JPH09200519A - Image processor and its method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To properly express a multi-gradation image obtained by a multi-value dither method with the PWM(pulse width modulation system) without deteriorating the image quality by selecting a dot generating method depending on a position of a pixel in a dither matrix. SOLUTION: Three figures to the left side of each of elements 1-18 in a dither matrix indicates the priority of dot generation in the matrix. The growing attribute L which is the growing direction of a pixel in each element indicates growing from the left end (left growing), the growing attribute C indicates growing from the center (center growing), and the growing attribute R indicates growing from the right (right growing). When a density of a noted pixel is zero, all elements are at a 0 level. As the density of a noted pixel increases, the element 1 changes with a prescribed density width as level 1, level 2, and level 3 in the center growing, then the element 2 changes as level 1-level 3 in the right growing. Furthermore, the element 3 grows in the left growing and the element 4 grows in the center growing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus and a method thereof, and more particularly to an image processing apparatus and a method thereof capable of forming a multi-gradation image by electrophotography.

[0002]

2. Description of the Related Art In recent years, image processing apparatuses such as printers and copying machines have been remarkably popularized, and accordingly, in terms of the performance of the image processing apparatuses, there has been a great improvement in the quality of output images. Also, as for the recording method, a device applying a large number of methods such as a silver salt method, a heat sensitive method, an electrophotographic method, an electrostatic recording method, and an inkjet method has been developed.

Further, in the image processing system, in consideration of the saving of the memory used in the image processing and the stability of the dot format, a method of binarizing the input multi-valued image data and outputting it is general. is there. 2 as one of the binarization methods
There is a value dither method.

However, with the progress of colorization of image processing apparatuses, it is necessary to express halftones and gradations in natural images such as photographs. In the above binary dither method, smooth and high quality output images are obtained. Couldn't get

As one of the methods for solving this problem, the multi-value dither method has been attracting attention. The multi-valued dither method is a dither method in which each square of a dither matrix has a plurality of threshold values, and therefore, as a result of the dither process, each pixel can have a plurality of values (levels).

In order to carry out this multi-valued dither method, naturally 1
A so-called multi-gradation recording method capable of recording three or more gradations per pixel is required. For example, in an electrophotographic laser beam printer (LBP), for example, pulse width modulation (PWM) (PW)
M) realizes this multi-tone recording method. Hereinafter, an example of PWM in LBP will be described with reference to FIG.

In the figure, the horizontal axis indicates the length of one pixel between the dotted lines. The vertical axis represents the analog voltage for each pixel and corresponds to the density level of "00h" to "ffh". In PWM, the laser drive signal is generated only while the analog voltage is higher than the triangular wave. Then, as shown in FIG. 13, the toner is deposited only on the portion of each pixel irradiated with the laser, and recording is performed. PWM
In this way, multi-gradation recording is realized in this way.

[0008]

By applying the multi-valued dither method as described above, pixels having an intermediate density appear. However, in the above-mentioned conventional PWM method, the dot area for each pixel grows from the center of the pixel. Therefore, when the result of the multi-valued dither processing is expressed by PWM, a gap is generated between the dots as shown in FIG. 14, for example. Therefore, the stability of dots is poor, and when pixels having the same gradation are vertically continuous, the gaps are continuously generated, resulting in vertical fine lines. As described above, the conventional image processing device has a problem that the image quality is deteriorated when the image obtained as a result of the multi-value dither is represented by the PWM method.

The present invention has been made to solve the above-mentioned problems, and it is possible to appropriately express a multi-gradation image obtained by the multi-value dither method by PWM without degrading the image quality. Image processing apparatus and method therefor.

[0010]

As one means for achieving the above object, the image processing apparatus of the present invention has the following configuration.

That is, input means for inputting a multivalued image signal and dither processing means for performing multivalued dither processing on the multivalued image signal based on a predetermined dither matrix to determine an output level for each pixel. And a pixel forming unit that forms each pixel with dots based on the output level, and a switching unit that switches a dot forming method in the pixel forming unit according to a pixel position in the dither matrix. To do.

