JP2913517B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for obtaining a high-quality recorded image.

[0002]

2. Description of the Related Art Conventionally, printers based on various principles have been proposed as output terminals of personal computers, workstations, and the like. In particular, a laser beam printer (hereinafter, referred to as LBP) using an electrophotographic process and a laser technique is used for recording. It is superior in terms of speed and print quality and is rapidly spreading.

On the other hand, the demand for full-color LBP is increasing in the market. However, in the case of full-color LBP, image data is to be output in addition to conventional characters and line drawings. It is necessary to perform image processing on the assumption of multi-tone output for the processing.

In general, in the case of an image output apparatus to which an electrophotographic process such as LBP is applied, there is a problem in the stability of the electrophotographic process itself. Tone can be secured.

A binary dither method is often used as a means for recording a halftone image by an image output device having an insufficient number of output gradations, such as an LBP or a normal thermal transfer printer. However, this binary dither method requires the use of a large-sized dither matrix in order to obtain sufficient gradation, and image quality degradation such as reduced resolution and occurrence of moire due to interference between halftone dots of the document and the dither pattern is required. There were problems such as occurrence.

In order to solve the above problems, a multi-value dither method has been conventionally proposed. This multi-value dither method will be described with reference to a block diagram showing the configuration of the image processing apparatus shown in FIG. For the sake of simplicity, it is assumed that the transmitted or stored image data has already been stored in the image memory 1.

The image memory 1 stores red R, green G, and blue B luminance data, each of which has an information amount of 8 bits × 3 = 24 bits per pixel. These pixel data are accessed by the main scanning direction counter 2 and the sub-scanning direction counter 3 via the address calculation unit 4, and the luminance data R, G, and B are read out from the beginning.

Since R, G, and B are luminance signals, the density conversion section 5 performs density conversion to convert them into density signals cyan C, magenta M, and yellow Y (the three primary colors of printing). For this conversion, a conversion table is usually set in a storage device such as a ROM or a RAM, and the contents are accessed using the luminance data value as an address. The actual table contents are, for example, conversion characteristics (horizontal axis: luminance and address, vertical axis:
Values based on the density and the contents of the memory are written.

The pixel data subjected to the density conversion is input to the color correction unit 6 in three colors. The color correction unit 6 performs under color removal (UCR... Under C) which is a well-known technique for density data.
olorRemoval). Black plate generation and masking are performed. The color correction unit 6 adds black (black:
BK) is added, and the information amount per pixel is actually 8 × 4
= 32 bits.

Next, these four-color data are transferred by the data selector 7, for example, in the order of BK, C, M, and Y if the transfer destination is the print engine 14 of a full-color printer.

On the other hand, of the address outputs of the main scanning direction counter 2 and the sub-scanning direction counter 3, the lower three bits are connected to a storage device 8 for storing a dither threshold matrix, and are uniquely determined by spatial coordinates of an image. An address for accessing this storage device 8 that outputs a determined threshold value can access a total of 6 bits, that is, 64 data. In this case, the dither threshold matrix stored in the storage device 8 may be, for example, the 8 × 8 dither threshold matrix shown in FIG.

The threshold value output from the storage device 8 is compared with the lower 6 bits of the density data 8 bits of the density level signal 10 input to the comparator 9 and output from the data selector 7. If the density data is greater than or equal to the threshold value, the comparator 9 outputs, for example, “1” as the comparison result 11. If the density data is smaller than the threshold value, for example, “0” is output as the comparison result 11, respectively.

On the other hand, the upper two bits of the density data output from the data selector 7 are connected so as to be input to a storage device 12 for redetermining pixel values, and are connected to a comparator 9.
And a total of 3 bits of data are accessed together with 1 bit of the comparison result 11 output from the CPU, and the final output value 13 is output.

FIG. 16 shows an example of an output value in the multi-value dither method. The final output when the upper two bits of the output of the data selector 7 are the density level signal 10 and the comparison output of the comparator 9 is the comparison result 11 is shown. An example of the value 13 is shown.

The above is the means to be taken when the multilevel dither is implemented in hardware. As shown in FIG. 16, the number of multilevel levels of the final output value 13 is 5 of 0, 3F, 7F, BF and FF.
One, that is, quinary dither.

In general, when outputting a full-color image with an image output device having a small number of multi-valued levels, as shown here,
The multi-value dither method and the like are widely used. For example, even if the number of outputtable gradations of the image output device itself is four, 8 ×
By combining a relatively large dither threshold matrix such as 8,

[0017]

(Equation 1)

A gradation can be obtained.

[0019]

As described in the prior art, an image output device such as an LBP or a thermal transfer printer having a small number of gradations of the process or the transfer principle itself includes the multi-value dither method. A pseudo area gradation technique is widely used.

For these, a dither threshold and a halftone type of matrix are devised (a threshold matrix is used to generate a plurality of dot concentrations in one matrix and to achieve both resolution and gradation), and image output. It has become possible to obtain a certain level of image quality by improving the resolution of the minimum recording dot of the device or by switching the dither matrix irregularly according to the density level.

However, even in the case of multi-valued dither, when it is desired to increase the number of gradations, the resolution is inevitably deteriorated. In addition, since an intermediate density level is used in one pixel in principle, the density unevenness of the recorded image is reduced. Easy to occur.

Further, in spite of the need for smoothness in the low gradation part due to the visual characteristics, since it has only a few discrete efficiency levels, when a recording pixel with the lowest density is formed on a white background, Roughness and texture are generated, and image quality is deteriorated especially in a low gradation part.

