JP2000253223A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an image without increasing memory capacity even in the case of an image whose image data of the four colors of Y, M, C and K cannot be stored in a memory. SOLUTION: An image forming device where the respective image forming units of Y, M, C and Bk are installed along an intermediate transfer belt in tandem is provided with a resolution conversion means converting the resolution of image data and a decision means for deciding whether an image memory is overflowed or not. When the picture memory is decided to be overflowed, image data whose resolution is dropped by the resolution conversion means is compressed and is stored in the image memory. Then, it is supplied for the formation of a color image.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an image forming apparatus for forming a color image by using a plurality of image forming units having different image forming colors. In particular, it relates to a tandem type image forming apparatus in which each image forming unit is provided along an intermediate transfer belt.

[0002]

2. Description of the Related Art FIG. 1 is a schematic view illustrating a tandem type image forming apparatus. In a tandem type image forming apparatus, Y
(Yellow), M (magenta), C (cyan), K (B
four image forming units (electrophotographic units including a photoconductor and a toner developing device) corresponding to four colors (k: black)
Are provided at predetermined intervals along the intermediate transfer belt,
Images are sequentially transferred from the image forming units to the intermediate transfer belt passing through the primary transfer position (the position where the image forming unit and the intermediate transfer belt are in contact with each other) with a time delay corresponding to the above-mentioned predetermined interval. Thus, the images of the respective colors are superimposed on the intermediate transfer belt, and a color image is formed. This color image is transferred onto the paper at a secondary transfer position (“transfer section” in the drawing; a position where the intermediate transfer belt contacts the paper), and the color image transferred onto the paper is fixed by the fixing section. .

In a tandem-type image forming apparatus, image data of each color of Y, M, C, and K is so arranged that each image forming unit can execute an image forming / transferring operation sequentially with a time delay corresponding to the predetermined interval. Are stored in the image memory, respectively, read out at predetermined timings corresponding to the transfer start times of the image forming units, and sent to the corresponding image forming units. Therefore, it is necessary to temporarily store image data for four colors of Y, M, C, and K in the image memory. However, from the viewpoint of cost reduction, the memory capacity cannot be increased so much. The image data of each color is stored by being compressed by a predetermined compression method.

[0004]

In the image forming apparatus, A
The maximum size of the output paper is determined such as three output possible, A4 output possible, etc. For example, in the case of a model capable of A3 output, the capacity of the image memory is Y, M, Each of the four colors C and K has a capacity capable of storing compressed image data for one image of A3 size. Further, the capacity beyond that is optional.

[0005] There are various types of images, such as photographic images and character images, and the compression ratios may differ greatly due to differences in their properties. For this reason, for example, compressed image data of an A3 size character image or a normal photographic image is converted to Y,
Even if the memory has a capacity capable of storing the four colors of M, C, and K, the compressed image data of the A3-size photographic image having special properties cannot be stored for the four colors of Y, M, C, and K. In some cases. In such a case, an error occurs in the conventional model and an image cannot be formed. Note that images with special properties that have extremely low compression ratios are rare, and installing a large-capacity memory for such rare images is difficult.
It is not a good idea in terms of cost.

In the present invention, for example, image data for four colors of Y, M, C, and K cannot be stored in a memory due to a small compression ratio, and an image cannot be formed due to an error in a conventional model. An object of the present invention is to make it possible to form an image without increasing the memory capacity even for such an image.

[0007]

