JPH0993436A - Image processing apparatus and method - Google Patents

Abstract

(57) An object of the present invention is to provide an image processing device and a method thereof that can obtain a good color image output by applying a multi-valued dither method. SOLUTION: When the power is turned on, a density measuring section 210 forms a predetermined patch image from a patch memory 208 on a transfer drum, measures the patch density, and obtains an intermediate level value (representative level value) of a predetermined number of prints. It is stored in the representative density memory 209. During normal image formation, the multi-valued image data is converted by the dither processing unit 201 into one of a predetermined number of levels by dithering with a predetermined bit depth, and the level converting unit 203 stores the level in the representative density memory 209. The image is formed by converting it into a representative level value.

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, for example, an image processing apparatus and a method thereof for forming a multi-tone color image.

[0002]

2. Description of the Related Art Conventionally, in an image processing apparatus such as a printer for forming a multi-valued image, it is general to reduce the data amount by binarizing the input multi-valued image data. As a result, the effect of reducing the capacity of the image memory, simplifying the apparatus configuration, stabilizing the dot formation, improving the quality of the formed image, and increasing the processing speed can be obtained.

As a method suitable for binarizing input multi-valued data, a binary dither method is known which is capable of preserving grayscale information while binarizing. In the binary dither method, a grayscale image is reproduced by changing the binarization threshold value for each square in a dither matrix in which one square corresponds to one pixel.

However, the recent development of image processing apparatuses is remarkable, and the demand for color image processing is increasing. In such a situation, when forming and outputting a halftone image or a gradation image of a natural image such as a photograph, it has been impossible to obtain a smooth and high-quality image by the conventional binary dither method.

As a method for properly reproducing such a color image, the multi-valued dither method has attracted attention. The multi-valued dither method is a method in which each square of the dither matrix has a plurality of threshold values and a plurality of possible values (levels) are set. Therefore, in the image processing apparatus to which the multi-valued dither method is applied, since the multi-valued data such as the input color photographic image can be converted into the multi-valued image data having a smaller number of levels, high-quality reproduction is possible. It will be possible.

[0006]

However, in the image processing apparatus to which the above-mentioned conventional multi-valued dither method is applied,
When high-resolution image output is performed, the intermediate level density in dither changes due to the environment due to instability of dot formation. Therefore, a pixel having a density different from the density intended by the operator is formed, which causes a problem that a stable grayscale image cannot be formed. This problem was particularly noticeable in color images.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an image processing apparatus and a method thereof that can obtain a good color image output by applying a multi-valued dither method. .

[0008]

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

That is, an image processing apparatus capable of forming a multi-valued image on a recording medium, a measuring means for forming a multi-valued sample patch image and measuring the patch density, and the measured patch density Holding means for calculating and holding a predetermined number of intermediate level values from the above, dither processing means for performing multi-value dither processing on the input multi-valued image data to determine the level, and dither processing means. It is characterized by further comprising level conversion means for converting the level to an intermediate level value held by the holding means, and forming means for forming an image based on the level value converted by the level conversion means.

For example, the patch density is measured by the measuring means at a predetermined timing, and a predetermined number of intermediate level values are calculated and held by the holding means.

For example, the predetermined timing is when the power of the apparatus is turned on.

For example, the forming means forms a color image, the measuring means forms a multi-valued sample patch image for each color and measures the patch density for each color, and the holding means for each color. It is characterized in that a predetermined number of intermediate level values are calculated from the patch densities measured in 1 above and are held for each color.

For example, the dither processing means determines the multi-valued image data to be one of four levels.

An image processing apparatus capable of forming a multi-valued image on a recording medium, the measuring means forming a multi-valued sample patch image and measuring the patch density, and the measured patch density. Holding means for calculating and holding a predetermined number of intermediate level values from the first and second multi-valued dither processing for the input multi-valued image data to determine one of the first level numbers. Dither processing means, second dither processing means for performing multi-value dither processing on the input multi-valued image data to determine one of the second level numbers, and the first dither processing means. Selecting means for selecting one of the level determined by the second dither processing means and the level determined by the second dither processing means, and converting the level selected by the selecting means into an intermediate level value held in the holding means. And level conversion means, and having a forming means for forming an image based on the converted level value by said level conversion means.

For example, the forming means forms a color image, the measuring means forms a multi-valued sample patch image for each color and measures the patch density for each color, and the holding means for each color. It is characterized in that a predetermined number of intermediate level values are calculated from the patch densities measured in 1 above and are held for each color.

For example, the holding means changes the number of intermediate levels to be held for each color to a number corresponding to the first level number or the second level number according to the distribution of the measured patch density. It is characterized in that the identification information of the intermediate level number is held at the same time.

For example, the selection means may select the level determined by the first dither processing means based on the intermediate level number identification information held in the holding means and the second level.
It is characterized in that any one of the levels determined by the dither processing means is selected for each color.