Further, control of so-called fogging, in which four adjacent dots after thermal fixing are completely fused with the growth of pixels, and reproduction of a high-density solid portion utilizing fogging cannot be compatible. In other words, setting the gradation level and the dither matrix that completely control the tumbling certainly improves the gradation in the high-density portion, but tends to lack the absolute density level, while allowing the tumbling. Although an absolute density level can be secured, the gradation in the high density region is impaired.

An object of the present invention is to solve the above-mentioned problems, to reduce degradation of resolution, to prevent generation of texture, to have excellent gradation in all density regions, and to suppress blurring particularly in a high density region. Another object of the present invention is to provide an image forming apparatus capable of obtaining a high-quality recorded image capable of securing an absolute maximum density.

[0025]

The image forming apparatus of the present invention SUMMARY OF THE INVENTION in accordance with the pixel position of each block composed of a plurality of pixels generated by dividing the input image data, respectively
A density level of the pixel to use meaning a conversion table for converting the dot recording density level for recording a dot, the image forming apparatus that performs gradation recording by converting the density level of the pixel in the dot recording density level Te, conversion table, at least, to convert the density levels of pixels, gradually becomes higher such dot recording density level in accordance with the concentration level of up to reaching the first concentration level pixels, the first density level After reaching the maximum density level, the pixel density level is converted to the dot recording density level when the pixel density level reaches the first density level, and when the pixel density level reaches the maximum density level, the pixel density level is converted to the maximum dot recording density level. The first conversion table and the pixel density level are converted to a dot recording density level at which dot printing is not performed until the pixel density level reaches the second density level. When the pixel density reaches the first density level in the first conversion table, the dot recording density gradually increases in accordance with the pixel density level at least up to the dot recording density level when the pixel density level reaches the first density level. And a second conversion table for converting to a level.

[0026]

According to the present invention, the low density level at which the visual resolution is not high at the pixel density level , that is, the second density level
If it is less than the degree level, the pixels in the block are
Dot recording density level that does not record each dot
And the density gradually increases according to the density level of the pixel.
To the pixel recording density level, and
High resolution level with high visual resolution,
If the density level is higher than the density level, the pixel
The more pixels in a block, the more pixels
The dot recording density level value corresponding to each
Can be converted to a higher value,
Density level is higher than the specified density level and lower than the maximum density level
In the case of, regardless of the pixel density level,
The value of the dot recording density level corresponding to the specific pixel of
The pixel when the density level of a pixel reaches a specific density level.
Dot recording density level, and the pixel density level is the highest density
If the level corresponds to a specific pixel in the block
The dot recording density level can be set to the maximum value.
You.

In addition, it is possible to obtain a high-quality recorded image which can suppress the wobble particularly in a high density region and secure an absolutely high density.

[0028]

FIG. 1 is a block diagram showing the configuration of an image forming apparatus according to an embodiment of the present invention. In the figure, reference numeral 15 denotes an image processing device, which is a density conversion unit 15-1, a conversion table 15;
-2, black / UCR section 15-3, color correction section 15-4, data selector
A digital data output device 16 is connected to the input side, and a print engine 14 is connected to the output side.

The digital data output device 16 receives an image signal from an image scanner or a video camera (not shown) as input, performs A / D conversion or predetermined image processing, or temporarily stores image data in a memory. It may be an interface of an image signal from a direct communication unit.

Next, the operation will be described with reference to FIGS.

Now, the print engine 14 is activated, and the digital data output device 16 starts transferring digital image data to the image processing device 15. The data to be subjected to image processing is a total of 24 bits of 8 bits for each color of RGB.
The RGB data input to the image processing device 15 is luminance data. The density conversion unit 15-1 converts the luminance data into density data, that is, C, M, and Y (cyan, magenta, and yellow), which are the three primary colors of printing. Is converted to

Generally, this conversion can be easily realized by writing the conversion table data on a storage device such as a RAM or a ROM constituting the conversion table 15-2 and accessing the data by appropriately offsetting the input data value. Normally, the density converter 15-1 uses a single color density, an overall density,
Contrast, base color control, etc. (density and color adjustment) can be performed.

The RGB (luminance) data is obtained by converting CMY after density conversion.
(Density) data 17, 18 and 19
Next, UCR (under color removal) and black plate generation are performed in the black / UCR unit 15-3 using the data 17, 18, and 19. UCR is CM
Data is reduced at a fixed rate with respect to the common amount of Y data 17, 18, and 19. Basically, this reduction amount is generated as a black plate. Originally, the purpose of UCR and black plate generation is to replace the common amount of CMY with black
(Toner).

However, in recent years, UCR and black plate generation are rarely performed purely for the purpose of saving toner. For example, the purpose is to prevent gradation deterioration in a high-density region, secure contrast, and secure a gray balance in a high-density region. And UCR
In addition, it is possible to positively change the amount of the black plate and output a higher quality image. After the UCR and black plate generation by the above processing, C data 20, M data 21, Y data 22, and Bk data 23 are generated.

Thereafter, the data other than the Bk data 23, which is an achromatic component, is input to the color correction unit 15-4. In the color correction unit 15-4, processing such as masking is performed on the chromatic components (CMY). The purpose of the masking is to correct the influence of the unnecessary absorption band of each color material.

For example, a C (cyan) color material has an unnecessary absorption band in a wavelength region other than C. Specifically, for example, Y (yellow)
It has a color component. Similarly, for M (magenta), Y
Is included. Therefore, when recording Y, it is necessary to reduce the Y component contained in C and M according to the density at which C and M are to be recorded. Usually 3 for CMY digital signal
A matrix operation of × 3 or an operation result is written in a storage device such as a ROM, and after accessing each color, addition and subtraction are performed to obtain a result.

