JPH01120966A - Multi-level gradation processing device for color picture - Google Patents

Abstract

PURPOSE:To reduce the memory bit number and to simplify a circuit by providing a means applying multi-level processing at every color, a means selecting the number of multi-level at every color and a means transmitting a processing data. CONSTITUTION:An original to be copied is decomposed into R, G, B by a color scanner 1 and then read. A shading correction circuit 2 corrects the uneven sensitivity of an image pickup element and an uneven lighting of light source. An MTF correction circuit 3 corrects the deterioration of the MTF characteristic especially at a high frequency region in an input system. The Y, M, C, Bk data subjected to color correction and UCR processing are subjected to multi-level processing of binary or above by a gradation processing circuit 6. The data subjected to gradation processing is sent to a printer 7, where the reproduced picture is outputted. Especially in an output system using a laser printer, the gradation processing data is sent indirectly to the printer 7 via the memory indirectly.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a multi-value gradation processing device for color images, and in particular, to a method for forming an intermediate gradation image using multi-value area gradation processing using a multi-value threshold. The present invention relates to a multi-value gradation processing device for color images.

(Prior art) In recent years, in various digital color image forming processes such as color copying machines, color printers, and color printing, multi-value area gradation processing using multi-value threshold values is used to create intermediate gradations using multiple colors. Image forming techniques are often employed. In such a multi-value gradation processing device for a color image, the greater the number of multi-value levels, the more the gradation is improved and the color image quality is improved.

However, if the number of multivalue levels is increased in order to improve gradation, the processing means will rapidly become complicated, and the number of processing circuits will increase, leading to an increase in cost. For example, when performing 3-bit signal processing on 8-level colors of yellow, magenta, cyan, and black, and
4 value levels: yellow, magenta, cyan, and black
If 2-bit signal processing is performed for color, the cost is 3:2. On the other hand, reducing the number of gradations is convenient in terms of cost, but the image quality suffers accordingly, and in order to maintain a certain level of image quality, the number of gradations cannot be reduced unnecessarily.

(Purpose) Therefore, the present invention aims to simplify the circuit by reducing the number of multi-value levels, that is, the number of memory bits, without impairing image quality, thereby reducing costs and speeding up processing. The object of the present invention is to provide a multi-value gradation processing device for color images.

(Structure) In order to achieve the above object, the present invention provides a color image in a digital image forming system that performs multi-value area gradation processing using a multi-value threshold value and forms a half-tone image using a plurality of colors. In the multi-value gradation processing @, the configuration includes means for performing multi-value processing for each color, means for selecting the number of multi-values for each color, and means for transmitting processing data. have.

In a color image multilevel gradation processing device having such a configuration, specific colors that do not affect image quality are processed using a reduced number of multilevel levels, and specific colors that do not affect image quality are processed using a reduced number of multilevel levels. Colors are now processed through an unreduced number of multi-value levels.

Examples of the present invention will be described in detail below.

First, if we consider what the number of gradations is, it means the change in brightness of each color. In other words, there are three indicators of color: brightness, hue, and saturation.
Among them, (1) hue is determined by a combination of the adhesion amounts of each color, for example, yellow, magenta, and cyan. Each color 1 is determined by the numerical ratio of color separation signals from a scanner or the like, which is determined by the arbitrary size of the dither processing matrix and the number of multivalue levels corresponding to one pixel of the matrix circle. For example, 4 of the 4x471 helix
At value level, 4X4X4 (level) x 3 (color) - 1
92 hues are obtained, and in the 8-value level of the 4.times.4 matrix, 4.times.4.times.8 (levels).times.3 (colors)-384 hues are obtained. As for hue, even 192 hues at four-value level have sufficient reproducibility. Moreover, in the case of area gradation, the brightness is changed at the same time as the hue depending on the ratio to the white background.

■Saturation indicates the ratio of the dominant wavelength of each color to white. For example, in an image created with each color of yellow, magenta, and cyan, the spectral reflectance of each color is wide, so it is necessary to control the saturation itself. is difficult and at the same time has no practical meaning. Further, the apparent saturation can be adjusted by adjusting the brightness, that is, the ratio of the amount of each color deposited.

From the consideration of (1) and (2) above, it is understood that gradation has a large meaning as the number of brightness levels.

Further, (2) Brightness refers to a change in brightness, and since the recording paper is a white background, this changes depending on the coverage of each color toner. However, each color toner has its own brightness, and even at 100% coverage, the brightness does not change significantly.