For example, the switching means is characterized by switching the growth direction of dots with respect to the density value for each pixel.

For example, the dither matrix has a growth direction attribute for each pixel position, and the switching means switches the growth direction of dots according to the growth direction attribute.

For example, the switching means switches the dot between central growth in which the dots grow from the center of the pixel, left growth in which the dots grow from the left end of the pixel, and right growth in which the dots grow from the right end of the pixel. Characterize.

For example, the dither matrix has a growth direction attribute so that dots of adjacent pixels are in contact with each other.

For example, in the dither matrix, the pixels located on the center line of the matrix are center-grown, the pixels located on the left half of the matrix are right-grown, and the pixels located on the right half of the matrix are left-grown. It is characterized by having attributes.

For example, the dither matrix is changeable.

For example, the pixel forming means is characterized by forming pixels by pulse width modulation.

For example, the switching means switches the triangular wave generated during the pulse width modulation.

For example, the switching means may switch the triangular wave to a triangular wave when the pixel grows at the center, a sawtooth wave that rises to the right when the pixel grows to the right, and a sawtooth wave that descends to the right when the pixel grows to the right.

As one method for achieving the above-mentioned object, the image processing method of the present invention includes the following steps.

That is, an input step of inputting a multi-valued image signal and a dither processing step of performing multi-valued dither processing on the multi-valued image signal based on a predetermined dither matrix to determine an output level for each pixel. And a switching step of switching a dot formation method according to a pixel position in the dither matrix, and a pixel formation step of forming each pixel by a dot based on the output level by the formation method selected in the switching step. It is characterized by having.

For example, the switching step is characterized by switching the growth direction of dots with respect to the density value for each pixel.

For example, the dither matrix has a growth direction attribute for each pixel position, and the growth direction of dots is switched in accordance with the growth direction attribute in the switching step.

For example, in the switching step, the dot is switched between central growth in which the pixel grows from the center, left growth in which the pixel grows from the left end, and right growth in which the dot grows from the right end. Is characterized by.

For example, the dither matrix has a growth direction attribute so that dots of adjacent pixels are in contact with each other.

For example, in the dither matrix, the pixels located on the center line of the matrix are center-grown, the pixels located on the left half of the matrix are right-grown, and the pixels located on the right half of the matrix are left-grown. It is characterized by having attributes.

For example, the dither matrix is changeable.

For example, in the pixel forming step, the pixel is formed by pulse width modulation.

For example, in the switching step, the triangular wave generated in the pulse width modulation is switched.

For example, in the switching step, the triangular wave is switched to a triangular wave when the pixel grows at the center, a sawtooth wave that rises to the right when the pixel grows to the right, and a sawtooth wave that descends to the right when the pixel grows to the left.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment according to the present invention will be described below in detail with reference to the drawings.

<First Embodiment> In the present embodiment,
In particular, an image processing apparatus using color electrophotographic technology will be described as an example. Further, in this embodiment, M (magenta), C (cyan), Y (yellow), and B are used as input image signals.
An example will be described in which an 8-bit image signal of K (black) is input in a frame-sequential manner and a 2-bit depth multi-value dither process using a 4 × 4 matrix is performed on the input image signal.

FIG. 1 shows the color LBP in this embodiment.
The structure of (laser beam printer) is shown. In FIG. 1, the transfer sheet P fed from the sheet feeding unit 101 is conveyed by the conveyance path 10.
The front end of the transfer drum 103 is held by the gripper 103f via the gripper 2 and is held on the outer periphery of the transfer drum 103. The latent image formed on the photosensitive drum 100 for each color by the optical unit 107 is developed by each color developing device Dy, Dc, Db, Dm and transferred to the paper 102 on the outer periphery of the transfer drum 103 a plurality of times. , A multicolor image is formed. After that, the transfer paper P is separated from the transfer drum 103 by the separation claw 113, fixed by the fixing unit 104, and ejected to the paper ejection tray 115.

