuit 6 will be described later. The coloring material density signal DY from the color correction circuit 7,
DM and DC are converted into corresponding equivalent neutral concentration signals by the END conversion circuit 8. The black amount determination circuit 9 generates a black signal for inking from these equivalent neutral density signals, and the under color removal circuit 10 generates a black signal from the equivalent neutral density signals of yellow, magenta, and cyan. Further, the equivalent neutral density signal is reconverted into coloring material density signals DY, DM, and DC in the inverse END conversion circuit 11. These coloring material density signals DY , DM , DC and the black signal from the black amount determination circuit 9 are subjected to gradation correction in accordance with the output characteristics of the color image output device 13 in the gradation correction circuit 12 . Finally, a color copy is obtained by attaching yellow, magenta, cyan, and black coloring materials to the paper according to the image of the original. Next, the adjustment of hue, brightness, and saturation using the HVC conversion circuit 3, HVC adjustment circuit 4, and HVC inverse conversion circuit 6 will be explained in detail. First, hue adjustment will be explained. In this embodiment, we focus on the fact that the position of the hue circle can be determined from the magnitude relationship of the density signals DR, DG, and DB.
The hue angle H is approximated from the density signals DR, DG, DB. Note that here, in order to simplify the explanation, the signal name and the signal value or hue angle are represented by the same symbol. The procedure for approximating the hue angle H will be explained below. First, let us consider approximation using reflectance expression. [0053] The red, green, and blue reflectance signals are RR and RG.
, RB and arrange them in order of size. That is, max(R
R , RG , RB ), mid( RR , RG , R
B), min(RR, RG, RB) are determined. For example, when RR > RG > RB, max(RR, RG, RB)=RR mi
d(RR,RG,RB)=RGmin(RR
, RG, RB)=RB. [0055] In the following explanation, max(RR
, RG , RB ), mid(RR , RG , RB
), min(RR, RG, RB) are simply expressed as max, mid, and min, respectively. [0056] min represents the white component. Therefore, max-min, mid-min excluding the white component
Which part of the hue range RY , GY , GC , BC , BM , RM belongs to which is divided by the six hue axes of red, yellow, green, cyan, blue, and magenta in the combination of ? is determined. Note that the angles of each hue axis are 0 degrees, 60 degrees, 120 degrees, and 18 degrees, respectively.
They are 0 degrees, 240 degrees, and 300 degrees. Table 1 shows the relationship between the magnitude of reflectance and the hue region. [Table 1] [0059] Here, if the hue angle ratio Hr is defined as follows: It is 0 when it is on the blue axis, and 1 when it is on the yellow, cyan, and magenta axes. Therefore, the reflectance signals RR, RG,
Based on the magnitude relationship of RB, it is possible to specify which of the six hue regions the hue belongs to, and furthermore, the hue angle H can be specified using the hue angle ratio Hr as H=F(Hr). Here, the function F is a function as follows. Next, the hue angle H thus defined is converted into a Munsell hue using the conversion table shown in Table 2. This is because the hue angle determined by the above-described calculation does not necessarily correspond accurately to the hue angle of the Munsell color wheel, and therefore requires correction. Also, intermediate angles are determined by interpolation. [Table 2] Next, the approximation based on reflectance expression is converted into approximation based on density. When the density is Di and the reflectance is Ri, Di = -log10 Ri, but the density Di is the absorptance Ai (=1-Ri).
If considered as an expression of Hr, the hue angle ratio Hr depending on density can be defined by the following equation with reference to equation (5). ##EQU00007## This change in the hue angle ratio Hr is also the same as in the case of approximation using reflectance expression. In addition, in the following explanation, max(DR, DG, DB), mid(
DR , DG , DB ), min(DR , DG ,
DB) are simply expressed as max, mid, and min, respectively. Further, approximation of hue angle by hue region division and function F can be performed in the same way as approximation by reflectance expression. In order to obtain the above hue angle H, the three color density signals DR, DG, DB from the RGB color separation circuit 2 are
is supplied to the hue conversion ROM 3H shown in FIG. Hue conversion ROM 3H has three color density signals DR.
, DG, and DB as inputs, and outputs the hue determined by the above calculation. Therefore, for example, a 6-bit hue signal H0 corresponding to the three color density signals DR, DG, DB is obtained from the hue conversion ROM 3H. This hue signal H0 is sent to the hue adjustment circuit 4H.
is supplied to The hue adjustment circuit 4H independently performs designated hue angle adjustment for each of the red, green, and blue regions, and is constructed from a look-up table type ROM. This hue adjustment circuit 4H includes a hue signal H0, a red area adjustment signal SR, a green area adjustment signal SG, and a blue area adjustment signal S.
B is input, and the hue is adjusted within the hue range shown in Table 3. [Table 3] [0076] If the input hue angle H belongs to any of the above three regions, the hue angle is adjusted according to the relationship H1 = H0 + 2 x fH. However, H1 is the hue angle after adjustment, and fH is the adjustment coefficient. The adjustment coefficient fH is determined by each area adjustment signal SR, SG, S from the operation panel unit 20.