Therefore, if the amount of adhered black toner, for example, black toner, is controlled, the dynamic range of brightness changes can be increased, and four-color reproduction becomes useful here.

As described above, gradation can be approximately represented by a change in brightness, and this depends on the number of gradations of the amount of adhesion of black, for example, black toner. In particular, due to UCR processing in which the most equal amounts of yellow, magenta, and cyan color toners are replaced with black toner, black has a large influence on brightness. This UCR processing determines the adhesion of black toner, but it first removes the brightness information from the original color, and then changes the hue to determine the adhesion of toner of other colors, such as yellow, magenta, and cyan. It can be said that this is a process in which the difference in the amount of black toner is used to represent the brightness, and the amount of attached black toner is used to represent the brightness.

Therefore, whereas conventionally a 3-bit, 8-level multi-level number is used for the four colors yellow, magenta, cyan, and black, we will maintain 3-bit processing only for black, and Even if 2-bit processing is performed, sufficient gradation can be obtained. The cost in this case is /' -1/ ×3/ -1/,
The cost of the processing circuit is reduced to 1/4.

Next, the processing circuit will be specifically explained based on the drawings.

FIG. 1 is a block diagram showing a copying system using the present invention. A document to be copied is separated into R, G, and B colors and read by a color scanner 1. In the shading correction circuit 2, sensitivity unevenness of the image carrier element, illumination unevenness of the light source, etc. are corrected. The MTF correction circuit 3 corrects deterioration in MTF characteristics of the input system, particularly in the high frequency range.

Examples of digital filters used for this MTF correction are shown in FIGS. 2(a) to 2(e). The γ correction circuit 4 corrects or converts the input data so that it has desired characteristics such as linear reflectance and linear density.

In addition, skin removal and the like are also performed at the same time.

The color correction UCR processing circuit 5 corrects the difference between the color separation characteristics of the input system and the spectral characteristics of the color materials of the output system. The color correction tJcR processing circuit 5 includes a color correction processing section that calculates each color material, for example, yellow, magenta, and cyan, necessary for faithful color reproduction, and a color correction processing section that calculates the color materials necessary for faithful color reproduction, and a portion where the three colors of yellow, magenta, and cyan overlap. and a UCR processing section for replacing the color with black.

The color correction process can be realized by executing the 71~ricks calculation as shown below.

In the above formula, B, G, and R indicate the number of corrections of B, G, and R. 7
1 to the ricks coefficient a is determined by the spectral characteristics of the input system and the output system (coloring material). Although a first-order masking equation is taken as an example here, by using a second-order term or a higher-order term such as 102, n, or yaku, color correction can be performed with high layer accuracy. Further, the calculation formula may be changed depending on the hue, or the Neugeba equation may be used. Either way, Y, M,
The value of C can be determined from the values of B, G, and R or B, G, and R.

FIG. 3 shows an example of a color correction circuit.

A ROM table is used here. That is,
Y, which is necessary to reproduce the colors represented by R, G, and B,
The amounts of M and C are calculated in advance, and the calculation results are used as RO.
M501, 502, and 503, respectively, and are configured to perform color correction processing in a table-reference manner. Furthermore, in color correction calculations, by taking advantage of the fact that complementary colors have the largest contribution, it is possible to reduce the number of bits of other colors compared to the number of bits of complementary color data, reducing ROM capacity without compromising calculation accuracy. . In the example shown in Figure 3, the complementary color is set to 7 bits, and the other colors are set to 5 bits x 2, so that 6-bit correction data can be obtained by addressing with a total of 17 bits of data. . As the ROM, a 1 megabit ROM having a configuration of 8 bits per word can be used. Further, it is also possible to configure the system using a plurality of ROMs each having a capacity of 256 kilobits or more.

On the other hand, the above [JCR processing is performed by calculating the following equation.

Y-Y-α-min(Y, M, C) tvl=M-a
-min(Y, M, C)C'=C-a-+n1n(
Y, M, C) 8=a-min(Y, M, C) In the above formula, α is a coefficient that determines the amount of tJcR,
When α=1, 100% UCR processing is performed.

α may be a constant value or may be variable depending on the concentration level. For example, if α is set close to 1 in high-density areas and close to O in highlight areas, the image in highlight areas 1 to 1 can be made smooth.

FIG. 4 shows an example of a UCR circuit.

Each of the Y, M, and C data after color correction is subtracted by Bkfjl calculated by the Bkff1 calculation circuit 504. Further, FIG. 5 shows an example of a Bkl calculation circuit. Comparator 5o5゜506o, J: and t't/kuta 507,508rY,M.