The developing devices Dy, Dc, Db, Dm for the respective colors are
Each end has a rotation axis, and each is held by the developing device selection mechanism unit 108 so as to be rotatable about the axis. As shown in FIG. 1, each color developing device Dy, Dc, Db, Dm is rotated for selecting a developing device while keeping its posture constant. After the selected developing device moves to the developing position, the developing device selection mechanism unit 108 is integrated with the selection mechanism holding frame 109, and the moving position is determined by the solenoid 109a around the fulcrum 109b in the direction of the photosensitive drum 100.

Next, the operation of the color LBP having the above configuration will be specifically described. First, the photosensitive drum 100 is uniformly charged to a predetermined polarity by the charger 111, and the latent image of, for example, M (magenta) color is formed on the photosensitive drum 100 by exposure with the laser beam light L emitted from the laser driving unit 106. Is developed, and M appears on the photosensitive drum 100.
A (magenta) color first toner image is formed. on the other hand,
The transfer paper P is fed at a predetermined timing, a transfer bias voltage (for example, +1.8 kV) having the opposite polarity (for example, positive polarity) to the toner is applied to the transfer drum 103, and the first toner image on the photosensitive drum 100 is formed. The transfer paper P is electrostatically attracted to the surface of the transfer drum 103 while being transferred to the transfer paper P. After that, the M (magenta) color toner remaining on the photosensitive drum 100 is removed by the cleaner 112, and the photosensitive drum 100 is prepared for the latent image formation and development process of the next color.

Next, a second latent image of C (cyan) color is formed on the photosensitive drum 100 by the laser beam L, and then a C (cyan) color developing device Dc is used to form a second latent image on the photosensitive drum 100. The second latent image is developed to form a second toner image of C (cyan) color. Then, the second toner image of C (cyan) color is transferred to the transfer paper P in alignment with the position of the first toner image of M (magenta) color that was previously transferred to the transfer paper P. In transferring the toner image of the second color, immediately before the transfer paper reaches the transfer portion, the transfer drum 1
A bias voltage (for example, +2.1 kV) higher than that at the time of transferring the first toner image is applied to 03. Similarly, Y
Third and fourth latent images of (yellow) color and K (black) color are sequentially formed on the photoconductor drum 100, and are sequentially developed by the developing devices Dy and Db, and are first transferred to the transfer paper P. Y (yellow) aligned with the toner image
The third and fourth toner images of color K and black (black) are sequentially transferred. As described above, 4
The color toner images are formed in an overlapping state.
Also in the transfer of the toner images of the third and fourth colors, a bias voltage higher than that at the time of transferring the second toner image to the transfer drum 103 immediately before the transfer paper reaches the transfer portion (for example, +2.5 kV, +3. 0 kV) is applied.

The reason why the transfer bias voltage is increased each time the toner image of each color is transferred is to prevent the transfer efficiency from being lowered. The main cause of the decrease in transfer efficiency is that, when the transfer paper separates from the photosensitive drum 100 after the transfer, the surface of the transfer paper is charged to a polarity opposite to the transfer bias voltage by air discharge (the transfer drum carrying the transfer paper). (The surface is also slightly charged.) Since the charged charges are accumulated for each transfer, the transfer electric field is reduced every transfer when the transfer bias voltage is constant.

In the present embodiment, during the transfer of the fourth color, when the leading edge of the transfer paper reaches the transfer start position (including immediately before and after that), an AC voltage of 5.5 kV (effective value, the same applies hereinafter). The frequency is 500 Hz, and a DC bias voltage (+3.0 kV) having the same polarity and the same potential as the transfer bias applied during the transfer of the fourth toner image is superimposed and applied to the charger 111. The photosensitive drum 100 is discharged. In the transfer of the fourth color, the charger 111 is operated when the leading edge of the transfer paper reaches the transfer start position in order to prevent uneven transfer. In particular, in the transfer of a full-color image, even if slight transfer unevenness occurs, it is conspicuous as a difference in color. Therefore, it is necessary to apply a required bias voltage to the charger 111 to perform the discharge operation as described above. Become.