B corresponds to, for example, the degree of adjustment from "0" to "6", and the relationship between the degree of adjustment and the adjustment coefficient fH is as shown in Table 4. [0077] [Table 4] [0078] The adjustment coefficient "+3" means maximum clockwise rotation of the hue; for example, in the case of a red region, the hue becomes yellowish. In addition, the adjustment coefficient "-3" means maximum left rotation of hue,
Similarly, in the red region, the hue is closer to purple. As mentioned above, only when a hue belongs to a specific hue region, hue adjustment is performed within that hue region, so that the hue can be adjusted without affecting other hues.
It becomes possible to adjust only the desired hue. Next, brightness adjustment will be explained. In this embodiment, three color density signals DR
, DG , DB , the visual density D is determined by the following approximate formula. D=α1 ×DR +α2 ×DG +α3 ×DB
=0.5×DR +0.45×DG +0.05×DB
Furthermore, from this visual density D, the brightness V is determined by the following approximate formula.
seek. ##EQU8## The above-mentioned approximation calculation is performed by the brightness conversion ROM 3V shown in FIG. Note that β is not limited to the above value, and as long as it is in the range of 2.30≦β≦2.45, the brightness V can be sufficiently approximated. The brightness conversion ROM 3V is a look-up table that inputs the three color density signals DR, DG, DB and outputs the brightness determined by the above calculation. Therefore, from the brightness conversion ROM 3V, 3
Brightness signal V according to color density signals DR, DG, DB
0 is obtained. This brightness signal V0 is applied to the brightness adjustment circuit 4V.
is supplied to The brightness adjustment circuit 4V is composed of a look-up table type ROM, and the brightness of the input V0
When the brightness of the output is set to V1, 0087
] [Equation 9] The following calculation is performed. [0089] Here, fV is the operating panel section 2.
This is the adjustment degree corresponding to the copy density adjustment signal SV from 0, and by changing it in the range of "2.5" to "5.5", for example, the brightness V is adjusted, and the copy density is adjusted as a result. be able to. Next, saturation adjustment will be explained. Here, the saturation C is determined from the previously determined maximum density value max, minimum value min, and brightness V using the following approximate formula. C=γ×V×(max-min) However, γ
:2.44 The above approximation calculation is performed using the saturation conversion ROM shown in FIG.
Performed by 3C. Note that γ is not limited to the above value,
If the range is 2.30≦γ≦2.60, the saturation C can be sufficiently approximated. The saturation conversion ROM 3C is a look-up table that receives the three color density signals DR, DG, and DB as input and outputs the saturation determined by the above calculation. Therefore, from the saturation conversion ROM 3C, 3
Saturation signal C according to color density signals DR, DG, DB
0 is obtained. This saturation signal C0 is sent to the saturation adjustment circuit 4C.
is supplied to The saturation adjustment circuit 4C is composed of a look-up table type ROM, and the saturation adjustment circuit 4C is composed of a ROM in a look-up table format, and the saturation adjustment circuit 4C
When the output saturation is C1, 0094
] [Equation 10] The following calculation is performed. Note that fC here is the operation panel section 2
It is an adjustment coefficient corresponding to the saturation adjustment signal SC from 0, and the saturation C can be adjusted by changing it in the range of "5" to "11", for example. As described above, the density information is first converted into saturation information, and this saturation information is adjusted, and then combined with hue and brightness information to be converted into a density signal, as described later. Therefore, even when the saturation is adjusted, the hue and brightness information are maintained. In this embodiment, the signals H1, V1, C1 from the HVC adjustment circuit 4 are supplied to the color correction circuit 5, and the signals V1, C1 are corrected for mapping. . That is, the mapping function fH (v0) shown in equations (2) and (4) explained in the section of action,
Conversion is performed for each hue based on gH (C). As shown in FIG. 7, the color correction circuit 5 includes a ROM for brightness mapping.
Equipped with 5V and chroma mapping ROM 5C. The brightness mapping ROM 5V has the formula (2)
Conversion data corresponding to is recorded in a look-up table format, and the hue signal H from the hue adjustment circuit 4H is
1 and the brightness signal V1 from the brightness adjustment circuit 4V are used as address inputs, and the converted brightness V2 is used as a data output. Furthermore, a switching signal SP for switching between negative paper mode and normal mode is supplied from the operation panel 20 as part of the address of the brightness mapping ROM 5V or as an independent control signal. In addition, ROM 5 for saturation mapping
Conversion data corresponding to equation (4) is recorded in look-up table format in C, and the hue adjustment circuit 4H
The hue signal H1 from and the saturation signal C1 from the saturation adjustment circuit 4C are used as address inputs, and the converted saturation C2
is used as the data output. Furthermore, the brightness mapping ROM