Conventionally, 3 × 3 linear masking (primary masking) has been the mainstream. However, primary masking is insufficiently effective, and recently, non-linear masking of second or higher order or color correction itself is performed in a black box. It is regarded as a mapping performed by
There are a number of color correction schemes for finding a mapping function outside of the CMY space.

Next, the C data 20, M data 21, and Y data 22 of the input data are converted into C 'data 24,
It is converted into M 'data 25 and Y' data 26. On the other hand, since the Bk data 23 is achromatic data, it does not participate in color correction.

The C 'data 24, M' data 25, Y 'data 26 (the above are chromatic data) and Bk data 23 (achromatic data) corrected by the color correcting section 15-4 are used as data selectors. Only one color data is selected by 15-5 and input to the gradation processing unit 15-6 to perform gradation processing of the image signal according to the present invention. Output image signal 37 after gradation processing (see FIG. 2)
Is sent to the print engine 14 to obtain a high-quality recorded image of the present invention.

Here, the contents of the gradation processing according to the present invention will be described in detail with reference to FIG. FIG. 2 is a block diagram showing the configuration of the gradation processing unit 15-6 in FIG.
Reference numeral 27 denotes a horizontal synchronizing signal generation circuit, which outputs a horizontal synchronizing signal 28 of an image. Here, as a source of the horizontal synchronizing signal, for example, in the case of a laser beam printer, a beam detect signal from a laser scanning optical system (not shown) may be subjected to waveform shaping and used. Reference numeral 29 denotes a horizontal binary counter which counts a horizontal synchronizing signal 28 and outputs a horizontal binary counter output 30 every time the horizontal synchronizing signal 28 is inputted.
Are switched between ON and OFF.

Reference numeral 31 denotes a vertical synchronizing signal generation circuit which outputs a vertical synchronizing signal 32 of an image. As a source of the vertical synchronizing signal, for example, a data transfer clock can be used as it is. Reference numeral 33 denotes a vertical binary counter, and a vertical synchronization signal 32
The ON / OFF of the vertical binary counter output 34 is switched every time the vertical synchronization signal 32 is input.

To explain each of the above signals in more detail,
FIG. 3 shows changes in the time axis direction of the horizontal synchronizing signal 28, the vertical synchronizing signal 32, the horizontal binary counter output 30, and the vertical binary counter output 34. The horizontal sync signal 28 is transferred every time one line of data is transferred.
Once every (line cycle), it occurs at the beginning of the line data, the vertical synchronization signal 32 is generated corresponding to each data in one line (pixel cycle), and the number of occurrences of the pixels included in one line Equal to the number. When these signals are counted by the respective binary counters 29 and 33, there are four combinations of the horizontal binary counter output 30 and the vertical binary counter output 34.

That is, as shown in FIG. 3, the state A is when the horizontal binary counter output = 0 and the vertical binary counter output = 0, and the state A is when the horizontal binary counter output = 1 and the vertical binary counter output = 0. State B, State C when horizontal binary counter output = 0, vertical binary counter output = 1, State D when horizontal binary counter output = 1, vertical binary counter output = 1
Respectively, the state of each counter output uniquely corresponds to the spatial position of the pixel, and the state of the counter output corresponding to each pixel position is state A, state B,
It is classified into state C and state D.

Hereinafter, if the codes A, B, C, and D are used as codes indicating pixel positions regularly arranged in space, the entire image is composed of pixels at the A position, the B position, the C position, and the D position. It can be divided into each area of 2 × 2 pixels.

Returning to FIG. 2, reference numeral 35 denotes a gradation conversion table. The input image signal 36, the horizontal binary counter output 30, and the vertical binary counter output 34 are used as memory addresses to store the pixel level after the gradation conversion into a table. With. The address line is
The input image signal 36 composed of 10 bits is assigned to the lower 8 bits, the vertical binary counter output 34 is assigned to the ninth bit, and the horizontal binary counter output 30 is assigned to the 10th bit.

Here, the gradation conversion table 35 is shown in FIG.
This will be described in further detail with reference to FIG. FIG. 5 shows the contents of the gradation conversion table 35. Input image signal 36
Is assumed to be 8 bits (256 levels), 256 addresses are required in the table to represent the gradation characteristics for one pixel. In the present embodiment, a plurality of pixels are regarded as one block and the priority is determined. For example, A, B,
Since there are four gradation characteristics classified into C and D, the total is 1024 addresses.

That is, in FIG.
Data representing the gradation characteristic of the pixel at the position A is stored at −0FFH, data representing the gradation characteristic at the position C at address 100H-1FFH, and data representing the gradation characteristic at the position B at address 200H-2FFH. , Address 300H-
3FFH stores data representing the gradation characteristics at the D position.

FIG. 6 is a graph showing the gradation conversion characteristics (horizontal axis: input image level, vertical axis: output) in this embodiment. In the present embodiment, the priority of the pixel at the position A (1) is set to the highest, and the position B (2) and the position C (3) are continued.
The priority of the pixel (4) is set to the lowest. A position
The gradation characteristics of the pixels at (1) and C position (3) include a first region (A) in which the output increases monotonically with respect to the input, and a constant density level lower than the saturation density with respect to the input. Second area
(B) and the third region where the saturation concentration is output with respect to the input (c)
Is set.

In the present embodiment, the gradation characteristic corresponding to the position A (1), that is, the pixel position having the highest priority, is that the output when the input image level is from 00H to 3FH (the first area (A)) is continuous. The output when the input image level is from 3FH to FBH (second area (a)) is kept at BFH, and when the input image level exceeds FBH (third area (c)), FFH is output. It is set as follows.