For example, the second level number is larger than the first level number.

For example, the first level number is 4,
The second level number is eight.

For example, the forming means forms a color image with magenta, cyan, yellow and black,
The first dither processing means determines the level for the multivalued image data of yellow, and the second dither processing means determines the level for the multivalued image data of other colors. .

For example, the patch density is measured by the measuring means at a predetermined timing, and a predetermined number of intermediate level values are calculated and held by the holding means.

For example, the predetermined timing is when the power of the apparatus is turned on.

Further, as one method for achieving the above-mentioned object, the image processing method of the present invention comprises the following steps.

That is, an image processing method in an image processing apparatus capable of forming a multivalued image on a recording medium, wherein a multivalued sample patch image is formed at a predetermined timing, the patch density is measured, and the patch density is measured. A predetermined number of intermediate level values are calculated from the measured patch densities and stored in advance, multilevel dither processing is applied to the input multilevel image data to determine the levels, and the determined levels are It is characterized in that it is converted into an intermediate level value held in advance and an image is formed based on the converted level value.

Further, in the image processing method in the image processing apparatus capable of forming a multi-valued image on a recording medium, a multi-valued sample patch image is formed at a predetermined timing, and the patch density is measured. A predetermined number of intermediate level values are calculated from the measured patch densities and stored in advance, and multi-value dither processing is applied to the input multi-value image data to obtain the first value.
Of any of the levels, the multilevel dither processing is performed on the input multivalued image data to determine any of the second levels, and the first dither processing means. Is selected or the level determined by the second dither processing means is selected, the selected level is converted into a pre-stored intermediate level value, and based on the converted level value. And an image is formed.

[0026]

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.

Color LBP FIG. 1 shows the configuration of a color LBP (laser beam printer) in this embodiment. In FIG. 1, the paper feeding unit 101
The transfer paper P fed from the transfer drum 1 is held at its front end by the gripper 103f via the conveying path 102, and the transfer drum 1
03 is held on the outer periphery. 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 discharged to the paper discharge tray 106 via the paper discharge unit 105, or the paper discharge tray 115.
Is discharged.

The developing devices Dy, Dc, Db and 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 structure will be specifically described. First, the photoconductor drum 100 is uniformly charged to a predetermined polarity by the charger 111, and a latent image of, for example, M (magenta) color is developed on the photoconductor drum 100 by exposure with the laser beam light L, and the photoconductor drum 100. A first toner image of M (magenta) color is formed on the upper surface. On the other hand, the transfer paper P is fed at a predetermined timing, a transfer bias voltage (for example, +1.8 kV) having a polarity (for example, positive polarity) opposite to that of the toner is applied to the transfer drum 103, and the first toner on the photoconductor drum 100 is applied. The image is transferred to the transfer paper P, and the transfer paper P is transferred to the transfer drum 103.
Is electrostatically adsorbed on the surface of. 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.

Then, 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.

A density sensor 50, which will be described in detail later, can measure the density of the toner image formed on the transfer drum 103.

Signal Processing Unit Next, a block configuration of the signal processing unit for generating a drive signal to be sent to the laser drive unit in the optical unit 107 based on the image data input in this embodiment is shown in FIG.
Shown in

In FIG. 2, reference numeral 201 denotes a dither processing unit, which performs multi-value dither processing of 4 × 4 2-bit depth, which will be described later, on 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 number determined by the dither processing unit 201 into a predetermined level in the output gradation based on the representative density level input from the density signal generation unit 204. . A density signal generator 204 outputs a corresponding representative density level from an internal representative density memory 209 in accordance with a color designation signal 205 indicating the type of color plane of image data currently being processed. A PWM unit 206 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. Reference numeral 210 denotes a density measuring unit, which generates data of a representative density memory 209 including a RAM by measuring an actually formed patch. Internal patch memory 20
Reference numeral 8 is composed of a ROM or the like and holds data for four colors for forming a sample patch for density measurement.
Reference numeral 300 denotes a CPU, which comprehensively controls the operation of the entire signal processing unit based on a control program stored in the ROM 301. 302 is a RAM, which is the CPU 300
Used as a work area for.

Density Measuring Unit 210 The density measuring unit 210 will be described in detail below with reference to FIGS. 3 to 7.

In the present embodiment, as described above, the 2-bit depth multivalued dither processing is performed. That is, the input 256-level (8-bit) multi-valued image data is represented by four representative density levels by the dither conversion. In this embodiment, the four levels are converted into the minimum density value, the maximum density value, and two density values in between.
The density measuring unit 210 stores these two density values in the typical density memory 209 as the typical density level data.

The method of creating the representative density level data in this embodiment will be described below.