Similar to 5V, a switching signal SP is supplied. When copying a normal color original, the switching signal SP is set to a low level, for example, and the brightness mapping ROM 5V and the chroma mapping ROM 5C output the input signals H1, V1 without conversion. ,C1
are output as they are as output signals H2, V2, and C2. Furthermore, when copying a color photograph formed on negative paper, the switching signal SP is set to a high level, and the equations (2) and (4) are set in the brightness mapping ROM 5V and the saturation mapping ROM 5C. ), the input signals H1, V1, C1 become the output signals H2, V2 based on the mapping functions fH (v0), gH (C) shown in
, C2. However, H2 = H1. [0101] If mode switching is not performed, bright colors on negative paper will become dull images with no gradation when copied, regarding medium and high density areas. FIG. 8 shows the relationship between the maximum saturation point before conversion and the maximum saturation point after conversion for each hue. In the figure, the ○ mark indicates the maximum saturation point before conversion (negative paper), and the × mark indicates the maximum saturation point after conversion (negative paper).
Indicates the maximum saturation point of color materials (color materials for color copies). The hue, brightness, and saturation signals H2, V2, and C2 adjusted and corrected as described above are HV
The signal is supplied to the C inverse conversion circuit 6, and converted again into red, green, and blue density signals DR, DG, and DB. That is, first, the density D is calculated from the brightness V, and max-min is calculated from the brightness V and the chroma C.
Then, the hue angle H is inversely transformed using a lookup table to obtain the hue angle ratio Hr [Equation 11] and hue region information. Furthermore, max-
max-mid is determined from min and hue angle ratio Hr. By the way, for each hue region, the density D,max
, mid, and min are given in the form of Table 5. [Table 5] Therefore, for each hue region, the density D, ma
From the values of x-min, max-mid, max, mid,
min can be obtained, and furthermore, DR , DG , D
Correspondence to B is also required. For example, the hue region RY, ie, DB
>DG >DR As an example of conversion, [0
111] [Formula 12] [0113] And these min, mid, max are
Each color density is assigned based on the previously determined hue region information. In this way, the hue, brightness, and saturation signals H, V, and C are again converted into the red, green, and blue density signals DR.
, DG, DB and supplied to the color correction circuit 7. [0115] In the above embodiment, in order to facilitate understanding, the explanation was divided into functional blocks.
Actually, from the color correction circuit 5 to the gradation correction circuit 12,
As shown in Figure 9, the ROM is used as a lookup table for each color material of yellow, magenta, cyan, and black.
It is composed of 40Y, 40M, 40C, and 40K. That is, a table is formed by integrating all intermediate calculation processes, and the hue, brightness, and saturation signals H1, V1, and C1 are input, and the yellow, magenta, and cyan signals are input to the color image output device 13. , number of dots for each color material of black NY , NM , NC , NK
is the direct output. Therefore, although density signals and the like are not generated in the circuit after the color correction circuit 5, they are incorporated in the table in the form of coefficients. In this embodiment, the color material signal is not limited to a signal that changes in an analog manner in direct correspondence with the density, but also includes the number of dots in the color image output device 13. [0117] In the above embodiment, the case where the color gamut of a color photograph is mapped to the color gamut of a color copy was explained as an example, but the invention is not limited to this. The present invention can also be applied to the case where the color reproduction gamut of a cathode ray tube is mapped to the color reproduction gamut of a color copy. Further, color image input devices include color television cameras, videotape recorders, computers, etc., color image readers, etc., and color image output devices include output devices using electrophotography, inkjet methods, etc. . [0118] The color copy referred to in this specification means
It is not limited to direct copies of manuscripts, but includes color images obtained in hard copy form;
Furthermore, it includes images (soft copies) displayed on a cathode ray tube. However, when displaying on a cathode ray tube,
Instead of converting to YMCK after HVC conversion, RGB
need to be converted to . That is, in the circuit shown in FIG. 6, from the color correction circuit 7 to the tone correction circuit 12, red,
It is sufficient to replace the green and blue density signals DR, DG, and DB with a circuit that converts them into RGB. Next, a second invention for correcting color tone deviation due to the reproducibility of coloring materials will be described. FIG. 10 shows an overall block diagram of a color copying machine to which the color correction method of the second invention is applied. The output of the color image input device 51 for reading a color original is supplied to an RGB color separation circuit 52, where it is converted into red, green, and blue density signals DR, DG, and DB. , magenta, and cyan colorant density signals DY, DM, and DC. The black amount determining circuit 54 generates a black signal for inking from these coloring material density signals DY, DM, and DC, and the under color removing circuit 55 generates a black signal for inking from these color material density signals DY, DM, and DC. Material concentration signal DY, DM