The gradation characteristic corresponding to the B position (2), that is, the pixel position having the next highest priority after the A position, outputs 00H when the input image level is less than 3FH, and changes the input image level from 3FH to 7FH. The output increases continuously until the input image level exceeds 7FH, and the output is fixed at FFH.

The gradation characteristic corresponding to the C position (3), that is, the pixel position having the second highest priority after the B position, outputs 00H when the input image level is less than 7FH, and outputs the input image level from 7FH to BFH. The output of (first area (A)) continuously increases, and the input image level changes from BFH to FBH (second area).
The output of the area (a) is kept at BFH, and when the input image level exceeds the FBH (third area (c)), FFH is set to be output.

The gradation characteristic corresponding to the D position (4), that is, the pixel position having the lowest priority, outputs 00H when the input image level is less than BFH, and outputs continuously from the input image level BFH to FFH. To FFH.

As described above, in the present embodiment, a plurality of pixels in the cut-out image block are assigned a priority to form a pixel, and a dot having a specific priority is monotonously formed. The characteristics of the gradation conversion table 35 are set so that an area that grows incrementally, an area that suppresses the complete growth of dots and saturates in a pseudo manner, and an area that completely grows the dots are generated.

Next, another embodiment of the present invention will be described with reference to FIGS.
This will be described below. In this embodiment, two pixels are regarded as one block and the priority is determined.

FIG. 7 is a block diagram showing the configuration of the gradation processing unit 15-6 in FIG. 1. The vertical synchronizing signal generation circuit 31 and the vertical binary counter 33 in FIG.
It comprises a horizontal synchronizing signal generation circuit 27, a horizontal binary counter 29 and a gradation conversion table 35.

The horizontal binary counter 29 is a horizontal synchronizing signal.
When the horizontal synchronization signal is input, the ON / OFF of the horizontal binary counter output 30 is switched,
A distinction is made between even lines (OFF) and odd lines (ON). Further, the gradation conversion table 35 has the pixel levels after gradation conversion into a table using the input image signal 36 and the horizontal binary counter output 30 as memory addresses.

Here, the gradation conversion table 35 is shown in FIG.
This will be described in more detail with reference to FIG. FIG. 8 shows the contents of the gradation conversion table 35. If the input image signal is 8 bits (256 levels), 256 addresses are required in the table to represent one gradation characteristic. In this embodiment, two gradation characteristics of an even line and an odd line are provided. Therefore, the total is 512 addresses.

That is, the address 00H-FFH represents the gradation characteristic of the even line, and the address 100H-1FFH
Represents the gradation characteristic of the odd line.

FIG. 9 is a graph showing gradation conversion characteristics in this embodiment. Now, when the image density is low, for example, when the input image level is less than 7FH in FIG. 9, the pixels of the odd-numbered lines are not recorded and the dot size of the pixels of the even-numbered lines is adjusted to reproduce the gradation. Is medium to high, for example, when the input image level is 7FH to F7H, the dots of the pixels of the even lines are recorded at a predetermined level lower than the maximum output, for example, CFH, and the pixels of the odd lines are recorded. Adjust the dot size to reproduce the gradation.
Further, when the image density is very high, for example, when the input image level is F8H or higher, the dots of the pixels of the even lines are recorded at the maximum output.

In other words, the priority is set for two pixels on the even-numbered line and the odd-numbered line which are adjacent in the horizontal direction, and the recording dots are grown with priority given to the pixels on the even-numbered line. A gradation conversion table is provided so that a dot grows monotonically and increases with respect to a high even-numbered line, a pseudo-saturated region where dots are suppressed from growing completely, and a region where dots grow completely. Set 35 characteristics.

By the way, in the electrophotographic printer, it is better to grow the dots of a specific pixel preferentially on the photoreceptor than to grow the dots of each pixel uniformly when forming the pixels, as described above. A strong electric field is generated in a micro area of the electrostatic latent image, and the gradation of a recorded image is improved.

In general, the correlation between adjacent pixels is very high in a natural image. Therefore, according to this embodiment, the priority is given to the growth of pixels in a block or the priority is given to the growth of each line, that is, the difference. And the gradation is improved at the latent image level. In addition, since a specific spatial frequency component is consequently superimposed on the image, for example, resistance to driving unevenness generated by the driving system is improved.

As described above, this embodiment is a novel means for superimposing noise having a specific spatial frequency component on an image, and the noise level is spatially determined as defined by, for example, a dither matrix. Affected by the periodic threshold value and is close to the analog of the pixel (for example, 2
A major feature is that it is derived from the density level itself (56 gradations), which is significantly different from the gradation reproduction method that provides a discrete noise level (for example, 4 gradations) such as the dither method.

In the above embodiment, the order of strict growth is not guaranteed for the pixels in a block. For example, even if the highest priority pixel does not completely grow in one block, The priority pixels may grow. In particular, in an image having undulations in data, for example, in a portion where the edge of an image such as a character or a line drawing is sharp, the growth order of the pixels may be reversed depending on the way of the block.

That is, the priority in this embodiment does not determine the order of growth of pixels, but merely defines the input density level at which each pixel grows. However, in a general natural image, the correlation between adjacent pixels is very high, and pixels involved in gradation reproduction (at the growth stage) are selected according to the input density level of a spatially macroscopic region to some extent. The same effect as when the growth order is specified can be obtained.