The minimum value of C is found, and the ROM is
Bkfi is determined using a table reference formula. Here, the configuration is such that tJCR processing is performed after color correction processing, but T'r,? Calculate the amount of Bk from the minimum value of m and B, and calculate R
, G, 1lllr minus Bk may be used to perform color correction processing.

Returning to Figure 1, Y, M, color corrected and UCR processed,
The C and Bk data are subjected to binary or more multivalue processing in the gradation processing circuit 6. A systematic dither method is generally used as a multi-value processing method, but other methods such as an error diffusion method can also be used. In the present invention,
In order to place particular importance on the gradation properties of Bk, gradation processing is performed using a larger number of multivalues than those of Y, M, and C. Here, B
Convert k to 8 values (3 bits/pel > , Y, M, C
We will perform quaternary processing (2 bits/pet). 3K to 8-value, Y, M depending on the required image quality
, C may be binarized.

FIG. 6 shows an example of a multi-value processing circuit using the organized dither method, in which the processing results are written in the memory address addressed by the dither matrix address and the image data, allowing the table reference method to be used. It is now possible to perform dither processing. This circuit is provided for each color of Y, M, C, and Bk, and can perform high-speed gradation processing by processing each color in parallel.

The gradation-processed data is sent to the printer 7, where a reproduced image is output. In particular, in an output system using a laser printer, the gradation processing data is sent to the printer 7 indirectly via the memory 8 rather than directly. Figure 7 shows
1 is a diagram showing the principle of a printer using one photoreceptor drum.

The Y, M, C, and 8 data processed in parallel by the image processing device are stored in the frame memory 701, and the data is sequentially read out from the memory one color at a time, and is sent to, for example, a laser writing unit of the printer 7. 702. Photosensitive drum 70 according to image data
The toner image formed on the transfer drum 704 is transferred to recording paper wrapped around a transfer drum 704. The recording paper is fixed to the transfer drum 704 until the four-color image is transferred. In this case, the color data initially sent to the printer 7 may be configured to be sent directly to the printer without involving the memory 8.

Conventionally, for example, Y, M, C, and Bk are all gradated to 3 bits and Qel, respectively, so if an A4 size document is processed at a sampling density of 16 pages/frame, it will require approximately 48 megabits. /color frame memory will be required. Therefore, the data for four colors becomes 192 megabits. In the present invention, for example, Y, M, C
Since each color is gradated to 2 bits, 'pet', the required capacity is 144 megabits (= 48 megabits × savings). Also, as mentioned above, if the first color does not go through the frame memory, Conventionally, 144 bits (=4
8 megabits/color x 3 colors), whereas in the present invention, if Bk, which has a large number of bits, is the first color, 9
6 megabits (= 48 megabits less).

FIG. 8 shows a four-drum type printer that is equipped with an independent image forming system for each color so as to enable high-speed output. In this case, the four colors will be outputted in parallel, but each drum 713.723, 733.7 will be printed on the recording paper.
There is a shift between each color by an interval of 43. In order to eliminate this deviation, it is necessary to provide a delay memory 705 for delaying data transfer to each write unit 712, 722, 732, 742 by a time corresponding to the drum interval. The required amount of delay memory 705 is based on the drum interval F.
(am), 64' (, -4+27+3F)
This is equivalent to . For example, if P is 100m+, sampling density is 16 lines/#, and feed is A4 horizontal feed (width in main scanning direction 2971M), 137 megabits (
-(297jIIIX 16 shoes/shoes)
x16 pieces/shoe) x3 bits). Using the present invention, if the first color is set to 3, which has a large number of bits, 6! Since it is only necessary to delay the corresponding amount with 2-bit data, 91 megabits (= (297ml+X 16 bottles/a+)
00#IX 6 x 16 lines/am) x 2 bits), saving memory.

In the above description, 3 bits are used for 3k, and 2 bits are used for Y, M, and C.

Needless to say, further savings can be achieved by setting M and C to 1 bit. Also, if 3k is the first color,
Frame memory and delay memory can be eliminated,
Even if the number of multi-values of Bk is increased by that amount, the cost of the memory will not increase.

The above describes the component pair Y, M, and C components in 3.
Next, the characteristics required between the Y, M, and C components will be explained. When expressing gradations using the systematic dither method, increasing the dither matrix increases the number of gradations, but the resolution decreases.