, DC subtract the black signal from. Each color material density signal SY , SM , SC after removing the undercolor is supplied to a compression circuit 56 . In this compression circuit 56, the outputs SY, SM, SC of the undercolor removal circuit 55 are supplied to each density comparator 61Y, 61M, 61C as shown in FIG. 11, and each coloring material density signal SY, SM , SC is yellow that can be reproduced by the color image output device 57 described later,
Value IYM corresponding to the maximum density of each color of magenta and cyan
It is compared with AX, IMMAX, and ICMAX, and the differences therebetween, that is, the excess amounts dY, dM, and dC are determined. [0124] dY = SY - IYMAX dM = SM - IMMAX dC = SC - ICMAX Then, in the maximum value detector 62, these excess amounts d
The maximum positive excess amount d among Y, dM, and dC is determined. d=max(dY, dM, dC) Then, this maximum excess amount d and a signal indicating which coloring material concentration has been saturated are supplied to the correction coefficient calculator 63, which corresponds to the maximum excess amount d. Maximum value SMAX of the density signal of the coloring material
A coefficient α is determined such that the value IMAX corresponds to the maximum reproducible density. [Equation 13] This coefficient α is calculated by the multiplier 64Y for each color material,
64M and 64C, the following calculations are performed, and compressed colorant density signals SY1, SM1, and SC1 are obtained. SY1=SY×α SM1=SM×α SC1=SC×α Colorant density signals SY1, SM1, SC from the compression circuit 56
1 and the black signal from the black amount determination circuit 54 are supplied to a color image output device 57, which outputs yellow, yellow, and black signals according to the image of the original.
Color copies can be obtained by attaching magenta, cyan, and black colorants to paper. Note that the block diagram shown in FIG. 11 is divided into blocks for each function in order to facilitate understanding of the operation of the compression circuit 56.
, SM , SC , the output colorant density signal SY1,
Since SM1 and SC1 are uniquely determined, the coloring material concentration signal S
Y, SM, SC are addresses, coloring material density signal S
It can also be configured with a look-up table type ROM that uses Y1, SM1, and SC1 as data. Furthermore, the preceding stage color correction circuit 53, black amount determination circuit 54, and undercolor removal circuit 5
It is also possible to create a ROM by integrating it with 5 etc. Next, the operation of the compression circuit 56 in the above color copying machine will be explained. Now, the yellow from the undercolor removal circuit 55,
The levels of the cyan and magenta coloring material density signals are SY, SM, and SC, as shown by the broken lines in FIG. is IYMAX,
Assume that IMMAX and ICMAX are. Note that here, to simplify the explanation, it is assumed that there is no density saturation in magenta and cyan. In the case of the example shown in FIG. 12, SY > IYMAX SM < IMMAX SC < ICMAX, so d=max(dY, dM, dC)=dY, where dY=SY-IYMAX. Therefore, in the multiplier 64Y, SY
×α However, when the calculation α=IYMAX/SY is performed, the level S
Y is compressed to level IYMAX. Also, the multiplier 6
For 4M and 64C, calculations of SM x α and SC x α are performed, and compression is performed at the same ratio (indicated by a solid line in the figure). [0134] Therefore, the yellow density does not become saturated on the color copy, and the ratio of the yellow, magenta, and cyan densities is maintained constant, so the hue of the original image does not change. do not have. Further, since the above-described compression operation is not performed on a document in which density saturation does not occur, sufficient density can be ensured in the reproduced image. [0135] As described above, according to the first invention, the color densities of red, green, and blue obtained by reading a color original are once converted into hue, brightness, and saturation; Saturation is mapped using a linear function, brightness is mapped using a non-linear function, and then converted back to color material density. As a result, it is possible to map the solid areas of the color original and the solid areas of the output color image in a corresponding manner, and the color gamut of the color photo and the color gamut of the output color image are different, and the maximum saturation It is possible to obtain a natural output color image even when the brightness is different. Further, in the second invention, when a coloring material density signal of a level that cannot be reproduced by the coloring material is input,
The level of that coloring material density signal is compressed to a reproducible level, and the levels of other coloring material density signals are also compressed at the same ratio. As a result, a color image can be reproduced without changing the hue even if the original image contains a high-density portion. Further, since the colorant density signal is not compressed for a normal document having an average density distribution, a color image with sufficient density can be reproduced.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining mapping from the color gamut of a color photograph to the color gamut of a color copy in the color correction method of the present invention.

