JPH09123524A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable to reduce the dissipation amounts of coloring materials while maintaining an excellent image by so controlling as to reduce the number of gradations while maintaining the linear gradation expression of the image formed on a sheet at the time of setting a toner save mode by providing a setting means of the toner save mode. SOLUTION: When an intermediate gradation processor 114 for executing the area gradation process of image data developed by an image memory 113 by a dither method to generate a binary bit image inputs a binary toner save mode signal from a host computer, it reduces the number of gradations of the image formed on a sheet by using a dither matrix for a toner save mode. That is, the dither matrix for the toner save mode is used at the time of setting the toner save (at the time of toner save mode signal being '0') by using a dither table memory, the doubled value of the threshold value allocated to the normal dither matrix is allocated, and the dissipated toner amount is reduced by half.

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 such as a laser beam printer.

[0002]

2. Description of the Related Art Conventionally, as a device for reproducing an image on a paper, a printer or a copying machine utilizing an electrophotographic system or an inkjet system has been known. These printers and copiers reproduce images by attaching coloring materials such as toner and ink onto paper. Even for the same one-page image, the number of characters and the thickness of the characters are different, or halftones such as those in photographs are included, so there are various conditions regarding the black-white ratio of the image. Therefore, even when reproducing the same one-page image, the consumption of the coloring material changes depending on the black-white ratio,
It will affect the running cost. In particular, when an image with a high black-white ratio (a lot of black) is reproduced, a relatively high cost is incurred even if it is only necessary temporarily such as test printing. In order to reduce such wasteful cost as much as possible, a device or a circuit having a toner save mode has been proposed or put into practical use (for example, H
L, a printer manufactured by Hewlett Packard
aser Jet4 or EET ASIC which is a circuit made by Destiny). In the toner save mode, the image data is thinned out every other dot so that the number of printed dots is approximately halved to reduce the consumption of the coloring material. For example, as shown in FIG.
When the image data indicating the character "A" is printed, the above-mentioned conventional device and circuit print out in the state as shown in FIG. In this case, the overall print result is an image with a low density, but it is effective in cases such as test printing and when image quality is not required so much (when it can be read as characters).

[0003]

The toner save mode described above functions more effectively for an image having a high black / white ratio (amount of toner or ink adhered). In a character image, the black-white ratio is about several percent. On the other hand, in a halftone image such as a photograph, the black-white ratio becomes 30% or more when the entire surface is printed, and it is not unusual that the black-white ratio exceeds 50%. That is, it can be said that the toner save mode is more effective for a halftone image. However, when the conventional toner save mode is set, the toner consumption amount is reduced by simple thinning processing of dot data. Therefore, it is necessary to perform thinning processing on a halftone image represented by the area gradation method. , The linearity of gradation expression is lost. This phenomenon will be described with reference to FIGS. The area gradation method is n
Handle a sub-matrix of × n dots as one pixel,
The gradation of each pixel of the halftone image is expressed by changing the ratio of black pixels in the sub-matrix. In the image forming apparatus adopting the area gradation method, binary bit image data is obtained by applying area gradation processing by a well-known dither method to image data of a halftone image in which the gradation of each dot is represented by multivalued data. To form. FIG. 3 shows a total of 16 dither patterns in the case of expressing the gradation of 16 gradations with a 4 × 4 dot sub-matrix as one pixel. When thinning processing for each dot is performed on each dither pattern, the result is as shown in FIG. 4 or 5. Since the dot data in each pattern is thinned out, it is natural that the number of gradations is reduced. Furthermore, it is understood that the linear gradation expression is lost because the density changes are not uniform in both cases, and the same density continues for many times or changes to the next density immediately. It

It is an object of the present invention to provide an image forming apparatus having a toner save mode for reducing the amount of coloring material consumed while maintaining a good image.

[0005]