Conversely, if the matrix is made smaller, the gradation will be reduced. Multi-leveling allows the gradation to be improved without increasing the matrix size. However, as the amount of data increases, the capacity of the puffer memory increases, which has a large effect on the resolution of characters, etc. It is. On the other hand, conversely, it is also conceivable to reduce the number of multivalues of components that have less influence on resolution.

In recent years, image information has become more and more colored, and text and line images are now generally colored not only in black but also in colors such as red, blue, and green. Therefore, it is necessary to maintain resolution for color components as well. However, among the Y, M, and C components, the Y component has a characteristic of being visually inconspicuous and having low resolution. Therefore, the multivalue number of Y component is C
, M components can also be reduced. In this case, among the colored characters, the green and red characters are C and Y, M and Y, respectively.
It is reproduced by overlapping color plates, but even if the resolution of Y is low, if the characters are reproduced so that the resolution of C and M colors is high, it is possible to recognize green or red characters. .

Here, as the amount of data after gradation processing (the number of bits per pixel), from the state where 3k is 3 bits and each of Y, M, and C is 2 bits, the number of bits of Y is reduced to [3k is 3 bits].
Let's calculate the memory capacity when Bit, M, and C are each 2 bits, and Y is 1 bit. First, for one drum, if 8k is used as the first color without going through memory, 80 megabits (= (297m x 16 pieces/frame))
× (210+Il+X16 lines/m)X (2+2+1
) bits), which is 17% (-0,17= (96 - then Then, by setting 8k as the first color and Y as the fourth color, the puff memory can be reduced the most. In this case, 68 megabits (= (297 + 111 +
IX 16 pieces/Nn) X (100m x 16 pieces/#l
ff1)
5% (=0.25= (9.
If Y is 1 bit, then 91 megabits (=(29
7mX16 lines/m＞X (100mX 16 lines/fn
In)×<1X3+2X3+3X1 bits), and the capacity is the same as when 3k is 3 bits and Y, M, and C are each 2 bits. In this way, by reducing the number of bits of the Y component (multi-value number), it is possible to reduce the memory size by D without deteriorating the image quality.

When transmitting data externally, such as by facsimile,
When saving (file) to an external storage device, the amount of data of the Bk component must also be considered. At this time, ignoring some deterioration in image quality, 3K is 3 bits, Y, M, C.
1 bit, 8k as 2 bits, Y, M, C as 1 bit.
By converting data into bits, it is possible to significantly reduce the amount of data. In this way, in a system that not only forms images such as copies but also transmits, stores, etc., multiple modes are provided, and in the case of a copy mode that requires higher image quality, 3-bit Bk, 3-bit Y, M.

In addition to setting C to 2 bits, if transmission and storage efficiency is important, 3k to 2 bits, Y, M.

It is convenient to configure the system so that gradation processing is performed using C as one bit. In addition, in transmission/storage mode, 3
Processing in which k is 3 bits and Y, M, and C are 1 bit, and B
It is also possible to adopt a configuration that enables processing in which k is 3 bits and Y, M, and C are 2 bits. In this case,
By adding processing conditions (number of gradations, image size, etc.) as a header to image data, the image can be reproduced without fail even when received data or saved data is read out.

FIG. 9 shows a block diagram of a reproduction system having copy modes. This system is controlled by a system controller 11 having a CPU. The operator can specify the mode and other processing conditions using switches (not shown) on the operation panel 12.

In the copy mode using the internal scanner, the scanner 13 is activated and the original is separated into R, G, and B colors and read.
The image processing unit 14 performs the above-mentioned shading correction, M
TF correction, γ correction, color correction, and UCR processing are performed, and multi-value processing is performed by the gradation processing circuit shown in FIG. At this time, according to the mode selection signal from the system controller 11, 3 is multivalued into 3 bits, and Y, M, and C are multivalued into 2 bits each. The next multiplexer 15 is for selecting data from the internal scanner 13 or data sent through the external 1/F 16. In copy mode, image data based on the data from the internal scanner 13 is selected. The next data converter 17 converts Bk3 bits, Y, M, and C to match the characteristics of the output system when external data with different numbers of bits (multiple values) is input.
is for converting each into 2-bit data. By addressing image data and a mode selection signal using ROM or RAM, conversion processing can be performed in a table-reference manner. The output data from this converter 17 is sent to a printer 19 via a memory 18 so that a reproduced image can be obtained.

