JPH0639185B2 - Color image reproduction method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a color image reproducing method for reproducing a color image by using colorants of a plurality of colors, and in particular, using a plurality of colorants having different coloring densities for at least one of the plurality of colors. The present invention relates to a color image reproduction method.

Hereinafter, a color ink jet printer will be described as an example of an apparatus that realizes such a method, but it goes without saying that the present invention can be applied to other types of image forming apparatuses such as a thermal transfer color printer or an electrophotographic printer. Absent.

<Prior Art and Description of Background> For reproducing a color image, it is general to use, for example, three colors of cyan, magenta, and yellow, or four colors obtained by adding black to these. In recent years, it has been proposed to use a plurality of coloring materials having different coloring densities for one color for the purpose of expanding the color reproduction range.

In the case of using two coloring materials of light and shade, since the coloring materials are switched depending on the reproduction density, the reproduction density range is dramatically expanded.

However, in the portion where the coloring material is switched, a minute dot formed by the dark and light coloring material has a difference in texture, and a so-called pseudo contour is generated in this portion, which deteriorates the quality of the reproduced image.

In consideration of this point, we previously proposed the method shown in FIG. 4 (a). In FIG. 4 (a), the horizontal axis represents the voltage applied to the ink jet head and the vertical axis represents the average optical density. This is the average optical density (hereinafter OD
Value) is changed from LO to L2, and the density reproduction range of dark ink is set to L
When 1 to L3, L1 <L2 is set, that is, the overlapping density reproducing range is provided in the density reproducing range, and the dots of the dark ink and the dots of the light ink are set by the dither method or the density pattern method in the overlapping density reproducing range. It is used to make it appear properly. With such a method, it is possible to significantly suppress the generation of pseudo contours and obtain a high quality image.

However, in order to further expand the reproduction range, it is necessary to increase the types of ink density used. As the number of types of ink density increases, the number of heads increases, which complicates the apparatus and increases the cost.

Therefore, it is conceivable that the overlapping density reproduction range is narrowed or the overlapping area is lost. However, in this case, a pseudo contour is likely to occur.

<Objects of the Invention> In view of the above points, the present invention prevents the occurrence of a false contour due to the difference in the density of the coloring material even if the overlapping density reproduction range is narrow or does not exist, and It is an object of the present invention to provide a color image forming method capable of reproducing a high quality color image.

More specifically, the invention described in claim 1 partially overlaps the density reproduction range by changing the dot size of the high density coloring material and the density reproduction range by changing the dot size of the low density coloring material, The density of the expanded overlapping reproduction range, which includes the overlapping density reproduction range and the density reproduction range that is reproduced with either the high-density coloring material or the low-density coloring material, is reproduced with the high-density coloring material dots and the low-density coloring material dots. It is reproduced by reproducing and reproducing at a ratio according to.

With this configuration, it is possible to widen the overall density reproduction range without widening the overlapping density reproduction range so much, and further, it is possible to prevent the generation of pseudo contours due to the difference in the specific density of the coloring material, and to improve the quality. The color image of can be reproduced.

On the other hand, according to the invention described in claim 2, the density reproduction range by varying the dot size of the high density coloring material and the density reproduction range by changing the dot size of the low density coloring material are separated from each other, and The density of the reproduction range between the lower limit of the reproduction range by and the upper limit of the reproduction range by the low-density coloring material is expressed by expressing dots of both coloring materials at a ratio according to the reproduction density.

With such a configuration, it is possible to set a wide density reproduction range without providing an overlapping reproduction density range, and further, it is possible to prevent the occurrence of a pseudo contour due to the difference in the specific density of the coloring material, and to achieve high quality. A color image can be reproduced.

<Description of Embodiments> FIG. 1 shows a block diagram of signal processing when the present invention is applied to an ink jet color printer.

