JP2011107546A - Image forming apparatus, image forming method and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To more accurately determine colors required for image formation, and appropriately suppress image formation using unneeded toner. <P>SOLUTION: The image forming apparatus 1 includes: a halftone image-processing part 64 to determine according to colors, a pulse width value corresponding to each of pixels based on image data; a pixel count part 65 to determine the presence or absence of dot formation based on the pulse width value corresponding to a pixel as a determination object, and a pulse width value corresponding to a pixel adjacent to the dot of the pixel as the determination object, and formed just before or just after the pixel as the determination object, with respect to the pulse width value corresponding to each of the pixels; and a CPU 52 to change colors and the number of colors used for the image formation based on the determination result by the pixel count part 65. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, an image forming method, and a program.

  Conventionally, there has been known an image forming apparatus capable of performing multicolor image formation using a plurality of different color toners, that is, so-called color image formation. In such a color image forming apparatus that includes black (K) as the toner to be used, an image that can be switched between color image formation and black-color image formation (monochrome image formation). There is a forming device.

  Furthermore, there is an image forming apparatus that performs color determination for determining whether to use multiple colors for image formation based on image data to be printed (for example, Patent Document 1). In such an image forming apparatus, in an image forming apparatus set to perform color image formation, monochrome image formation is performed on image data (monochrome image data) formed in a single color, It is possible to suppress waste of time and consumption of members associated with the operation of each unit for image formation using toner other than black.

JP 2007-251740 A

  However, color determination by a conventional image forming apparatus is insufficient, and multicolor toner may be used for an image formed with a single black color.

  Some image forming apparatuses employ a multi-tone reproduction method using pulse width modulation (PWM). In this method, multi-tone reproduction is obtained by changing the dot size of each pixel forming an image according to the pulse width. The pulse width in the multi-tone reproduction method using PWM is individually set for each of a plurality of color toners based on the input image data. The multi-tone reproduction method using PWM can be used in combination with other multi-tone reproduction methods, for example, tone reproduction by pixel density adjustment or dither processing.

  FIG. 15 shows an example of the relationship between the pulse width, the development threshold, and the dots to be formed. As shown in the figure, dot formation by the multi-tone reproduction method using PWM depends on the pulse width. If the pulse width falls below a predetermined threshold (for example, the duty ratio is 25 [%]), no dot is formed. The pulse functions as a signal for driving a laser diode (Laser Diode, LD) for performing an exposure process necessary for forming a dot, but a predetermined period from when the pulse is input until the laser diode is driven. There is a delay time. For this reason, when the laser diode is driven with a pulse width less than a predetermined threshold, the amount of light for performing the exposure process cannot be obtained, and no dot as a toner image is formed.

FIG. 16 is a graph showing an example of the relationship between the development threshold and the area of dots to be formed. H, I, and J shown in FIG. 16 correspond to the dots H, I, and J shown in FIG.
As shown in the above description and FIGS. 15 and 16, when the pulse width is below a predetermined threshold, no dots are formed.

  On the other hand, when the pulse width is less than the predetermined threshold, dots are not necessarily formed. For example, when there is a pixel whose pulse width is equal to or greater than the predetermined threshold immediately before a pixel whose pulse width is less than the predetermined threshold, the delay time for driving the laser diode is also applied to the pixel whose pulse width is lower than the predetermined threshold. Dots can be formed without being affected. Thus, the relationship between the pulse width at the target pixel and the presence or absence of dot formation is not uniquely determined, and is affected by the pulse width at the pixel adjacent to the target pixel.

  However, in the conventional color determination, color determination is simply performed based on the presence or absence of a pulse regardless of whether or not a toner image is actually formed as a dot. That is, in the conventional color determination, when there is a pulse for dot formation other than black, the input image data is always determined as color image data. For this reason, in the conventional image forming apparatus based on color determination, an operation for forming a color image may be performed even on an image on which dots other than black are not formed at the time of output. In some cases, the consumption of the operation of each part related to image formation and the waste of time for the operation are caused.

  In particular, in an image forming apparatus that superimposes an image formed by a plurality of types of toner on a single photosensitive drum, a waste of time for color image formation occurs more remarkably. As an example of such an image forming apparatus, a so-called CMYK one-drum four-cycle image forming apparatus using cyan (C), magenta (M), yellow (Y), and black (black (K)) toners is given. . In the case of an image forming apparatus of a 1-drum 4-cycle system, in the case of monochrome image formation, processing that requires only one rotation of the photosensitive drum for black color, and in the case of color image formation, depending on the number of types of toner. It is necessary to rotate the photosensitive drum a number of times, in this case, four rotations of the photosensitive drum. That is, in such an image forming apparatus, in the case of color image formation, it is necessary to rotate the photosensitive drum by the number of times proportional to the type of toner as compared with monochrome image formation. For this reason, when image formation by color image formation is performed even though dots other than black are not formed, rotation of the photosensitive drum for toner unnecessary for actual dot formation is involved. Reduce productivity.

  The above problem is not limited to monochrome / color distinction, and there is a problem that image formation using toner unnecessary for image formation may be performed in image formation using a plurality of colors.

  An object of the present invention is to more accurately determine a color necessary for image formation and suitably suppress image formation using unnecessary toner.

  According to the first aspect of the present invention, an electrostatic latent image is formed by optically scanning the photosensitive member in the main scanning direction by driving the laser light source in accordance with the pulse width of the pulse width modulation signal for each of the plurality of toners. An image forming apparatus for developing an image by developing the electrostatic latent image with toner, a pulse width determining unit that determines a pulse width of a pulse width modulation signal for each pixel of image data, and a determination target Is determined based on the pulse width of the pulse width modulation signal corresponding to the determination pixel and the pulse width of the pulse width modulation signal corresponding to the adjacent pixel adjacent to the determination pixel in the main scanning direction. A determination unit that determines whether or not a dot is formed on the determination pixel, and a switching control unit that switches a color and the number of colors used for image formation based on a determination result of the determination unit. To.

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the determination unit applies the determination pixel to a value based on a pulse width of a pulse width modulation signal corresponding to the determination pixel. For the first value obtained by adding a value based on the pulse width of the pulse width modulation signal output immediately before the corresponding pulse width modulation signal and the value based on the pulse width of the pulse width modulation signal corresponding to the determination pixel Generating a second value obtained by adding a value based on the pulse width of the pulse width modulation signal output immediately after the pulse width modulation signal corresponding to the determination pixel, and generating the first value and the second value. And a predetermined threshold value, and the number of dots formed for each color is counted based on a comparison result between the first value and the second value and the predetermined threshold value, and the switching control unit Dot formation of each color counted by the part And switches the color and color number used in the image forming based on.

  According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, at least one of the first value and the second value is greater than a predetermined threshold value. In this case, it is counted that dots are formed in the determination pixel.

  A fourth aspect of the present invention is the image forming apparatus according to the second or third aspect, wherein the switching control unit performs image processing based on a comparison result between the number of dot formations of each color and a predetermined number of dot formations. The color and the number of colors used for formation are switched.

  A fifth aspect of the present invention is the image forming apparatus according to any one of the second to fourth aspects, wherein colors of the plurality of toners include colors other than black and black, and the switching control unit Is characterized in that the color used for image formation is switched to one of black or a plurality of colors based on the number of dots formed in colors other than black.

