JP2729030B2 - Image processing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing method having a plurality of color processing modes.

[0002]

2. Description of the Related Art Various types of color image forming apparatuses are known, and among them, those using a laser printer for performing image exposure on a photoreceptor with a laser beam and developing it to obtain an image are often used. Such a laser printer has advantages such as high print quality and high speed, and is widely used as an output device such as a color copier or a normal printer. In such a laser printer, modulation for making the emission time of the laser correspond to the magnitude of the image signal,
An image is formed by performing so-called pulse width modulation.

In performing pulse width modulation in this way, by changing the synchronization of the reference pulse, it is possible to select a plurality of color processing modes having different gradation reproducibility as shown in FIGS. In the reproducibility a shown in the figure, the whole can be reproduced sharply, but the highlight part is crushed. In the reproducibility of b, the gradation of the highlight portion is good, but it is difficult to form a sharp image as a whole.

[0004]

However, when the output characteristics of the image forming apparatus change due to aging,
Since color processing data cannot be created corresponding to each of the plurality of color processing modes, there is a problem that optimal color processing cannot be performed for each mode.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described circumstances, and has as its object to enable optimal color processing to be performed in each color processing mode even when output characteristics of an image forming apparatus change. I do.

[0006]

[Means for Solving the Problem and Function] To achieve the above object, the present invention provides a first color processing for performing color processing using first color processing data corresponding to a first image output condition. A second color processing mode for performing color processing using second color processing data corresponding to a second image output condition, wherein the first image output condition and the second image output The first color processing data and the second color processing data are obtained by calculating based on data obtained by reading a standard image corresponding to each mode output by the output unit according to each condition.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on embodiments.

FIG. 5 is a schematic configuration diagram of a device of a color image reading means used in the present embodiment. In the figure, 1
Reference numeral 2 denotes an original table glass on which the original 1 is placed. Reference numeral 3 denotes an illumination device, and reference numeral 4 denotes an imaging element array. 5 is an infrared cut filter, 6 is a contact type CCD color sensor (hereinafter, referred to as CCD), and 7 is an optical system unit.

First, the process of reading a color original will be described. When a copy key (not shown) is depressed, the illumination device 3 illuminates the document 1, and reflected light from the document passes through the imaging element array 4 and the infrared cut filter 5. And CCD6
An original image is formed thereon, and the optical system unit 7 scans the original in the direction of the arrow. As shown in FIG. 6, red (R), green (G), and blue (B) filters are regularly attached to the CCD 6 for each pixel.

As the document is scanned, the electric signal from the CCD 6 is processed by the signal processing circuit shown in FIG.

In FIG. 1, 6R, 6G and 6B are CCDs 6
The signals from the above R, G and B elements are shown. Next, the R, G, and B signals are introduced into an A / D conversion circuit 11 and a LOG conversion circuit 12, and are converted into Y signals in a digital signal form.
₁ , M ₁ and C ₁ signals.

[0012] The Y _1, M _1, C ₁ signal is introduced into the image signal generating circuit 13 for performing under color removal (hereinafter referred to as UCR) and black generation processing, the processing shown in the following equation, Y _2, Generate M ₂ , C ₂ , and Bk ₃ signals. (Y
₁ , M ₁ , C ₁ ) _min −k ₄ > 0 Y ₂ = Y ₁ −k ₃ (Y ₁ , M ₁ , C ₁ ) _min M ₂ = M ₁ −k ₃ (Y ₁ , M ₁ , C ₁ ) _min C ₂ = C ₁ -k ₃ (Y ₁ , M ₁ , C ₁ ) _min Bk ₃ = k ₃ (Y ₁ , M ₁ , C ₁ ) _min + k ₂ (i) (Y ₁ , M _1, C _{1) min} -k when _{4 ≦ 0 Y 2 = Y 1} M 2 = M 1 C 2 = C 1 Bk 3 = k 1 (Y 1, M 1, C 1) min + k 2 ... (ii) Here, (Y ₁ , M ₁ , C ₁ ) _min is the minimum signal among Y ₁ , M ₁ , and C ₁ , and k ₁ , k ₂ , k ₃ , and k ₄ are constants.

As described above, calculating (Y ₁ , M ₁ , C ₁ ) _min and reducing the amount of the YMC color material is a UCR operation, k ₃ is a coefficient for determining the UCR amount, and k ₄ Is UCR
Is the coefficient that determines the point at which

[0014] Depending on the UCR amount, a inking operation adding black toner (or black ink, etc.) instead of the respective color materials, the point where coefficient k ₁ determines the inking amount, k ₂ begins inking Is a coefficient that determines

Here, k ₁ and k ₃ , or k ₂ and k ₄ are different from each other, but may be the same.