According to the first aspect of the present invention, there is provided an image forming apparatus for forming a color image using a plurality of image forming units each having a different image forming color. Control means for controlling input of image data for each image forming color and output of image data to each image forming unit, an image memory for storing image data, and resolution conversion for converting the resolution of image data Means for storing image data in an image memory, reading image data corresponding to the output request from the image memory in response to an output request from the control means, and outputting the image data to an image forming unit corresponding to the output request Means for determining whether or not the image memory overflows. The control means, when it is determined that the image memory overflows, performs resolution conversion. After reducing the resolution of the image data by, passed to the memory control unit, it is an image forming apparatus according to claim. According to a second aspect of the present invention, in the first aspect, the image forming colors are four colors of Y, M, C, and Bk, and the control means reduces the resolution of the image data of 600 dpi to 400 dpi by the resolution conversion means. An image forming apparatus characterized in that: The invention according to claim 3 is characterized in that Y, M,
An image forming apparatus for forming a color image using an image forming unit for four colors of C and Bk, wherein a resolution converting means for converting the resolution of image data and a determination for determining whether or not an image memory overflows. Means, and when it is determined that the image memory overflows, to store the image data whose resolution has been reduced by the resolution conversion means in the image memory, and to provide the image formation by each image forming unit,
An image forming apparatus characterized in that: According to a fourth aspect of the present invention, there is provided an image forming apparatus for forming a color image using a plurality of image forming units each having a different image forming color, wherein an image forming operation is controlled and image data of each image forming color is formed. Control means for controlling input and output of image data to each image forming unit, an image memory for storing image data,
Color / monochrome conversion means for generating single-color image data from one or more image-forming image data, storing the image data in an image memory, and requesting the output in response to an output request from the control means Memory control means for reading image data corresponding to the image memory from the image memory and outputting the image data to the image forming unit corresponding to the output request, and determining means for determining whether or not the image memory overflows, the control means comprising: If it is determined that the image memory overflows, the single-color image data generated by the color / monochrome conversion unit is passed to the memory control unit, and the single-color image data should be output to any one of the image forming units. Is set in the memory control means. The invention according to claim 5 is characterized in that Y, M, C, B
An image forming apparatus for forming a color image using an image forming unit for four colors of k, wherein color / monochrome conversion means for generating image data for one color from image data for one or more image forming colors And determining means for determining whether or not the image memory overflows. When it is determined that the image memory overflows, the image data for a single color generated by the color / monochrome conversion means is stored in the image memory. An image forming apparatus for performing image formation by any one of the image forming means.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [1] Overall Configuration: FIG. 1 shows a configuration example of a tandem-type image forming apparatus according to an embodiment, and FIG. 2 shows a configuration example of a control unit in FIG. The illustrated image forming apparatus includes four image forming units (P / C units) corresponding to four colors of Y, M, C, and K, and each image forming unit has a predetermined interval along the intermediate transfer belt. It is provided in.
Each image forming unit is a unit for forming an image by an electrophotographic method, and includes a photosensitive drum, a toner developing device, and the like. The intermediate transfer belt is rotated at a constant speed in the counterclockwise direction in the figure. Each image forming unit is provided with an image signal from the control unit. Based on the image signal, an image formed by each image forming unit by the electrophotographic method is sequentially transferred from each image forming unit onto the intermediate transfer belt. Transfer (primary transfer) is performed. The timing at which the image signals are transferred to the respective image forming units is controlled so that the shifts at the predetermined intervals are compensated and the images of the respective colors are superimposed on the intermediate transfer belt without any shift. A color image formed by superimposing the images of each color on the intermediate transfer belt is transferred (secondarily transferred) onto a sheet at a transfer unit shown in the figure. Further, the color image transferred onto the paper is fixed by a fixing unit, and the paper after the image is fixed is discharged to a paper discharge unit.

The control unit includes a CPU 10, a ROM 11 in which a control program for the CPU 10 is stored, a RAM 12 also used as a work area, an interface 21 to which print data (image data) is inputted from outside, and an external input. A raster converter 14 for converting the converted image data into raster format data (however, the raster data may be externally input), and a color / color converter for converting one or more color image data into single color image data. A monochrome conversion unit 15, a resolution conversion unit 16 for converting the resolution of the image data, and an image data provided from the interface unit 21, the raster conversion unit 14, the color / monochrome conversion unit 15 or the resolution conversion unit 16,
A compression / decompression unit 13 for decompressing data read from the image memory, image data compressed by the compression / decompression unit 13 is written to the image memory, and compressed image data from the image memory is read and sent to the compression / decompression unit 13. A memory unit 17 having an image memory, and a DMA controller 1 for transferring image data expanded by the compression / expansion unit 13 to an image forming unit (any one of Y, M, C, and K).
8 and an I / O port 22.

