Image forming apparatus and image processing apparatus

Abstract

To provide a technique which increases the storage property of a product.SOLUTION: An image formation apparatus comprises: setting means which sets any one of a plurality of operation modes; processing means which performs image processing to image data according to the operation mode set by the setting means; and image formation means which forms an image on a sheet on the basis of the image data after the image processing by the processing means. When a first mode of the plurality of operation modes is set, the image processing by the processing means comprises: determining an object pixel whose gradation value is equal to or greater than a first threshold and an adjacent pixel adjacent to the object pixel and different from the object pixel in the image data; setting a first gradation value smaller than the first threshold as the gradation value of the object pixel; and setting a second gradation value larger than the first gradation value as the gradation value of the adjacent pixel.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer, or an LED printer. [Background technology]

[0002] Electrophotographic image forming apparatuses perform image processing on image data to maintain the quality of the images they form. Patent Document 1 discloses a configuration for correcting the gradation values ​​of pixels corresponding to areas other than the edges of the area to which toner is attached in order to suppress the occurrence of partial image loss. Patent Document 2 discloses a configuration for changing the exposure pulse width based on the result of density detection in order to suppress toner consumption while maintaining image quality. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3596903 [Patent Document 2] Patent No. 6248907 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, importance has been attached to the preservability (archival ability) of the output (sheet on which an image is formed) of an image forming apparatus. An output with high preservability means, for example, an output in which the formed image is unlikely to be significantly lost even when repeatedly viewed or carried around. If the preservability is low, characters and the like may become unreadable due to loss of the image.

[0005] Patent Documents 1 and 2 disclose image processing for maintaining the image quality of the final product, but do not disclose image processing for improving the preservation stability of the final product.

[0006] The present invention provides a technique for improving the preservation quality of a product. [Means for solving the problem]

[0007] According to one aspect of the present invention, an image forming apparatus includes a setting means for setting one of a plurality of operating modes, a processing means for performing image processing on image data in accordance with the operating mode set by the setting means, and an image forming means for forming an image on a sheet based on the image data after the image processing by the processing means, and when a first mode of the plurality of operating modes is set, the image processing by the processing means includes determining a target pixel in the image data having a gradation value equal to or greater than a first threshold value and an adjacent pixel that is different from the target pixel and adjacent to the target pixel, setting a first gradation value smaller than the first threshold value as the gradation value of the target pixel, and setting a second gradation value greater than the first gradation value as the gradation value of the adjacent pixel. Effect of the Invention

[0008] According to the present invention, the preservation of the product can be improved. [Brief description of the drawings]

[0009] [Figure 1] 1 is a block diagram of an image forming apparatus according to an embodiment. [Diagram 2] 5A and 5B are diagrams showing examples of screens displayed on an operation unit. [Diagram 3] FIG. 2 is a diagram illustrating the configuration of an image forming unit according to an embodiment. [Figure 4] 4 is a flowchart of image processing according to one embodiment. [Diagram 5] FIG. 4 is an explanatory diagram of image processing according to an embodiment. [Figure 6] 11 is a diagram illustrating how the toner that adheres varies depending on the image processing. [Figure 7] 1 is a diagram illustrating the difference in resistance to friction due to differences in the type of adhering toner. [Figure 8] FIG. 13 is a diagram illustrating how the amount of toner that adheres varies depending on the number of continuous pixels. [Figure 9]5 is a diagram illustrating how toner adheres differently depending on the charging voltage and the developing voltage. [Figure 10] 5A to 5C are diagrams illustrating the effects of the embodiment. [Figure 11] 5A to 5C are diagrams illustrating the effects of the embodiment. [Figure 12] 1A and 1B are diagrams showing characters used to explain image processing according to an embodiment. [Figure 13] 4 is a flowchart of image processing according to one embodiment. [Figure 14] FIG. 4 is an explanatory diagram of image processing according to an embodiment. [Figure 15] 11 is a diagram illustrating how the toner that adheres varies depending on the image processing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0011] First Embodiment FIG. 1 is a configuration diagram of an image forming apparatus 100 according to this embodiment. A control unit 51 controls the entire image forming apparatus 100. A processor (CPU) 51a of the control unit 51 controls the image forming apparatus 100 by executing a control program. A read-only memory (ROM) 51c holds the control program executed by the CPU 51a and various parameters used by the CPU 51a to control the image forming apparatus 100. Note that other non-volatile memories such as flash memory can be used instead of the ROM 51c. A random access memory (RAM) 51b is a volatile memory, and is used to store temporary data and the like in the control of the image forming apparatus 100.

[0012] The image memory 52 stores image data of an image formed by the image forming apparatus 100. For the image memory 52, for example, a storage device such as a hard disk drive (HDD) can be used. The image memory 52 may be an internal device of the image forming apparatus 100 or an external device that communicates with the image forming apparatus 100 via a network. The operation unit 53 provides a user interface. A user can operate the image forming apparatus 100 via the operation unit 53. In addition, the user can recognize the state of the image forming apparatus 100 from the contents displayed on a display unit 53w (FIG. 2) of the operation unit 53.

