JP3431687B2 - Image forming apparatus and image forming system - 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 forming apparatus and an image forming system for expressing a halftone by a multi-valued dither method.

[0002]

2. Description of the Related Art Conventionally, in image processing, one pixel (dot) of a dither matrix for dither processing is further subdivided into multi-valued outputs to further improve gradation reproducibility. It has been known. A conventional example of performing density correction in image processing using such a multi-valued dither processing method is disclosed in, for example, Japanese Patent Laid-Open No. 3-80768. This is because the first correction means for performing density correction based on dither processing is applied to the data input from the outside in the image forming apparatus, and the multi-valued image data corrected by the first correction means. A multi-valued dither processing means for performing multi-valued dither processing using a dither matrix (hereinafter referred to as a multi-valued dither matrix), and multi-valued dither data obtained by the multi-valued dither processing means based on output characteristics of a printer Second density correction
And the correction means of. In this case, since the correction for the dither processing (first correction means) and the correction for the printer characteristics (second correction means) are performed independently, it is possible to replace the multi-value dither processing means or the printer. Also, it is possible to easily deal with various combinations of dither processing and printer characteristics.

[0003]

In the case of the above-mentioned conventional example, the density correction by the second correction means is one for the multivalued image data after the multivalued dither processing by the multivalued dither processing means. The γ correction curve (density correction curve), that is, a single γ characteristic corresponding to one pixel of the multi-valued dither matrix after dither processing is repeatedly used for all pixels. However, since the γ characteristics of each dot of the multi-valued dither matrix are actually different from each other, accurate density correction cannot be performed by making one γ correction curve correspond to all pixels. Also, providing the image forming apparatus with multi-valued dither processing means and various correction means increases the number of parts and increases cost, and the image density correction processing takes time.

[0004]

According to a first aspect of the present invention, multi-value dither processing is performed on an image signal input from the outside using a multi-value dither matrix of multi-value dither processing means, and the multi-value dither processing is performed. An image forming apparatus that expresses a halftone by multi-valued image data obtained by dithering has at least two density correction curves corresponding to the γ characteristics of each dot of the multi-valued dither matrix that performs multi-valued dithering. A γ correction table is provided.

According to a second aspect of the invention, in the first aspect of the invention, there is provided a density correction curve switching means for switching the density correction curve of the γ correction table for each multi-valued dither matrix.

According to a third aspect of the invention, in the first aspect of the invention, the density correction curve of the γ correction table is a curve corresponding to at least the first matrix level of the multi-valued dither matrix.

According to a fourth aspect of the present invention, a computer having multi-value dither processing means for performing multi-value dither processing on the input image signal using the multi-value dither matrix, and each of the multi-value dither matrices. A γ correction table having at least two density correction curves corresponding to the γ characteristics of dots is provided.

According to a fifth aspect of the invention, in the fourth aspect of the invention, there is provided a data combination transfer means for transferring a plurality of multivalued image data after multivalued dither processing in combination.

[0009]

According to the present invention, the optimum density correction curve corresponding to the γ characteristic of each dot of the multi-valued dither matrix is selected from among the plurality of density correction curves in the γ correction table, and the multi-valued correction curve is selected. It is possible to correct the image data.

According to the second aspect of the present invention, the density correction curve is switched for each multivalued dither matrix.
Even when the multi-valued dither processing means is exchanged, it is possible to perform the γ correction suitable for the combination of the respective matrices.

According to the third aspect of the present invention, the density correction curves corresponding to the first matrix level of the multi-valued dither matrix and the other matrix levels are used, so that the reproducibility of the highlight portion is improved. It becomes possible to do.

According to the present invention, the multi-valued dither processing is performed in the computer, and then the multi-valued image data obtained by the multi-valued dither processing is processed in the image forming apparatus. It is possible to select the density correction curve corresponding to the γ characteristic of each dot and perform the density correction.