3A to 3D show the patch memory 2
FIG. 10 is a diagram showing M, C, Y, and K color patches held in 08. For example, in FIG. 3A, “M1”, “M2”, and “M6” indicate magenta patches for the entire image area, and although they are omitted midway,
Six patches of M6 are sequentially arranged at a predetermined interval.
Similarly, in each of (b) to (d) of FIG.
.About.C6, Y1 to Y6, and K1 to K6. These six patches of each color have predetermined density levels, and for example, each level (0
0h to ffh). In the example shown in FIG. 4, the density levels are set so that the density differences between the patches are equal.

In this embodiment, when the LBP main body is powered on, the operation in the concentration measurement mode is started first. In the density measuring unit 210, first, the magenta patch data shown in FIG. 3A is read from the patch memory 208, and the PWM unit 206 generates a laser drive signal by predetermined pulse width modulation. Then, based on the drive signal, latent images corresponding to the patches M1 to M6 are formed on the photosensitive drum 100 as described above. Nothing is formed in the area other than the patch shown in FIG.

In this way, when a magenta latent image is first formed, the latent image is developed through the above process and a magenta toner image is formed. In the density measurement mode, since the transfer paper P is not conveyed at this stage, the toner image is directly formed on the patch portion on the transfer drum 103.

FIG. 5 is a schematic perspective view of the vicinity of the transfer drum 103 at this stage. 5, the same components as those in FIG. 1 described above are denoted by the same reference numerals, and the description thereof will be omitted. Reference numeral 50 denotes a concentration sensor included in the concentration measuring unit 210, which includes a light emitting unit and a light receiving unit for measuring the concentration. The density sensor 50 emits a light beam to each of the six patches 603 formed on the transfer drum 103 and receives the reflected light to measure each density.

When the measurement of the magenta patch is completed, the density measuring unit 210 next reads the cyan patch shown in FIG. 3B from the patch memory 208 and forms a cyan toner image on the transfer drum 103. . here,
The area where the cyan patch shown in FIG. 3B is to be formed does not overlap with the magenta area shown in FIG. The formed 6 cyan patches are measured by the density sensor 50, and thereafter, similarly, formation and measurement of a yellow patch shown in FIG. 3C and formation and measurement of a black patch shown in FIG. 3D. I do.

FIG. 6 shows the appearance of the surface of the transfer drum 103 at the time when the density measurement of the four colors of M, C, Y and K is completed. It can be seen that the toner images of the respective colors are formed side by side without overlapping each other.

FIG. 7 shows the result of the density measurement of the above-mentioned magenta patch. The horizontal axis of FIG. 7 is the input density level,
That is, it is the set density value of the patch, and the vertical axis is the actually measured density value. In FIG. 7, six measurement points are plotted, and the concentration curve 801 is obtained by interpolating the measurement points.

Here, the maximum density value (sixth patch measurement value) in the density curve 801 is defined as dmax, and the dm
The density of division points when ax is divided into three parts is dα and dβ.
Next, the input density levels corresponding to the densities dα and dβ of the density curve 801 are set to the representative density levels α and β, respectively.
As described above, in the magenta density curve 801,
Maximum density dmax and divided density points dβ, dα, and d
Representative density levels α and β corresponding to β and dα are obtained.
In the density measuring unit 210, the density signal generating unit 20 sets the representative density levels α and β in magenta as Mα and Mβ.
4 is stored in the representative density memory 209.

Similarly, the values of the representative density levels α and β are obtained for cyan, yellow and black, respectively.
Therefore, the representative density memory 209 stores the values of the representative density levels α and β for each of M, C, Y, and K colors. Above,
End the concentration measurement mode.

As described above, in the present embodiment,
Since the representative density level data for each color is obtained by actual measurement when the power of the LBP is turned on, it is possible to obtain an appropriate representative density value corresponding to the secular change of the LBP.

Density Signal Generation Unit 204 In the density signal generation unit 204, the representative density levels α and β for each color are stored in the representative density memory 209 after the density measurement mode is completed. Then, in the normal mode (the mode in which an image is actually formed on the transfer paper P based on the input image data), the representative density level α of the corresponding color is determined according to the content of the color designation signal 205 controlled by the CPU 300. The value of β is output to the level conversion unit 203. That is, when the color designation signal 205 specifies magenta, the representative density levels Mα and Mβ of magenta are output to the level conversion unit 203, and when cyan is specified, the values of the representative density levels Cα and Cβ of cyan are output to the level conversion unit 203. It

Dither Processing Unit 201 Next, the dither processing unit 201 will be described in detail with reference to FIGS. 8 to 11. As described above, the dither processing unit 201 performs 2-bit depth multi-value dither processing by a 4 × 4 matrix on the input 8-bit data of each color. In the case of 2-bit depth multi-value dither processing, input multi-value data is converted into 4-level gradation, and hence these 4 levels will be referred to as levels a to d in order of increasing density.