  A sixth aspect of the present invention is the image forming apparatus according to the fifth aspect, wherein the plurality of toners correspond to each color for color printing including black and at least cyan, magenta, and yellow, and the switching is performed. The control unit switches the color used for image formation to one of black or black and color for color printing based on the number of dots formed for each color printing color.

  A seventh aspect of the present invention is the image forming apparatus according to any one of the first to sixth aspects, wherein the pulse width determining unit performs dither processing based on the image data, The pulse width of the pulse width modulation signal corresponding to each pixel of the image is determined for each color.

  According to an eighth aspect of the present invention, an electrostatic latent image is formed by optically scanning the photoconductor in the main scanning direction by modulating and driving a laser light source for each of the plurality of toners according to the pulse width of the pulse width modulation signal. An image forming method for forming an image by developing the electrostatic latent image with toner, a step of determining a pulse width of a pulse width modulation signal for each pixel of image data, and a determination pixel to be determined The determination pixel based on the pulse width of the pulse width modulation signal corresponding to the determination pixel and the pulse width of the pulse width modulation signal corresponding to the adjacent pixel adjacent to the determination pixel in the main scanning direction. And a step of determining whether or not dots are formed, and a step of switching colors and the number of colors used for image formation based on the determination result.

  According to a ninth aspect of the present invention, there is provided a program according to a ninth aspect of the present invention, wherein for each of a plurality of toners, a laser light source is modulated and driven in accordance with the pulse width of a pulse width modulation signal to optically scan the photosensitive member in the main scanning direction. An image forming apparatus for forming and developing an image by developing the electrostatic latent image with toner, means for determining a pulse width of a pulse width modulation signal for each pixel of image data, and a determination target For the pixel, the determination is made based on the pulse width of the pulse width modulation signal corresponding to the determination pixel and the pulse width of the pulse width modulation signal corresponding to the adjacent pixel adjacent to the determination pixel in the main scanning direction. It is characterized by functioning as means for determining whether or not a dot is formed on a pixel, and means for switching the color and number of colors used for image formation based on the determination result. .

  According to the present invention, it is possible to more accurately determine a color necessary for image formation and to appropriately suppress image formation using unnecessary toner.

1 is a main configuration diagram of an image forming apparatus 1 according to an embodiment of the present invention. 3 is a block diagram illustrating a configuration of a control unit 50. FIG. It is a data block diagram which shows an example of a structure of a band frame. It is a data block diagram which shows an example of a frame header area | region. 3 is a block diagram showing a configuration of a band image processing unit 60. FIG. It is explanatory drawing which shows the structure of the halftone image process part 64, and the structure of the dither screening process which used pulse width modulation together. It is explanatory drawing which shows an example of a dither threshold value array. It is a table which shows an example of the relationship between the input value of the total part 644, and an output value. It is explanatory drawing which shows an example of the data before and behind the dither screening process which used pulse width modulation together. FIG. 9A is an explanatory diagram showing an example of pixel data before dither screening processing using pulse width modulation together, and FIG. 9B is an example of pulse width values after dither screening processing using pulse width modulation together. It is explanatory drawing which shows. 3 is a circuit diagram showing a configuration of a pixel count unit 65. FIG. It is a table which shows the relationship between the input and output of encoder ENC. It is explanatory drawing which is an example of the data before and behind the dither screening process which used pulse width modulation together, and shows the case where the pixel used as an effective pixel is included. FIG. 12A is an explanatory diagram showing an example of pixel data including effective pixels, and FIG. 12B shows the pulse width value when the dither screening process is performed on the pixel data of FIG. It is explanatory drawing which shows an example. It is an example of data before and after the dither screening process using pulse width modulation together, and is an explanatory diagram showing a case where a pixel that becomes an effective pixel is not included. FIG. 13A is an explanatory diagram showing an example of pixel data not including effective pixels, and FIG. 13B is a pulse width value when the dither screening process is performed on the pixel data of FIG. It is explanatory drawing which shows an example. It is a flowchart which shows the flow of a process at the time of performing the dither screening process combined with the pulse width modulation by software processing, and the count process of the number of effective pixels. It is explanatory drawing which shows an example of the relationship between a pulse width, a development threshold value, and the dot formed. It is a graph which shows an example of the relationship between a development threshold value and the area of the dot formed.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an image forming apparatus 1 according to an embodiment of the present invention.
The image forming apparatus 1 includes a document reading unit 2, an operation panel 3, a paper feed cassette 5, a transfer unit 20, a fixing unit 40, and a control unit 50.

The document reading unit 2 reads a document and generates image data. The document reading unit 2 is an image scanner that irradiates a document with light, receives the reflected light by an image sensor, and converts it into an electrical signal. The document reading unit 2 has a translucent document table, and reads a document placed on the document table. The electrical signal generated by the document reading unit 2 undergoes analog-digital conversion and is converted into image data as a digital signal. This image data can be used as print image data.
Although not shown, the document reading unit 2 may include another document reading mechanism. For example, an automatic paper feeder (ADF, Auto Document Feeder) may be provided to automatically and continuously read a plurality of documents set on a document table.

  The operation panel 3 functions as an input device that enables various inputs to the image forming apparatus 1 and also functions as a display device that displays various information related to the image forming apparatus 1.

The paper feed cassette 5 is a tray for storing paper P. The image forming apparatus 1 forms an image on the paper P.
When an image is formed on the paper P, the paper P is conveyed by a roller provided in the image forming apparatus 1 to form an image. In the configuration shown in FIG. 1, the paper P is transported upward from the paper feed cassette 5 one by one by the paper feed roller 11 and the separating roller 12, transported by the timing rollers 13 and 14, and guided to the transfer unit 20.

The transfer unit 20 transfers the toner image onto the paper P.
The transfer unit 20 includes an intermediate transfer belt 21, image forming units 30C, 30M, 30Y, and 30K, and a cleaning device 39.
The intermediate transfer belt 21 is a ring-shaped belt, and is supported by rollers 22, 23, 24 and transfer rollers 25, 25, 25, 25 provided on the inner side thereof, and is driven along the direction of arrow V shown in FIG. Is done.

  The image forming units 30C, 30M, 30Y, and 30K form toner images of respective colors of CMYK and transfer (primary transfer) to the intermediate transfer belt 21. The image forming unit 30C is a cyan (C) toner image, the image forming unit 30M is a magenta (M) toner image, the image forming unit 30Y is a yellow (Y) toner image, and the image forming unit 30K is black (K). That is, a black toner image is formed and transferred to the intermediate transfer belt 21.

Each of the image forming units 30C, 30M, 30Y, and 30K includes a photosensitive drum 31, a laser scanning unit 32, and a developing unit 33.
The photosensitive drum 31 rotates in the clockwise direction (sub-scanning direction) in the figure, and its outer peripheral surface is uniformly charged by a charger (not shown). The laser scanning unit 32 forms an electrostatic latent image on the photosensitive drum 31 by optically scanning the photosensitive drum 31 in the main scanning direction by deflecting laser light generated by modulating a laser light source by a polygon mirror. . The developing unit 33 performs a developing process for forming a toner image by attaching toner to the photosensitive drum 31 on which the latent image is formed.

  The photosensitive drums 31 of the image forming units 30C, 30M, 30Y, and 30K are provided at positions facing the individual transfer rollers 25, 25, 25, and 25, respectively. The toner images of the respective colors formed on the photosensitive drum 31 are transferred (primary transfer) to the intermediate transfer belt 21 by the electric field applied by the transfer roller 25.