According to the invention of claim 5, a plurality of multi-valued image data are combined and transferred, that is, for example, multi-valued dithered 4-bit multi-valued image data is converted into 8 bits and transferred. It is possible to shorten the data transfer time.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIGS. First, the configuration of this system will be described with reference to FIG. This system is a color laser printer 1 (see FIG. 15) as an image forming apparatus that outputs color images of four colors (C, M, Y, K), and a computer connected to the color laser printer 1 by Ethernet. And a personal computer 2. However, in FIG. 1, only the printer unit 3 of the color laser printer 1 is shown, and the printer unit 3 receives a bitmap image signal (hereinafter referred to as an image signal) and a control signal from the personal computer 2. 4 and a printer engine 5 connected to the printer controller 4. The printer engine 5 controls each unit (not shown) of the printer unit 3 based on an image signal sent from the printer controller 4 to output an image. The overall structure of the color laser printer 1 will be described later.

Next, the structure of the printer unit 3 will be described with reference to FIG. Hereinafter, a procedure for performing multi-valued dither processing and its density correction (γ correction) centering on magenta (M) of the four colors will be sequentially described. Now, the image signal a from the personal computer 2 is an 8-bit signal for each color (256 gradations), and a multi-value dither processing circuit 6 (ROM) as multi-value dither processing means.
Will be sent to. Multi-value dither processing is performed in the multi-value dither processing circuit 6 to form a 4-bit image signal b (multi-value image data). In this multi-valued dither processing circuit 6, the value of the multi-valued dither matrix for multi-valued dither processing that is referred to changes every time a pixel or line advances, so the ternary counters 7, 8 corresponding to the size of the multi-valued dither matrix are used. Is provided. These ternary counters 7 and 8 are adapted to perform a counting operation by the pixel synchronization clock signal c and the line synchronization clock signal d generated by the count designation value from the personal computer 2.

FIG. 3 shows an example of the structure of a multivalued dither matrix M for multivalued dither processing. The size of the multi-valued dither matrix M here is 3 × 3, each pixel (dot) in the frame can be changed in 16 steps, and the dot arrangement order is from number up to, and the order from the smallest number The dots are struck from and formed. In addition, FIGS. 4A to 4I
Indicates each matrix level of the multi-valued dither matrix M, the first matrix level of (a) corresponds to dot arrangement, the second matrix level of (b) corresponds to dot arrangement, The same applies to the ninth matrix level (i). In addition, in FIG.
The circles in the frame indicate change dots that change in 16 steps, and the crosses in the frame indicate all illuminated dots. The number of gradations per color that can be represented by such a multi-valued dither matrix M is 13
It becomes 6.

Then, as shown in FIG. 1, the 4-bit image signal b subjected to the multi-valued dither processing is temporarily stored in the frame memory 9, and then the γ correction table 10 (ROM).
Sent to. In this γ correction table 10, the image signal b
Is subjected to γ correction processing for the γ characteristic of the multi-valued dither matrix M by the density correction curve stored in the table (details will be described later), and becomes an image signal e of 8 bits for each color. In this case, the γ correction table 10 shows the image signal b
Since the .gamma. Correction data to be referred to changes every time when is moved in the main scanning direction and the sub scanning direction, ternary counters 11 and 12 corresponding to the .gamma. Correction data matrix size are provided. These ternary counters 11 and 12 are counted by the main scanning clock signal f and the sub scanning clock signal g for reading from the frame memory 9. And
An 8-bit image signal (LD drive signal) e for each color obtained by performing γ correction in the γ correction table 10 in this manner
Is sent to the LD controller 13, so that desired image output control can be performed.

Here, the multi-level dither processing will be described in more detail. The level of the 8-bit image signal a sent to the multi-level dither processing circuit 6 is 0 to 255, and this image signal a is divided into 9 in the multi-level dither processing circuit 6 as shown in FIG. , Each of these divided levels is assigned to each dot arrangement position of the multi-valued dither matrix M. That is, as shown in FIG.
At the dot arrangement position, the level of the image signal a is 0 to 27.
Is converted to a 4-bit signal of 0 to 15 and all are converted to 15 when the level is 28 or more. At the dot arrangement position, 0 when the level of the image signal a is 28 to 55.
Are converted to 4-bit signals of .about.15, all are converted to 0 when the level is 27 or less, and all are converted to 15 when the level is 56 or more. Below, other dot placement positions ~
The same conversion is performed for the above, and as a result, as shown in FIG. 7, all the image signals a input from the outside become signals converted into 4 bits (4 bit signals b). The multi-valued image data converted into 4 bits in this way is stored in the frame memory 9 as described above.