After that, when the leading end of the transfer paper P on which the four color toner images have been superposed and transferred approaches the separation position, the separation claw 113 is formed.
Approach, the leading end thereof comes into contact with the surface of the transfer drum 103, and the transfer paper P is separated from the transfer drum 103. The tip of the separation claw 113 keeps contact with the transfer drum surface,
After that, it separates from the transfer drum 103 and returns to the original position. As described above, from the time when the leading end of the transfer paper reaches the transfer start position of the final color, the charger 111 moves the rear end of the transfer paper
3 is operated until the transfer sheet 3 is separated, the charge accumulated on the transfer paper (the polarity opposite to the toner) is removed, the separation of the transfer paper by the separation claw 113 is facilitated, and the air discharge at the time of separation is reduced. The trailing edge of the transfer paper is the transfer end position (photosensitive drum 10
0, the transfer bias voltage applied to the transfer drum 103 is turned off (ground potential) when it reaches the nip formed by the transfer drum 103. At the same time, the bias voltage applied to the charger 111 is also turned off.

The separated transfer paper P is fixed to the fixing device 10.
4, where the toner image on the transfer paper is fixed and discharged onto the discharge tray 115. The above is the printing process in the color LBP used in this embodiment.

An image processing unit 130 is input to the laser driving unit 106 by applying dither processing and PWM to a multi-valued image signal input from an external device such as a host computer (not shown). Generate a drive signal. Reference numeral 105 denotes an operation unit, which includes an operation panel such as an LCD, LEDs and the like, and various keys and the like for inputting commands from the operator and notifying the state of the apparatus.

Next, FIG. 2 shows a detailed configuration of the image processing unit 130 described above. In FIG. 2, reference numeral 201 denotes a dither processing unit, which performs multi-bit dither processing of 2-bit depth on the input 8-bit multi-value image data of each color. Reference numeral 202 is an image buffer, and 203 is a level conversion unit, which converts the level determined by the dither processing unit 201 into a predetermined level in the output gradation. A PWM unit 204 performs pulse width modulation on the finally obtained multi-valued image signal to generate and output a laser drive signal to the laser drive unit 106. Further, 300 is a CPU, and the image processing unit 13
The operation of the whole 0 is comprehensively controlled based on the control program stored in the ROM 301. A RAM 302 is used as a work area of the CPU 300.

Here, FIG. 3 shows an example of a dither matrix for realizing the 4-gradation dither used in the dither processing section 201. In the matrix of FIG. 3, the numbers on the upper right of each cell indicate the cell number, and cells 1 to 18 exist.
Also, the three numbers on the left side of each square indicate the priority of dot formation in the matrix. The smaller the number indicating the priority is, that is, the higher the priority is, the lower the dither threshold is set. Therefore, three thresholds are stored for each square. Hereinafter, Th1,
Th2, ... Th54. Further, “L”, “C”, and “R” shown in the lower right of each square are growth attributes indicating the growth direction of the pixels of the square, and “L” is grown from the left end (hereinafter, left growth). , “C” means growing from the center (center growing), and “R” means growing from the right end (right growing). Further, in the present embodiment, the values of 4 gradations that can be taken by each pixel are set to “level 0”, “level 1”,
These are "Level 2" and "Level 3".

Below, the result of multi-value dither processing by such a dither matrix for the input image signal is
This will be described with reference to FIG. For example, if the density value of the pixel of interest is "0", all the cells of the dither matrix are "level 0", but as the density of the pixel of interest increases, the cell 1 grows to "level 1" and "level 1." 2 "," level 3 "and change in a predetermined density range (Fig. 4a), and then the mass 2
Changes from "level 1" to "level 3" with right growth (Fig. 4b). Subsequently, the mass 3 grows by left growth (FIG. 4c),
The mass 4 grows by central growth (FIG. 4d). Subsequent cells also change in the same manner, and all cells become "level 3" at the maximum density.