[0013] FIG. 2 shows an example of a screen displayed on the display unit 53w when the display unit 53w is a touch panel display. The user can perform various settings related to printing (image formation) via the display unit 53w. For example, the user can set the sheet type (paper type) and the sheet size (paper size). The user can also set the image quality (print quality) via the icon 53a. In this embodiment, the image quality that can be set is normal quality or archive quality. Archive quality means an image quality with higher preservability than normal quality. Note that the image quality that can be set may include qualities other than normal quality and archive quality. In the following description, the operation mode of the image forming apparatus 100 when normal quality is set is referred to as normal mode, and the operation mode of the image forming apparatus 100 when archive quality is set is referred to as archive mode.

[0014] Returning to Fig. 1, in order to form an image, the control unit 51 reads out image data stored in the image memory 52 and outputs it to the image processing unit 500. The image processing unit 500 performs image processing on the image data according to the operation mode, and outputs the image data after the image processing to the image forming unit 50. The image forming unit 50 forms an image on a sheet based on the image data after the image processing, and outputs a product. Note that the sheet means a medium on which an image is formed, such as a recording paper, and the product means a sheet on which an image is formed by the image forming apparatus 100.

[0015] FIG. 3 is a configuration diagram of the image forming unit 50. During image formation, the photoconductor 1 is rotated clockwise in the drawing. The charging roller 2 outputs a charging voltage to charge the surface of the photoconductor 1 to a uniform charging potential. The exposure device 3 exposes the photoconductor 1 based on input image data to form an electrostatic latent image on the photoconductor 1. The development roller 4 of the development device outputs a development voltage to attach toner to the electrostatic latent image on the photoconductor 1, thereby forming a toner image on the photoconductor 1. Meanwhile, the sheet P stored in the cassette 11 is fed to the conveying path by the feed roller 7 and conveyed to a position facing the photoconductor 1. The transfer roller 5 outputs a transfer voltage to transfer the toner image on the photoconductor 1 to the sheet P. The fixing device 6 heats and presses the sheet P to which the toner image has been transferred, thereby fixing the toner image to the sheet P. After the toner image is fixed, the sheet P is discharged to a discharge tray 15 by a discharge roller 14.

[0016] FIG. 4 is a flowchart showing image processing in the image processing unit 500 during image formation. When image formation starts, image data stored in the image memory 52 is input to the image processing unit 500. In the following, it is assumed that this image data is 8-bit bitmap data. That is, it is assumed that the image data is data indicating the coordinates of each pixel and the gradation value of each pixel in 8 bits. Therefore, the gradation value is any value from 0 to 255. In this example, it is assumed that the gradation value 0 is white, the gradation value 255 is black, and the gradation values ​​1 to 254 are intermediate colors between white and black. It is to be noted that the density of intermediate colors increases as the gradation value increases. In the following description, the conveying direction of the sheet P is referred to as the sub-scanning direction, and the direction perpendicular to the sub-scanning direction in the plane of the sheet P is referred to as the main scanning direction.

[0017] In S10, the image processing unit 500 determines whether the archive mode or the normal mode is set. If the normal mode is set, the image processing unit 500 performs normal image processing on the image data in S15. The normal image processing is, for example, screen processing, smoothing processing, edge enhancement processing, etc. Note that the image processing may not be performed and S15 may be skipped.

[0018] On the other hand, when the archive mode is set, the image processing unit 500 identifies a target pixel in S11. In this example, the target pixel is a pixel whose gradation value is equal to or greater than a first threshold value. In this example, the first threshold value is 235. In S12, the image processing unit 500 changes the gradation value of the target pixel to a first gradation value that is smaller than the first threshold value. In this example, the first gradation value is 125.

[0019] Next, the image processing unit 500 specifies adjacent pixels in S13. The adjacent pixels are pixels adjacent to the target pixel among the pixels not selected as the target pixel in the sheet P. The adjacent direction may be either the main scanning direction or the sub-scanning direction. The image processing unit 500 changes the gradation value of the adjacent pixel to a second gradation value larger than the first gradation value in S14. In this example, the second gradation value is 255. It is possible to set a condition that the gradation value of the adjacent pixel is equal to or smaller than a second threshold value. In other words, it is possible to configure the adjacent pixel to be a pixel adjacent to the target pixel among the pixels not selected as the target pixel and having a gradation value equal to or smaller than the second threshold value. The second threshold value is smaller than the first gradation value, and may be, for example, 0 or a value close to 0.

[0020] After executing the process of S14, the image processing unit 500 executes normal image processing in S15 in the same manner as when the normal mode is selected. However, when the process of S14 is executed, the process of S15 can be skipped and the process of S16 can be executed. By executing normal image processing, even if the image to be formed contains a graphic image portion expressed in intermediate gradation values, the graphic image portion is subjected to normal image processing. In either case, the image processing unit 500 outputs the image data processed in S16 to the image forming unit 50.