Next, the structure according to the present invention will be described in sequence. 8A to 8I show the γ characteristic in magenta for each matrix level of the multi-valued dither matrix M. As can be seen from FIG. 8, the shape of the γ characteristic differs depending on each matrix level. Therefore, in order to perform the optimum density correction in the multi-valued dither processing method, γ correction for each matrix level is required. However, in the conventional γ correction, as described in the above-mentioned conventional example, only one γ correction curve is made to correspond to all the pixels, and the optimum density correction cannot be performed.

Therefore, in this embodiment, the γ correction table 1
Multivalued dither matrix M for performing multivalued dither processing in 0
At least two density correction curves 14 (γ correction curves) corresponding to the γ characteristics of each dot of (1) are provided (claim 1
Corresponding to the described invention). The details will be described below. FIG. 9 shows the density correction curve 14 per matrix level.
(The vertical axis represents the density value, the horizontal axis represents the output level value), the density value is normalized to 0 to 255, and the output level value is in the range of 0 to 255. The output density γ here is set to 1. In this case, as in FIG. 5, the output level values (16
Value). Such a density correction curve 14 is 1
This is for determining the output level value for each matrix level, and in the actual magenta color, since the output is by a matrix of 3 × 3, nine density correction curves 14 are required. Individual density correction curves 14 are required. The output level value data thus determined is stored in the γ correction table 10.

FIG. 10 shows an image signal (4 bit signal: density value) b converted into 4 bits by multi-value dither processing,
An example of converting the 8-bit LD drive signal e (output level value) using the density correction curve 14 of the γ correction table 10 will be shown. Here, an example is shown in which the dot arrangement of the first matrix level and the dot arrangement of the second matrix level are converted. FIG. 11 collectively shows the two matrix levels. Therefore, in this way, a plurality (at least two) of the density correction curves 1 in the γ correction table 10
By including 4, the optimum density correction curve 14 corresponding to the γ characteristic of each dot of the multi-valued dither matrix M can be selected from these density correction curves 14 to correct the multi-valued image data. As a result, the optimum density correction can be performed on the multivalued image data.

In the present embodiment, the output level value data is 36 in total for the four colors. However, if there is no large difference in the γ characteristics of the multi-valued dither matrix M between each level and between each color, the output level is set. By sharing the value, the number of data of the output level value can be reduced. In this case, the density correction curve 14 of the γ correction table 10 may be a curve corresponding to at least the first matrix level of the multi-valued dither matrix M (corresponding to the invention of claim 3).
That is, for example, in FIG. 8, since there is a large difference between the γ characteristic of the first matrix level of (a) and the γ characteristic of the other matrix levels, the first matrix level of (a) The fifth matrix level (e) is selected as another matrix level, and γ correction is performed using these two γ correction data. Therefore, at least the first matrix level is selected as the matrix level of the multi-valued dither matrix M, and two density correction curves 14 corresponding to the first matrix level and the other matrix level (fifth matrix level) are thus selected. Using γ
By performing the correction, it is possible to improve the reproducibility in the highlight portion and thereby eliminate the pseudo contour. In FIG. 9, an example in which the output density γ is set to 1 has been described, but the output density γ can be arbitrarily determined by arbitrarily setting the interval of the density values.

Further, in this embodiment, the γ correction table 1
A density correction curve switching means (not shown) for switching the density correction curve 14 of 0 for each multi-valued dither matrix may be provided (corresponding to the invention of claim 2). As a result, even when the multi-valued dither processing means is exchanged, γ correction suitable for the combination of the respective matrices can be performed, whereby a good image can be created.

Next, a second embodiment of the present invention will be described with reference to FIGS. The description of the same parts as those in the first embodiment described above will be omitted, and the same reference numerals will be used for the same parts. In the configuration of the printer unit 3 of FIG. 1 described above, the multi-value dither processing and the γ correction are separate circuits (the multi-value dither processing circuit 6 and the γ correction table 1).
However, such processing may be performed simultaneously in one circuit. That is, in this embodiment, a multi-value dither processing / γ correction circuit (ROM) 15 for simultaneously performing the multi-value dither processing and the γ correction is provided in the preceding stage of the frame memory 10.
Was set up.