(I) Y ₂ , M ₂ , C represented by the formula (ii)
_The method of determining the constants k ₁ and k ₂ for generating the Bk ₃ signal and the Bk ₃ signal, that is, the coefficient of the bleeding will be described below.

Here, i indicates each patch (for example, i = 0 to 63) on a standard color original (described later). And Y ',
M ′ and C ′ correspond to Y ₁ , M ₁ and C ₁ in FIG.
Bk _i indicates the amount of black at each point on the color image used in determining K ₁ and K ₂ , and i is the number of each point.

K ₁ and k ₂ are determined with φ as the black evaluation function.

[0019]

[Outside 1] Therefore,

[0020]

[Outside 2]

Here, if the above equation is expressed as [E] [F] = [G], the coefficients k ₁ and k ₂ are obtained from F = E ⁻¹ · G. E ^-1 is the inverse matrix of E.

The Y ₂ , M ₂ , C ₂ , and Bk ₃ signals thus generated by the image signal generation circuit are output to the color correction circuit 14.
, And performs the arithmetic processing shown in the following equation to obtain Y ₃ , M ₃ ,
The signals are C ₃ and Bk ₃ . In the present embodiment, Bk ₂ = Bk ₃ because the color correction circuit 14 does not perform any arithmetic processing on the Bk signal.

[0023]

[Outside 3]

Here, a _{11 to} a ₃₃ are masking coefficients for color correction, respectively. The above processing is controlled by a CPU (central processing unit) 20, and a RAM (random access memory) 21 and a ROM (random access memory) 22 store arithmetic programs and data.

These Y ₃ , M ₃ , C ₃ , and Bk ₃ signals are
The image is visualized by a color image forming means (printer 17) such as a color thermal transfer printer, a color inkjet printer, and a color laser beam printer.

FIG. 3 shows an example of a circuit used for forming an image by performing the pulse width modulation.
The digital color image signal D _Vin from the interface 16 is latched by the latch 1 by the video clock VCLK and synchronized. This image signal D _Vin is converted by a D / A converter 102 into an analog video signal AV. D
The output of the / A converter 102 is converted to a voltage level by a resistor 103 and then input to one input terminal of two comparators 104 and 113. In this example, two systems of triangular wave generation circuits each having an integration circuit as a basic configuration are prepared, and J / K flip-flops 105 and 113 which synchronize with VCLK and divide the frequency of PHCLK and TXCLK which are different from each other by two are provided. Integrate the output of. Where TXC
The frequency of LK is the frequency of a where the resolving power is important. On the other hand, the frequency of PHCLK is the frequency (where a>
b) is set. 50% of these divided
The duty clock is supplied to the variable resistors 107a and 107b and the capacitors 108a through buffers 106 and 114, respectively.
A triangular wave is formed by the integrating circuit constituted by 108b. And capacitors 109a and 109b, variable resistors 110a and 1
The bias is adjusted by 10b, and the protection resistors 111a, 111
b and the other input terminals of the above-mentioned comparators 104 and 115 through the buffer amplifiers 112a and 112b, are compared with the analog video signal AV, and become two systems of pulse width modulation signals M and N. These two signals M and N are input to the switch 118, and are switched to M for a character original and to N for a photographic original by a selector circuit 118 by a control signal 123 from the CPU 20.

The switching is performed by the operation of the CPU 20. The operation of the CPU 20 will be described with reference to FIG.

First, the state of the mode setting switch 26 is captured (# 1), and it is determined whether the photograph mode or the character mode is set (#).
3). Here, the photograph mode is a b in FIGS. 7 and 8 described later.
And the character mode is a mode corresponding to the reproducibility shown in FIGS. 7 and 8a described later. When the photograph mode is set in # 3, the switch 25 is set to the b side (# 5), and the switch 118 is switched to the NAND gate 117 side (# 7). #
If the character mode is set in step 3, switch 25
Is set to the a side (# 9), and the switch 118 is set to the NAND gate 1
Switch to the 16 side (# 11). Accordingly, color correction can be performed by changing the masking coefficient and performing the CR operation according to the gradation reproducibility of the printer 17.

Here, the characteristic required of the color image reading means is that the image output when the same color original is read becomes the same.