A video signal, for example, a composite video signal including R, G, B color signals and a synchronizing signal is input to the video signal interface 1. Here, the signals are synchronized and sample-held by the sample-hold circuit. This signal is guided to the AD conversion circuit 2 in the next stage, and the gradation signals of the image signals R, G, B are converted into digital signals. This digital signal is stored in an appropriate number of lines in the next line memory 3. Here, the line is generally taken in the vertical direction, but it is clear that it may be taken in the horizontal direction. Next, the data in the line memory is subjected to processing such as color conversion, γ conversion, masking processing, and undercolor removal for each pixel by the image processing circuit 4, and generally converted into cyan, magenta, yellow and black signals. Then, it is converted into the applied voltage value of each head and input to the head driver 6. Each ink jet head 9 discharges an amount of ink corresponding to the magnitude of the applied voltage to express hue and density.

On the other hand, the system controller 5 that controls the printer sequence generates a head drive signal, a carriage motor drive signal, and a paper feed signal at the timing corresponding to the input image signal, and the head driver 6, the carriage motor driver 7, and the paper motor, respectively. The ink is supplied to the feed motor driver 8, and the ink jet head 9, the carriage motor and its mechanism 10, and the paper feed motor and its mechanism 11 are controlled at the initial timing to print the reproduced image of the input video signal on the recording medium.

FIG. 2 is a detailed block diagram of the image processing circuit 4 of FIG.

The R, G, B signals are converted into cyan, magenta, and yellow density signals C ₀ , M ₀ , Y ₀ by the CMY converter 21. For example
_{C 0 = -log 10 R, M} 0 = -log 10 G, processing of Y ₀ = -log ₁₀ B is performed.

Next, the γ conversion unit 22 performs γ conversion.

For example Is performed.

Next, the under color removal circuit 23 determines whether or not the black is used. If the black is used, the black component is subtracted from the other C ₁ , M ₁ , and Y ₁ .

For example _{BK 2 = α {Min (C} 1, M 1, Y 1)} + β and then, using a loiter larger than the value _{L B} there are _{BK 2, C 2 = C 1} -BK 2 M 2 = M 1 - Let BK ₂ Y ₂ = Y ₁ −BK ₂ .

Further, when the BK _{2 <L B} is _{C 2 = C 1 M 2 =} M 1 Y 2 = Y 1.

Further, masking processing is performed in the masking circuit 24 in consideration of unnecessary absorption of ink, and the densities C ₃ , Y ₃ , M ₃ ,
BK ₃ is required.

Generally _{C 3 = f C (C 2} , M 2, Y 2, BK 2) M 3 = f M (C 2, M 2, Y 2, BK 2) Y 3 = f Y (C 2, M 2, Y ₂ , BK ₂ ) BK ₃ = f _BK (C ₂ , M ₂ , Y ₂ , BK ₂ ) is performed.

The following matrices can be used as the functions f _C , f _M , f _Y , and f _BK .

The matrix coefficient Mij is obtained by processing the data obtained by actual color reproduction by the least square method.

Then, in the head selection control unit 25, the densities C ₃ , M ₃ , Y ₃ , and
Determine which density of ink to use according to BK ₃ ,
It outputs the selection signals CC, MC, YC, and BKC of each head for the dark and light inks.

Then, in the voltage value conversion unit 26, the densities C ₃ , M ₃ , Y ₃ , B of each color are
The applied voltage values V _C , V _M , V _Y and V _BK of each head are output to the head driver 27 by K ₃ and the selection signals CC, MC, YC and BKC. The head driver 27 applies an analog voltage corresponding to the voltage values V _C , V _M , V _Y and V _BK to each ink jet head selected by the selection signals CC, MC, YC and BKC.