The multi-value dither processing / γ correction circuit 15 changes the values of the multi-value dither matrix for multi-value dither processing and the γ correction matrix to be referred to each time a pixel or a line advances, so that these multi-value dither Ternary counters 16 and 17 according to the sizes of the matrix and the γ correction matrix
Is provided. These ternary counters 16, 17
The pixel sync clock signal c and the line sync clock signal d generated by the count designation value from the personal computer 2 are used for counting. Then, the multi-value dither processing / γ correction circuit 15
The 8-bit image signal b that has been subjected to the multi-value dither processing and the density correction processing in (1) is temporarily stored in the frame memory 9 and then
The counters 18 and 19 which are counted by the main scanning clock signal f and the sub-scanning clock signal g for reading from the frame memory 9 are sent to the LD controller 13 to control the desired image output. be able to.

FIG. 13 shows a method of determining the output level value. In this case, the total number of output value determination matrices 20 is 256 from MO to M255. In a printer for four-color output, such an output value determination matrix 2
4 sets of 0 are required. The density correction curve 14 (γ correction curve) determined by the output determination method is recorded at each dot arrangement position (see FIG. 3) of these matrices.
The details will be described below. When the value of the input image signal a is 0 to 27, the dot at the dot arrangement position changes in 16 levels determined by the density correction curve 14 as shown in FIG. When it becomes above, the output level value takes 255. Further, when the value of the image signal a is 28 to 55, the dot at the dot arrangement position changes in 16 steps, and when it is 27 or less, it becomes 0, and when it is 56 or more, it becomes 255. Hereinafter, the output value determination matrix 20 is similarly created. Then, assuming that the value of the input image signal a is 54/255, the M54 matrix is selected from the output value determination matrix 20. As a result, the output level value of the pixel at the dot arrangement position becomes 255/255, the output level value of the pixel at the dot arrangement position becomes 241/255, and the output level values of the pixels at the other dot arrangement positions to 0/255. Become. Thus, even if the multi-valued dither processing and the γ correction are performed at the same time, it is possible to obtain the same effect as that of the first embodiment described above.

Next, a third embodiment of the present invention will be described with reference to FIG. The description of the same parts as those of the first and second embodiments described above will be omitted, and the same reference numerals will be used for the same parts. In the first and second embodiments described above, the multi-valued dither processing is performed by the printer unit 3, but the present invention is not limited to this, and may be performed by the personal computer 2 side. Therefore, in the present embodiment, on the personal computer 2 side as shown in FIG. 14A, a multi-value dither processing means for performing multi-value dither processing on the input image signal a using the multi-value dither matrix M. A multi-valued dither processing circuit 21 (ROM) is provided (claim 4).
Corresponding to the described invention). Further, an even dot latch circuit 22 and an odd dot latch circuit 23 are provided in the subsequent stage of the multi-value dither processing circuit 21 as a data combination transfer means for transferring a plurality of multi-value image data after multi-value dither processing in combination. (Corresponding to the invention of claim 5).

In such a configuration, the 8-bit image signal a and the matrix coordinate signal X of the X and Y coordinates are used.
a and Xb are input to the multilevel dither processing circuit 21. A multi-value dither matrix M for multi-value dither processing is set in the multi-value dither processing circuit 21, and the multi-value dither-processed image data is a 4-bit image signal h.
Becomes Of the 4-bit image signal h, the even-numbered image signal is latched by the even-numbered dot latch circuit 22, the odd-numbered image signal is latched by the odd-numbered dot latch circuit 23, and converted into the 8-bit image signal i. I
It is sent from the / F driver 24 to the printer unit 3.