Hereinafter, an example of a masking coefficient which is a correction means for that purpose will be described.

First, a standard color original whose chromaticity coordinates are known is read by the optical system unit 7 shown in FIG.
Y ₃ , M ₃ , C ₃ , Bk _{3 of the} color correction circuit 14 shown in FIG.
For example, in this embodiment, the masking coefficient is determined by the following least square method so that the signal has the target value. Here Y ', M', C 'is Y ₁ in FIG. 1, M
₁ and C ₁ , respectively, and Y _i is a target value, respectively.
i is the number of each point on the color image used in determining the masking coefficient. In this embodiment, 64 points are selected as described later.

The evaluation functions for yellow, magenta, and cyan are denoted by φ _Y , φ _M , and φ _C , respectively. Then, for yellow,

[0033]

[Outside 4] Similarly, when magenta and cyan are performed, the following expression is obtained.

[0034]

[Outside 5]

Here, if the above equation is expressed as [C] [A] = [D], then A = C ^-1 · D, so that the coefficients a ₁₁ and a ₃₃ are obtained. Note that C ^-1 is the inverse matrix of C.

The calculation for obtaining a _{11 to} a ₃₃ is CP
U20, and the new coefficients a ₁₁ to
a ₃₃ and a ₄₄ are input to the color correction circuit 14.

Thus, the output of Y, M, C, and Bk when a standard color original is read can be controlled to always approach the target value.

Incidentally, the above masking coefficients a _{11 to} a ₃₃
The following is a description of the standard color original used for obtaining the original.

The full-color image forming means normally performs color reproduction using three primary color toners of yellow, magenta and cyan.

The image signal from the color image reading means is transmitted to the color image forming means at a predetermined gradation level (for example, 256 gradations). In the color image forming means used at this time, each of the three primary colors can reproduce 256 gradations, and the number of colors obtained by mixing a single color and a plurality of colors is 256 × 256 × 256. Ideally, the reproducibility of each of these colors is determined and the masking coefficient is determined, but this makes the calculation extremely complicated. Therefore, among the colors of the standard color original,
It is preferable to use a standard color manuscript by a reference character generator, which is selected so as to maximize highlight range reproduction, single-color density reproduction, and color reproduction range which are important in forming a color image.

As an example of a standard color original, in this embodiment, 0, 32, 128, and 25 of the yellow gradation levels are used.
6 (here 0 is white, 256 is a solid color), four levels are provided with emphasis on reproduction on the highlight side, and similarly, four levels of magenta and cyan are also prepared. Then, a masking coefficient was determined using a pattern having 64 (= 4 × 4 × 4) colors as a standard color original by combining these four levels for each color.

FIGS. 7 and 8 show the color reproducibility when pulse width modulation is performed by comparing each of the reference signals a and b of the pulse width modulation means with the digital color image signal to form an image. Show.

[0043] In FIGS. 7 and 8, is used to represent the color reproduction ^{(L *, a *, b} *) a in the expression ^* b ^*
FIG. 4 is a diagram showing coordinates and looking down from the L ^* axis.
In this figure, the image density and the saturation tend to increase as the distance from the origin increases. The area (A) surrounded by a straight line is the color reproduction range of the color image forming means, the mark で is the chromaticity coordinate of the halftone output by the reference signal a, and the mark で is the chromaticity coordinate of the halftone output by the reference signal b. is there.

As can be seen from FIGS. 7 and 8, when the image is output using the reference signal a, the saturation of the halftone image is lower than when the image is output using b. Here, in the figure, the shorter the distance from the origin, the lower the saturation (saturation). That is, b
When is used, the saturation reproduction of the halftone is better than that of a, and a vivid color reproduction can be achieved. On the other hand, in the case of a, FIGS. 4, 7 and 8
Thus, it can be seen that the color reproduction range is more emphasized on the higher density side than b. This is suitable for reproducing characters and the like as described above.

Therefore, as the evaluation color used when determining the masking coefficient of the color correction means, it is preferable to emphasize the highlight for b and select a color having a higher density than b for a.

The standard color original (B) output at b is read, an image signal is formed using the masking coefficient (b) obtained for the image output using the reference signal b, and the color image forming means uses the reference signal b. The chromaticity of the reproduced color is indicated by a mark in FIG. Further, using the masking coefficient (b), the standard color original (A) output with the reference signal a is read, and the chromaticity of the color reproduced by the reference signal a is indicated by a square in FIG.