The fact that the order of pixel growth is not completely determined minimizes the degradation of resolution. For example, in a method in which pixel data in a block is processed by some method, and data is rearranged with priority given to each pixel position, image data does not actually exist (or the value is There is a possibility that the data is weighted at (small) locations, and the resolution is definitely degraded. However, in the present embodiment, for example, if the pixel value is somewhat large and the density is high even in the case of a line image or the like (characters and line images are mostly output at the maximum density), the target pixel surely grows. The resolution will not be degraded at all.

Also, in this embodiment, almost analog density control can be performed on the visual characteristics in all the density areas, so that high density recording dots do not suddenly appear on a white background, and particularly in a smooth image such as a natural image. On the other hand, it is possible to suppress the rough feeling in the low gradation part and to greatly improve the gradation in the low gradation part.

That is, for a smooth image, a characteristic is emphasized particularly in a low gradation area, and a characteristic is emphasized for a portion represented by a normal high density such as a character or a line image. It shows the characteristics.

On the other hand, in the process of suppressing the growth of pixels of a specific priority, the phenomenon of fogging in which dots of adjacent pixels overlap in the middle to high density region is efficiently suppressed, and the gradation in this density region is improved. .

The process of printing dots at the highest density level when the input image level of a pixel with a higher priority is very high corresponds to, for example, only the case where very high density reproduction is required in a high density solid area. . Therefore, according to the present embodiment, it is possible to suppress so-called occurrence of fogging, in which four adjacent dots after thermal fixing are completely fused together with the growth of pixels, and to reproduce a high-density solid portion by actively using fogging. It is possible to achieve both.

As described above, in this embodiment, the gradation level of the high-density portion is improved by setting the gradation level and the dither matrix in which the problem of the conventional technique is suppressed, but the absolute density is increased. If the level tends to be insufficient, and on the other hand, the blur is allowed, the absolute density level can be secured, but on the other hand, the conventional disadvantage that the gradation property in the high density area is impaired is effectively solved. be able to.

What has been described above is that, for example, in the case where priority is given to 2 × 2 pixels and gradation expression is performed, the three regions (monotonic to input) described in detail for the pixel with the highest priority It is also preferable to set a first area in which the output increases incrementally, a second area in which a constant density level lower than the saturation density is output with respect to the input, and a third area in which the saturation density is output with respect to the input. Setting three regions for pixels other than the lowest pixel can also be easily changed.

That is, the above three areas can be set to an arbitrary number of pixels, an arbitrary priority pixel, and an arbitrary position pixel. Further, the output level of the second region which is a pseudo saturation region can be set. These changes only change the contents of the gradation conversion table and can be made very easily.

Although the present embodiment has been described in detail by setting 2 × 2 blocks, the present invention can be implemented regardless of the size of the blocks. In addition, the number of pixels in which dots are preferentially grown is not limited to one, but may be plural. This change involves changing the number of count bits of the counter according to the size of the block (the size in each direction may be different), securing a gradation conversion table area for the number of counter output states, and It can be realized simply by describing the contents of the table.

In the other embodiments described with reference to FIGS. 7 to 9, priority is given to two adjacent pixels for the sake of simplicity. However, the present invention is not limited to this. The number of pixels on which dots can be preferentially grown is not limited to one, but may be plural.

For example, when priority is given to four lines to perform gradation expression, the dot with the highest priority is not completely grown, and the dot other than the dot with the lowest priority is not completely grown. Can also be easily changed.

Further, the relationship between an arbitrary line and the priority can be easily performed by rewriting the gradation conversion table, and the priority can be easily set for each line. These changes can be made very easily only by changing the number of bits of the counter and the table area and table contents corresponding to the number of output statuses of the counter.

Further, according to the present embodiment, the high priority dot and the low priority dot are each one as a result, and a group is formed, so that the resolution is almost maintained.
Even if priorities are set for four lines of line data, the block size to which visual modulation is performed is 1
× 4, and the resolution is kept very high as compared with the case where a 4 × 4 dither matrix is adopted by a normal dither method.

Next, a laser beam printer (LBP) employing the gradation processing described in each of the above embodiments will be described in detail with reference to FIGS.

An LBP for forming a color image by applying an electrophotographic process technology selectively forms an image by irradiating a light beam corresponding to each color onto a photoreceptor having a photoconductive layer. A plurality of electrostatic latent images respectively corresponding to the specific components are developed with respective predetermined toners, and a single color toner image is superposed to form a color image on one transfer material. ing.

FIG. 10 is a side sectional view showing the structure of a main part of the LBP.
FIG. 11 is a perspective view of a main part of the photoconductor reference detection mechanism, and FIG. 12 is a perspective view of a main part of the intermediate transfer body reference detection mechanism.

In FIG. 10, reference numeral 38 denotes a belt-shaped photoreceptor, and selenium (Se) or an organic photoconductor (OPC) is formed on the outer peripheral surface of a belt base material such as a closed loop resin having a joint 38a.
And the like are coated in a thin film form. The belt-shaped photoreceptor 38 is supported by two photoreceptor transport rollers 39a and 39b so as to form a vertical plane, and the photoreceptor transport roller is rotated by a drive motor (not shown), and the belt-shaped photoreceptor is rotated. 38 is rotated in the direction of arrow A.

A charger 40, an exposure optical system 41, black (BK), cyan (C), magenta (M), and yellow ( Y)
Developing units 42BK, 42C, 42M, and 42Y, an intermediate transfer unit 43, a photoconductor cleaning device 44, a static eliminator 45, and a photoconductor reference detection sensor 46.