FIG. 14B shows the configuration of the printer unit 3, in which the 8-bit image signal i sent from the personal computer 2 is an even dot latch circuit 25 and an odd dot latch circuit 26. Is divided into 4-bit image signals b and γ correction is performed by the γ correction table 10 using the density correction curve 14 as described above. In the γ correction table 10, the value of the γ correction matrix to be referred to changes each time a pixel or a line advances, so that the ternary counter 27 corresponding to the size of the multi-valued dither matrix,
28 are provided. These ternary counters 27, 2
8 is adapted to perform a counting operation by the pixel synchronization clock signal c and the line synchronization clock signal d generated by the designated count value from the personal computer 2. The 8-bit image signal b subjected to the density correction processing in the γ correction table 10 is temporarily stored in the frame memory 9, and then the main scanning clock signal f for reading from the frame memory 9 and the sub scanning clock signal f. Counters 29 and 3 which are counted by the signal g
It is sent to the LD controller 13 by 0, and thereby desired image output control can be performed.

As described above, the multi-valued dither processing can be performed on the personal computer 2 side.
It is not necessary to provide a circuit for performing multi-valued dither processing on the printer side as in the conventional case, and thus the configuration on the printer side can be simplified. Further, by transferring a plurality of multi-valued image data in combination, that is, by converting multi-valued dither-processed 4-bit multi-valued image data into 8-bits and transferring the data, the data transfer time can be shortened. it can.

Next, the overall structure of the color laser printer 1 having the printer section 3 will be schematically described with reference to FIG. The color laser printer 1 is a CMYK four-color printer that uses a 3 × 3 multivalued dither matrix M and performs multivalued dither processing with 16 values per pixel. Input image level value is 0 to 255 (8 bits) per color
And the output level value is 0 to 255 (8 bits). Hereinafter, the structure of the printer will be sequentially described. First, when the image signal a (color signal) output from the personal computer 2 is input to the printer unit 3, L
An LD (not shown) is drive-controlled by an LD drive signal e sent from the D controller 13. The laser beam generated from this LD is a polygon mirror 3 rotated by a drive motor 31 a in the laser writing unit 31.
1b is rotated and scanned, the optical path is bent by the reflection mirror 31e via the fθ lens 31c and the condenser lens 31d in order, and the circumference of the photoconductor belt 34, which has been previously neutralized by the lamp 32 and uniformly charged by the charger 33, is used. The surface is exposed to light to form an electrostatic latent image. Here, the image pattern to be exposed is a monochromatic image pattern when the desired full-color image is separated into yellow, magenta, cyan, and black. The electrostatic latent images thus formed are developed into yellow, magenta, cyan, and black by the rotary developing devices 35, 36, 37, and 38, and are visualized into a monochromatic image to form a monochromatic image. The monochromatic image formed on the photoconductor belt 34 is transferred onto the intermediate transfer belt 39 that rotates counterclockwise while contacting the photoconductor belt 34.
Yellow, magenta, formed on the photosensitive belt 34,
The cyan and black monochromatic images are sequentially superposed on the surface of the intermediate transfer belt 39. The images of yellow, magenta, cyan, and black superimposed on the intermediate transfer belt 39 are fed from the paper feed table 40 to the paper feed roller 41 and the registration roller 4.
A transfer roller 44 transfers the transfer paper 43 to the transfer paper 43 that has been conveyed through the transfer roller 2. After the transfer is completed, the transfer paper 43 is fixed by the fixing device 45, so that a good full-color image can be obtained.

In the above embodiments, the color laser printer 1 is used. However, any printer capable of multi-value output (16 values) of the minimum pixel in the multi-value dither method may be used. An inkjet printer or a thermal printer can be applied.

[0033]

According to the first aspect of the present invention, the multi-value dither processing is performed on the image signal input from the outside by using the multi-value dither matrix of the multi-value dither processing means, and the multi-value dither processing is performed. In an image forming apparatus that expresses a halftone by the obtained multi-valued image data, a γ correction table having at least two density correction curves corresponding to the γ characteristics of each dot of the multi-valued dither matrix that performs multi-valued dither processing is provided. Since it is provided, it is possible to select the density correction curve corresponding to the γ characteristic of each dot of the multi-valued dither matrix, and thus it is possible to perform the optimum density correction on the multi-valued image data.

According to a second aspect of the present invention, in the first aspect of the present invention, since the density correction curve switching means for switching the density correction curve of the γ correction table for each multivalued dither matrix is provided, the multivalued dither processing means is provided. Even when they are exchanged, it is possible to perform the γ correction suitable for the combination of the respective matrices, and thus it is possible to create a good image.