First, the algorithm of the multi-valued dither processing in this embodiment will be described. FIG. 8 shows an example of the 4 × 4 dither matrix used in this embodiment. For convenience of explanation, the 16 cells of the matrix are numbered “1” to “16” in ascending order of their threshold values. Further, FIG. 9 shows the distribution of thresholds in the matrix. FIG. 9 shows the thresholds of the respective levels in the cells “1” to “16” of the dither matrix in the density values 00h to ffh. For example, 1a, 1b, 1c represent the thresholds in the cell of “1”. The density x of the input data in "1" is converted to the corresponding density level by comparison with the threshold value. That is, if x <1a, level a, 1a
If ≦ x <1b, it is converted to level b, if 1b ≦ x <1c, it is converted to level c, and if 1c ≦ x, it is converted to level d. Similarly,
2a to 2c correspond to the cell "2", and 1c <2a.
Similarly thereafter, 16a to 16c correspond to the cell "16",
16c is the maximum threshold. The values 1a to 16c are stored in the ROM 12 which will be described later in the dither processing unit 201.
It is stored in 04.

By setting a plurality of thresholds stepwise for each cell of the dither matrix in this manner, the following density change can be obtained. That is, as the density of the image increases, the density level of the cell "1" first increases from the level a to the level b, the level c, and the level d, and then the density level of the cell "2" changes from the level a to the level b and the level. c, level d. That is, in FIG. 9, if the density value x of the input data is the point P1, the cell "1" is converted to the d level, and the other cells "2" to "16" have the level a.
Is converted to If the density value x of the input data is the point P2, the cell "1" is converted to the d level, the cell "2" is converted to the c level, and the other cells "3" to "16" are converted to the a level. Therefore, the density value x of the input data is the point P3, that is, ff
All the cells of "1" to "16" are converted to the level d by h.

FIG. 10 shows the detailed structure of the dither processing unit 201 for realizing the above-described multi-valued dither algorithm, and the image forming processing in the normal mode will be described.

In FIG. 10, the input 8-bit data is input to each of the 16 mass processing units 1101 to 1116. Here, for example, a detailed configuration of the mass 1 processing unit 1101 is shown in FIG. In the mass 1 processing section 1101, first, the values of 1 a, 1 b, and 1 c are input from the ROM 1204 to the comparators 1201 to 1203, respectively, and the comparator 12
In 01 to 1203, the input 8-bit data is compared with 1a, 1b and 1c, respectively, and the result is input to the level generating section 1205. In the level generator 1205,
In accordance with the comparison result, a 2-bit signal indicating any one of the levels a to d described above is generated and output. Square 2
The processing units 1102 to 16 and the processing unit 1116 also have the same configuration as the processing unit 1101.

As described above, all 32-bit level signals output from the mass processing units 1101 to 1116 in FIG. 10 are input to the selector 1118. On the other hand, in the counter 1117 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 current pixel position of the input data is sent to the selector 1118. The selector 1118 refers to the pixel position signal to select and output the corresponding one (2 bits) from the level signals sent from the mass processing units 1101 to 1116.

As described above, the dither processing section 20.
At 1, the input 8-bit data is converted to an appropriate level (any of levels a to d) and a 2-bit signal (level signal) indicating the level is output.

Level conversion unit 203 2 output from the dither processing unit 201 as described above
The bit level signal 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. On the other hand, the level conversion unit 203
From the density signal generating unit 204 described above, one of the MCYK representative density levels α,
β is input. Then, in the level conversion unit 203, the level signal (2
Bit) is converted into a level (8 bits) corresponding to a laser drive signal for actually forming an image as shown below.

Level a → 00h Level b → α Level c → β Level d → ffh That is, the 2-bit signal indicating the level sent from the dither processing unit 201 is converted into the 8-bit signal indicating the actual output level value. To do.

The converted 8-bit signal is sent to the PWM unit 206.
Output to In the PWM unit 206, the 8-bit signal is converted into an analog voltage by a D / A converter (not shown),
By comparing with a triangular wave by an analog comparator (not shown), pulse width modulation is performed and converted into a PWM signal. Then, the PWM signal drives the laser in the laser drive unit, and the transfer paper P is printed by the above-described process.

The process in the above-mentioned normal mode is as follows.
It is performed in the order of M, C, Y, K according to the color designation signal 205.

As described above, according to the present embodiment, since the intermediate level converted by the multi-value dither processing can be set appropriately according to the environment, it is possible to always form a high-quality image with stable density. It will be possible.

In the present embodiment, an example in which the 2-bit multi-valued dither processing is preferably performed has been described, but the present invention is not limited to this example, and multi-valued dither processing of 3 bits or more can be easily performed. Is applicable to.

<Second Embodiment> The second embodiment of the present invention will be described below.
An embodiment will be described. The basic configuration of the image processing apparatus according to the second embodiment is the same as that of the first embodiment described above, and thus the description thereof will be omitted.