[0021] FIG. 5 is an explanatory diagram of image processing in the image processing unit 500. In FIG. 5(A) to FIG. 5(C), each square cell represents a pixel. FIG. 5(A) visually represents the gradation value of the pixel in the portion corresponding to the character "A" when forming an image including a plurality of characters on the sheet P. The gradation value of the pixels constituting the character is a value greater than the first threshold value (235), and the gradation value of the other pixels is 0 (or a value close to 0). That is, the pixels shown in black in FIG. 5(A) are all target pixels. FIG. 5(B) visually represents the image data after processing the image data shown in FIG. 5(A) when the normal mode is used. The color of each pixel corresponds to the gradation value. That is, the darker the color of the pixel, the higher the gradation value. It is noted that FIG. 5(B) is also an image actually formed in the portion shown in FIG. 5(A) when the normal mode is used. As shown in FIG. 5(B), when the normal mode is used, an image according to the original gradation value is formed.

[0022] On the other hand, Fig. 5(C) visually represents the image data after processing the image data shown in Fig. 5(A) when the archive mode is used. Note that Fig. 5(C) is also the image that is actually formed in the portion shown in Fig. 5(A) when the archive mode is used. As shown in Fig. 5(C), the gradation value of the target pixel F is changed to 125 in S12, resulting in a low density. Also, the gradation value of the adjacent pixel E adjacent to the target pixel F is changed to 255 in S14, resulting in a high density.

[0023] Figure 6(B) shows a cross-section of the sheet P before fixing by the fixing device 6 along the line G shown in Figure 6(A). Note that the left side of Figure 6(B) is the cross-section when image formation is performed in the normal mode, and the right side is the cross-section when image formation is performed in the archive mode. In the case of the archive mode, the height of the toner adhering to the sheet P is lower than that in the normal mode, and the width of the toner adhering to the sheet in the main scanning direction becomes wider. That is, Ha > Hb and Wa < Wb. Here, Ha and Wa are the height and the width in the main scanning direction of the toner adhering to the sheet P when the normal mode is selected, respectively, and Hb and Wb are the height and the width in the main scanning direction of the toner adhering to the sheet P when the archive mode is selected, respectively. Note that in the case of the archive mode, the width of the toner in the main scanning direction becomes wider than that in the normal mode because the tone value of adjacent pixels is changed to 255, and the same applies in the sub-scanning direction. Therefore, in the case of the archive mode, the width of the toner adhering to the sheet P in the sub-scanning direction also becomes wider than that in the normal mode. Thus, in the case of the archive mode, the area of the toner adhering to the sheet P becomes larger than that in the normal mode. Note that Figure 6(B) shows the toner before fixing, but the same applies after fixing.

[0024] Subsequently, the influence of the differences in the height and area of the toner on the storability (archivability) will be described. Figure 7 shows a state in which a frictional force is applied to the toner after fixing on the sheet P with the eraser rubber V. Note that the toner in Figure 7(A) is higher and has a smaller area than the toner in Figure 7(B). That is, Figure 7(A) corresponds to the toner of the product formed in the normal mode, and Figure 7(B) corresponds to the toner of the product formed in the archive mode. When a frictional force is applied to the surface of the toner T with the eraser rubber V, a shear stress Y is applied to the toner T and the interface between the sheet P and the toner T.

[0025] The shear stress Y is proportional to the frictional force and inversely proportional to the area of ​​the toner T. In other words, the shear stress Y in the case of FIG. 7(B) is smaller than that in the case of FIG. 7(A). In the archive mode, the area of ​​the toner T is larger than in the normal mode, so the shear stress Y is smaller than in the normal mode. Therefore, the archive mode is more resistant to frictional forces than the normal mode. Also, in the archive mode, the area of ​​the toner is larger than in the normal mode, so even if part of the toner that makes up the characters is lost due to the shear stress Y, it is possible to prevent a significant decrease in readability compared to the normal mode.

[0026] Furthermore, a low height of the toner T means that the amount of toner per unit area is small. Therefore, the lower the height of the toner T, the smaller the heat capacity of the toner T. The smaller the heat capacity, the easier it is for the heat of the fixing device 6 to be transmitted to the interface between the toner T and the sheet P, and the adhesion of the toner T to the sheet P and the binding force between the toner particles become stronger. Therefore, in the archive mode, the adhesion of the toner T to the sheet P and the binding force between the toner particles can be made stronger than in the normal mode, and the toner can be made stronger against frictional forces than in the normal mode.

[0027] As described above, in the archive mode, the toner height is lower than in the normal mode, and the toner area (area of ​​the area where toner is continuously attached) is larger than in the normal mode. This configuration makes it possible to form a product that is highly resistant to wear. Therefore, by using the archive mode, it is possible to form a product that is highly storable (archivable) and that is less likely to cause character misrecognition and ensures readability.