This dither matrix is held in the ROM 301 in the form of a table as shown in FIG. 5, for example. Of course, the format in which the dither matrix is held is not limited to the example shown in FIG.

FIG. 6 shows a detailed configuration of the dither processing unit 201 for realizing the above algorithm.

In FIG. 6, the input 8-bit data is input to each of the 18 mass processing units 601 to 618.
Here, for example, a detailed configuration of the mass 1 processing unit 601 is shown in FIG. In the mass 1 processing unit 601, first, the ROM 70
4 to Th1, Th2, and Th3 values are 701-
Each of the comparators 703 to 703 inputs the input 8-bit data to Th1 and Th, respectively.
2, Th3, and the result is input to the level generation unit 705. The level generation unit 705 generates and outputs a 2-bit signal indicating any of the above-mentioned "level 0" to "level 3" according to the comparison result. The mass 2 processing units 602 to 18 are also the mass 1 processing units 601.
It has the same configuration as.

Thus, in FIG. 6, each mass processing unit 6
All the 36-bit level signals output from 01 to 618 are input to the selection unit 605. On the other hand, in the counter 604 that counts the number of pixels of the input image data in the main scanning direction and the sub scanning direction, a signal indicating the pixel position of the current input data is sent to the selection unit 605. Selection unit 605
Then, by referring to the pixel position signal, the corresponding one (2 bits) is selected from the level signals sent from the mass processing units 601 to 618, and is output as the level signal 608.

At this time, the LCR signal 609 is simultaneously output from the selection unit 605. The LCR signal 609 is “C” when the selection unit 605 selects the signal of the cell 1 and “R” when the signal of the cell 2 is selected.
"L", "C", set for each square as shown in
A 2-bit signal indicating any value of "R" is output.
As described above, in the dither processing unit 201, the mass is converted to an appropriate level (any of levels 0 to 3) based on the input 8-bit data, and the 2-bit signal (level signal) indicating the level is converted. ) 608 and a 2-bit signal 609 indicating the dot growth direction of the square.

The 2-bit level signal 608 output from the dither processing unit 201 as described above is temporarily held in the image buffer 202 and input to the level conversion unit 203 at an appropriate timing in synchronization with the image clock. . Then, in the level conversion unit 203, the dither processing unit 20
The level signal (2 bits) output from 1 is converted into a level (8 bits) signal corresponding to the laser drive signal for actually forming an image as described below.

Level 0 → 00h Level 1 → a Level 2 → b Level 3 → ffh where a and b are predetermined 8-bit values of 00h <a <b <ffh obtained from the output density characteristics at the time of image formation. is there.

That is, the level conversion unit 203 converts the 2-bit signal indicating the level sent from the dither processing unit 201 into the 8-bit signal indicating the actual output level value.

Then, the converted 8-bit signal is PWM
It is output to the section 204. The PW is shown below in the block diagram of FIG.
A detailed configuration of the M unit 204 will be shown and described.

In FIG. 8, 805 is a latch, 806 is a D / A converter, 807 is an adder, 808 is a triangular wave generator, 809 is a selector, and 811 is an inverter.

In the triangular wave generating section 808, the L triangular wave generating section 808L, the C triangular wave generating section 808C, and the R triangular wave generating section 808R respectively provide three types of triangular waves indicated by 901a to 901c in FIGS. Generate. That is, 901a and 901c are sawtooth waves. Or later,
901a shown in (a) of FIG. 9 is an L triangular wave, and (b) of FIG.
901b shown in FIG. 9 is a C triangular wave, and 901b shown in FIG.
c is called an R triangular wave. In FIGS. 9A to 9C, the distance between the vertical lines on the horizontal axis corresponds to the length of one pixel. In addition, the vertical axis represents the analog voltage for each pixel, from "00h"
It corresponds to the density level of "ffh".