In the above-mentioned first embodiment, the multi-value dither processing with 2-bit depth has been described.
In the embodiment, an example in which dither processing with a depth of 3 bits is further combined will be described.

Signal Processing Unit FIG. 12 shows a block configuration of the signal processing unit in the second embodiment. In FIG. 12, reference numeral 1301 denotes a 2-bit dither processing unit, which performs 2-bit depth multi-value dither processing on input 8-bit multi-value image data. Reference numeral 1302 denotes a 3-bit dither processing unit, which performs 3-bit depth multivalue dither processing on input 8-bit multivalue image data. 1303 is an image buffer, 13
Reference numeral 05 is a level conversion unit, and each dither processing unit 1301,
The level number determined in 1302 is converted into a predetermined level in the output gradation based on the representative density level input from the density signal generator 1306. 1306
Is a density signal generator, which outputs a corresponding representative density level from the internal representative density memory 1313 in accordance with a color designation signal 1308 indicating the type of color plane of the image data currently being processed. A PWM unit 1307 performs pulse width modulation on the finally obtained multi-valued image signal and generates and outputs a laser drive signal to the laser drive unit. Also, 13
Reference numeral 11 denotes a density measuring unit, which generates data of the representative density memory 1313 by measuring the actually formed patch. The internal patch memory 1312 holds data for four colors for forming a sample patch for density measurement. Further, 1330 is a CPU that comprehensively controls the operation of the entire signal processing unit based on a control program stored in the ROM 1331. A RAM 1332 is used as a work area of the CPU 1330.

Concentration Measuring Unit 1311 The concentration measuring unit 1311 in the second embodiment generates the data of the representative concentration memory 1313 similarly to the concentration measuring unit 210 in the first embodiment described above. Is characterized in that a representative density level corresponding to 2-bit dither and a representative density level corresponding to 3-bit dither are generated for each color.

Hereinafter, a method of creating representative density level data in the second embodiment will be described.

First, in the density measurement mode, similarly to the first embodiment described above, the patches of the respective colors shown in FIGS. 3A to 3D held in the patch memory 1312 are formed, and the density is measured. And obtain a concentration curve. An example of the density curve is shown in FIG. 13, and a method of setting the representative density level in the second embodiment will be described.

In the second embodiment, the densities corresponding to the input density levels 33h, 66h, 99h and cch in the density curve are d1, d2, d3 and d4 in order. Then, it is determined whether or not the following relationships are satisfied with respect to the predetermined threshold values th1 and th2.

D4-d1> th1 and d3-d2> th2 Then, if this relationship is satisfied, it is determined that 2-bit dither processing is to be performed on the color plane, and if not, 3-bit dither processing is to be performed. The thresholds th1 and th
No. 2 is stored in the ROM 1331 and may be appropriately set according to the characteristics of the device, the usage environment, and the like. Of course, the threshold value may be stored in the RAM 1332 so that it can be changed by an operation unit (not shown).

Further, the representative density memory 1313 simultaneously stores a 1-bit identification signal indicating which of 2-bit dither processing and 3-bit dither processing is to be performed. However, the processing for determining the dither type described above is performed in magenta, cyan,
The 2-bit dither processing is unconditionally performed for the black color plane only and for yellow because of the characteristic that the visibility is low, that is, it is difficult for the human eye to see.

In the density curve shown in FIG. 13, since the gradient of the intermediate density level is steep, d1 to d4 satisfy the above equation. In this case, that is, the reproduction of halftones is low,
It is not appropriate to set many gradations. Therefore, the number of levels converted by the multi-value dither processing is the same as the above-mentioned first level.
It is assumed to be expressed by 2 bits (4 levels) as in the embodiment.

On the other hand, another example of the concentration curve is shown in FIG.
In the concentration curve of FIG. 14, since the gradient of the intermediate concentration level is gentle, d1 to d4 do not satisfy the above equation. In such a case, that is, since the reproducibility of the halftone is relatively high, more gradations can be set. Therefore, the number of levels converted by the multi-value dither processing is represented by 3 bits (6 levels in this embodiment).

As the representative density level for the 2-bit dither processing, two values (α, β) are determined by the method of dividing dmax into three, as in the first embodiment described above. A method for determining the representative density level for the 3-bit dither processing will be described below with reference to FIG.

In the case of the 3-bit dither processing, as shown in FIG. 15, first, the respective densities obtained by dividing dmax into seven equal parts are sequentially d.
Then, the input density levels corresponding to da to df in the density curve are stored in the representative density memory 1313 as the representative density levels a to f. That is, in the 3-bit dither processing, six representative density levels are set.

As described above, in the second embodiment, in the magenta, cyan, and black color planes, two representative density levels α, are set depending on whether 2-bit dither processing or 3-bit dither processing is performed. β or one of the six representative density levels a to f is stored in the representative density memory 1313 together with the identification signal.