  Although not shown, each of the image forming units 30C, 30M, 30Y, and 30K has a cleaning device for cleaning the toner and residual charge remaining on the photosensitive drum 31 after the transfer. A toner image is formed and transferred.

The intermediate transfer belt 21 transfers (secondary transfer) the toner image transferred (primary transfer) by the image forming units 30C, 30M, 30Y, and 30K to the paper P. The toner image transferred (primary transfer) to the intermediate transfer belt 21 is transferred (secondary transfer) to the paper P by an electric field applied by a transfer roller 26 provided at a position facing the roller 22.
The intermediate transfer belt 21 after the secondary transfer is cleaned by a cleaning device 39. The transfer unit 20 transfers an image to one or a plurality of sheets P by repeating primary transfer, secondary transfer, and cleaning.

The fixing unit 40 performs a fixing process on the paper P to which the toner image is transferred. The fixing unit 40 includes a heating roller 41 and a roller 42.
The heating roller 41 has a configuration for generating heat inside thereof, and heats the sheet P that contacts the outer peripheral surface by heating the outer peripheral surface. The roller 42 is provided at a position facing the heating roller 41 across the conveyance path of the paper P, and sandwiches the paper P in cooperation with the heating roller 41. The paper P sandwiched between the heating roller 41 and the roller 42 is heated and pressurized, and the toner image transferred by this heating and pressure is fixed on the paper P.

  The paper P after the fixing process is conveyed by the paper discharge rollers 15 and 16 and discharged onto the paper discharge unit 4.

  The image forming apparatus 1 of the present embodiment can perform image formation by either a monochrome method or a color method. The monochrome method is image formation using only black (K) toner. The color system is image formation using toners of four colors of cyan (C), magenta (M), yellow (Y), and black (K).

In the case of monochrome image formation, primary transfer by the image forming units 30C, 30M, and 30Y is not performed.
The image forming units 30 </ b> C, 30 </ b> M, and 30 </ b> Y of the present embodiment are provided to operate so as to be able to switch proximity / separation with respect to the intermediate transfer belt 21. In the case of monochrome image formation, the image forming units 30C, 30M, and 30Y move to positions separated from the intermediate transfer belt 21, and do not transfer the toner image to the intermediate transfer belt 21 (primary transfer). In the case of monochrome image formation, each part of the image forming units 30C, 30M, and 30Y does not perform an operation for forming a toner image.

  The control unit 50 determines whether to use a monochrome method or a color method based on the print image data, and controls the operation of each unit of the image forming apparatus 1 based on the determination result.

FIG. 2 shows the configuration of the control unit 50.
The control unit 50 includes a data receiving unit 51, a CPU 52, a RAM 53, a ROM 54, a band image processing unit 60, a frame memory 70, and a PWM decoding unit 80. Each unit of the control unit 50 is connected by a bus 90.

  The data receiving unit 51 receives print image data. Specifically, the data receiving unit 51 is connected to the document reading unit 2 and an external device, and receives print image data. The form of connection between the data receiving unit 51 and the transmission source of print image data such as the document reading unit 2 or an external device is not particularly limited. For example, it may be a dedicated bus, a connection based on a known serial or parallel transfer standard, or a network connection. Examples of connections based on known serial or parallel transfer standards include RS-232C, SCSI, USB, IEEE 1394, and the like. These connection forms do not preclude the use of connection methods invented in the future regardless of wired / wireless.

The CPU 52 performs various processes by software processing. The CPU 52 reads a program, data, and the like corresponding to the processing contents from the ROM 54, executes the processing, and controls the operation of each unit according to the processing result.
The RAM 53 stores programs and data read by the CPU 52 and parameters generated by the processing of the CPU 52.
The ROM 54 stores various programs and data read by the CPU 52.

The CPU 52 performs various processes for forming an image on the paper P based on a print job instructing image formation by the image forming apparatus 1 and print image data serving as original data of an image formed by the print job. There is no limitation on the input method of the print job. For example, a case where the print job is input from an external device or a case where the print job is input via the operation panel 3 is used.
The CPU 52 divides the print image data received by the data receiving unit 51 into a plurality of bands, performs a conversion process for dividing the print image data into CMYK color planes, and inputs the converted image data to the band image processing unit 60. Data input from the CPU 52 to the band image processing unit 60 is image data in band units separated for each color of cyan (C), magenta (M), yellow (Y), or black (B).

  The band image processing unit 60 generates data for a pulse width modulation signal having a pulse width value for forming dots based on the band unit image data input from the CPU 52 and stores the data in the frame memory 70. The band image processing unit 60 is individually provided for each color of cyan (C), magenta (M), yellow (Y), or black (B), and image data in a band unit corresponding to each color is input. The band image processing unit 60 for each color generates a pulse width value for forming a dot corresponding to each color and stores it in the frame memory 70.

In addition, the band image processing unit 60 counts the number of pixels (effective pixels) formed as dots during image formation from the generated pulse width value, and outputs the count value.
Note that a pixel is a minimum unit constituting image data that is digital data. A dot is a point formed on a paper P by toner, and an image formed on the paper P is a combination of a plurality of dots.

The pulse width value generated by the band image processing unit 60 is stored as a part of data constituting the band frame.
The frame memory 70 stores a band frame.
The PWM decoding unit 80 generates and outputs a drawing pulse signal for performing laser irradiation by the laser scanning units 32 of the image forming units 30C, 30M, 30Y, and 30K based on the pulse width value included in the band frame.

Next, the band frame will be described.
The band frame is data generated corresponding to each band-unit image data of each color generated based on the print image data. The image formed on the paper P is based on the band frame of each color corresponding to the band unit image data of each color generated from the image constituting one page.

FIG. 3 shows an example of the configuration of the band frame.
The band frame has a frame header area F1 and a frame data area F2.
The frame header area F1 is a data area including information indicating various attributes of the band frame.

FIG. 4 shows an example of the frame header area F1.
The frame header area F1 includes a print page ID, a frame number, a frame width, a frame length, a frame pixel count value, a frame data start address, and a next frame address start address.

The print page ID is a unique ID assigned to an image page configured using the band frame.
The frame number is a number indicating the printing order of the band frame in an image configured using the band frame. For example, in the case of a band frame printed at the beginning of a page, 1 is set as the frame number.
The frame width is information indicating the size of a predetermined direction (width direction) of an image formed by the band frame, and is indicated by, for example, the number of pixels.
The frame length is information indicating the size of one direction (length direction) orthogonal to a predetermined one direction of an image formed by the band frame, and is indicated by the number of lines, for example.

  The frame pixel count value is a count value indicating the number of effective pixels among the pixels included in the band frame. The count value of the number of effective pixels set as the frame pixel count value is counted by the band image processing unit 60 and set by the CPU 52.

The frame data start address is the head address of the frame data area F2 of the band frame.
The next frame address start address is the head address of the frame header area F1 of the next band frame. In the case of a band frame corresponding to a portion printed at the end of one page, an invalid value (for example, -1) is set as the next frame address start address.

  The frame data area F2 is a data area for storing the pulse width value of each pixel of the band frame. The pulse width value of each pixel stored in the frame data area F2 is based on each pixel of the band-unit image data from which the band frame is generated. The pulse width value generated by the band image processing unit 60 is stored in the frame data area F2.