In the reception mode, image data sent from an external transmitter is received according to header information, and a reproduced image is output from the printer 19 through the multiplexer 15, data converter 17, and memory 18. In the reception mode, first, header information is received through the signal line H, and the information is analyzed by the system controller 11. According to the header information, the system controller 11 generates a more detailed mode selection signal. The image data entered after the header is
The signal is input to the data converter 17 through the multiplexer 15. Here, the image data is converted into image data of a predetermined number of bits according to the mode selection signal described above, and is sent to the printer 19. In this mode, the system controller 11 controls the scanner 13 so that it does not operate.

Next, in the transmission mode, the internal scanner 13 is operated in the same manner as in the copy mode, and the original is separated into R, G, and B colors and read, and the image processing section 14 performs the above-mentioned shading correction, MTF correction, and γ correction. , color correction, and IJcR processing, and multivalue processing is performed by the gradation processing circuit shown in FIG. At this time, gradation processing is performed using the number of bits (multi-value number) according to the processing conditions selected by the operator through the operation panel 12. System controller 11
The processing conditions are transmitted as header information prior to the image data. Thereafter, the image data that has undergone predetermined gradation processing is transmitted. At this time, the printer 19 is controlled not to operate.

The exchange of data with the external storage device is basically the same as in the transmission mode or reception mode, so the explanation will be omitted.

As explained above, according to this embodiment, in the gradation processing of a color image, by gradation processing the Y, M, and C components to a number of bits less than 3, the amount of data can be reduced while maintaining high image quality. can be reduced. As a result, it is possible not only to save frame memory and delay memory, but also to shorten operation time and save memory when transmitting and storing data. Furthermore, by making the number of multi-values of gradation processing variable, it is possible to further shorten operating time and save memory during transmission and storage.

By increasing the number of bits of lζ and Bk, not only the gradation but also the resolution of black characters in particular can be improved. Furthermore, since the control circuits of the Y, M, and C capture units are simpler than those for Bk, the cost of the writing system can also be reduced.

Although the above embodiment relates to a laser printer,
The present invention can also be applied to other output devices such as thermal head printers using one head or multiple heads. Furthermore, the present invention is not limited to a four-color full-color copying system, but can also be applied to, for example, a two-color copying system, red and black, in which gradation processing is performed by using a large number of bits for black and a small number of bits for red. That is, in a system that reproduces images in multiple colors, by reducing the number of bits for gradation processing of a specific color, the amount of data can be reduced without deteriorating image quality.

Note that the present invention is applicable not only when using a matrix of a fixed size (for example, when using a 4 x 4 matrix), but also when changing the matrix size to keep the number of reproduced tones constant. It is possible. In other words, when using a matrix of a fixed size, the number of tones reproduced will change depending on the number of bits used, but the resolution of the image is determined by the matrix size, so to achieve high resolution, the matrix The smaller the size, the better. However, the human eye's response to horizontal colors is weak, so even if the resolution is reduced, there is little deterioration in image quality. Moreover, as shown in FIG. 7, the horizontal color requires the most delay memory, so by reducing 3 bits to 2 bits for the horizontal color, a very large memory cost reduction effect can be obtained.

(Effects) As described above, the present invention includes means for performing multivalue processing for each color, means for selecting the number of multivalues for each color, and means for transmitting processing data. The number of multivalue levels, that is, the number of memory pins 1 to 1 can be reduced without impairing the image quality of both the character part and the picture part of a color original, resulting in a significant cost reduction.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a copying system using the present invention, FIGS. 2(a) to (e) are explanatory diagrams showing examples of digital filters used for MTF correction, and FIG. 3 is a diagram of a color correction circuit. FIG. 4 is a block diagram showing an example of an LICR circuit, FIG. 5 is a block diagram showing an example of a Bkl calculation circuit, and FIG. 6 is a multi-value processing circuit using systematic dither method. A block diagram showing an example, FIG. 7 is a diagram showing the principle of construction of a printer using one photosensitive drum, FIG. 8 is a diagram of the principle of construction of a four-drum printer, and FIG. 9 is a copying system with copy mode FIG.

Claims (1)

[Claims] 1. Multi-value gradation processing of a color image in a digital image forming system that performs multi-value area gradation processing using a multi-value threshold value to form an intermediate gradation image using a plurality of colors. Multi-value gradation of a color image, characterized in that the device comprises means for performing multi-value processing for each color, means for selecting the number of multi-value conversion for each color, and means for transmitting processing data. Processing equipment. 2. Image forming colors are composed of four colors: yellow, magenta, cyan, and black, and the multi-value number of at least one of yellow, magenta, and cyan is set to be smaller than black. A multi-value gradation processing device for color images according to claim 1.