The crosses in FIG. 9 and the circles in FIG. 10 correspond to the crosses and circles in FIGS. 7 and 8, respectively. From FIGS. 9 and 10, when the reference signal b is used in the image forming means, almost uniform reproduction can be performed for all colors.
When the reference signal a is used, the color reproduction of the highlight portion is not good.

Similarly, FIG. 11 shows a reproduction of the standard color original (B) with the reference signal b using the masking coefficient (a) with respect to the reference signal a.
FIG. 12 shows a reproduction of the standard color original (A) by using the reference signal a. According to FIGS. 11 and 12, when the reference signal b is used for the masking coefficient (a), the reproduction of the highlighted portion is not good.

This is the color of the highlight part which is difficult to reproduce in FIG. On the other hand, when the reference signal a is used, good color reproduction can be obtained.

As described above, an image is formed with the color image signal converted by the masking coefficient (a) and the reference signal a, and the color image signal and the reference signal b converted by the masking coefficient (b) are formed. If the image formation is performed in this way, good image quality can always be obtained.

Next, the effect of changing the image signal generation circuit 13 according to the gradation reproducibility as in the present embodiment will be described with reference to FIG.

FIG. 13 is a diagram showing the difference in color reproducibility depending on the reference signals a and b. The diagram shown on the a ^* and b ^* surfaces as shown in FIG. 7 shows the saturation and lightness (L ^* ). Shown in the coordinates of

In FIG. 13, the symbol .largecircle. Indicates the coordinate of the reference signal "a" and the coordinates of the halftone outputted in yellow, magenta and cyan in single color or mixed color, and the symbol "x" indicates the coordinates of the halftone output in the reference signal "b".
As is clear from FIG. 13, when the reference signal b is output, the brightness of the highlight portion is lower than in the case of a, and there are many brighter reproduced colors.

In other words, it can be said that the number of colors (including achromatic colors) that can be reproduced using the single color or the mixed colors of yellow, magenta, and cyan in the brightness direction in the highlight portion is larger when the reference signal b is used. Conversely, using the reference signal a,
The number of colors obtained as a single color or a mixed color is larger in the high density part than in b.

Therefore, the coefficients k ₁ , k ₂ , k ₃ , and k ₄ which determine the amount of UCR and the amount of blemishes are calculated as described above.
Using the image output with the reference signal a, the value is obtained (this is kn
(A)), (n = 1, 2, 3, 4)), kn
When an image is output with the reference signal b using (a), the image becomes dark as a whole. This is because, as shown in FIG. 13, since the reference signal b has more reproduced colors in the lightness axis direction in the highlight portion, the UCR and the SMI can be used for colors that can be reproduced without using the Bk signal. It depends on what is being done.

Conversely, the URC obtained for the reference signal b
Also, when an image is output with the reference signal a using the coefficient kn (b) of the blurring, the Bk signal is small and sufficient black cannot be reproduced.

As described above, when the image is output with the reference signal a using the UCR and the coefficient kn (a) for embossing, high brightness can be reproduced. When the image is output with the reference signal b using kn (b). , Vivid color reproduction of the highlight part is possible.

As described above, mainly in the case of a character original, the masking coefficient (a) and the coefficient k
n (a) to form an image using the color image signal converted and generated and the reference signal a, and mainly in the case of a photographic original, the masking coefficient (b), the UCR and the summing coefficient,
If an image is formed using the color image signal converted and generated by kn (b) and the reference signal b, an image quality faithful to the original can always be obtained. In the above-mentioned kn (a) and kn (b), n can be arbitrarily selected, and may be UCR only, Sumi only, or UCR and Sumi.

The selection of the combination of the masking coefficient, the UCR coefficient, the bleeding coefficient, and the reference signal described above may be performed by, for example, switching between the character mode and the photographic mode using the operation unit. Alternatively, the original may be automatically recognized by the color image reading means and the reference signal may be switched, and accordingly, the switching between the masking coefficient, the UCR, and the summing coefficient may be performed.

[Other Embodiments] In the present embodiment, a color image forming apparatus using pulse width modulation has been described, but another embodiment will be described below.

In a color image forming apparatus of the type in which the tone is reproduced by changing the light emission amount of the laser and modulating the dot area, when the number of lines is switched, the same problems occur in the tone reproduction and color reproduction. it is obvious.