Density Signal Generating Unit 1306 The density signal generating unit 1306 of the second embodiment outputs a representative level signal according to the color designation signal 1308 in the normal mode, like the density signal generating unit 204 of the first embodiment described above. However, the number of representative level signals stored in the representative density memory 1313 is different from that in the first embodiment.

For example, when the color designation signal 1308 designates magenta, the representative density levels Mα, Mβ or Ma of magenta stored in the representative density memory 1313.
~ Mf is output according to the identification signal. Subsequently, if cyan is designated, the representative density levels of M, C, and K are output to the level conversion unit 1305, such as cyan representative density levels Cα, Cβ, or Ca to Cf. As described above, for yellow, the representative density level Y
α and Yβ are output. At the same time, a bit identification signal 1309 indicating whether to perform 2-bit dither processing or 3-bit dither processing is output to the level conversion unit 1305 and the selector 1303 for each color.

3-bit Dither Processing Unit 1302 Next, the operation of the 3-bit dither processing unit 1302 will be described in detail. The 2-bit dither processing unit 130
Since 1 is the same as the dither processing unit 201 in the above-described first embodiment, description thereof will be omitted.

First, the algorithm of the 3-bit dither processing in the second embodiment will be described.

In the 3-bit dither process, since there are 8 levels of gradation with a depth of 3 bits, the 8 levels will be referred to as levels a to h in the order of increasing density. The dither matrix is the same as in the case of 2 bits and is as shown in FIG. FIG. 16 shows the distribution of thresholds in the matrix. FIG. 16 shows that at the density values 00h to ffh,
The thresholds of the respective levels in the cells "1" to "16" of the dither matrix are shown. For example, 1a to 1g show the thresholds in the cell "1", and the density x of the input data in the cell "1" is the threshold value. Is converted into the corresponding density level. That is, if x <1a, level a, 1a
If ≦ x <1b, the level is converted to level b. If 1g ≦ x, the level is converted to level h. Similarly, 2a to 2g correspond to the cell "2", and 1g <2a. Similarly, 16a to 16
g corresponds to the cell “16”, and 16 g is the maximum threshold value. The values 1a to 16g are stored in the ROM 1808 described later in the dither processing unit 201.

By thus setting a plurality of thresholds stepwise for each square of the dither matrix, the following density change can be obtained. That is, as the image density increases, the density level of the cell "1" first increases from the level a to the level b, ..., Level h, and then the cell "2".
, The level of the density increases from the level a to the level b ,. The density value of the input data is ffh
Then, all the cells of "1" to "16" are converted to the level h.

The detailed configuration of the 3-bit dither processing unit 1302 for realizing the 3-bit dither algorithm described above is shown in FIG. 17, and the image forming process in the normal mode will be described.

In FIG. 17, the input 8-bit data is input to each of the 16 mass processing units 1901-1916. Here, for example, the detailed configuration of the mass 1 processing unit 1901 is shown in FIG. In the mass 1 processing unit 1901, first, the values of 1a to 1h are read from the ROM 1808 by 180, respectively.
1 to 1807 are input to the comparators 1801 to 1801.
At 1807, the input 8-bit data is set to 1
The result is input to the level generating unit 1809. The level generating unit 1809 generates and outputs a 3-bit signal indicating any one of the above levels a to h according to the comparison result. Mass 2 processing unit 190
The 2nd to 16th cell processing unit 1916 is also a 1st cell processing unit 1901.
It has the same configuration as.

As described above, each of the mass processing units 1901-19
All 48-bit level signals output from 16 are input to the selector 1918. On the other hand, in the counter 1917 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 selector 1918. Selector 1
At 918, by referring to the pixel position signal, the corresponding one (3 bits) is selected and output from the level signals sent from the mass processing units 1901-1916.

As described above, in the 3-bit dither processing section 1302, the input 8-bit data is converted into an appropriate level (any one of the levels a to h) and the 3-bit signal ( Level signal) is output.

Level converter 1305 The 8-bit data input in the second embodiment is 2
After being processed by the bit dither processing unit 1301 or the 3-bit dither processing unit 1302, it is input to the selector 1303. A bit identification signal 1309 that specifies either 2-bit dither / 3-bit dither processing is input to the selector 1303 from the density signal generation unit 1306. If the signal indicates 2-bit dither, the 2-bit dither processing unit is input. If the 2-bit level signal from 1301 indicates 3-bit dither, the 3-bit level signal from the 3-bit dither processing unit 1302 is selected.