The CPU 10 communicates with another CPU (not shown) for controlling the overall operation of the image forming apparatus, and controls the operation timing of each block in accordance with the timing of the overall operation. Among the functions performed by the control means described in the claims, the function of “controlling the image forming operation” is performed by the another CPU, and the function of “controlling the input of image data and the output of image data” is performed by the CPU 10. Plays. Color/
The monochrome conversion unit 15 has a function of generating single-color image data of only lightness based on four-color image data of Y, M, C, and K. Alternatively, a function of selecting and outputting image data of a certain color (for example, “K”) from image data of four colors of Y, M, C, and K may be provided. The resolution conversion unit 16 converts, for example, 600 dpi image data into 40
This is a block for converting to 0 dpi or 300 dpi. A known process can be adopted as the resolution conversion process.

When the image forming apparatus is a printer connected to a computer or the like or a facsimile connected to a communication line via a modem, the image data input to the interface 21 is a computer or a modem. This is the data sent from. On the other hand, when the image forming apparatus is mounted on a copying machine, the image data input to the interface 21 is the data transmitted from a scanner unit that is integrally mounted with the image forming apparatus and configures the copying machine. It is. In any case, before being input to the interface 21 of the image forming apparatus, Y, M, C, K
It is assumed that the image data is converted into image data for printing.

[2] Outline of Operation: Next, the operation of the image forming apparatus according to the first embodiment will be described with reference to the flowcharts shown in FIGS. 3 (a), 4, 6 and 8. I do. The operation of the image forming apparatus according to the second embodiment will be described with reference to flowcharts shown in FIGS. 3B, 5, 7, 8, and 9.

[2-1] First Embodiment: FIG.
FIG. 4 is a flowchart showing the entire processing of the first embodiment executed by the CPU 10, FIG. 4 is a flowchart showing the data input processing (S01) of FIG. 3A, and FIG.
FIG. 8 is a flowchart showing the memory count processing (S03) of FIG. 3A, and FIG. 8 is a flowchart showing the tandem control output processing (S07) of FIGS. 3A and 3B. First
In the image forming apparatus of the present embodiment, the following operations are realized by the processing shown in these flowcharts.

In the initial input mode, Y, M, C, K
Are input, compressed, and stored in the image memory. At the time of this storage, the memory counter is counted, and it is determined whether or not the image memory overflows based on the count value of the memory counter.

If the image memory does not overflow, normal operation is performed. That is, Y, M, C, K
Tandem operation in which the four color images are sequentially transferred onto the intermediate transfer belt with a time delay corresponding to the arrangement interval of the image forming units, and are successively transferred onto the intermediate transfer belt to form a color image by being superimposed on the intermediate transfer belt. Is performed.

When the image memory overflows, image data of four colors of Y, M, C, and K is input again,
After each of the resolutions is reduced, they are compressed and stored in the image memory. Thereafter, a tandem operation of forming a color image by transferring and superimposing the four color images on the intermediate transfer belt as in the case where the image memory does not overflow is performed.

[2-2] Second Embodiment: FIG.
FIG. 5 is a flowchart showing the entire processing of the second embodiment executed by the CPU 10, FIG. 5 is a flowchart showing the data input processing (S51) of FIG. 3B, and FIG.
FIG. 8 is a flowchart showing a memory count process (S53), FIG. 8 is a flowchart showing a tandem control output process (S07) of FIGS. 3A and 3B, and FIG. 9 is a monochrome diagram of FIG. It is a flowchart which shows a control output process (S59). In the image forming apparatus according to the second embodiment, the following operations are realized by the processing shown in these flowcharts.

In the initial input mode, Y, M, C, K
Are input, compressed, and stored in the image memory. At the time of this storage, the memory counter is counted, and it is determined whether or not the image memory overflows based on the count value of the memory counter.

If the image memory does not overflow, normal operation is performed. This operation is the same as in the first embodiment, and a description thereof will be omitted.

When the image memory overflows, image data of four colors of Y, M, C, and K is input again,
From these, monochromatic image data (image data with only brightness)
Is generated. Note that image data of a certain color may be selected. A monochrome operation of forming a single color (for example, “K”) image based on the image data after color / monochrome conversion (image data of only brightness) is performed.

[3] Details of processing procedure and operation: The first
Each operation of the second embodiment is realized by the CPU 10 setting each block in FIG. 2 at the following timing according to the following procedure.