[0028] In addition, as shown in Fig. 6B, in the archive mode, the uniformity of the toner height is higher than in the normal mode. The reason for this and the effect of the uniformity of the toner height on the storage property (archive property) are explained below.

[0029] Fig. 8 shows the exposure potential of the photoconductor 1 and the toner adhering to the photoconductor 1 when, for example, a pixel with a gradation value of 255 is exposed. Here, the exposure potential of the photoconductor 1 is the potential of the exposed area of ​​the surface of the photoconductor 1 exposed by the exposure device 3, that is, the potential of the area where an electrostatic image is formed. Note that Fig. 8 shows the exposure potential and the toner adhering when the number of consecutive pixels with a gradation value of 255 in the main scanning direction is 1, 6, and 20. In Fig. 8, Vd is the charging potential (the potential of the non-exposed area of ​​the photoconductor 1), Vdc is the development potential (the potential due to the development voltage), and VI is the target potential of the exposure area.

[0030] The ideal exposure potential is one that falls to the target potential VI over the entire pixel. Toner adheres to the photoconductor 1 in the portion of its surface where the exposure potential is lower than the development potential Vdc. The greater the difference between the exposure potential and the development potential Vdc, the greater the amount of adhered toner (the height of the toner). Therefore, when the exposure potential falls to the target potential VI over the entire pixel, the toner adheres to the photoconductor 1 at the same height over the entire pixel. For example, as shown in FIG. 8, when the number of consecutive pixels with a gradation value of 255 in the main scanning direction is 20, the exposure potential of the photoconductor 1 approaches the ideal state. However, the smaller the number of consecutive pixels, the greater the deviation between the actual exposure potential and the ideal exposure potential. For example, as shown in FIG. 8, when only one pixel has a gradation value of 255 and the other pixels have a gradation value of 0, the exposure potential of the pixel with a gradation value of 255 does not reach the target potential VI, and only falls below the development potential Vdc at the center of the pixel. This is because the light intensity of the light spot formed on the photoconductor 1 by the exposure device 3 to expose the photoconductor 1 is not uniform across the entire area of ​​the light spot, but has the characteristic of becoming weaker from the center of the light spot toward the edges. From Figure 8, it can be seen that the smaller the number of consecutive pixels to which toner is to be attached, the less uniform the height of the toner becomes, and the toner adheres so as to gradually become higher from the edges to the center of the electrostatic latent image. It can also be seen that the smaller the number of consecutive pixels to which toner is to be attached, the shorter the width of the adhered toner becomes than the ideal width.

[0031] For example, in a character image or a line image, the number of consecutive pixels to which toner is attached in the main scanning direction or the sub-scanning direction is small, so the height of the toner in the center is lower than that of the ends, resulting in low uniformity of the toner height. In such a case, in the fixing device 6, the center of the toner image, which has a high toner height, inhibits the application of heat and pressure to the ends of the toner image, which has a low toner height, and the fixing property of the ends is reduced. On the other hand, when the uniformity of the toner height is high, the fixing device 6 sufficiently heats and presses the entire area including the center and ends of the toner image. Therefore, the higher the uniformity of the toner height, the higher the fixability of the toner to the sheet P. In addition, since the fixability of the toner to the sheet P is uniform in the entire area of ​​the toner image, it is less likely to fall off even if it is worn.

[0032] In this embodiment, in order to ensure uniformity in toner height, the gradation value of the adjacent pixel is made larger than the corrected gradation value of the target pixel. This is because the exposure potential of the edge of the electrostatic latent image tends to be higher than the potential of the center, as shown in Fig. 8. By making the gradation value of the edge of the electrostatic latent image higher, the exposure potential of the edge can be quickly reduced, and uniformity in toner height can be ensured.

[0033] The lower the gradation value of the target pixel F, the lower the toner height can be, but if the toner height is too low, it will look like a hollow. If the toner height is too low, the amount of toner will be reduced, the binding force between the toners will be weak, and the fixability of the toner to the sheet P may deteriorate. Therefore, the appropriate gradation value after correction of the target image F varies depending on the image forming conditions such as the development voltage, the line width of the image, etc. For example, for a toner image composed of a width of about 6 pixels such as the character "A" used in the description of this embodiment, the gradation value after correction can be set to about 30 to 70% or 40 to 60% of the maximum gradation value. The gradation value of the adjacent pixel E is adjusted so that the toner height at the edge of the toner image is approximately the same as that at the center. Therefore, it is set higher than the gradation value after correction of the target pixel F, for example, between 50 to 100% of the maximum value.

[0034] In this embodiment, the target pixel F is a pixel whose gradation value is equal to or greater than the first threshold value. However, if all pixels whose gradation values ​​are equal to or greater than the first threshold value are treated as target pixels and their gradation values ​​are reduced, it may not be possible to obtain a desired image for graphic images composed of dark gradation values.