FIG. 3 shows details of the circuit of the head driver 27 shown in FIG. The ink jet head control will be specifically described with reference to FIG. 3 by taking cyan signal processing as an example. Digital signal from the voltage value converter 26 shown in FIG.
V _C is input to a DA conversion off DAC that modulates the head applied voltage and generates a voltage V _H corresponding to the digital signal.
The head selection signal CC output from the head selection control section 25 is also input to one input terminal of the AND gate G3 and one input terminal of the AND gate G2 through the inverter G1. When the signal CC is low level, the head H1 is selected, and when the signal CC is high level, the head H2 is selected. A head drive pulse is input from the system controller 5 to the other input terminals of the AND gates G2 and G3. Now, the driving of the head H1 when the signal CC is at the low level will be described. Since one input end of the AND gate G2 is at high level, when the head drive pulse goes high, the output of the AND gate G2 goes high and the output of the buffer G4 goes high. Therefore, the transistor T _r 3 is turned on and the transistor T _r 1 is also turned on. Here, the voltage V _H is applied to the head H1 via the resistor R3.
As a result, the piezoelectric vibrator contracts in the inner diameter direction of the glass tube, and colored droplets are ejected. The discharge amount of the colored droplets is controlled by the voltage V _H.

The transistor T _r 2 is off because the output of the inverter G6 is low level. Next, when the pulse becomes low level, the transistor T _r 1 is turned off contrary to the above,
When T _r 2 is turned on, the charge charged in the head H1 is discharged through the resistor R4, and the piezo element is restored to the original state. Ink jetting is controlled as described above.

Next, the detailed operation of the head selection control unit 25 will be described.

FIGS. 5 (a), (b) and (c) show the characteristics of the reproduced average optical density (hereinafter referred to as OD value) with respect to the head applied voltage of each of the cyan, magenta and yellow inks of the first embodiment. KC is dark cyan ink, UC is light cyan ink, KM is dark magenta ink, UM is light magenta ink, and HY is yellow ink. To explain cyan, as shown in FIG. 5 (a), the light cyan ink UC has an OD value of 0.10 to 0.5.
0 can be reproduced, and the dark cyan ink KC can reproduce an OD value of 0.40 to 1.12. OD value L
Since the range between 1'and L2 ', that is, 0.40 to 0.50, can be expressed by either dark or light ink, this darkness reproduction range is called an overlapping density reproduction range.

Since the overlapping density reproduction range is very narrow, a pseudo contour is likely to occur at the density joint. Therefore, in this embodiment, the density range L that cannot be originally reproduced with dark ink is L.
A (0.35) to L1 '(0.40), the density range L2' (0.50) to LB (0.
55) and an enlarged overlap density reproduction range including the overlap density reproduction range are set, and in the density reproduction range, the dots of the dark ink and the dots of the light ink are made to appear at a ratio according to the density. Therefore, in the area below the OD value LA, only the dots of light ink are made to appear in a size according to the size of the reproduced OD value, and in the area above the OD value LB, only the dots of dark ink are reproduced. Both of them are shown in the enlarged overlapping density reproduction range of the OD values LA to LB. In the present embodiment, the expanded overlapping density reproduction range is expanded to both sides of the overlapping density reproduction range, but it may be set to only one side.

FIG. 6 shows a detailed circuit of the head selection control unit 25 for realizing such a method.

In the figure, 28 and 29 are binary counters, 30 is a ROM (Read Only Memory) storing dither patterns, 3
Reference numeral 1 is a magnitude comparator.

A clock C _X generated when the head moves in the main scanning direction is input to the counter 28, and a clock C _Y generated when the head moves in the sub scanning direction is input to the counter 29.

The ROM 30 stores a 2 × 2 dither pattern as shown in FIG. 7 (a), and the coordinates (X, Y) of (0,0), (0, 0, 0, 1),
(1, 0) and (1, 1) are selected, and the threshold value corresponding to the coordinates is input to one input terminal of the comparator 31. Then, the digital value C ₃ indicating the reproduction density of cyan is input to the other input terminal. Comparator 31 is C ₃
If> C _D , the signal CC is set to high level, and if C ₃ ≦ C _D, it is set to low level.

For example, when C ₃ changes as shown in Fig. 7 (b), Fig. 7 (c)
Dots are formed as shown.