If the present invention is applied to such an apparatus, a masking coefficient and / or a UCR coefficient and a smoothing coefficient of the image reading means are prepared and switched in accordance with the switching of the number of lines of the image forming means. , Faithful reproduction of image quality becomes possible.

Also, in a color image forming apparatus using a density modulation method for forming a specific pixel by a fixed laser spot or an ink jet and selecting lighting of each pixel, for example, a well-known dither pattern method, When the dither pattern is changed, the tone reproduction, the color reproduction, etc. also change. Therefore, by applying the present invention, the masking coefficient of the image reading means and the UC
By preparing and switching between the R coefficient and the smoothing coefficient, it is possible to reproduce faithful image quality.

In this embodiment described above, the processing means for performing the undercolor processing of the supplied color image signal is shown in FIG.
, The under color removal operation, that is, k ₃ (Y ₁ , M ₁ , C ₁ ) _min is subtracted from the signal of each color, and the summing operation is performed, that is, K ₁ (Y
₁ , M ₁ and C ₁ ) are used, but the present invention does not necessarily require both operations, and may rely on only one of the operations. These operations may be performed electrically or optically.

The color image processing means which has a predetermined gradation reproducibility and processes a color image in accordance with the signal subjected to the undercolor processing by the processing means is capable of processing a signal obtained by performing pulse width modulation. We used a color laser printer that performs image formation according to the requirements and a color thermal transfer printer and a color inkjet printer that can change the gradation reproducibility.
A device that simply processes an image signal electrically without necessarily forming an image and outputs the signal to an external printer may be used.

In this embodiment, a color laser printer, a color thermal transfer printer or a color ink jet printer is used as the color image forming means for changing the image forming conditions. However, the present invention is not limited to this. In other words, any means capable of changing the image forming conditions, such as changing the latent image forming conditions and developing conditions, may be used.

Also, means for controlling the processing state of the lower color processing means in accordance with the gradation reproducibility of the color image processing means is switched by switching the state of the switch circuit 118 shown in FIG. Is switched to the CPU 20.

[0068]

As described above, according to the present invention,
Even if the output characteristics of the image forming apparatus change with time, environmental conditions, or the like, it is possible to create optimal color processing data for each color processing mode.

Therefore, color processing can always be performed on an input image using the most suitable color processing data for each color processing mode, and a high-quality output image reflecting each color processing mode can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an apparatus according to an embodiment of the present invention.

FIG. 2 is a flowchart illustrating an example of an operation of a CPU 20.

FIG. 3 is a block diagram showing an example of an internal circuit of the printer 17.

FIG. 4 is a diagram showing a difference in tone reproduction characteristics of the printer 17;

FIG. 5 is a diagram showing an example of an image reading device including a CCD 6.

FIG. 6 is a diagram showing an example of an enlarged view of a surface of a CCD 6;

FIG. 7 is a diagram illustrating a part of chromaticity coordinates of a standard color original.

FIG. 8 is a diagram illustrating a part of chromaticity coordinates of a standard color original.

FIG. 9 is a diagram illustrating an example of chromaticity of a standard color original and chromaticity coordinates after reproducing an image thereof.

FIG. 10 is a diagram illustrating an example of chromaticity of a standard color original and chromaticity coordinates after reproducing an image thereof.

FIG. 11 is a diagram showing an example of chromaticity of a standard color original and chromaticity coordinates after reproducing an image thereof.

FIG. 12 is a diagram illustrating an example of chromaticity of a standard color original and chromaticity coordinates after reproducing the image.

FIG. 13 is a diagram illustrating an example of a relationship between lightness and saturation when an image of a standard color original is reproduced.

[Explanation of symbols]

 Reference Signs List 11 A / D conversion circuit 12 LOG conversion circuit 13 Black extraction and UCR circuit 14 Color correction circuit

Claims (4)

(57) [Claims] 1. A first color processing mode for performing color processing using first color processing data corresponding to a first image output condition, and a second color processing mode corresponding to a second image output condition A second color processing mode for performing color processing using data, and reading a standard image corresponding to each mode output by an output unit according to each of the first image output condition and the second image output condition. An image processing method, wherein the first color processing data and the second color processing data are obtained by calculating based on the obtained data. 2. The image processing apparatus according to claim 1, wherein the first image output condition and the second image output condition have different resolutions.
The image processing method described in the above. 3. The image processing method according to claim 1, wherein the first color processing mode is a photograph mode, and the second color processing mode is a character mode. 4. The image processing method according to claim 1, wherein the image output condition is a gradation number conversion process.