[0035] Therefore, when the number of consecutive pixels in the main scanning direction and the sub-scanning direction whose gradation value is equal to or greater than the first threshold value is equal to or greater than a predetermined value, these consecutive pixels are not considered as target pixels. In other words, the target pixels can be pixels whose gradation value is equal to or greater than the first threshold value and whose number of consecutive pixels in at least one of the main scanning direction and the sub-scanning direction is smaller than a predetermined number (second consecutive number). As described in FIG. 8, such a solid image with a large number of consecutive pixels has high toner uniformity and a large area, so that it has a sufficiently high resistance to friction. Also, unlike character images and line images, it has a high resistance to image loss, so problems are unlikely to occur even if it is not considered as a target pixel.

[0036] Furthermore, when type information indicating the type of image is included in the input image data, the type information can be used to determine the target pixel. Specifically, when a photographic image, a graphic image, a line image, or a character image is indicated by the image data, the pixels constituting the line image and the character image and having gradation values ​​equal to or greater than the first threshold value can be selected as the target pixel.

[0037] As described above, when the archive mode is selected, image processing is performed so that the toner area is increased and the toner height is decreased compared to the normal mode. By increasing the toner area, the shear stress Y applied to the toner and the interface between the sheet P and the toner is decreased, and the occurrence of image loss due to frictional force can be suppressed. In addition, by lowering the toner height, the toner can be sufficiently heated in the fixing device 6, and the adhesion force of the toner to the sheet and the binding force between the toner particles are increased, and the occurrence of image loss due to frictional force can be suppressed. Furthermore, by increasing the uniformity of the toner height, the entire toner can be uniformly heated, and the adhesion force of the toner to the sheet and the binding force between the toner particles are increased, and the occurrence of image loss due to frictional force can be suppressed. With this configuration, a product with high storability can be formed.

[0038] Second Embodiment Next, the second embodiment will be described with respect to differences from the first embodiment. In the first embodiment, when the archive mode is selected, the toner area is increased, the toner height is decreased, and the uniformity of the toner height is increased compared to the normal mode. In this embodiment, the same effect as the first embodiment is achieved by adjusting the charging voltage and the developing voltage.

[0039] FIG. 9 shows the exposure potential of the photoconductor 1 and the toner adhering to the photoconductor 1 when six consecutive pixels with a gradation value of 255 in the main scanning direction are exposed. The left side of FIG. 9 shows the case of the normal mode. In the normal mode, the maximum value of the toner height is H6a, and the width of the toner in the main scanning direction is W6a. In the normal mode, the height of the toner at the ends in the main scanning direction adhering to the photoconductor 1 is lower than that of the center. The center of FIG. 9 shows the case where the absolute value of the charging voltage is lower in the archive mode than in the normal mode. By lowering the charging voltage, the charging potential Vd of the photoconductor 1 is also lower. By lowering the charging potential Vd, the exposure potential is easily reduced. Since the toner adheres to the photoconductor 1 in the area where the exposure potential is lower than the development potential Vdc, the width of the toner in the main scanning direction is W6b, which is larger than W6a in the normal mode. Although the toner height H6b is approximately the same as H6a, which is the maximum value in the normal mode, the uniformity of the toner height is higher than in the normal mode by adjusting the charging potential Vd.

[0040] The right side of Fig. 9 shows a case where, in addition to adjusting the charging voltage in the center of Fig. 9, the absolute value of the developing voltage is also made lower than in the normal mode. By making the absolute value of the developing voltage lower than in the normal mode, the developing potential Vdc becomes smaller. The height of the toner adhering to the photoconductor 1 becomes smaller as the difference between the developing potential Vdc and the exposure potential becomes smaller, so the toner height H6c can be made lower than the toner height H6a in the normal mode.

[0041] As described above, in the archive mode, by adjusting both the exposure voltage and the development voltage, the toner area can be increased, the toner height can be decreased, and the uniformity of the toner height can be increased compared to the normal mode. As shown in FIG. 9, by adjusting only the exposure voltage, the toner area can be increased and the uniformity of the toner height can be increased compared to the normal mode, but the toner height cannot be decreased. However, by increasing the toner area and increasing the uniformity of the toner height compared to the normal mode, the preservability of the product can be increased compared to the normal mode. When only the development voltage is adjusted, the toner height can be decreased, and therefore the preservability of the product can be increased compared to the normal mode. In this embodiment, the image processing unit 500 executes only normal image processing.

[0042] Next, the effects of the first and second embodiments will be described. FIG. 10 shows the results of the wear test. The wear test was performed by forming an image including the letter "A" and pressing an eraser against the image with a predetermined force, and then reciprocating 100 times at a speed of one reciprocation per second over a 50 mm section including the letter "A" of the product, and then judging the state of the letter "A" on the product. The test was performed on the product formed in the archive mode of the first embodiment, the product formed in the archive mode of the second embodiment, and the product formed in the normal mode. In the archive mode of the second embodiment, only the development voltage was adjusted.