Next, details of the band image processing unit 60 will be described.
FIG. 5 shows the configuration of the band image processing unit 60.
The band image processing unit 60 includes band buffers 61 and 62, a control signal generation unit 63, a halftone image processing unit 64, and a pixel count unit 65.

The band buffers 61 and 62 store image data in band units. The band unit image data input from the CPU 52 is sequentially stored alternately in the band buffers 61 and 62.
As shown in FIG. 5, switches 66 and 67 are provided for the input and output of the band buffers 61 and 62, respectively, so that the band buffer for input and output can be switched. The switches 66 and 67 are controlled so that input to one of the band buffers 61 and 62 and output from the other can be performed. For example, when band-unit image data is being input to the band buffer 61, the band-unit image data stored in the band buffer 62 is read by the halftone image processing unit 64.

  The control signal generation unit 63 individually inputs various signals to the halftone image processing unit 64, the pixel count unit 65, and the switches 66 and 67. For example, the operation control of the switches 66 and 67 is performed by a control signal from the control signal generation unit 63. In addition, signals related to operation control of the band image processing unit 60 are output by the control signal generation unit 63.

  The halftone image processing unit 64 reads out image data in band units from the band buffers 61 and 62, and performs dither screening processing in combination with pulse width modulation for halftone reproduction for each pixel included in the image data in band units. To output a pulse width value for a pulse width modulation signal for halftone reproduction using pulse width modulation (PWM). The pulse width value output by the halftone image processing unit 64 is a pulse of a signal for forming dots corresponding to each pixel of image data obtained by performing dither processing on band-unit image data on the paper P. A value indicating the width. The pulse width value is stored in the frame memory 70.

  In the case of the present embodiment, the band-unit image data is image data indicating the density value of one pixel with 8 [bit] gradation. The pulse width value that has been subjected to the dither screening process using pulse width modulation by the halftone image processing unit 64 indicates the density value of one pixel in a gradation of 4 [bits].

  The pixel counting unit 65 counts the number of effective pixels for each band frame based on the pulse width value input from the halftone image processing unit 64.

Next, details of the dither screening process using pulse width modulation will be described.
FIG. 6 shows the structure of the halftone image processing unit 64 and the mechanism of the dither screening process using pulse width modulation together.
The halftone image processing unit 64 includes a pixel position generation unit 641, an array reference address generation unit 642, a threshold determination unit 643, and a totaling unit 644.

  The pixel position generation unit 641 calculates the in-page position of the pixel to be processed based on the band-unit image data and the control signal input from the control signal generation unit 63. The in-page position is a pixel position in the print image data that is the source of the image data in band units.

  The array reference address generation unit 642 determines the reference position of the pixel to be processed based on the in-page position, and outputs it to the threshold value determination unit 643. The reference position is the position of a pixel in an area formed by n × m [pixel] that is a unit of the dither screening process. The array reference address generation unit 642 of this embodiment determines a reference position in a block of 25 pixels of 5 × 5 [pixels].

  The threshold discriminating unit 643 compares the density value of the pixel to be processed with the threshold value of the reference position of the dither threshold array corresponding to the reference position of the pixel to be processed, and the magnitude is sent to the totaling unit 644 as a comparison result. Output.

FIG. 7 shows an example of the dither threshold value array.
In the present embodiment, as shown in FIG. 7, the dither threshold value array is provided as an array corresponding to 25 pixels of 5 × 5 [pixels]. The dither threshold value array is provided according to the number of pixels as a unit of the dither screening process, and is not limited to the example shown in FIG.

  The dither threshold value array has a threshold value set for each pixel of the block that is a unit of the dither screening process. As shown in FIG. 7, the thresholds set in the dither threshold array and corresponding to each pixel of the block that is a unit of the dither screening process are individual threshold values. The threshold determination unit 643 compares the density value (8 [bit]) of the pixel to be processed with the threshold corresponding to the reference position of the pixel to be processed, and compares the magnitudes. The comparison process of the threshold value determination unit 643 in the present embodiment is based on a comparator (CMP). If the density value (8 [bit]) of the pixel to be processed is larger than the threshold value corresponding to the reference position of the pixel to be processed, the comparator (CMP) outputs True (1), otherwise False Outputs (0).

As shown in FIG. 7, 15 dither threshold value arrays No. 1 to No. 15 are provided. In each of the dither threshold array No1 to the dither threshold array No15, different threshold values are set for at least one pixel or more.
As illustrated in FIG. 6, the threshold determination unit 643 includes threshold determination blocks B1 to B15 that individually perform comparison processing for each dither threshold array of dither threshold array No1 to dither threshold array No15. The density value (8 [bit]) is compared with the threshold value corresponding to the reference position of the pixel to be processed, and the magnitude is compared.

  The totaling unit 644 outputs the pulse width value (4 [bit]) of the pixel to be processed based on the comparison results by the threshold determination blocks B1 to B15 of the threshold determination unit 643.

FIG. 8 shows an example of the relationship between the input value and the output value of the counting unit 644.
The output value by the totalization unit 644 of the present embodiment is based on the relationship between the input value and the output value shown in FIG. For example, when the input from the threshold determination block B15 is True (1), the counting unit 644 sets the pulse width value of the pixel to be processed to 15 regardless of the input contents of the other threshold determination blocks B1 to B14. To do. The other input contents are similarly based on the determination table shown in FIG.

FIG. 9 shows an example of data before and after the dither screening process using pulse width modulation. FIG. 9A shows an example of pixel data before dither screening processing using pulse width modulation together, and FIG. 9B shows an example of pulse width values after dither screening processing using pulse width modulation together.
As shown in FIG. 9A, when the density value is 83 for all the pixels of 5 × 5 [pixel], the threshold values set in the dither threshold array No1 to the dither threshold array No15 shown in FIG. Based on the comparison result, the pulse width value after the dither screening process using pulse width modulation takes the value shown in each pixel in FIG.

  The pulse width value output by the halftone image processing unit 64 is stored in the frame memory 70 as data constituting a band frame.

Next, counting of the number of effective pixels will be described.
FIG. 10 shows the configuration of the pixel count unit 65.
The pixel count unit 65 includes registers REG1, REG2, REG2a, REG2b, REG3, adders ADD1, ADD2, comparators (comparators) CMP1, CMP2, encoder ENC, and counter CNT.

The registers REG1, REG2, REG2a, REG2b, and REG3 store the pulse width value of each pixel.
The registers REG2, REG2a, and REG2b store the pulse width value of a pixel (determination target pixel) to be determined as to whether or not the pixel is an effective pixel.
The register REG1 stores a pulse width value corresponding to the pixel immediately after the determination target pixel. The pulse width value corresponding to the pixel immediately after the determination target pixel is a pulse width corresponding to the pixel adjacent to the determination target pixel and output immediately before the pulse width corresponding to the determination target pixel. A value indicating the width.
The register REG3 stores a pulse width value corresponding to the pixel immediately before the determination target pixel. The pulse width value corresponding to the pixel immediately before the determination target pixel is a pulse width corresponding to a pixel adjacent to the determination target pixel and is output immediately after the pulse width corresponding to the determination target pixel. A value indicating the width.

Adders ADD1 and ADD2 add two input values and output the result.
The adder ADD1 receives the values of the registers REG1 and REG2. That is, the adder ADD1 adds the pulse width value corresponding to the pixel immediately after the determination target pixel to the pulse width value corresponding to the determination target pixel, and outputs the added value.
The adder ADD2 receives the values of the registers REG2 and REG3. That is, the adder ADD1 adds the pulse width value corresponding to the pixel immediately before the determination target pixel to the pulse width value corresponding to the determination target pixel, and outputs the added value.