Here, the characteristics of each triangular wave will be described with reference to FIG. In FIG. 9, 902 is a voltage corresponding to the input density. The input concentration value is
It is converted into four values by the dither processing unit 201 in the previous stage, and “00h”, “a”, “b”,
Since the density level has been converted to one of "ffh", the voltage 902 is "00h", "a", "b", "ff".
A value corresponding to any one of the density levels of “h” is taken, but here, the case of corresponding to the density level “b” will be described. In the PWM unit 204, the voltage of the laser is 9
02 is irradiated only for a period higher than each of the triangular waves 901a to 901c. Therefore, since the toner is recorded and recorded only on the portion irradiated with the laser for each pixel, (a) in FIG.
In the cases of to (c), only the portions indicated by 903a to 903c are recorded. That is, in the case of the L triangular wave shown in FIG. 9A, dots grow from the left side of the pixel, and in the case of the C triangular wave shown in FIG. 9B, dots grow from the center of the pixel, and In the case of the R triangular wave shown in c), it can be seen that dots grow from the right side of the pixel.

In FIG. 8, the input 8-bit image signal is first synchronized with the rising edge of the image clock PCLK by the latch circuit 805, converted into an analog voltage by the D / A converter 806, and then the analog comparator 8
07.

On the other hand, in the triangular wave generator 808, the above-mentioned L triangular wave and C are synchronized with the image clock PCLK.
A triangular wave and an R triangular wave are generated and input to the selector 809. The selector 809 selects any one of the L triangular wave, the C triangular wave, and the R triangular wave according to the LCR signal 810 input from the dither processing unit 201, and inputs them to the analog comparator 807.

In the analog comparator 807,
By comparing the two signals of the analog voltage and the triangular wave, a pulse width modulated signal is output. And
By inverting the signal with the inverter 811, the PWM signal input to the laser driving unit 106 is obtained.

The PWM signal obtained as described above drives the laser in the laser driving unit 106, and a multi-tone image is recorded on the transfer paper P by the above-described process.

The process in the image processing unit 130 described above is sequentially performed for each color of M, C, Y and K.

As described above, according to this embodiment, when PWM is applied to a pixel after multi-value dither processing, the growth direction of the pixel according to the density value is changed according to the pixel position in the dither matrix. By classifying into left-growth, center-growth, and right-growth, a halftone image can be stably expressed, and a higher quality output image can be obtained.

<Second Embodiment> The second embodiment of the present invention will be described below.
An embodiment will be described. Since the device configuration in the second embodiment is the same as that in the above-described first embodiment, the description thereof will be omitted.

In the second embodiment, the setting of the dither matrix is different from that of the first embodiment described above. FIG. 10 shows an example of the dither matrix used in the second embodiment. In the matrix of FIG. 10, as in FIG. 3, the numbers in the upper right of each cell indicate the cell number, and cells 1 to 18
Exists. Also, the three numbers on the left side of each square indicate the priority of dot formation in the matrix. The smaller the number indicating the priority is, that is, the higher the priority is, the lower the dither threshold is set. Therefore, three thresholds are stored for each square. Hereinafter, the thresholds are set to Th1, Th2, ... Th54 in ascending order. Further, “L”, “C”, and “R” shown in the lower right of each cell indicate the growth direction of the pixel of the cell, and “L” indicates growth from the left end (hereinafter, left growth), “C”. Grows from the center (central growth),
“R” means growing from the right end (right growth).
In addition, the values of 4 gradations that can be taken by each pixel are set in ascending order of “level 0”, “level 1”, “level 2”, and “level 3”.
And

Below, the result of multi-valued dither processing by such a dither matrix for the input image signal is
This will be described with reference to FIG. For example, if the density value of the target pixel is “0”, all the cells of the dither matrix are “level 0”, but as the density of the target pixel increases,
First, the pixels of the cells 1 and 2 grow in order from the center (FIG. 11a). Then, when the cells 1 and 2 grow to "level 3," the cells 3, 4, 5, 6, 7, and 8 are next grown, the cells 3 and 5 are grown to the right, the cells 4 and 6 are grown to the left, and the cells 7, 8 is central growth and grows sequentially (FIG. 11b). When all of the above 6 cells become "Level 3", then the cells 9, 10,
11 and 12, the cells 9 and 11 grow to the right, and the cells 10 and 12
Grows leftward sequentially (FIG. 11c). And finally,
Squares 13, 14, 15, 16, 17, and 18 are squares 1
3 and 15 are right growth, squares 14 and 16 are left growth, square 1
Nos. 7 and 18 are sequentially grown by central growth, and all the cells reach "level 3" at the maximum concentration.