[0043] The wear test results of ◯, △, and × in FIG. 10 indicate the degree of readability of the letter "A." Specifically, a state in which readability is ensured is indicated as ◯, a state in which there is no readability is indicated as ×, and a state in which readability is poor is indicated as △. FIG. 11(A) is an example of a result of ◯, FIG. 11(B) is an example of a result of △, and FIG. 11(C) is an example of a result of ×. As shown in FIG. 10 and FIG. 11, the results of both the first and second embodiments are better than the normal mode, and the products are resistant to wear and have high preservation properties.

[0044] <Third embodiment> Next, the third embodiment will be described with a focus on the differences from the first embodiment. Character images and line images can be composed of lines of various thicknesses. FIG. 12 visually shows the gradation value of the image data of each pixel for forming the character "3". Each square cell indicates a pixel, and the gradation value of the pixels constituting the character is a value greater than the first threshold value (235), and the gradation value of the other pixels is 0 (or a value close to 0). In FIG. 12, reference symbol J indicates a part of the character "3" composed of a line width of 1 pixel to 3 pixels. As described with reference to FIG. 8, the thinner the line, the lower the width and height of the toner that adheres. For example, the character "A" used in the description of the first embodiment is composed of a line with a width of about 6 pixels. However, if the gradation value of the target pixel is lowered as in the first embodiment for an extremely thin line such as 1 pixel to 3 pixels, the height becomes too low, and it may not be possible to improve the fixation of the toner to the sheet P.

[0045] Therefore, in this embodiment, the target pixels are classified into first target pixels and second target pixels, and the gradation value of the first target pixels is corrected in the same manner as in the first embodiment, but the gradation value of the second target pixels is not corrected. Note that the second target pixels are pixels among the target pixels that satisfy a predetermined criterion, and the first target pixels are pixels among the target pixels other than the second target pixels. In the following example, it is assumed that the predetermined criterion is satisfied when the pixel is an edge pixel among the target pixels. In other words, the edge pixel among the target pixels is regarded as the second target pixel.

[0046] 13 is a flowchart showing the image processing of this embodiment. In S20, the image processing unit 500 determines whether the archive mode or the normal mode is set. If the normal mode is set, the image processing unit 500 executes normal image processing on the image data in S26, as in the first embodiment. On the other hand, if the archive mode is set, the image processing unit 500 identifies a target pixel in S21. As in the first embodiment, the target pixel is a pixel whose gradation value is equal to or greater than the first threshold value. In this example, the first threshold value is set to 235.

[0047] In S22, the image processing unit 500 classifies the target pixel into a first target pixel and a second target pixel (exclusion pixel). In this example, the second target pixel is an edge pixel among the target pixels. In other words, in this example, the second target pixel is a pixel adjacent to an adjacent pixel among the target pixels. Note that a target pixel in which at least one of four adjacent pixels in the main scanning direction and the sub-scanning direction is equal to or less than the third threshold value can also be set as the second target pixel. Note that the third threshold value is smaller than the first gradation value, and can be set to, for example, 0 or a value close to 0. The first target pixel is a pixel among the target pixels other than the second target pixel. In S23, the image processing unit 500 changes the gradation value of the first target pixel to a first gradation value smaller than the first threshold value. In this example, the first gradation value is set to 125. On the other hand, in this embodiment, the gradation value of the second target pixel is not changed, and therefore, the gradation value is the same as that in the normal mode.

[0048] Next, the image processing unit 500 specifies adjacent pixels in S24. The adjacent pixels are pixels adjacent to the target pixel among the pixels not selected as the target pixel in the sheet P. The adjacent direction may be either the main scanning direction or the sub-scanning direction. The image processing unit 500 changes the gradation value of the adjacent pixel to a second gradation value larger than the first gradation value in S25. In this example, the second gradation value is 200. The adjacent pixel may be provided with a condition that the gradation value is equal to or less than a second threshold value. In other words, the adjacent pixel may be a pixel adjacent to the target pixel among the pixels not selected as the target pixel and having a gradation value equal to or less than the second threshold value. The second threshold value may be smaller than the first threshold value, for example, 0 or a value close to 0. The third threshold value and the second threshold value may be the same value.

[0049] After executing the process of S25, when the normal mode is selected in S26, the image processing unit 500 executes normal image processing in the same manner as in the case where the normal mode is selected. However, when the process of S25 is executed, the process of S26 can be skipped and the process of S27 can be executed. By executing normal image processing, even if the formed image has a graphic image portion represented by intermediate gradation values mixed therein, normal image processing is performed on the graphic image portion. In any case, the image processing unit 500 outputs the image data after being processed in S27 to the image forming unit 50.

[0050] FIG. 14 visually shows the image data after processing of the image data shown in FIG. 12 output by the image processing unit 500 when the archive mode is selected. Reference numeral F is the first target pixel, and the gradation value has been changed from 255 to 125. Reference numeral F' is the second target pixel, and the gradation value remains 255. Reference numeral E is an adjacent pixel, and the gradation value has been changed from 0 to 200. The thin line portion indicated by reference numeral J is such that the gradation value is maintained while the gradation values of the surrounding adjacent pixels are increased.