An image forming apparatus of the present invention executes setting means for setting a toner save mode and area gradation processing to convert multivalued image data into binary data and output it. A halftone processing unit that reduces the number of gradations while maintaining the linear gradation expression of the image formed on the paper when the toner save mode is set by the setting unit, compared to when the mode is not set; An image forming unit that forms an image on a sheet based on the binary data output from the processing unit. The image forming apparatus of the present invention expresses a halftone image by the area gradation method, and when the toner save mode is set, the dots of the image are not thinned and are formed on the paper as compared to when the toner save mode is not set. The feature is that the number of gradations of the image is reduced. As a result, the consumption of the coloring material is reduced while maintaining the linear gradation expression. Further, preferably, the halftone processing unit includes a memory for storing a threshold value set for each dot in the dither matrix used in the area gradation processing, and stored in the memory when the toner save mode is set. When the toner save mode is set by the setting unit, the number of gradations of the image formed on the sheet by the image forming unit is reduced by switching the threshold value.
Further, preferably, the halftone processing unit includes a buffer that reduces the multi-valued image data to half the value, and executes the area gradation processing.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A laser beam printer 100 which is an embodiment of an image forming apparatus of the present invention will be described below with reference to the accompanying drawings. FIG. 6 shows the configuration of a laser beam printer 100 that adopts the area gradation method. The signal processing unit 110 is sent from the host computer 200 via the host interface 50.
The bit image data is subjected to area gradation processing by the dither method to form binary bit image data, and the formed binary bit image data is output to the exposure unit 1. The exposure unit 1 emits a laser beam based on the input binary bit image data. The laser beam emitted from the exposure unit 1 exposes the surface of the photoconductor 3 via the mirror 2 as shown by the arrow. The photoreceptor 3 uniformly charged by the charger 4 forms an electrostatic latent image on the surface of the photoreceptor 3 when exposed by the exposure unit 1. The developing device 5 attaches toner as a coloring material to the electrostatic latent image formed on the surface of the photoconductor 3 to form a visible image.
The toner attached to the photoconductor 3 is transferred by the transfer charger 8 onto the transfer paper conveyed from the paper feed cassette 6 by the conveyance roller 7. The toner image transferred to the copy paper is fixed by the fixing device 9 and then discharged to the paper discharge tray 10. The potential remaining on the surface of the photoconductor 3 after the transfer operation is removed by the eraser 11. Since the image forming operation of the electrophotographic laser beam printer is well known,
Detailed explanation is omitted.

FIG. 7 is a block diagram of the signal processor 110. The CPU 110 develops image data representing one dot as 8-bit data in the image memory 113 based on the image data received from the host computer 200 via the host interface 50. Besides this, CPU
Reference numeral 111 denotes a control of the entire signal processing unit 110 and a printer engine (exposure unit 1, a motor (not shown) that drives the photosensitive drum 3), a charger 4, a developing device 5, a conveyance roller 7, a transfer charger 8, and a fixing device. 9 and eraser 11
Etc.) and communication control. The ROM 112 has a CPU
A program executed by 111 is stored. The image memory 113 is composed of a DRAM and has a CP
In addition to storing the image data expanded in U111, C
It is also used as a work area for the PU 111. The halftone processing unit 114 performs area gradation processing by the dither method on the image data expanded in the image memory 113 to generate a binary bit image. A binary toner save mode signal is input from the host computer 200 to the halftone processing unit 114. When the toner save mode is set, the value of the toner save mode signal input from the host computer 200 is 0. As will be described later, when the toner save mode is set (when the value of the toner save mode signal is 0), the halftone processing unit 114 does not set the toner save mode (when the value of the toner save mode signal is 1). The number of gradations of the image formed on the paper is reduced as compared with the above, and the processing of reducing the toner consumption amount is performed.

FIG. 8 shows the first circuit of the halftone processing unit 114.
An example will be described. In this circuit, a dither matrix for toner save mode is provided in addition to the normal dither matrix. When setting the toner save mode, the number of gradations of the image formed on the paper is reduced by using the dither matrix for the toner save mode. The image data representing one dot in 8 bits transferred from the image memory 113 is input to the comparator 120. When the host computer 200 sets the toner save mode to 0,
When the toner save mode is not set, the toner save mode signal of 1 is output to the dither table memory 121 via the host interface 50. The dither table memory 121 stores the threshold values of the normal dither matrix and the dither matrix for the toner save mode. When the toner save mode is not set (when the toner save mode signal is 1), the normal dither matrix is stored. Dither matrix (Fig. 9
When the toner save is set (when the toner save mode signal is 0), the dither matrix for the toner save mode (see FIG. 10) is used. 2-bit address counters 122 and 1 for main scanning and sub-scanning, respectively
23 counts up with the dot synchronization clock and the line synchronization clock, respectively. The dither table memory 121 outputs to the comparator 120 the threshold value at the address specified by the count value of each of the counters 122 and 123 in the dither matrix specified based on the value of the toner save mode signal. The comparator 120 compares the value of the image data with a threshold value and outputs binary bit image data indicating black if the value of the image data is larger than the threshold value and white otherwise. .