The comparators CMP1 and CMP2 compare the two input values and output the comparison result.
The comparator CMP1 compares the output value of the adder ADD1 with a predetermined threshold value RegThv. The predetermined threshold value RegThv is a threshold value for determining whether or not the determination target pixel is an effective pixel. The comparator CMP1 outputs True (1) when the output value of the adder ADD1 is larger than a predetermined threshold value RegThv, and outputs False (0) otherwise. That is, the comparator CMP1 outputs True (1) when the added value of the pulse width value corresponding to the determination target pixel and the pulse width value corresponding to the pixel immediately after the determination target pixel is larger than the predetermined threshold value RegThv. Otherwise, False (0) is output.
The comparator CMP2 compares the output value of the adder ADD2 with a predetermined threshold value RegThv. The comparator CMP1 outputs True (1) when the output value of the adder ADD2 is larger than a predetermined threshold value RegThv, and outputs False (0) otherwise. That is, the comparator CMP2 outputs True (1) when the sum of the pulse width value corresponding to the determination target pixel and the pulse width value corresponding to the pixel immediately before the determination target pixel is greater than the predetermined threshold RegThv. Otherwise, False (0) is output.

  If the pulse width value of each pixel does not exceed a predetermined threshold value RegThv, a dot cannot be formed only by a signal corresponding to the pulse width value of only that pixel. However, in some cases, a dot corresponding to the pixel can be formed by continuation with a signal corresponding to the pulse width value output immediately before or after the pixel. The comparators CMP1 and CMP2 perform determination by comparing the sum of the pulse width value corresponding to the determination target pixel and the pulse width value corresponding to the pixel immediately before or after the determination target pixel and a predetermined threshold RegThv. An output value for determining whether or not a dot is formed by a signal having a pulse width value corresponding to the target pixel is output as True (1) or False (0).

  The encoder ENC outputs the output results of the comparators CMP1 and CMP2 and the input contents from the register REG2b based on a predetermined rule.

FIG. 11 shows the relationship between the input and output of the encoder ENC. In the following description, the input from the comparator CMP1 is input A, the input from the comparator CMP2 is input B, and the input from the register REG2b is input C.
As shown in FIG. 11, the encoder ENC has a case where at least one of the inputs A and B is True (1) and the value of the input C is not 0 (4 [bit] notation is not “0000”). True (1) is output to, otherwise False (0) is output. That is, the encoder ENC outputs False (0) when both the inputs A and B are False (0) or when the value of the input C is 0 (4 [bit] notation is “0000”). To do.

  The output result of the encoder ENC indicates whether or not the determination target pixel is an effective pixel. When the output result of the encoder ENC is True (1), the determination target pixel is an effective pixel, and a dot is formed during image formation. When the output result of the encoder ENC is False (0), the determination target pixel is not an effective pixel, and no dot is formed during image formation. Hereinafter, the case where the determination target pixel becomes an effective pixel and the case where it does not become an example are illustrated using FIGS. 12A and 12B and FIGS. 13A and 13B.

FIG. 12 shows an example of data before and after the dither screening process using pulse width modulation, and includes a case where a pixel to be an effective pixel is included. FIG. 12A shows an example of pixel data including effective pixels, and FIG. 12B shows an example of a pulse width value when the dither screening process is performed on the pixel data of FIG.
In the pixel data shown in FIG. 12A, when the density values of the pixels P1 to P4 are 83, based on the comparison result with the threshold values set in the dither threshold array No1 to the dither threshold array No15 shown in FIG. The pulse width value after the dither screening process using pulse width modulation takes the value shown in each pixel in FIG.
Here, the pulse width value after the dither screening process is 3 for the pixel P2. When the predetermined threshold RegThv is 3, the pulse width value of the pixel P2 does not exceed the predetermined threshold RegThv. However, the pulse width value of the pixel P1 immediately before the pixel P2, that is, the pixel P1 adjacent to the left side of the pixel P2, is 15, and the sum of the pulse width value of the pixel P1 and the pulse width value of the pixel P2 is 18. For this reason, the sum of the pulse width value of the pixel P1 and the pulse width value of the pixel P2 exceeds a predetermined threshold value RegThv, and the encoder ENC outputs True (1). That is, the pixel P2 is determined as an effective pixel. Such a pattern is the case of the pattern Pt1 shown in FIG.

FIG. 13 shows an example of data before and after the dither screening process using pulse width modulation, and shows a case where pixels that are effective pixels are not included. FIG. 13A shows an example of pixel data not including effective pixels, and FIG. 13B shows an example of a pulse width value when the dither screening process is performed on the pixel data of FIG. 13A. .
In the pixel data shown in FIG. 13A, when the density values of the pixels P5 to P8 are 83, based on the comparison result with the threshold values set in the dither threshold array No1 to the dither threshold array No15 shown in FIG. The pulse width value after the dither screening process using pulse width modulation takes the value shown in each pixel in FIG.
Here, for the pixel P7, the pulse width value after the dither screening process is 3. When the predetermined threshold RegThv is 3, the pulse width value of the pixel P7 does not exceed the predetermined threshold RegThv. The pulse width value of the pixel immediately before the pixel P7, that is, the pixel P9 adjacent to the left of the pixel P7 is 0, and the sum of the pulse width value of the pixel P9 and the pulse width value of the pixel P7 is Both are 3. For this reason, the added value of the pulse width value of the pixel P7 and the pulse width value of the pixel P9 does not exceed the predetermined threshold value RegThv. Further, the pulse width value of the pixel immediately after the pixel P7, that is, the pixel P8 adjacent to the right of the pixel P6 is 0, and the sum of the pulse width value of the pixel P7 and the pulse width value of the pixel P8 is 3. . For this reason, the added value of the pulse width value of the pixel P7 and the pulse width value of the pixel P8 does not exceed the predetermined threshold value RegThv. In this way, the pulse width value of the pixel P7 does not exceed the predetermined threshold RegThv even if the pulse width values corresponding to the pixels before and after the pixel P7 are added, and the encoder ENC outputs False (0). That is, it is determined that the pixel P7 is not an effective pixel. Such a pattern is the case of the pattern Pt2 shown in FIG.

  The predetermined threshold value RegThv is determined in advance. The predetermined threshold RegThv is a value determined by the characteristics of the image forming apparatus 1, and is a value obtained experimentally.

The counter CNT counts the output result of the encoder ENC.
The counter CNT of this embodiment counts the number of true (1) among the output results of the encoder ENC. That is, the counter CNT counts the number of effective pixels. The counter CNT counts the result of determining whether or not the pulse width value corresponding to all the pixels included in one band-unit image data is a valid pixel, and outputs the count value. The output count value is set to the frame pixel count value of the band frame corresponding to the band unit image data.

  The counter CNT outputs the number of effective pixels included in all the pixels included in the image data in one band unit and is reset, and starts counting for the image data in the next band unit.