The 2-bit or 3-bit level signal output from the selector 1303 is temporarily held in the image buffer 202 and input to the level conversion unit 1305 at an appropriate timing in synchronization with the image clock. On the other hand, the level conversion unit 1305 has two of MCYK according to the color designation signal 1308 from the density signal generation unit 1306 described above.
The representative density levels α and β for bits or the representative density levels a to f for 3 bits are input. At this time, the bit identification signal 130 indicating 2-bit dither / 3-bit dither
9 is also input at the same time.

In the level converter 1305, if the bit identification signal 1309 indicates 2 bits, the level signal (2 bits in this case) input from the selector 1303 is used as a laser drive signal for actually forming an image. Is converted to a level (8 bits) corresponding to the following.

Level a → 00h Level b → α Level c → β Level d → ffh On the other hand, if the bit identification signal 1309 indicates 3 bits, the level signal (3 bits in this case) input from the selector 1303 is Convert as shown below.

Level a → 00h Level b → a Level c → b Level d → c Level e → d Level f → e Level g → f Level h → ffh That is, 2-bit dither processing section 1301 or 3-bit dither processing section 1302 The 2-bit or 3-bit signal indicating the level sent from the
Convert to bit signal.

The converted 8-bit signal is sent to the PWM unit 130.
7 is output. In the PWM unit 1307, the 8-bit signal is converted into an analog voltage by a D / A converter (not shown) and compared with a triangular wave by an analog comparator (not shown) to perform pulse width modulation and convert it into a PWM signal. Then, the PWM signal drives the laser in the laser drive unit, and the transfer paper P is printed by the above-described process.

The processing in the above-mentioned normal mode is as follows.
It is performed in the order of M, C, Y, K according to the color designation signal 1308.

As described above, according to the second embodiment,
In addition to the effect obtained in the above-described first embodiment, by switching the multi-value dither processing of a plurality of bit depths according to the color plane, it is possible to express more multi-gradation and obtain a higher quality image. Can be formed.

In the second embodiment, the combination of the 2-bit dither processing and the 3-bit dither processing has been described. However, the present invention is not limited to this example, and a plurality of bit depths can be obtained with the same idea. It is possible to combine the multi-valued dither processing of. For example, the optimum combination may be determined in consideration of the processing speed and memory capacity of the device.

Further, the combination is not limited to the combination of multi-valued dithers only. For example, it is possible to perform normal binary dithering only for yellow and perform multi-valued dithering of 2 bits or more for other colors.

Further, in each of the embodiments, the concentration measurement mode has been described as operating when the power of the LBP main body is turned on, but of course, the concentration measurement mode is performed at an arbitrary timing by an operation from an operation unit (not shown). May be.

The present invention may be applied to a system composed of a plurality of devices such as a host computer, an interface and a printer, or to an apparatus composed of a single device such as a copying machine.

It goes without saying that the present invention can also be applied to the case where it is implemented by supplying a program to a system or an apparatus. In this case, the storage medium storing the program according to the present invention constitutes the present invention. Then, by reading the program from the storage medium to the system or device, the system or device operates in a predetermined manner.

[0103]

As described above, according to the present invention, since the intermediate level converted by the multi-value dither processing can be appropriately set according to the environment, it is possible to always form a high-quality image with stable density. Is possible.

Further, by switching a plurality of multi-valued dither processings of different levels for each color plane, an image with higher gradation can be obtained.

[0105]

[Brief description of drawings]

FIG. 1 is a color LBP according to an embodiment of the present invention.
FIG.

FIG. 2 is a block diagram showing a configuration of a signal processing unit in the present embodiment.

FIG. 3 is a diagram showing an example of patches of each color according to the present embodiment.

FIG. 4 is a diagram showing a relationship between patch numbers and density levels in this embodiment.

FIG. 5 is a diagram showing how the concentration is measured in the present embodiment.

FIG. 6 is a diagram showing each color patch formed on the transfer drum in the present embodiment.

FIG. 7 is a diagram showing an example of a magenta density curve in the present embodiment.

FIG. 8 is a diagram showing an example of a dither matrix in the present embodiment.

FIG. 9 is a diagram showing a dither threshold value according to the present embodiment.

FIG. 10 is a block diagram showing a detailed configuration of a dither processing unit in this embodiment.

FIG. 11 is a block diagram showing a detailed configuration of a mass 1 processing unit in the present embodiment.

FIG. 12 is a block diagram showing a configuration of a signal processing unit in a second embodiment according to the present invention.

FIG. 13 is a diagram showing an example of a concentration curve in the second embodiment.

FIG. 14 is a diagram showing an example of a concentration curve in the second embodiment.

FIG. 15 is a diagram for explaining a method of acquiring a threshold value of 3-bit dither processing in the second embodiment.

FIG. 16 is a diagram showing a threshold value of 3-bit dither in the second embodiment.

FIG. 17 is a block diagram showing a detailed configuration of a 3-bit dither processing unit in the second embodiment.

FIG. 18 is a block diagram showing a detailed configuration of a mass 1 processing unit in the second embodiment.