This dither matrix is held in the RAM 302 in the form of a table as shown in FIG. 12, for example, and can be appropriately changed as desired by the operator.

As described above, in the second embodiment, by setting the dither matrix in a well-balanced manner, halftone pixels generated by multi-valued dither can be represented in a well-balanced manner. Further, by appropriately setting the dither matrix according to the image to be formed, an image in which halftones are flexibly expressed can be obtained.

Needless to say, even if the matrix size or the number of gradations per pixel is changed, an appropriate dither matrix can be set as in the above embodiment.

Even if the present invention is applied to a system composed of a plurality of devices (eg, host computer, interface device, reader, printer, etc.), a device composed of one device (eg, copying machine or facsimile machine). etc)
May be applied to. Therefore, the image data may not be inputted only from the external device, but the image processing device itself may be provided with a scanner or the like, and the document image may be inputted from the scanner.

Further, an object of the present invention is to supply a storage medium having a program code of software for realizing the functions of the above-described embodiments to a system or apparatus, and to supply a computer (or CPU or MPU of the system or apparatus).
) Can be achieved by reading and executing the program code stored in the storage medium.

In this case, the program code itself read from the storage medium realizes the function of the above-described embodiment, and the storage medium storing the program code constitutes the present invention.

As a storage medium for supplying the program code, for example, a floppy disk, hard disk, optical disk, magneto-optical disk, CD-ROM, C
DR, magnetic tape, nonvolatile memory card, ROM
Etc. can be used.

Further, by executing the program code read by the computer, not only the functions of the above-described embodiment are realized, but also the OS or the like running on the computer is actually executed based on the instruction of the program code. It goes without saying that a case where a part or all of the processing of (1) is performed and the functions of the above-described embodiments are realized by the processing is also included.

Furthermore, after the program code read from the storage medium is written in the memory provided in the function expansion board inserted into the computer or the function expansion unit connected to the computer, based on the instruction of the program code, It goes without saying that a case where the CPU or the like included in the function expansion board or the function expansion unit performs a part or all of the actual processing and the functions of the above-described embodiments are realized by the processing is also included.

[0076]

As described above, according to the present invention, when pulse width modulation is applied to a pixel after multi-value dither processing,
By classifying the pixel growth direction according to the density value into left growth, center growth, and right growth according to the pixel position in the dither matrix, it is possible to stably express halftone images and A high quality output image can be obtained.

Further, by appropriately changing the setting of the dither matrix, halftone pixels generated by multi-value dither can be expressed in a well-balanced manner, and halftones can be flexibly expressed according to an image to be formed. It will be possible.

[0078]

[Brief description of drawings]

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

FIG. 2 is a diagram showing a detailed configuration of an image processing unit in the present embodiment.

FIG. 3 is a diagram showing an example of a dither matrix in this embodiment.

FIG. 4 is a diagram showing a state of pixel growth in the present embodiment.

FIG. 5 is a diagram showing an example of holding a dither matrix in the present embodiment.

FIG. 6 is a block diagram showing a detailed configuration of a dither processing unit in this embodiment.

FIG. 7 is a block diagram showing a detailed configuration of a mass 1 processing unit in the present embodiment.

FIG. 8 is a block diagram showing a detailed configuration of a PWM unit in the present embodiment.

FIG. 9 is a diagram showing a triangular wave generated in the present embodiment.

FIG. 10 is a diagram showing an example of a dither matrix in the second embodiment according to the present invention.

FIG. 11 is a diagram showing a state of pixel growth in the second embodiment.