9 and 10 show an example of a normal dither matrix stored in the dither table memory 121 and a dither matrix for the toner save mode. FIG.
In the case of the normal dither matrix shown in FIG. 8, 8, 24, 40, 56, ...
6, 232, and 248 are set, and thresholds that increase in 16 units are set. On the other hand, in the case of the dither matrix for toner save mode shown in FIG. 10, 16, 48,
, 80, ..., And a value that is twice the threshold value assigned to the normal dither matrix is assigned. In addition,
If the value obtained by doubling the threshold value assigned to each dot of the normal dither matrix exceeds 255 (maximum value), the threshold value of 255 is assigned. When the threshold value is 255, the comparator 120 outputs binary bit image data indicating white for all image data. By using the dither matrix shown in FIG. 10 for the toner save mode, the number of gradations (density) of the image reproduced on the paper is halved. As a result, when the toner save mode is set, the amount of toner consumed can be reduced by half while maintaining the linear gradation on the low density side. The ratio of the threshold value assigned to the dither matrix for the toner save mode to the threshold value assigned when the toner save mode is not set is
The number is not limited to double, and may be determined according to the image quality required when the toner save mode is set. That is, if the reduction in toner consumption is important, the magnification may be increased, and if the image quality is desired to some extent even when the toner save mode is set, the magnification may be decreased. In either case, the linear gradation expression is maintained.

Next, a laser beam printer according to another embodiment will be described. FIG. 11 shows a circuit of the halftone processing unit 114 of FIG. 7 in this embodiment. FIG.
The configuration of the laser beam printer other than the halftone processing unit 114 shown in FIG. 1 is the same as that already described, and therefore, redundant description will be omitted here. In the present embodiment, when the toner save mode is set, binary bit image data is output by using the data of half the value of the image data. This reduces the number of gradations of the image formed on the paper.
In the halftone processing unit 114, the image data transferred from the image memory 113 is input to the two data buffers 150 and 151. Image data is 1
A dot is represented by 8 bits. Data obtained by shifting the 8-bit image data by 1 bit in the lower direction, that is, the lower 2nd bit data is input to the data buffer 150 as the lower 1st bit data. Bit data having a value of 0 is input to the upper 1 bit of the data buffer 150. As a result, the value of the data input to the data buffer 150 becomes half the value of the image data.
The data buffers 150 and 151 are provided with inverters 170 and 171 at their control terminals, and output the held image data to the comparator 152 in response to the input of the signal of value 1. The toner save mode signal inverted by the inverter 170 is input to the control terminal of the data buffer 150. The toner save mode signal inverted by the inverter 172 and further inverted by the inverter 171 is input to the control terminal of the data buffer 151. When the toner save mode signal is 1, that is, when the toner save mode is not set, the data buffer 1
The signal 1 is input to 51. The data buffer 151 outputs the held image data to the comparator 152. When the toner save mode signal is 0, that is, when the toner save mode is set, the data buffer 15
A signal having a value of 1 is input to 0. Data buffer 150
Compares the stored image data with the comparator 152.
Output to The dither table memory 153 stores threshold values of a dither matrix as shown in FIG. 9, for example. The dither table memory 153 has 2-bit counters 154 and 155 for each of main scanning and sub-scanning.
The output from is input as the address of the dither matrix. The dither table memory 153 outputs the threshold value of the input address to the comparator 152. The comparator 152 compares the image data input from the data buffer 150 or 151 with the threshold value data input from the dither table memory 153. When the image data is larger than the threshold value, black is output. ,
Otherwise, the binary bit image data indicating white is output. The data input to the comparator 152 from the data buffer 150 when the toner save mode is set has a value (density) that is one half of the image data, so the density of the dither processing is also halved.
As a result, it is possible to reduce the toner consumption amount by half while maintaining the linear gradation expression.

FIG. 12 shows a dither pattern used in the dither table memory 153 when the toner save mode is set. When the dither pattern shown in FIG. 12 is compared with the normal dither pattern used when the toner save mode is not set as shown in FIG. 3, the toner save is larger than the normal dither pattern used when the toner save mode is not set. It can be seen that the number of dither patterns used when setting the mode is reduced by half. This is because when the toner save mode is set, the value of the image data output from the data buffer 150 is half the value when the toner save mode is not set. FIG.
Is a graph showing the relationship between the input density (the value of 8-bit image data) and the print density (the gradation reproduced on the paper). From this graph, the linear gradation expression is achieved even when the toner save mode is set. Is understood to maintain. In this example, the value of the image data input to the dither table memory 153 when the toner save mode is set is half the value when the toner save mode is not set, but the value is not limited to this and the toner save mode is not limited. It may be determined according to the image quality required when setting the mode. That is, if the reduction in toner consumption is important, the magnification may be increased, and if the image quality is desired to some extent even when the toner save mode is set, the magnification may be decreased. In either case, the linear gradation expression is maintained.