[Explanation of symbols]

 201 dither processing unit 202, 1304 image buffer 203, 1305 level conversion unit 204, 1306 image signal generation unit 206, 1307 PWM unit 208, 1308 patch memory 1301 2-bit dither processing unit 1302 3-bit dither processing unit 1303 selector

Claims (16)

[Claims] 1. An image processing apparatus capable of forming a multi-valued image on a recording medium, the measuring means forming a multi-valued sample patch image and measuring the patch density, and the measured patch density. Holding means for calculating and holding a predetermined number of intermediate level values from the above, dither processing means for performing multi-value dither processing on the input multi-valued image data to determine the level, and dither processing means. Image processing, which comprises: a level converting means for converting the obtained level into an intermediate level value held by the holding means; and a forming means for forming an image based on the level value converted by the level converting means. apparatus. 2. The image processing apparatus according to claim 1, wherein the patch density is measured by the measuring means at a predetermined timing, and a predetermined number of intermediate level values are calculated and held by the holding means. . 3. The image processing apparatus according to claim 2, wherein the predetermined timing is when the power of the apparatus is turned on. 4. The forming unit forms a color image, the measuring unit forms a multi-valued sample patch image for each color and measures the patch density for each color, and the holding unit for each color. 2. The image processing apparatus according to claim 1, wherein a predetermined number of intermediate level values are calculated from the patch densities measured in step S6 and stored for each color. 5. The image processing apparatus according to claim 1, wherein the dither processing unit determines the multi-valued image data to be one of four levels. 6. An image processing apparatus capable of forming a multi-valued image on a recording medium, the measuring means forming a multi-valued sample patch image and measuring the patch density, and the measured patch density. Holding means for calculating and holding a predetermined number of intermediate level values from the holding means; and first holding means for carrying out multivalued dither processing on the input multivalued image data to determine one of the first level numbers.
Second dither processing means, and second multi-value dither processing for input multi-valued image data to determine any one of the second level numbers.
Dither processing means, selecting means for selecting one of the level determined by the first dither processing means and the level determined by the second dither processing means, and the level selected by the selecting means. An image processing apparatus comprising: a level conversion unit that converts the intermediate level value held by the holding unit; and a forming unit that forms an image based on the level value converted by the level conversion unit. 7. The forming unit forms a color image, the measuring unit forms a multi-valued sample patch image for each color and measures the patch density for each color, and the holding unit for each color. 7. The image processing apparatus according to claim 6, wherein a predetermined number of intermediate level values are calculated from the patch densities measured in step S6 and stored for each color. 8. The holding unit changes the number of intermediate levels to be held for each color to a number corresponding to a first level number or a second level number according to the distribution of the measured patch densities, 8. The image processing apparatus according to claim 7, wherein the identification information of the intermediate level number is held at the same time. 9. The selection means is determined by the second dither processing means and the level determined by the first dither processing means based on the intermediate level number identification information held by the holding means. 9. The image processing apparatus according to claim 8, wherein any one of the different levels is selected for each color. 10. The image processing apparatus according to claim 6, wherein the second level number is larger than the first level number. 11. The number of first levels is four and the number of second levels is eight.
The image processing apparatus according to any one of the preceding claims. 12. The forming means comprises magenta, cyan,
A color image is formed using yellow and black, the level is determined by the first dither processing unit for yellow multivalued image data, and the second dither processing unit is set for the multivalued image data of other colors. The image processing apparatus according to claim 10, wherein the level is determined by 13. The measuring device measures the patch density at a predetermined timing, and the holding device calculates and holds a predetermined number of intermediate level values.
The image processing apparatus according to any one of the preceding claims. 14. The image processing apparatus according to claim 13, wherein the predetermined timing is when the power of the apparatus is turned on. 15. An image processing method in an image processing apparatus capable of forming a multi-valued image on a recording medium, comprising forming a multi-valued sample patch image at a predetermined timing, measuring the patch density, and A predetermined number of intermediate level values are calculated from the measured patch densities and stored in advance, and multilevel dither processing is applied to the input multilevel image data to determine the levels, and the determined levels are calculated. An image processing method, comprising: converting to an intermediate level value held in advance; and forming an image based on the converted level value. 16. An image processing method in an image processing apparatus capable of forming a multi-valued image on a recording medium, comprising forming a multi-valued sample patch image at a predetermined timing, measuring the patch density, and A predetermined number of intermediate level values are calculated from the measured patch densities and stored in advance, and multilevel dither processing is applied to the input multilevel image data to determine any of the first level levels. The multi-valued dither processing is performed on the input multi-valued image data to determine any one of the second level numbers, and the level determined by the first dither processing means and the second level. Selecting one of the levels determined by the dither processing means, converting the selected level into a pre-stored intermediate level value, and forming an image based on the converted level value. Picture Processing method.