[3-1] First embodiment: [3-1-1] Data input processing (FIG. 4). In the data input process of step S01 in FIG. 3A, a process for inputting image data sent from the outside from the interface 21, performing raster conversion, compressing the image data, and storing the image data in the image memory is performed. In the re-input mode, image data sent from the outside is input from the interface 21, and after raster conversion, the resolution is reduced, and the raster data after resolution conversion is compressed and stored in the image memory. Is performed.

That is, in the initial input mode (S101: initial input), if there is a data input (S111: YES),
The fact that the image data of each color should be raster-converted is set in the raster conversion unit 14 (S113), and the fact that the data after raster conversion should be compressed is set in the compression / decompression unit 13 (S11).
7) The information indicating that the compressed data should be written to the image memory is set in the memory unit 17 (S119). As a result, the set processing is executed in the raster conversion unit 14, the compression / decompression unit 13, and the memory unit 17.

In the re-input mode (S101: re-input), if there is a data input (S121: YES), the fact that image data of each color should be raster-converted is set in the raster conversion unit 14 (S123), The resolution conversion unit 1 determines that the resolution of the converted data should be converted and the target resolution.
6 (S125), and the fact that the raster data after resolution conversion is to be compressed is set in the compression / decompression unit 13 (S12).
7) The information indicating that the compressed data should be written to the image memory is set in the memory unit 17 (S129). Thereby, the raster conversion unit 14, the resolution conversion unit 16, the compression / decompression unit 1
3. In the memory unit 17, the set processing is executed.
Note that the target resolution may be configured such that the resolution conversion unit 16 holds a predetermined constant value.

[3-1-2] Memory count processing (FIG. 6). In the memory counting process of step S03 in FIG. 3A, it is determined whether or not the image memory has overflowed based on the count value of the writing counter that is counted when the compressed image data is stored in the image memory. The input mode is set according to the determination result.

When the writing to the image memory is completed (S2
01: YES), and reads the write counter (S20)
3) Based on the count value, it is determined whether or not the memory has overflowed (S205).

If no overflow has occurred (S20)
5: NO), the input mode is initialized to “1” (S20)
7). Thus, at the time of the next image input, input in the initial input mode is performed. That is, the input is performed at a standard resolution that does not reduce the resolution.

If an overflow has occurred (S20)
5: YES), and set the input mode to “2” (S21)
1). That is, an input mode for re-inputting image data from the outside, reducing the resolution, compressing the image data, and storing the compressed image data in the image memory is set. Further, in order to input the image data for the re-input mode, a data re-input request to an external device (scanner, computer, or the like) is set (S21).
5).

[3-1-3] Tandem control output processing (FIG. 8).
In the tandem control output process of step S07 in FIG. 3A, image data of four colors of Y, M, C, and K are
Processing for outputting to the corresponding image forming units is performed with a time delay corresponding to the arrangement interval of each of the image forming units M, C, and K. The delay counter described below measures the elapsed time from the time when a reference image formed at a predetermined position upstream of the image forming position on the intermediate transfer belt has passed the position of the reference position detection sensor (see FIG. 1). It is a counter to do.

If an output request has been issued (S301: Y
ES), check whether the delay counter is operating (S)
303). Since it is not operating at first (S303: N
O), it waits until the reference image passes the position of the reference position detection sensor (S305). When the reference image is detected by the reference position detection sensor (S305: YES), the delay counter is started (S307).

When the count value of the delay counter reaches a value to output the Y image data (S311: YES), the output of the Y image data is set (S313). This allows
The memory unit 17 reads the Y image data from the image memory, the compression / decompression unit 13 decompresses the read Y image data, and the DMA controller outputs the decompressed Y image data to the Y imaging unit. The count value “CT-Y” of the delay counter that gives the timing at which the Y image data is to be output is determined by the distance measured along the intermediate transfer belt between the reference position detection sensor and the Y image forming unit, and the intermediate transfer This value is determined by the belt speed and the distance between the reference image and the leading end of the image measured along the intermediate transfer belt.