Here, each of the color developing units 42BK to 42Y stores a toner corresponding to each color. The toner color is selected by selecting the developing unit corresponding to each color by rotating the detachable cams 48BK, 48C, 48M, and 48Y rotatably supported at both ends by the body of the body in accordance with the color selection signal. For example, 42B
This is performed by bringing K into contact with the belt-shaped photoconductor 38. The remaining unselected developing units 42C, 42M, and 42Y are separated from the belt-shaped photoconductor 38.

The charger 40 includes a charging wire 40a made of a tungsten wire or the like, a shield plate 40b made of a metal plate, and a grid plate 40C. When a high voltage is applied to the charging line 40a, the charging line 40a causes a corona discharge to uniformly charge the belt-shaped photoconductor 38 via the grid plate 40C.

Reference numeral 47 denotes an exposure light beam of image data emitted from the exposure optical system 41. In LBP, this exposure light beam 47
Is obtained by applying an image signal, which has been subjected to light intensity modulation or pulse width modulation by a laser drive circuit (not shown) from a gradation converter to a semiconductor laser (not shown), to obtain a belt-shaped photosensitive member. A plurality of electrostatic latent images are formed on 38, each corresponding to a particular one of the plurality of predetermined color components.

As shown in FIG. 11, the photoconductor reference detecting sensor 46 detects the position of the seam 38a of the belt-shaped photoconductor 38, and a seam is formed at one end of the belt-shaped photoconductor 38.
A photoconductor reference mark 38b such as a slit disposed at a predetermined position with respect to 38a is detected.

A photoreceptor clutch mechanism 49 is a driving source.
The rotation of the photoconductor 38 is controlled by turning on / off the power from a power supply (not shown), and is provided on the drive shaft of the photoconductor transport roller 39b.

Returning to FIG. 10, the intermediate transfer body unit 43 includes a seamless loop belt-shaped intermediate transfer body 43A made of a conductive resin or the like, and two intermediate transfer bodies supporting the intermediate transfer body. Body transfer rollers 43B and 43C and an intermediate transfer body 43A
In order to transfer the toner image on the belt-shaped photoconductor 38, the image forming apparatus has an intermediate transfer body transfer roller 43D disposed opposite the belt-shaped photoconductor 38 with the intermediate transfer body 43A interposed therebetween.

Here, the surface peripheral length L1 of the belt-shaped photosensitive member 38
Is nominally equal to the surface peripheral length L2 of the intermediate transfer member 43A, but is set such that the relationship of L1 ≦ L2 always holds within the range of the variation.

Next, as shown in FIG. 12, 43E is an intermediate transfer member.
An intermediate transfer member reference detection sensor that detects the reference position of 43A, and detects the reference position with an intermediate transfer member reference mark 43a such as a slit disposed at one end of the intermediate transfer member 43A.

Returning to FIG. 10, reference numeral 43F denotes an intermediate transfer member cleaning device for scraping off residual toner on the intermediate transfer member 43A, and the intermediate transfer member is formed while the composite image is being formed on the intermediate transfer member 43A. The transfer member 43A is separated from the transfer member 43A, and comes into contact only when the transfer member 43A is used for cleaning.

Reference numeral 50 denotes a transfer body cassette containing a transfer material 51. The transfer materials 51 are sent out one by one from a transfer body cassette 50 to a sheet transport path 53 by a half-moon-shaped paper feed roller 52.

Reference numeral 54 denotes a registration roller for temporarily stopping the transfer material 51 in order to make the transfer material 51 coincide with the position of the composite image formed on the intermediate transfer member 43A. I have. Reference numeral 56 denotes a transfer roller for transferring the composite image formed on the intermediate transfer member 43A to the transfer material 51. The intermediate transfer member 43A is used only when the composite image is transferred to the transfer material 51.
Contact rotation.

Reference numeral 57 denotes a heat roller 57a having a heat source inside.
And a pressure roller 57b.The fixing device is composed of a heat roller 57a and a pressure roller 57b.
Is fixed to the transfer material 51 by pressure and heat in accordance with the rotation of the holding member to form a color image.

The LBP configured as described above is:
The operation will be described below.

The belt-shaped photosensitive member 38 and the intermediate transfer member 43A are
Each of them is driven by a drive source (not shown), and is controlled so that their peripheral speeds become the same constant speed. Further, an image forming area of the intermediate transfer body 43A is set in advance by an intermediate transfer body reference detection sensor 43E for detecting an intermediate transfer body reference mark 43a for determining a reference position, and a belt-shaped photosensitive member 38 is set in this area. Is adjusted so that the seam 38a of the intermediate transfer member 43a does not overlap with the intermediate transfer body transfer roller 43D, and is driven in synchronization.

In this state, first, a high voltage is applied to the charging line 40a in the charger 40 connected to the high voltage power supply to cause corona discharge, and the surface of the belt-shaped photosensitive member 38 is uniformly -700V to -800.
Charge to about V.

Next, the belt-shaped photoreceptor 38 is rotated in the direction of arrow A, and a laser corresponding to a predetermined one of a plurality of color components, for example, black (BK) is formed on the uniformly charged surface of the photoreceptor 38. When the beam is irradiated with the exposure light beam 47, the irradiated portion on the belt-shaped photoreceptor 38 loses its charge and an electrostatic latent image is formed. At this time, the electrostatic latent image is transferred to a belt-like photosensitive position at a predetermined position in the image area on the intermediate transfer member 43A by a signal from the intermediate transfer reference detection sensor 43E for detecting the reference position of the intermediate transfer member 43A. It is formed avoiding the seam 38a of the body 38.