Operation control of each part of the pixel count unit 65, including a count value output command and reset for the counter CNT, is based on a control signal of the control signal generation unit 63. Hereinafter, a data flow and a processing flow in the operation of the pixel count unit 65 will be described.
The pulse width value corresponding to each pixel is first input to the register REG1, and then sequentially shifted to the registers REG2 and REG3 by clock control based on a control signal from the control signal generation unit 63. When new pulse width values are input to the registers REG1 and REG2, the pulse width values stored in the registers REG1 and REG2 are shifted to REG2 and REG3, respectively. That is, the pulse width values corresponding to three consecutive pixels are stored in the registers REG3, REG2, and REG1 in order from the pulse width value corresponding to the previously formed pixel. With reference to the pixel corresponding to the pulse width value of the register REG2, the pixel corresponding to the pulse width value stored in the register REG3 is the immediately preceding pixel, and the pixel corresponding to the pulse width value stored in the register REG1 is the immediately following pixel. Become.

  When the clock is engraved by the control signal, the pulse width values of the registers REG1 and REG2 are input to the adder ADD1, the pulse width values of the registers REG2 and 3 are input to the adder ADD2, and the pulse width value of the register REG2 is input to the register REG2a. Is entered. Adders ADD1 and ADD2 add the two input pulse width values.

  When the clock is further cut thereafter, the output value of the adder ADD1 is input to the comparator CMP1, the output value of the adder ADD2 is input to the comparator CMP2, and the pulse width value of the register REG2a is input to the register REG2b. At this time, a predetermined threshold value RegThv is also input to the comparators CMP1 and CMP2. The comparators CMP1 and CMP2 compare the pulse width value after addition input from the adders ADD1 and ADD2 with a predetermined threshold value RegThv.

  Thereafter, when the clock is further engraved, the output values of the comparators CMP1 and CMP2 and the pulse width value of the register REG2b are input to the encoder ENC. The encoder ENC determines whether or not the pixel corresponding to the pulse width value of the register REG2 is an effective pixel by performing output based on the predetermined rule.

When the clock is further engraved thereafter, the output value of the encoder ENC is input to the counter CNT. Thereafter, similarly, the processing is repeated until the determination as to whether or not the pulse width value corresponding to all the pixels of the image data of one band unit is an effective pixel is completed.
When the counting of the number of effective pixels of one band unit of image data is completed and output, each unit of the pixel counting unit 65 is initialized by a control signal.

Next, the image forming process by the CPU 52 will be described.
As described above, the CPU 52 performs various data processing on the print image data received by the data receiving unit 51 based on the print job, divides the print image data for one page into a plurality of bands, and performs CMYK. Are input to the band image processing unit 60 for each color.

When the input of the band unit image data for one band is completed, the CPU 52 issues a command for performing dither screening processing using pulse width modulation in combination with the band unit image data and counting the number of effective pixels. 60. Hereinafter, a dither screening process using pulse width modulation by the band image processing unit 60 and a command for counting the number of effective pixels will be referred to as “band image processing”.
Prior to the instruction for performing the band image processing, the CPU 52 generates data of the frame header area F1 of the band frame corresponding to the image data of the band unit and stores it in the frame memory 70. At the time of the instruction, the CPU 52 inputs the bandwidth, the band length, and the frame data start address to the band image processing unit 60. After the instruction, the CPU 52 outputs the next band unit image data to a band buffer in which the band unit frame processed by the instruction is not stored.

After completing the band image processing for one band unit image data, the band image processing unit 60 notifies the CPU 52 of the completion of the band image processing. In this embodiment, the control signal generation unit 63 is responsible for receiving commands from the CPU 52 and notifying the CPU 52, but a dedicated configuration may be provided separately.
When the notification of completion of the band image processing is received, the CPU 52 acquires the count value of the number of effective pixels by the pixel count unit 65, and uses the acquired count value as the frame pixel count value of the band frame corresponding to the image data of the band unit. Set.

The CPU 52 repeats output of band unit image data, band image processing, and setting of the count value of the number of effective pixels for all band unit image data corresponding to one page of print image data.
When there are a plurality of print pages, the CPU 52 repeats the same processing in order from the print image data of the first printed page.

  After completing the band image processing and setting of the count value of the number of effective pixels for all the band-unit image data corresponding to the print image data for one page, the CPU 52 performs cyan (C ), The frame headers of all band frames of magenta (M) and yellow (Y) are scanned, and determination based on the frame pixel count value is performed. If there is at least one band frame of cyan (C), magenta (M), or yellow (Y) satisfying a predetermined condition (for example, 1 or more) that satisfies the frame pixel count value, the CPU 52 changes the color of the image of the one page. Judged to be an image. When there is no cyan (C), magenta (M), or yellow (Y) band frame that satisfies a predetermined condition (for example, 1 or more) as the frame pixel count value, the CPU 52 converts the one-page image into a monochrome image. It is determined that

  The CPU 52 performs printing by a color method when printing a color image, and performs printing by a monochrome method when printing a monochrome image. The CPU 52 controls the operation of each part of the PWM decoding unit 80 and the image forming apparatus 1 according to the printing method to form an image on the paper P. In the case of the color method, the CPU 52 operates the PWM decoding unit 80 and the image forming units 30C, 30M, 30Y, and 30K for each color of CMYK. In the monochrome system, the CPU 52 operates only the black (K) PWM decoding unit 80 and the image forming unit 30K.

  The PWM decoding unit 80 sequentially reads out data for one page composed of a plurality of band frames stored in the frame memory 70 based on an instruction from the CPU 52. Then, the PWM decoding unit 80 compares the pulse width values before and after adjacent to each pulse width value for each pulse width value included in the band frame, and brings the output pixel closer to the pixel having the larger pulse width. Is added with pulse signal output position information. The PWM decoding unit 80 generates and outputs a drawing pulse signal for performing laser irradiation by the laser scanning unit 32. Laser irradiation using the drawing pulse signal is performed according to a synchronization signal for synchronizing the operations of the respective units that perform image formation by the image forming apparatus 1.

The image forming apparatus 1 according to the present embodiment can change a color system / monochrome system switching pattern by setting.
For example, as described above, a print setting that uses a color method for a color image and a monochrome method for a monochrome image can be set to be applied to each page individually.

  As another setting, when print image data having a plurality of pages is printed, the pages after the page that is a color image can be set to print in the color method regardless of the determination result of the color image / monochrome image. . For example, when the first page is a monochrome image, the CPU 52 performs control for image formation by the monochrome method and starts image formation. If there is a page determined to be a color image in the second page and thereafter, control for image formation by the color method is performed for the pages after that page regardless of the determination result of the color image / monochrome image. At this time, the process for determining the color image / monochrome image may be omitted for the pages after that page.

The operation of bringing the image forming units 30C, 30M, and 30Y close to / separated from the intermediate transfer belt 21 requires an operation time for switching the proximity / separation. For this reason, in printing of print image data having a plurality of pages, if the proximity / separation of the image forming units 30C, 30M, and 30Y is switched for each page, the time until image formation is completed due to the switching operation time. This may increase the productivity of the image forming apparatus 1. Therefore, when there is a page that has been determined to be a color image, the switching operation time is determined by performing control for image formation by the color method for pages subsequent to that page regardless of the determination result of the color image / monochrome image. This makes it possible to switch between the monochrome method and the color method while maintaining the productivity of the image forming apparatus 1 at a high level.
In particular, in the case of an image forming apparatus used in a corporate office or the like, the printed image is often a monochrome image. For this reason, first, image formation is started in the monochrome system, and when there is a color image, switching to the color system is performed, thereby maintaining the productivity of the image forming apparatus 1 at a high level and switching between the monochrome system / color system. It is possible to reduce the consumption of the image forming units 30C, 30M, and 30Y.