FIG. 12 is a diagram showing an example of holding a dither matrix in the second embodiment.

FIG. 13 is a diagram showing the principle of PWM in a conventional image processing apparatus.

FIG. 14 is a diagram showing a state of pixel growth in conventional PWM.

[Explanation of symbols]

 130 image processing unit 201 dither processing unit 202 image buffer 203 level conversion unit 204 PWM unit 300 CPU 301 ROM 302 RAM

Claims (20)

[Claims] 1. An input means for inputting a multi-valued image signal, and a dither processing means for performing multi-valued dither processing on the multi-valued image signal based on a predetermined dither matrix to determine an output level for each pixel. And a pixel forming unit that forms each pixel with dots based on the output level, and a switching unit that switches a dot forming method in the pixel forming unit according to a pixel position in the dither matrix. Image processing device. 2. The image processing apparatus according to claim 1, wherein the switching unit switches a dot growth direction with respect to a density value for each pixel. 3. The dither matrix has a growth direction attribute for each pixel position, and the switching means switches the growth direction of dots according to the growth direction attribute. Image processing device. 4. The switching means switches between a center growth in which a dot grows from a pixel center, a left growth that grows from a left end of a pixel, and a right growth that grows from a right end of a pixel. The image processing apparatus according to claim 3, characterized in that 5. The image processing apparatus according to claim 4, wherein the dither matrix has a growth direction attribute so that dots of adjacent pixels are in contact with each other. 6. In the dither matrix, the pixels located on the center line of the matrix are center-grown, the pixels located on the left half of the matrix are right-grown, and the pixels located on the right half of the matrix are left-grown. The image processing apparatus according to claim 4, wherein the image processing apparatus has an attribute. 7. The image processing apparatus according to claim 1, wherein the dither matrix is changeable. 8. The image processing apparatus according to claim 4, wherein the pixel forming unit forms pixels by pulse width modulation. 9. The image processing apparatus according to claim 8, wherein the switching unit switches a triangular wave generated during the pulse width modulation. 10. The switching means outputs the triangular wave,
10. The image processing apparatus according to claim 9, wherein the pixel is switched to a triangular wave when growing at the center, a sawtooth wave that rises to the right when growing to the right, and a sawtooth wave that descends to the right when growing to the left. 11. An input step of inputting a multi-valued image signal, and a dither processing step of performing multi-valued dither processing on the multi-valued image signal based on a predetermined dither matrix to determine an output level for each pixel. And a switching step of switching the dot forming method according to the pixel position in the dither matrix, and a forming method selected in the switching step,
A pixel forming step of forming each pixel by dots based on the output level. 12. The image processing method according to claim 11, wherein in the switching step, a dot growth direction with respect to a density value is switched for each pixel. 13. The dither matrix has a growth direction attribute for each pixel position, and in the switching step, the dot growth direction is switched according to the growth direction attribute. The described image processing method. 14. In the switching step, the dots are switched between central growth that grows from the center of the pixel, left growth that grows from the left end of the pixel, and right growth that grows from the right end of the pixel. Claim 1 characterized by the above-mentioned.
3. The image processing method according to 3. 15. The image processing method according to claim 14, wherein the dither matrix has a growth direction attribute so that dots of adjacent pixels are in contact with each other. 16. In the dither matrix, the pixels located on the center line of the matrix are center-grown, the pixels located on the left half of the matrix are right-grown, and the pixels located on the right half of the matrix are left-grown. 15. The image processing method according to claim 14, which has an attribute. 17. The image processing method according to claim 11, wherein the dither matrix is changeable. 18. The image processing method according to claim 14, wherein in the pixel forming step, pixels are formed by pulse width modulation. 19. The image processing method according to claim 18, wherein in the switching step, a triangular wave generated in the pulse width modulation is switched. 20. In the switching step, the triangular wave is switched to a triangular wave when the pixel grows in the center, a sawtooth wave that rises to the right when the pixel grows to the right, and a sawtooth wave that descends to the right when the pixel grows to the left. Image processing method.