[0051] FIG. 15(B) shows a cross-section of the sheet P at the line G shown in FIG. 15(A) before fixing by the fixing device 6. The left side of FIG. 15(B) is a cross-section when image formation is performed in the normal mode, and the right side is a cross-section when image formation is performed in the archive mode. Similar to the first embodiment, in the case of the archive mode, the height of the toner adhering to the sheet P is lower than that in the normal mode, and the width of the toner adhering to the sheet P in the main scanning direction becomes wider. That is, Ha>Hb and Wa<Wb. Here, Ha and Wa are the height and the width in the main scanning direction of the toner adhering to the sheet P when the normal mode is selected, respectively, and Hb and Wb are the height and the width in the main scanning direction of the toner adhering to the sheet P when the archive mode is selected.

[0052] Fig. 15(C) shows a cross section of sheet P taken along line K shown in Fig. 15(A) before fixing by fixing device 6. Note that the left side of Fig. 15(C) is a cross section when image formation is performed in normal mode, and the right side is a cross section when image formation is performed in archive mode. The width Wbj of toner adhering to sheet P in archive mode is larger than the width Waj of toner adhering to sheet P in normal mode. On the other hand, the height Hbj of toner adhering to sheet P in archive mode is slightly larger than the height Haj of toner adhering to sheet P in normal mode.

[0053] However, when comparing the height Ha of the toner adhering to sheet P at the position of line G with the height Haj at the position of line K in the normal mode, Haj is much smaller than Ha. On the other hand, the height Hb of the toner adhering to sheet P at the position of line G and the height Hbj at the position of line K in the archive mode are substantially equal. In other words, the archive mode can make the toner height more uniform than the normal mode. Also, in some areas, the toner height is slightly higher than in the normal mode, but in other areas the toner height can be made lower than in the normal mode. Therefore, the archive mode can improve the fixation of toner to sheet P.

[0054] As described above, in this embodiment, even if an image portion made up of extremely fine lines is included, the fixation of toner onto the sheet P is ensured, and a product with high storability can be formed.

[0055] In this embodiment, the predetermined criterion for determining that a pixel is the second target pixel is that the pixel is an edge pixel among the target pixels. However, instead of the edge pixel, the pixel that constitutes a thin line among the target pixels can be classified as the second target pixel. Specifically, the width of the line constituted by the target pixels in the main scanning direction and the sub-scanning direction, that is, the number of consecutive pixels, is determined based on the image data. Then, if the number of consecutive pixels in at least one of the main scanning direction and the sub-scanning direction is smaller than a predetermined number (first consecutive number) of, for example, 4 pixels or 5 pixels, the pixel that constitutes the line can be classified as the second target pixel. In the first embodiment, it was described that the pixel whose gradation value is equal to or greater than the first threshold value and whose number of consecutive pixels in at least one of the main scanning direction and the sub-scanning direction is smaller than a predetermined number (second consecutive number) is the target pixel. Here, the second consecutive number is a value larger than the first consecutive number.

[0056] <Other> In each of the above embodiments, when the archive mode is selected, the fixing temperature of the fixing device 6 can be increased and the process speed of the image formation, that is, the conveying speed of the sheet P can be slowed down. By increasing the fixing temperature and slowing the process speed, the toner can be sufficiently heated and the fixability of the toner to the sheet P can be improved. Furthermore, when the archive mode is selected, in addition to performing the image processing described in the first and third embodiments, the developing voltage and / or the charging voltage can be lowered as described in the second embodiment, compared to the normal mode.

[0057] Furthermore, the present invention can be realized as an image processing device that performs the image processing described in the first and third embodiments. The image processing device transmits image data after image processing to an image forming device to form an image.

[0058] [Other embodiments] The present invention can also be realized by a process in which a program for implementing one or more of the functions of the above-described embodiments is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. The present invention can also be realized by a circuit (e.g., ASIC) that implements one or more of the functions.

[0059] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0060] 53: operation unit, 500: image processing unit, 50: image forming unit

Claims

1. A setting means for setting one of a plurality of operation modes; a processing means for performing image processing on image data in accordance with the operation mode set by the setting means; an image forming unit that forms an image on a sheet conveyed in a conveying direction based on the image data after the image processing by the processing unit; Equipped with an image forming apparatus characterized in that, when a first mode of the plurality of operating modes is set, the image processing by the processing means includes determining a target pixel in the image data whose gradation value is equal to or greater than a first threshold value and an adjacent pixel that is different from the target pixel and is adjacent to the target pixel, setting a first gradation value that is smaller than the first threshold value as the gradation value of the target pixel, and setting a second gradation value that is greater than the first gradation value as the gradation value of the adjacent pixel.