Further, as another setting, it is possible to set to always print in the color method regardless of the contents of the print image data. In the case of this setting, processing for determining a color image / monochrome image may be omitted.
The setting of the color / monochrome switching pattern described above is merely an example, and other settings may be performed.
The color / monochrome switching pattern can be set using the operation panel 3.

  In this embodiment, since the effective pixel number of black (K) is not used for determination of the printing method, the configuration of the pixel counting unit 65 and the effective pixel number of the band image processing unit 60 of black (K) are used. Processing related to counting may be omitted.

  As described above, according to the present embodiment, the halftone image processing unit 64 determines the pulse width value corresponding to each pixel for each color, and the pixel count unit 65 becomes the determination target for the pulse width value corresponding to each pixel. Whether or not the pixel is an effective pixel is determined based on the pulse width value corresponding to the pixel and the pulse width value corresponding to the pixel immediately before or immediately after the pixel to be determined. Accordingly, it is possible to accurately determine the presence or absence of dot formation for each pixel taking into account the influence of the immediately preceding or immediately following pixels. For this reason, for each color, it is possible to more accurately determine the color necessary for image formation according to the presence or absence of dots to be formed. Then, the CPU 52 switches the color and the number of colors used for image formation based on the count value of the effective pixel number based on the discrimination result of the pixel count unit 65, such as switching between the color method and the monochrome method. Accordingly, it is possible to suitably suppress image formation using unnecessary toner, such as printing on a monochrome image by a color method.

  Further, the adder ADD1 obtains a value obtained by adding the pulse width value corresponding to the pixel to be determined and the pulse width value corresponding to the pixel immediately after the pixel to be determined, and the adder ADD2 A value obtained by adding the pulse width value corresponding to the pixel to be determined and the pulse width value corresponding to the immediately preceding pixel adjacent to the pixel to be determined is obtained. Thereafter, the counter CNT counts the number of effective pixels based on the comparison result between the added values output from the adders ADD1 and ADD2 by the comparators CMP1 and CMP2 and a predetermined threshold value RegThv. Then, the CPU 52 switches the color and the number of colors used for image formation based on the count value of the number of effective pixels, such as switching between the color method and the monochrome method. Thus, for each color, it is possible to more accurately determine the color necessary for image formation according to the number of dots to be formed. In addition, it is possible to suitably suppress image formation using unnecessary toner, such as printing on a monochrome image by a color method.

  Further, the counter CNT adds a pulse width value corresponding to the pixel to be determined and a pulse width value corresponding to the immediately preceding pixel adjacent to the pixel to be determined, or a pulse corresponding to the pixel to be determined. A pixel to be judged is counted as an effective pixel when at least one of the value obtained by adding the width value and the pulse width value corresponding to the pixel immediately after the pixel to be judged is greater than a predetermined threshold RegThv To do. As a result, although the pulse width value of a single pixel does not exceed a predetermined threshold value RegThv for a certain pixel, a pulse width capable of forming dots by making it continuous with the pulse width corresponding to the immediately preceding or immediately following pixel is effective. Can be counted as Then, even if the pulse width corresponding to the pixel immediately before or immediately after the adjacent pixel is continued, a pulse width that cannot form a dot is not counted as an effective pixel, so that it is erroneously determined as a color image based on the dot that is not formed. Can be reliably deterred.

  Further, the CPU 52 determines a color corresponding to a band frame whose frame pixel count value satisfies a predetermined condition (for example, 1 or more) as a color used for image formation. As a result, the color and the number of colors used for image formation can be accurately controlled.

  Further, the image forming engine of the image forming apparatus 1 uses black (black (K)) and a toner other than black, and the CPU 52 determines whether the monochrome method / the color is based on the frame pixel count value of the band frame corresponding to the color other than black. Switch the color system. This makes it possible to switch between the color method and the monochrome method based on the number of dots formed using toner other than black. For the black (K) band image processing unit 60, the configuration of the pixel counting unit 65 and the processing related to the counting of the effective pixel number can be omitted.

  Further, the image forming engine of the image forming apparatus 1 uses black (black (K)) and cyan (C), magenta (M), and yellow (Y) toner, and the CPU 52 performs cyan (C), magenta (M). ) And yellow (Y) are switched between the monochrome method and the color method based on the frame pixel count value of the band frame. Accordingly, it is possible to switch between the color method and the monochrome method based on the number of dots formed using cyan (C), magenta (M), and yellow (Y) toner. For the black (K) band image processing unit 60, the configuration of the pixel counting unit 65 and the processing related to the counting of the effective pixel number can be omitted.

  Further, the halftone image processing unit 64 performs a dither screening process using pulse width modulation together, and outputs a pulse width value corresponding to each pixel of the image subjected to the dither screening process. This makes it possible to accurately determine the presence / absence of dot formation for each pixel, taking into account the influence of the immediately preceding or immediately following pixel for each pixel in printing that performs dither screening processing using pulse width modulation in conjunction with image formation. it can. In addition, it is possible to suitably suppress image formation using unnecessary toner, such as printing on a monochrome image by a color method.

  The embodiment of the present invention should be considered that the embodiment disclosed this time is illustrative and not restrictive in all respects. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  For example, the control of the printing method based on the count value of the number of effective pixels of each color is not limited to the switching between the color method and the monochrome method, and the image forming unit to be operated is flexibly switched according to the color and the number of colors used for image formation. Can do. For example, there is a method of forming an image with only a color having an effective pixel number of 1. In the case of this control, for example, when only the number of effective pixels is 1 or more for cyan (C) and black (B), the color method is not used, and two-color printing only for cyan (C) and black (B) is performed. . The same applies to other color combinations, and image formation using one color other than black (B) and image formation using three colors are also possible.

  By switching the image forming unit to be operated according to the color and the number of colors used for image formation, it is possible to suppress consumption due to the operation of the image forming unit unnecessary for image formation.

  Further, the toner is not limited to CMYK, and may be toner by other color reproduction methods. For example, the toner may be a color reproduction method using a color space such as CMY, RGB, or the like. It may include toner of a color that does not depend on a specific color space, such as a spot color. Colorless or transmissible toner, toner capable of recognizing a printed image by invisible light such as ultraviolet rays, or the like may be used.

  The above-described embodiment is a tandem type image forming apparatus, but the present invention can also be applied to other types of image forming apparatuses. Examples of other image forming apparatuses include a rotary image forming apparatus.

  In the above-described embodiment, when the count value of the number of effective pixels is 1 or more among the band frames of each color other than black constituting one page, it is determined to be treated as a color image. The count value of the number of effective pixels can be an arbitrary value. The same applies not only to the switching between the monochrome method and the color method, but also when the image forming unit to be operated is flexibly switched according to the color and the number of colors used for image formation. In addition, various numerical value settings such as the number of pixels of n × m [pixels] and the dither threshold value array illustrated in the embodiment are merely examples, and can be appropriately changed.

  In the above-described embodiment, the dither screening process using the pulse width modulation and the counting process of the effective pixels by the band image processing unit 60 are performed by dedicated hardware, but the dither screening process using the pulse width modulation is also used. One or both of the effective pixel count processing may be performed by software processing. In the case of software processing, a dither screening process using pulse width modulation and a program and data corresponding to a count process of the effective pixel count are stored in a storage device such as a ROM, and the CPU reads the program from the ROM and performs these processes. be able to.