Steps S311 and S313 described above are for Y image data, but the same applies to M, C, and K image data. That is, when the count value of the delay counter becomes a value at which M image data / C image data / K image data should be output (S315 / S319 / S323: Y
ES), output of M image data / C image data / K image data is set (S317 / S321 / S325). Thereby, the memory unit 17 reads the M image data / C image data / K image data from the image memory, and compresses the read M image data / C image data / K image data /
The expansion unit 13 expands, and the DMA controller outputs the expanded M image data / C image data / K image data to the M imaging unit / C imaging unit / K imaging unit.
Also, the count values “CT-M” and “CT-
The meaning of "C" and "CT-K" is the same as above,
The distance between the reference position detection sensor and the M image forming unit / C image forming unit / K image forming unit measured along the intermediate transfer belt, the speed of the intermediate transfer belt, and the distance between the reference image and the image front end, respectively. It is determined by the distance measured along the intermediate transfer belt. When the output of the K image signal is set as described above, the count of the delay counter is cleared (stopped) (S327).

[3-2] Second embodiment: [3-2-1] Data input processing (FIG. 4). In the data input process of step S51 in FIG. 3B, a process for inputting image data sent from the outside from the interface 21, performing raster conversion, compressing the image data, and storing it in the image memory is performed. In the re-input mode, image data sent from the outside is input from the interface 21, converted into image data of only a single color brightness after raster conversion, and raster data after monochrome conversion is compressed and stored in an image memory. Is performed for storing the information.

The processing in the initial input mode according to the second embodiment is the same as the processing in the initial input mode according to the first embodiment, and a description thereof will be omitted.

On the other hand, in the re-input mode (S101: re-input), if there is a data input (S121: YES), the fact that the image data of each color should be raster-converted is set in the raster conversion unit 14 (S123), and the raster is output. The color / monochrome conversion unit 15 sets that the converted data is to be converted to monochromatic image data (S165), and indicates that the raster data that has been converted to monochrome data is to be compressed.
(S127), and the fact that the compressed data should be written to the image memory is set in the memory unit 17 (S12).
9). As a result, the raster conversion unit 14, the color / monochrome conversion unit 15, the compression / decompression unit 13, and the memory unit 17 execute the set processing. As described above, a method of generating single-color image data (image data of only brightness) from four colors of Y, M, C, and K may be adopted.
May be adopted. In the latter case, the processing is simplified.

[3-2-2] Memory count processing (FIG. 7). In the memory counting process of step S53 in FIG. 3B, it is determined whether or not the image memory overflows based on the count value of the writing counter counted when storing the compressed image data in the image memory. The mode is set according to the determination result.

The determination of the overflow of the image memory is the same as the determination in the first embodiment, and the description is omitted.

If no overflow has occurred (S20)
5: NO), and initializes the input mode to “1” (S20)
7) The output mode is set to "1" (S259). That is, an initial input mode for inputting image data of four colors,
Sets the tandem control output mode for outputting color image data. The initialization processing for setting the input mode to “1” is a setting for the next image input.

If overflow has occurred (S20)
5: YES), and set the input mode to “2” (S21)
1) The output mode is set to "2" (S263). That is, a re-input mode in which four-color image data is re-input to generate, compress, and store monochrome image data, and a monochrome image is formed by outputting single-color (for example, “K”) image data. Set the monochrome control output mode.
Further, in order to input the image data for the re-input mode, a data re-input request to an external device (scanner, computer, etc.) is set (S215).

[3-2-3] Output processing. [3-2-3-1] Tandem control output processing (Fig. 8). FIG. 3 (b)
The tandem control output process of step S07 of the first
Since this is the same as the tandem control output process of step S07 in FIG. 3A of the embodiment, the description is omitted.

[3-2-3-2] Monochrome control output processing (FIG. 9). In the monochrome control output process of step S59 in FIG. 3B, a process for outputting K image data to the K image forming unit is performed. The following delay counter is the same as that described above, and a description thereof will be omitted.

If an output request has been made (S401: Y
ES), check whether the delay counter is operating (S)
403). Since it is not operating at first (S403: N
O), it waits until the reference image passes the position of the reference position detection sensor (S405), and when the reference image is detected by the reference position detection sensor (S405: YES), starts the delay counter (S407).

When the count value of the delay counter reaches a value to output K image data (S411: YES), the output of K image data is set (S413). This allows
The memory unit 17 reads the K image data from the image memory, the compression / decompression unit 13 decompresses the read K image data, and the DMA controller outputs the decompressed K image data to the K imaging unit. When the output of the K image signal is set as described above, the count of the delay counter is cleared (stopped) (S427).