Next, a laser beam printer according to the third embodiment will be described. This embodiment differs from the other embodiments only in the halftone processing unit 114. FIG.
Reference numeral 4 denotes a circuit of the halftone processing unit 114. In this embodiment, the same members as those described above are designated by the same reference numerals, and detailed description thereof will be omitted. This circuit is different from the first and second embodiments in that an error diffusion method is applied when performing halftone reproduction. That is, the 8-bit image data output from the data buffers 150 and 151 is added to the comparator 161 via the adder 160.
Is input to The comparator 161 stores a threshold value and compares the input image data with the threshold value. In this circuit, this threshold value is 127. When the image data is larger than the threshold value, 8-bit image data (11111111) indicating black is
Otherwise, 8-bit image data (00000000) indicating white is output to the binarization circuit 162. The binarization circuit 162 converts 8-bit image data indicating black to bit data (1) indicating black, converts 8-bit image data indicating white to bit data (0) indicating white, and outputs the bit data (0). To do. In addition, the comparator 161
The image data output from is input to the adder 163. Image data that has not passed through the comparator 161 is also input to the adder 163, and the difference between the two is output to the error filter 164 as an error. The error filter 164 performs a process of distributing this error to surrounding pixels. Next, the image data of the pixel input to the adder 160 is added to the error data corresponding to this pixel output from the error filter 164, and input to the comparator 161. Comparator 1
The output from 61 is input to the binarization circuit 162 and the addition circuit 163 as described above, and the same processing as the above-described processing is performed. Also in the present embodiment, by using the data buffers 150 and 151 by switching according to the value of the toner save signal, it is possible to switch between normal halftone processing and halftone processing for toner saving.
The linear gradation expression is maintained regardless of which of the data buffers 150 and 151 is used.

Next, a laser beam printer according to the fourth embodiment will be described. This embodiment differs from the other embodiments only in the halftone processing unit 114. FIG.
Indicates a circuit of the halftone processing unit 114. This circuit also reproduces halftones by the error diffusion method. In this circuit,
The comparator 165 stores a threshold value for normal halftone processing and a threshold value for halftone processing for toner saving. Further, the comparator 165 inputs the toner save mode signal, and switches and uses these two threshold values. In the present embodiment, these thresholds are 127 in the normal mode and 200 in the toner save mode. The halftone processing of the error diffusion method in this circuit is the same as that in the third embodiment, and therefore detailed description thereof is omitted. Also in this embodiment, the comparator 1
By switching the threshold used at 65,
The normal halftone process and the halftone process for toner saving can be switched and performed. Even if either of the two threshold values is used, the linear gradation expression is maintained.

Although the laser beam printer has been described above as an example, the present invention is not limited to the ink jet printer,
The form of image formation does not matter as long as it is a device such as a thermal printer that consumes a coloring material to form an image. In particular, the above-described halftone processing can be effectively applied to an image forming apparatus that expresses a halftone image in an area gradation method. When applied to inkjet printers and thermal printers,
When the toner save mode (ink save mode) is set, it is possible to reduce the consumption of ink as a coloring material while maintaining linear gradation expression. Further, since the number of times the inkjet head or the thermal head is driven can be reduced, the life of the head can be extended. Further, regarding a laser beam printer that performs reversal development, it is possible to reduce the number of times the laser emits light when the toner save mode is set, and it is possible to extend the life of the laser light source, for example, a semiconductor laser. Also,
In the laser beam printer described above, the area gradation processing using the dither method and the switching of the dither matrix and image used when setting the toner save mode are performed using hardware, but they are also performed by software processing in the CPU 111. be able to. Further, in the above-described embodiment, as the area gradation processing, the simple dither processing and the dither processing of the error diffusion method have been described as examples, but the invention is not limited to this, and the area gradation processing using the density pattern method is not limited to this. May be Further, in the above-described embodiment, the toner save mode is set on the host computer side, but it may be set on the printer side. For example, the printer has a switch for setting the toner save mode,
The toner save mode setting may be switched in response to the operation of this switch by the user.