On the other hand, the developing device 42BK in which the black toner contributing to the development is stored is a separation / contact cam based on a color selection signal.
It is pushed in the direction of arrow B by the rotation of 48BK and comes into contact with the belt-shaped photoconductor 38. With this contact, toner adheres to the electrostatic latent image formed on the photoreceptor 38 to form a toner image, and the development is completed. Developing device 42BK after development is a separation / contact cam
By the 180 ° rotation of 48BK, the photosensitive drum 38 is moved from the contact position to the belt-like photosensitive member 38 to the separated position.

The toner image formed on the belt-like photosensitive member 38 by the developing device 42BK is transferred to the intermediate transfer member 43A by an intermediate transfer member transfer roller arranged in contact with the belt-like photosensitive member 38 for each color.
The image is transferred by applying a high voltage to 43D. Residual toner not transferred from the belt-shaped photoreceptor 38 to the intermediate transfer member 43A is removed by the photoreceptor cleaning device 44, and the charge on the belt-shaped photoreceptor 38 from which the residual toner has been scraped off by the static eliminator 45. Is removed.

Next, for example, when the color of cyan (C) is selected, the contact / separation cam 48C is rotated, and this time, the developing device 42C is pushed in the direction of the belt-shaped photosensitive member 38 so as to contact the photosensitive member 38, and
(C) Development is started. In the case of a copying machine or a printer using four colors, the above-described developing operation is sequentially repeated four times, and the toner images of four colors BK, C, M, and Y are superposed on the intermediate transfer member 43A to form a composite image.

In the composite image formed in this manner, the paper transfer roller 56, which has been separated so far, comes into contact with the intermediate transfer member 43A, and a high pressure is applied to the paper transfer roller 56 and the pressure is transferred from the transfer material cassette 50 by the pressure. The image is transferred onto the transfer material 51 sent along the paper transport path 53 at a time. Subsequently, the transfer material 51 onto which the toner image has been transferred is sent to the fixing device 57, where it is fixed by the heat of the heat roller 57a and the clamping pressure of the pressure roller 57b, and is output as a color image.

The residual toner on the intermediate transfer member 43A that has not been completely transferred onto the transfer material 51 by the paper transfer roller 56 is removed by the intermediate transfer member cleaning device 43F.

The intermediate transfer member cleaning device 43F is at a position away from the intermediate transfer member 43A until a single composite image is obtained, and the composite image is obtained. After being transferred to 51, a contact state is established, and residual toner is removed.

With the above operation, recording of one image is completed, and a high quality color recorded image is obtained.

The printer is not limited to the electrophotographic system using a laser beam according to the present embodiment, but can be implemented by a thermal transfer system or an ink jet system. In addition, the same electrophotographic type such as an LED type or a liquid crystal shutter type may be used.

In this embodiment, a full-color printer in which tone reproduction is important has been described. However, a monochrome printer may be used. Further, in the present embodiment, the color image is superimposed on the intermediate transfer member. However, the color image may be superimposed on the photoreceptor or on transfer paper.

[0109]

As described above, in the image forming apparatus of the present invention , the visual resolution is high at the pixel density level.
However, when the gradation characteristics are important at low density levels,
Do not record dots for multiple pixels in the
The dot recording density level and the pixel density level
Convert to a dot recording density level that gradually increases
By doing so, the density corresponding to each pixel in the block
Combine levels into specific pixels and correspond to this specific pixel
Recording can be performed using the
When forming an image, all dots are the same according to density.
The resolution is lower than the image forming apparatus that records in
However, the desired density can be stably maintained by improving the gradation characteristics.
Obtainable. At the same time, the pixel density level
If the visual resolution is high but the gradation characteristics are very important
If the density level of the pixel is not
The more pixels in a block, the more pixels
The dot recording density level value corresponding to each
By converting to a higher value,
The density levels corresponding to these pixels into a specific pixel
To perform recording using dots corresponding to this particular pixel.
From the state, the density level corresponding to each pixel in the block
Make the bell correspond to each pixel, and
The pixel density level changes to a state where printing is performed with the corresponding dots.
As the bell gets higher,
The higher the density level, the lower the resolution and the higher the
High resolution and low gradation characteristics (image type
Resolution and gradation characteristics)
The transition from low to high concentration levels
As a result, it is possible to form an optimal image according to the visual characteristics, and to obtain a high-quality recorded image. In addition, the pixel density
If the level is higher than the specified density level and lower than the maximum density level
In this case, regardless of the pixel density level,
The value of the dot recording density level corresponding to the
Dot when the density level reaches a specific density level
Set the recording density level to the maximum density level of the pixel.
In the case of a pixel, the dot corresponding to a specific pixel in the block
By setting the value of the recording density level to the maximum value, for example,
Large dots, such as when using toner to record dots
It is completely fusing adjacent dots after thermal fixing with the composed
High-density levels due to suppression of so-called
It is possible to achieve both the improvement of the tonal characteristics and the reproduction of a high-density solid portion by positively utilizing the blur, and a high-quality recorded image can be obtained.

[Brief description of the drawings]

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

FIG. 2 is a block diagram showing a configuration of a tone processing unit of head 1;

FIG. 3 is a diagram illustrating an output waveform diagram and a counter output illustrating an operation of each circuit in FIG. 2;

FIG. 4 is a diagram illustrating a state of a counter output corresponding to each pixel position in FIG. 2;

FIG. 5 is a diagram showing the contents of a gradation conversion table of FIG. 2;

FIG. 6 is a graph showing the gradation conversion characteristics of FIG.

FIG. 7 is a block diagram illustrating a configuration of a gradation processing unit according to another embodiment of the present invention.