FIG. 2 is a graph showing an example of conversion characteristics in the saturation direction and lightness direction in the present invention.

FIG. 3 is a diagram for explaining a procedure for determining lightness that provides maximum saturation in an intermediate hue portion.

FIG. 4 is a graph showing an example of a mapping function related to brightness.

FIG. 5 is a diagram for explaining mapping from the color gamut of a color photograph to the color gamut of a color copy in an intermediate hue portion.

FIG. 6 is a block diagram showing an example of the overall configuration of a color copying machine to which the color correction method of the present invention is applied.

FIG. 7 is a block diagram around the HVC adjustment circuit.

FIG. 8 is a diagram illustrating the mapping state of the maximum saturation point in each hue.

FIG. 9 is a block diagram showing a look-up table type ROM that directly outputs the number of dots for each color material in a color image output device from each signal of hue, brightness, and saturation.

FIG. 10 is a block diagram showing an example of the overall configuration of a color copying machine to which the color correction method of the present invention is applied.

FIG. 11 is a block diagram showing a configuration example of a compression circuit.

FIG. 12 is a diagram for explaining the compression operation of color material density signals in the same compression circuit.

FIG. 13 is a graph showing the difference between the color reproduction gamut of a color photograph and the color reproduction gamut of a color copy.

FIG. 14 is a graph for explaining a conventional color correction method.

FIG. 15 is a diagram for explaining the relationship between a coloring material density signal and a density that can be reproduced by a coloring material.

FIG. 16 is an explanatory diagram showing the density of each coloring material when the coloring material density signal is not compressed.

[Explanation of symbols]

Claims (2)

[Claims] 1. The color densities of red, green, and blue obtained by reading a color original are converted into hue, brightness, and saturation, and for each hue, the maximum chroma part in the color reproduction gamut of the original is an output color image. The color gamut of the original is mapped to the color gamut of the output color image by converting the brightness and saturation so that they match the maximum chroma of the color gamut of the image, and the hue, brightness, and chroma after mapping are 1. A color correction method for a color image output device, characterized in that the color density is converted into a coloring material density, and a color image is output based on the coloring material density. [Claim 2] The levels of the yellow, magenta, and cyan colorant density signals are compared with the level corresponding to the maximum density reproducible by the corresponding colorants, and the level of any of the colorant density signals is determined to be reproducible. When the level corresponding to the maximum density is exceeded, the level of the colorant density signal of the color with the largest excess amount is compressed to the level corresponding to the maximum reproducible density, and the colorant density signal of the other two colors is compressed to the level corresponding to the maximum reproducible density. A color correction method for a color image output device characterized by compressing density signal levels at the same ratio.