FIG. 14 is a flowchart showing a processing flow when a dither screening process using pulse width modulation by software processing and an effective pixel count process are performed.
First, the CPU divides the print image data into a plurality of bands, converts the print image data into CMYK colors, and generates band-unit image data (step S1). Next, the CPU performs a dither screening process using pulse width modulation on the image data of each color band unit based on preset dither threshold value array data (step S2), and the pulse corresponding to each pixel. Get the width value. Next, based on the pulse width value obtained in step S2, the CPU is effective based on the pulse width value corresponding to the pixel to be determined and the pulse width value corresponding to the pixel immediately before and after adjacent to the pixel. The number of pixels is counted (step S3) to obtain a count value. Then, the CPU stores a band frame including the pulse width value obtained in step S2 and the count value of the number of effective pixels obtained in step S3 in the frame memory 70 (step S4).
The input / output values of the dither screening process using the pulse width modulation in step S2 and the effective pixel count process in step S3 are the dither screening process and the step using the pulse width modulation realized by hardware in the above-described embodiment. This is no different from the effective pixel count processing in S3.

In the dither screening process by software processing and the count process of the number of effective pixels, priority order of determination in some determination processes may be provided.
For example, when the pixel density exceeds the threshold value of the dither threshold value array No15, the comparison process between the pixel density and the threshold value using the dither threshold value array No1 to No15 shown in FIG. 7 is compared with the other dither threshold value arrays No1 to No14. Also, the pixel density always exceeds the threshold. Similarly, if the pixel density exceeds the threshold of the higher numbered dither threshold array, the pixel density always exceeds the threshold of the lower numbered dither threshold array. Therefore, the comparison process between the pixel density and the threshold value of the dither threshold value array starts from the threshold value of the dither threshold value array having a larger number, and when the pixel density exceeds the threshold value, the pulse width value is calculated based on the number of the dither threshold value array. It is also possible to omit the comparison process with the threshold values of other dither threshold value arrays.
In addition, in the counting process of the number of effective pixels, when the pulse width value corresponding to the pixel to be determined exceeds a predetermined threshold value RegThv alone, an addition value with the pulse width corresponding to the preceding and subsequent pixels is obtained. First, it is determined whether it is an effective pixel. First, it is determined whether or not the pulse width value corresponding to the pixel to be determined alone exceeds a predetermined threshold value RegThv. The sum of the pulse width and the predetermined threshold value RegThv may be compared.

  In the case of software processing, in addition to the same effects as in the case of hardware processing, further algorithm optimization can be realized.

In the above description, the example in which the ROM is used as the computer-readable medium of the program according to the present invention has been disclosed, but the present invention is not limited to this example. As other computer-readable media, a non-volatile memory such as a flash memory and a portable recording medium such as a CD-ROM can be applied.
Further, a carrier wave (carrier wave) is also applied to the present invention as a medium for providing program data according to the present invention via a communication line.

20 transfer unit 30C image forming unit 30K image forming unit 30M image forming unit 30Y image forming unit 31 photosensitive drum 32 laser scanning unit 33 developing unit 50 control unit 52 CPU
60 Band image processing unit 61, 62 Band buffer 63 Control signal generation unit 64 Halftone image processing unit 65 Pixel count unit 66, 67 Switch 70 Frame memory 80 PWM decoding unit 641 Pixel position generation unit 642 Array reference address generation unit 643 Threshold discrimination Section 644 Total section ADD1, ADD2 Adder B1-B15 Threshold determination block CMP1, CMP2 Comparator CNT Counter ENC Encoder

Claims (9)

For each of the plurality of toners, a laser light source is modulated and driven in accordance with the pulse width of the pulse width modulation signal, whereby the photosensitive member is optically scanned in the main scanning direction to form an electrostatic latent image. An image forming apparatus that forms an image by developing with
A pulse width determining unit that determines the pulse width of the pulse width modulation signal for each pixel of the image data by color;
For a determination pixel to be determined, a pulse width of a pulse width modulation signal corresponding to the determination pixel, a pulse width of a pulse width modulation signal corresponding to an adjacent pixel adjacent to the determination pixel in the main scanning direction, A determination unit for determining whether or not a dot is formed in the determination pixel based on
A switching control unit that switches colors and the number of colors used for image formation based on the determination result of the determination unit;
An image forming apparatus comprising: The discrimination unit is a value based on the pulse width of the pulse width modulation signal output immediately before the pulse width modulation signal corresponding to the determination pixel with respect to the value based on the pulse width of the pulse width modulation signal corresponding to the determination pixel. Of the pulse width modulation signal output immediately after the pulse width modulation signal corresponding to the determination pixel with respect to the first value obtained by adding the above and the value based on the pulse width of the pulse width modulation signal corresponding to the determination pixel. Generating a second value obtained by adding a value based on a pulse width, comparing the first value and the second value with a predetermined threshold, and comparing the first value and the second value. And the number of dots formed for each color based on the result of comparison with a predetermined threshold,
The image forming apparatus according to claim 1, wherein the switching control unit switches colors and the number of colors used for image formation based on the number of dots formed for each color counted by the determination unit.   The determination unit, when at least one of the first value and the second value is larger than a predetermined threshold, counts that a dot is formed in the determination pixel. The image forming apparatus according to 2.   4. The image formation according to claim 2, wherein the switching control unit switches colors and the number of colors used for image formation based on a comparison result between the number of dot formations of each color and a predetermined number of dot formations. 5. apparatus. The colors of the plurality of toners include black and colors other than black,
The switching control unit switches a color used for image formation to one of black or a plurality of colors based on the number of dots formed in colors other than black. The image forming apparatus described in the item. The plurality of toners correspond to black and each color for color printing including at least cyan, magenta, and yellow,
The switching control unit switches a color used for image formation to one of black or black and a color for color printing based on the number of dots formed for each color for color printing. The image forming apparatus described.   The pulse width determination unit performs dither processing based on the image data, and determines a pulse width of a pulse width modulation signal corresponding to each pixel of the image after dither processing for each color. The image forming apparatus according to any one of claims 1 to 6. For each of the plurality of toners, a laser light source is modulated and driven in accordance with the pulse width of the pulse width modulation signal, whereby the photosensitive member is optically scanned in the main scanning direction to form an electrostatic latent image. An image forming method for forming an image by developing with
Determining the pulse width of the pulse width modulation signal for each pixel of the image data by color;
For a determination pixel to be determined, a pulse width of a pulse width modulation signal corresponding to the determination pixel, a pulse width of a pulse width modulation signal corresponding to an adjacent pixel adjacent to the determination pixel in the main scanning direction, Determining whether or not a dot is formed on the determination pixel based on
A step of switching colors and the number of colors used for image formation based on the determination result;
An image forming method comprising: For each of the plurality of toners, a laser light source is modulated and driven in accordance with the pulse width of the pulse width modulation signal, whereby the photosensitive member is optically scanned in the main scanning direction to form an electrostatic latent image. A computer of an image forming apparatus that forms an image by developing with
Means for determining the pulse width of the pulse width modulation signal for each pixel of the image data by color;
For a determination pixel to be determined, a pulse width of a pulse width modulation signal corresponding to the determination pixel, a pulse width of a pulse width modulation signal corresponding to an adjacent pixel adjacent to the determination pixel in the main scanning direction, Means for determining whether a dot is formed on the determination pixel based on
Means for switching the color and the number of colors used for image formation based on the determination result;
A program characterized by functioning as

Images