[0044]

According to the present invention, since image data for four colors of Y, M, C, and K cannot be stored in the memory, an image which cannot be formed due to an error in the conventional model is generated. Even so, an image can be formed without increasing the memory capacity.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a configuration example of an apparatus according to an embodiment.

FIG. 2 is a block diagram showing a configuration example of a control unit of the apparatus shown in FIG.

FIG. 3 is a flowchart showing an entire process in a CPU 10 of FIG. 2, where (a) shows the control of the first embodiment;
(B) shows the control of the second embodiment.

FIG. 4 is a flowchart showing a data input process (S01) of FIG.

FIG. 5 is a flowchart showing a data input process (S51) of FIG. 3 (b).

FIG. 6 is a flowchart showing a memory count process (S03) of FIG.

FIG. 7 is a flowchart showing a memory count process (S53) of FIG. 3 (b).

FIG. 8 is a flowchart showing a tandem control output process (S07) of FIGS. 3 (a) and 3 (b).

FIG. 9 is a monochrome control output process of FIG. 3B (S59).
5 is a flowchart showing the process.

[Explanation of symbols]

 10 CPU P / C unit Imaging unit

Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat II (reference) H04N 1/46 H04N 1/46 C F term (reference) 2C087 AA15 AC08 BC07 BD24 BD35 BD40 DA02 2H027 EE08 FA28 FA30 2H030 AD07 AD11 5C073 BB02 BC02 CE01 CE06 5C079 HA13 HB03 LA26 LA31 LA37 MA00 MA02 MA11 NA10 NA13 PA02

Claims (5)

[Claims] 1. An image forming apparatus for forming a color image by using a plurality of image forming units each having a different image forming color, wherein the image forming apparatus controls an image forming operation and inputs image data for each image forming color. Control means for controlling output of image data to each image forming unit; image memory for storing image data; resolution conversion means for converting the resolution of image data; and storing image data in the image memory. And memory control means for reading image data corresponding to the output request from the image memory in response to an output request from the control means and outputting the image data to the image forming unit corresponding to the output request; and determining whether the image memory overflows. Determination means for determining whether the image memory overflows. If the image memory is determined to overflow, the control means reduces the resolution of the image data by the resolution conversion means. An image forming apparatus that transfers the image data to a memory control unit after the image forming apparatus is moved down. 2. The image forming apparatus according to claim 1, wherein the image forming colors are four colors of Y, M, C, and Bk, and the control unit reduces the resolution of the image data of 600 dpi to 400 dpi by the resolution converting unit. An image forming apparatus comprising: 3. An image forming apparatus for forming a color image using an image forming unit for four colors of Y, M, C, and Bk, wherein a resolution converting means for converting the resolution of image data and an image memory are provided. Determining means for determining whether or not the image memory overflows. If it is determined that the image memory overflows,
An image forming apparatus comprising: storing image data whose resolution has been reduced by a resolution conversion means in an image memory and providing the image data by each image forming unit; 4. An image forming apparatus for forming a color image using a plurality of image forming units each having a different image forming color, wherein the image forming apparatus controls an image forming operation and inputs image data for each image forming color. Control means for controlling the output of image data to each image forming unit; an image memory for storing image data; and generating single-color image data from one or more image-forming color image data A color / monochrome conversion unit for storing image data in an image memory, and reading out image data corresponding to the output request from the image memory in response to an output request from the control unit, and an image forming unit corresponding to the output request Memory control means for outputting to the image memory; and determination means for determining whether or not the image memory overflows. The control means, when it is determined that the image memory overflows, In this case, the image data for a single color generated by the color / monochrome conversion means is passed to the memory control means,
An image forming apparatus, wherein a setting to output the monochrome image data to one of the image forming units is set in a memory control unit. 5. An image forming apparatus for forming a color image using an image forming unit for four colors of Y, M, C, and Bk, wherein one or two or more image data for image forming colors are used for a single color. Color / monochrome converting means for generating the image data of the above, and determining means for determining whether or not the image memory overflows. If it is determined that the image memory overflows,
An image forming apparatus, comprising: storing image data for a single color generated by a color / monochrome conversion unit in an image memory and forming an image by any one of the image forming units.