It is also possible to provide means for detecting the remaining amount of the coloring material in the printer, and to automatically switch to the toner save mode when this means detects that the coloring material is running out. Specifically, in the case of a laser beam printer, a sensor for detecting the remaining amount of toner is provided in the developing device, and if the remaining amount of toner is found to be less than a predetermined amount by the output from this sensor, the toner save mode is set. Run. As the sensor for detecting the remaining amount of toner, a weight sensor, a photo sensor, or the like can be applied. When the developing device contains a two-component developer, the magnetic permeability of the developer may be detected to detect the remaining amount. A more specific method of detecting the remaining amount is well known, and therefore the description thereof is omitted here. In the case of an inkjet printer or a thermal printer, a sensor for detecting the remaining amount of ink may be provided in the ink tank. In this way, when the toner save mode is automatically set according to the remaining amount of the coloring material, even if the remaining amount of the coloring material is less than the predetermined amount, the user can obtain a high quality image. Image output may be required. Therefore, it is desirable that the user can cancel the setting of the toner save mode. Specifically, a switch for releasing the toner save mode may be provided, and the toner save mode may be released in response to the switch being turned on. Alternatively, the printer may not be provided with a switch, and the toner save mode may be released in response to a command from the host computer. In the example of the above-described embodiment, the toner save mode signal may be set to 1 in response to the cancellation command from the user.

[0016]

According to the image forming apparatus of the present invention, the halftone processing unit switches the threshold value set for each dot in the dither matrix used in the area gradation processing, and multivalued image data is displayed. The halftone data is used to execute the area gradation processing, and when the toner save mode is set, the number of gradations of the image reproduced on the paper is reduced as compared with the case where the toner save mode is not set. With this, while maintaining the linear gradation expression,
Toner consumption can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing bitmap data developed based on character image data.

FIG. 2 is a diagram showing a state of an image formed on a sheet when the bitmap data shown in FIG. 1 is thinned out for each dot and output to a printer engine.

FIG. 3 is a diagram showing a total of 16 patterns of sub-matrix in the case of expressing a gradation of 16 gradations with a 4 × 4 dot sub-matrix as one pixel.

FIG. 4 is a diagram showing a result when thinning processing for each dot is performed on the data of each pattern in FIG.

FIG. 5 is a diagram showing a result when thinning processing for each dot is performed on the data of each pattern of FIG.

FIG. 6 is a sectional view of a laser beam printer that employs an area gradation method.

FIG. 7 is a block diagram of a signal processing unit.

FIG. 8 is a circuit diagram of an example of a halftone processing unit.

FIG. 9 is a diagram showing a normal dither matrix.

FIG. 10 is a diagram showing a dither matrix for toner save mode.

FIG. 11 is a circuit diagram of a halftone processing unit according to the second embodiment.

FIG. 12 is a diagram of a dither pattern used when the toner save mode is set.

FIG. 13 is a graph showing the relationship between input density (value of 8-bit image data) and print density (gradation reproduced on paper).

FIG. 14 is a circuit diagram of a halftone processing unit according to the third embodiment.

FIG. 15 is a circuit diagram of a halftone processing unit according to a fourth embodiment.

[Explanation of symbols]

 100 ... Laser beam printer 110 ... Signal processing unit 111 ... CPU 112 ... ROM 113 ... Image memory 114 ... Halftone processing unit 120, 152 ... Comparator 121, 153 ... Dither table memory 122, 123, 154, 155 ... Counter 150, 151 ... Data buffers 170, 171, 172 ... Inverter 200 ... Host computer

Claims (3)

[Claims] 1. A setting means for setting a toner save mode, and area gradation processing is executed to convert multi-valued image data into binary data for output, and when the toner save mode is set by the setting means, Based on the halftone processing unit that reduces the number of gradations while maintaining the linear gradation expression of the image formed on the paper as compared to when the mode is not set, and the binary data output from the halftone processing unit. An image forming apparatus, comprising: an image forming unit that forms an image on a sheet. 2. The image forming apparatus according to claim 1, wherein the halftone processing unit includes a memory that stores a threshold value set for each dot in a dither matrix used in area gradation processing, When the toner save mode is set, the threshold value stored in the memory is switched so that when the toner save mode is set by the setting unit, the image forming unit forms the image on the sheet as compared to when the mode is not set. An image forming apparatus characterized by reducing the number of gradations of an image. 3. The image forming apparatus according to claim 1, wherein the halftone processing unit is provided with a buffer that reduces multi-valued image data to a half value, and executes area gradation processing. Image forming apparatus.