The circles in FIG. 7 (c) indicate minute dots formed with light ink, Indicates fine dots formed by dark ink. Since the size of the minute dot is variably controlled by the density value C ₃ , as shown in FIG. 7 (c), when C ₃ is 0.37 and 0.5.
The size of dots formed by the dark ink and the light ink is different from that in the case of 2. However, when C ₃ is 0.5 or more, the size of the light ink dot is saturated to the maximum, and when C ₃ is 0.4 or less, the size of the dark ink dot is saturated to the minimum. FIG. 8 shows the relationship between the OD value to be reproduced and the voltage applied to the head. In each case, the applied voltage is set to the maximum voltage that forms the maximum dot or the minimum voltage that forms the minimum dot in the expanded portion of the expanded overlapping density reproduction range.

In this way, in the enlarged overlapping region, by separately printing the dark dots and the light dots, a desired OD value can be obtained on average, and it is possible to make a considerable difference in the density of the ink itself, and the reproducible range is expanded. In addition, the image quality is improved by eliminating the discomfort at the joints of light and shade dots.

Next, a second embodiment of the present invention will be described. Dark cyan ink KC 'and light cyan ink U used in the second embodiment.
The characteristic of the reproduced OD value with respect to the head applied voltage of C'is
Shown in the figure. As shown in FIG. 9, the light ink UC 'can reproduce an OD value of 0.10 to 0.50, and the dark ink KC' can reproduce an OD value of 0.82 to 1.30. Therefore, the intermediate region LC (0.
50) to LD (0.82) cannot be originally reproduced.

Therefore, in the present embodiment, such an intermediate area is reproduced by causing the minimum dot of dark ink and the maximum dot of light ink to appear at a ratio according to the density. The structure of the head control unit 25 for realizing such a method can be realized by FIG. 6 as in the first embodiment. However, the contents of the ROM 30 that generates the dither pattern are different. The contents of the ROM 30 in the second embodiment are shown in FIG. Therefore, when the digital value C ₃ indicating the reproduced OD value changes as shown in FIG. 10 (b), for example, dots are formed as shown in FIG. 10 (c).

Also in FIG. 10 (c), ○ indicates a minute dot formed by light ink, Indicates fine dots formed by dark ink. The size of the minute dots changes according to the OD value to be reproduced in the reproducible range of each ink, but does not change in the intermediate region.

FIG. 11 shows the relationship between the OD value to be reproduced and the voltage applied to the head. In the intermediate region, as shown in FIG. 11, the head applied voltage is the maximum voltage that forms the minimum dot in the case of dark ink, and the minimum voltage that forms the maximum dot in the case of light ink.

In this way, even if the OD value cannot be reproduced originally, the dark ink and the light ink are appropriately ejected, the desired OD value is obtained on the average, and it is possible to make a considerable difference in the density of the ink itself, and the reproducible range. Greatly expands. In addition, the image quality is improved by eliminating the discomfort at the joints of light and shade dots.

In the above description of the first and second embodiments, the case of cyan has been described, but magenta can be similarly realized.
Further, in the first embodiment, the density of yellow is one kind, but it may be plural kinds. Further, although the case where two different ink densities for each color are used has been described, the present invention can also be applied to the case where cyan has three different ink densities.

Moreover, the dither matrix is not limited to the case of 2 × 2, and is 3
Not limited to x3, 4x4, or nxn, nxm (n and m are different integers) may be used, or a modified dither pattern may be used. Further, by changing the size of the diamatrix depending on the joint, 3 × 3 may be used for the darker joint and 2 × 2 may be used for the thinner joint. In addition to the dither, a density pattern method for forming a plurality of dots for one input pixel can be used.