FIG. 8 is a diagram showing contents of a gradation conversion table of FIG. 7;

FIG. 9 is a graph showing the gradation conversion characteristics of FIG.

FIG. 10 is a side sectional view showing a configuration of a main part of a laser beam printer in which the present invention is implemented.

11 is a perspective view of a main part of the photoconductor reference detection mechanism of FIG. 10;

12 is a perspective view of a main part of the intermediate transfer member reference detection mechanism of FIG. 10;

FIG. 13 is a block diagram illustrating a configuration of an image processing apparatus using a conventional multilevel dither method.

14 is a graph showing a density conversion characteristic in the density conversion unit in FIG.

FIG. 15 is a diagram illustrating an example of a conventional 8 × 8 dither threshold matrix.

16 is an example of an output value in the multi-value dither method obtained from FIG.

[Explanation of symbols]

14… Print engine, 15… Image processing device, 15-1…
Density converter, 15-2: Conversion table, 15-3: Black / UC
R section, 15-4: color correction section, 15-5: data selector,
15-6: gradation processing section, 16: digital data output device,
27: horizontal synchronization signal generation circuit, 29: horizontal binary counter,
31: vertical synchronization signal generation circuit, 33: vertical binary counter, 35: gradation conversion table.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-58-24270 (JP, A) JP-A-62-110363 (JP, A) JP-A-59-189780 (JP, A) JP-A-1- 269367 (JP, A) JP-A-59-163959 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H04N 1/40-1/409 H04N 1/46 H04N 1/60

Claims (3)

(57) [Claims] 1. A according to the pixel position in each of a plurality of pixels generated by dividing the input image data composed blocks, the dot recording density level for the density level of each of the pixel recording of dots to use meaning the conversion table for converting into, an image forming apparatus that performs gradation recording by converting the density level of the pixel in the dot recording density level, the pre-Symbol conversion table, at least, the density level of the pixel Until the first density level is reached, the dot recording density level is gradually increased in accordance with the density level of the pixel, and after the first density level is reached, immediately before reaching the maximum density level. Converting the density level of the pixel to a dot recording density level when the density level reaches the first density level; First to convert 1
And a conversion table for converting the density level of the pixel into a dot recording density level at which dot printing is not performed until the density level reaches the second density level, and when the density level reaches the second density level, the first conversion In the table, up to the dot recording density level when the density level of the pixel reaches the first density level, at least the dot recording density level is gradually increased according to the density level of the pixel. An image forming apparatus comprising: a conversion table. 2. One by one based on input image data.
An image forming apparatus for performing gradation recording by changing the size of one dot, wherein the density level of the image data is converted to a density level for changing the size of each dot and printing dots. having a plurality of conversion tables, and with the concentration level after conversion for conversion before the concentration levels are different in the conversion table each prior Symbol process conversion table going concentration levels of at least the image data becomes high, the image data Until this density level reaches a first density level lower than the maximum density level, the density level is changed to a density level at which a dot of a first size smaller than the maximum size is recorded according to the density level of the image data. The density is converted to a density level at which dots are recorded so as to gradually increase, and the density level of the image data is The density level of the image data is converted to a density level at which dots of the first size are recorded irrespective of the density level of the image data after reaching the density level of 1 and immediately before reaching the maximum density level; A first conversion table for converting the density level of the image data to a density level at which a dot of the maximum size is recorded when the density level reaches the maximum density level, and until the density level reaches the second density level. When the density level of the image data reaches the second density level, at least the first size in the first conversion table is changed according to the density level of the image data. Conversion table for converting to a density level at which dot recording is performed so that the density level gradually increases up to the density level at which the dot is recorded. Conversion processing means comprising: a block dividing means for dividing the image data into a plurality of blocks by dividing the image data at a position where the image data is recorded so that one block is composed of a plurality of pixels; and a position of the pixel in the block. Different conversion tables are sequentially selected according to the density table, and a density level for performing dot recording is determined according to the density level of the image data according to the conversion table, and the dot size is determined according to the density level. An image forming apparatus comprising: a recording unit that performs recording by changing the image forming apparatus. 3. An image forming apparatus for performing gradation recording by changing a dot size corresponding to each pixel on a one-to-one basis based on input pixel data, wherein the density level of the pixel data is set to one. having a plurality of conversion tables for converting the density level for recording dots changing the size of dots, and with the concentration level after conversion for conversion before the concentration levels are different in each said conversion table, before Symbol conversion table At least in the process of increasing the density level of the pixel data, the density level of the pixel data is increased according to the density level of the pixel data until the density level reaches a first density level lower than the highest density level. Dot recording so as to gradually increase to a density level for recording a dot of a first size smaller than the size of the dot The density level of the pixel data is converted to the first density level after the density level of the pixel data reaches the first density level and immediately before reaching the maximum density level regardless of the density level of the pixel data. A first conversion table for converting the density level of the pixel data into a density level for printing a dot having a maximum size when the density level of the pixel data reaches the maximum density level; Converting the density level to a density level at which dot printing is not performed until the density level reaches the second density level; and when the density level of the pixel data reaches the second density level, the density level of the pixel data is changed. Gradually increases at least up to the density level at which the dots of the first size are recorded in the first conversion table. A conversion processing means including a second conversion table for converting into a density level at which dots are recorded; and a plurality of blocks by dividing the pixel data at a position where the pixel data is recorded so that one block is composed of a plurality of pixels. And a block dividing means for sequentially selecting the different conversion tables according to the positions of the pixels in the block to correspond to each other, and printing dots according to the density level of the pixel data according to the conversion table. And a recording means for determining the density level of the image and changing the size of the dot in accordance with the density level for recording.