According to the invention of claim 3, in the invention of claim 1, the density correction curve of the γ correction table is a curve corresponding to at least the first matrix level of the multivalued dither matrix. It is possible to improve the reproducibility in the light section, which makes it possible to obtain a clear image without pseudo contours.

According to a fourth aspect of the present invention, there is provided a computer having a multilevel dither processing means for performing multilevel dither processing on the input image signal by using the multilevel dither matrix.
Since the γ correction table having at least two density correction curves corresponding to the γ characteristics of each dot of the multi-valued dither matrix is provided, the multi-valued dither processing can be performed on the computer side. As described above, it is not necessary to provide the image forming apparatus side with the multi-valued dither processing means, and the configuration of the image forming apparatus side can be simplified.

According to a fifth aspect of the invention, in the fourth aspect of the invention, the data combination transfer means for transferring a plurality of multivalued image data after multivalued dither processing in combination is provided, so that the data transfer time is shortened. In this way, the image creation work can be performed efficiently.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a printer unit in a color laser printer which is a first embodiment of the present invention.

FIG. 2 is a block diagram showing a schematic configuration of the present system.

FIG. 3 is a schematic diagram showing a dot arrangement order of a multi-valued dither matrix.

FIG. 4 is a schematic diagram showing each matrix level of a multi-valued dither matrix.

FIG. 5 shows an 8-bit image signal converted into 9 by multi-value dither processing.
It is a schematic diagram which shows a mode that it was divided.

FIG. 6 is a block diagram of an input image signal obtained by multi-value dither processing.
It is a schematic diagram which shows a mode that it converts into a bit signal.

FIG. 7 is a schematic diagram showing a matrix of a 4-bit conversion example by the multi-valued dither processing of FIG.

FIG. 8 is a characteristic diagram showing a γ characteristic for each matrix level in magenta.

FIG. 9 is a characteristic diagram showing how an output level value is determined by a density correction curve.

FIG. 10 is a schematic diagram showing how a 4-bit signal is converted into an LD drive signal by density correction processing.

FIG. 11 is a schematic diagram showing a matrix of 8-bit conversion examples by the density correction processing.

FIG. 12 is a block diagram showing a configuration of a printer unit that is a second embodiment of the present invention.

FIG. 13 is a schematic diagram showing a method of determining an output level value in a matrix form.

FIG. 14 shows a third embodiment of the present invention,
(A) is a block diagram showing the configuration on the computer side,
(B) is a block diagram showing a configuration on the printer side.

FIG. 15 is a vertical sectional side view showing the overall configuration of the image forming apparatus.

[Explanation of symbols]

2 computer 3 image forming apparatus 6 multi-valued dither processing means 10 γ correction table 14 density correction curve 15 multi-valued dither processing means (γ correction table) 21 multi-valued dither processing means 22, 23 data combination transfer means

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-189068 (JP, A) JP-A-61-150574 (JP, A) JP-A-62-206965 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) H04N 1/407 G06T 5/00 100

Claims (5)

(57) [Claims] 1. Multivalued dither processing is performed on an image signal input from the outside using a multivalued dither matrix of multivalued dither processing means, and multivalued image data obtained by this multivalued dither processing is used. In an image forming apparatus that expresses a halftone, an image characterized by including a γ correction table having at least two density correction curves corresponding to the γ characteristics of each dot of the multivalued dither matrix that performs the multivalued dither processing. Forming equipment. 2. The image forming apparatus according to claim 1, further comprising density correction curve switching means for switching the density correction curve of the γ correction table for each multi-valued dither matrix. 3. The image forming apparatus according to claim 1, wherein the density correction curve of the γ correction table is a curve corresponding to at least the first matrix level of the multivalued dither matrix. 4. A computer provided with a multivalued dither processing means for performing multivalued dither processing on an input image signal by using a multivalued dither matrix, and to correspond to γ characteristics of each dot of the multivalued dither matrix. An image forming system including an image forming apparatus having a γ correction table having at least two density correction curves. 5. The image forming system according to claim 4, further comprising data combination transfer means for transferring a plurality of multivalued image data after multivalued dither processing in combination.