Further, in the present embodiment, the data of C ₃ , M ₃ , Y ₃ , and BK ₃ is realized by hardware using a dither ROM, but it may be realized by software using a microcomputer. Good. Further, as shown in FIG. 12, 2 in FIG.
1, 22, 23, and 24 are calculated in advance, the result is stored in the ROM 71, and by referring to the table,
Y ₃ , M ₃ , C ₃ , and BK ₃ can be obtained. FIG. 8 shows a case where different ink densities of three types of cyan, three types of magenta, one type of yellow, and one type of black are used. Then, the OD values of Y, M, C, and K four colors are output from the ROM 71 in 6 bits and input to the voltage conversion tables 72 to 75.
To the voltage conversion tables 72 to 75, in addition to the respective OD value data, the 8-bit signal is input by adding the "2" mode of each of the main scanning direction address X and the sub scanning direction address Y, and the result is the voltage. It is output as a value. Among them, magenta and cyan have three kinds of inks having different densities, and therefore signals for distinguishing them are output as CC ′ and MC ′. When CC '= 0, the head for the lightest ink is used, when CC' = 1, the head for an intermediate density ink is used, and when CC '= 2, the head for the darkest ink is used.

<Explanation of Effects> As described above, according to the present invention, the pseudo contour is generated even when the overlapping density reproduction ranges do not exist in the density reproduction ranges of the coloring materials having different densities or when the overlapping density reproduction ranges exist, but are narrow. It is possible to prevent the above, and it is possible to significantly widen the density reproduction range without increasing the number of coloring materials having different densities. Therefore, a high-quality image having high gradation can be realized very easily, which is industrially advantageous.

[Brief description of drawings]

1 is a control block diagram of the color ink jet printer, FIG. 2 is a detailed circuit diagram of the image processing circuit of FIG. 1, and FIG.
The figure shows the detailed circuit diagram of the head driver 27, FIG. 4 and FIG.
(a), (b), (c) are head applied voltage-OD value characteristic chart, 6th
The figure shows the detailed circuit diagram of the head selection controller, Fig. 7 (a), (b),
FIG. 8 (c) is a diagram for explaining the operation of the head selection control unit of the first embodiment, FIG. 8 is a diagram showing the relationship between the OD value and the applied voltage, and FIG.
The figure shows the head applied voltage -O of the cyan ink of the second embodiment.
FIG. 10 (a), (b), (c) is a diagram for explaining the operation of the head selection control unit of the second embodiment, and FIG. 11 is a second diagram.
FIG. 12 is a diagram showing the relationship between the OD value and the applied voltage in this embodiment, and FIG. 12 is an image processing circuit diagram of another example. In the figure, 25 is a head selection control unit, 30 is a ROM,
31 is a comparator, KC and KC 'are dark cyan ink,
UC and UC 'represent light cyan ink, respectively.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical display location // B41J 2/525 (72) Inventor Shinaki Sakurada Canon 770 Shimonoge, Takatsu-ku, Kawasaki-shi, Kanagawa Canon (56) Reference JP 58-65666 (JP, A) JP 53-102034 (JP, A) JP 57-76559 (JP, A) JP 57-129749 ( JP, A)

Claims (2)

[Claims] 1. A color image reproduction method using a plurality of colorants having different intrinsic densities for at least one of a plurality of colors used for color image reproduction, the size of dots formed by a high density colorant. Part of the density reproduction range obtained by varying the density reproduction range obtained by changing the size of dots formed by the low-density coloring material having a lower specific density than the high-density coloring material. , The overlapping density reproduction range and the density of the enlarged overlapping density reproduction range including the density reproduction range reproduced by one of the high density coloring material and the low density coloring material adjacent to the overlapping density reproduction area, And a color image reproduction method characterized by reproducing dots of low-density coloring material at a ratio according to the reproduction density. 2. A size of dots formed by a high-density coloring material in a color image reproducing method using a plurality of coloring materials having different intrinsic densities for at least one of the plurality of colors used for color image reproduction. And the density reproduction range obtained by changing the size of the dots formed by the low-density coloring material whose specific density is lower than that of the high-density coloring material. , The density of the reproduction range between the lower limit of the reproduction range of the high-density coloring material and the upper limit of the reproduction range of the low-concentration coloring material, depending on the reproduction density of the high-density coloring material dots and the low-density coloring material dots. A color image reproduction method characterized by reproducing by displaying it at a certain ratio.