2. A setting means for setting one of a plurality of operation modes; a processing means for performing image processing on image data in accordance with the operation mode set by the setting means; an image forming unit that forms an image on a sheet conveyed in a conveying direction based on the image data after the image processing by the processing unit; Equipped with an image forming apparatus characterized in that, when a first mode of the plurality of operating modes is set, the image processing by the processing means determines a target pixel in the image data whose gradation value is equal to or greater than a first threshold value, an adjacent pixel that is different from the target pixel and is adjacent to the target pixel, and an excluded pixel among the target pixels that satisfies a predetermined criterion, and sets a first gradation value smaller than the first threshold value as the gradation value of pixels among the target pixels excluding the excluded pixel, and sets a second gradation value greater than the first gradation value as the gradation value of the adjacent pixel.

3. 3. The image forming apparatus according to claim 2, wherein when the first mode is set, the gradation values ​​of the excluded pixels are not changed in the image processing by the processing means.

4. A setting means for setting an operation mode to one of a plurality of operation modes including a first mode and a second mode; a processing means for performing image processing on the first image data in accordance with the operation mode set by the setting means; an image forming unit that forms an image on a sheet transported in a transport direction based on the second image data after the image processing by the processing unit; Equipped with An image forming apparatus characterized in that, in the second image data when the second mode is set, a pixel having a gradation value equal to or greater than a first threshold is defined as a target pixel, and a pixel that is different from the target pixel and adjacent to the target pixel is defined as an adjacent pixel, wherein, in the second image data when the first mode is set, the gradation value of the target pixel is a first gradation value that is smaller than the first threshold, and the gradation value of the adjacent pixel is a second gradation value that is greater than the first gradation value.

5. A setting means for setting an operation mode to one of a plurality of operation modes including a first mode and a second mode; a processing means for performing image processing on the first image data in accordance with the operation mode set by the setting means; an image forming unit that forms an image on a sheet transported in a transport direction based on the second image data after the image processing by the processing unit; Equipped with an image forming apparatus characterized in that, in the second image data when the second mode is set, a pixel whose gradation value is equal to or greater than a first threshold value is defined as a target pixel, a pixel that is different from the target pixel and is adjacent to the target pixel is defined as an adjacent pixel, and a pixel among the target pixels that satisfies a predetermined criterion is defined as an excluded pixel, wherein, in the second image data when the first mode is set, the gradation values ​​of pixels among the target pixels excluding the excluded pixel are a first gradation value that is smaller than the first threshold value, and the gradation values ​​of the adjacent pixels are a second gradation value that is greater than the first gradation value.

6. 6. The image forming apparatus according to claim 5, wherein the gradation value of the excluded pixel in the second image data when the first mode is set is the same as the gradation value of the excluded pixel in the second image data when the second mode is set.

7. 7. The image forming apparatus according to claim 2, wherein the excluded pixel is the target pixel adjacent to the adjacent pixel.

8. 7. The image forming apparatus according to claim 2, wherein the excluded pixels are the target pixels whose consecutive number in at least one direction among the transport direction and a direction perpendicular to the transport direction is smaller than a first predetermined number.

9. The image forming apparatus according to any one of claims 1 to 8, characterized in that the target pixel is a pixel having a gradation value equal to or greater than a first threshold value, and a number of consecutive pixels in at least one of the transport direction and a direction perpendicular to the transport direction that is smaller than a second predetermined number.

10. 9. The image forming apparatus according to claim 1, wherein the target pixel is a pixel that constitutes a character image or a line image and has a gradation value equal to or greater than a first threshold value.

11. 11. The image forming apparatus according to claim 1, wherein the adjacent pixel is different from the target pixel, adjacent to the target pixel, and has a gradation value equal to or less than a second threshold value.

12. The image forming apparatus according to claim 11 , wherein the second threshold value is smaller than the first gradation value.

13. A setting means for setting one of a plurality of operation modes; a processing means for performing image processing on image data in accordance with the operation mode set by the setting means; Equipped with an image processing device characterized in that, when a first mode of the plurality of operating modes is set, the image processing by the processing means includes determining a target pixel in the image data whose gradation value is equal to or greater than a first threshold value and an adjacent pixel that is different from the target pixel and is adjacent to the target pixel, setting a first gradation value smaller than the first threshold value as the gradation value of the target pixel, and setting a second gradation value larger than the first gradation value as the gradation value of the adjacent pixel.

14. A setting means for setting one of a plurality of operation modes; a processing means for performing image processing on image data in accordance with the operation mode set by the setting means; Equipped with an image processing device characterized in that, when a first mode of the plurality of operating modes is set, the image processing by the processing means determines a target pixel in the image data whose gradation value is equal to or greater than a first threshold value, an adjacent pixel that is different from the target pixel and is adjacent to the target pixel, and an excluded pixel among the target pixels that satisfies a predetermined criterion, and sets a first gradation value smaller than the first threshold value as the gradation value of pixels among the target pixels excluding the excluded pixel, and sets a second gradation value greater than the first gradation value as the gradation value of the adjacent pixel.

Images