JP2006159480A - Image forming device and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve character reproducibility especially for color characters regardless of the sizes of the characters. <P>SOLUTION: Whether printing is directly performed by a basic resolution, or whether an area gradation process is performed are switched in response to character attributes such as character sizes. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming method and apparatus, and more particularly to a color image forming apparatus and method.

  In recent years, an opportunity to input to a computer and output by a printer instead of handwritten input of a document has been widely used mainly in general offices. In addition, the demand for color image printing has increased with the increase in the speed and price of computers and printers, and there have been many opportunities to create documents such as catalogs and insurance policies that have been created by printing. FIG. 13 is a diagram illustrating a flow of a conventional image forming method. Referring to the figure, the user forms an image on the application software 100 operating on the computer, and the types of the image are roughly classified into three. That is, the image is a natural image input by an optical scanner or a digital camera, a character input by a user using a keyboard, or a graphic such as a figure input by a pointing device such as a mouse. These images may be mixed on one document depending on the application software 100 used.

  Thus, the image created by the user on the application software 100 is processed as print data by the printer driver 101 installed on the same computer as the computer on which the application software 100 is normally installed. The print driver 101 creates print data suitable for the image forming apparatus to be used. When a laser printer capable of high-speed printing is used, the print driver 101 is described in a page description language such as Adobe PostScript. The print data created by the print driver 101 is sent to the image forming apparatus and converted into a bitmap by the bitmap processing unit 102. Here, the bitmapping will be briefly described. Normally, since characters and graphic images are described by commands defined in the page description language, they cannot be printed by the image forming apparatus as they are. Therefore, it is necessary to decode the command and convert it into a bitmap that can be printed by the image forming apparatus. Further, since the image which is bitmap information from the beginning has a large amount of information, the image is compressed by the print driver and decompressed by the image forming apparatus.

  The bit-converted data is color-converted by the color conversion unit 103. Here, the color conversion is performed by converting an RGB signal matched to the normal monitor color into a YMCK signal matched to the image forming apparatus. The data converted into the YMCK signal is subjected to gradation correction by the gradation correction unit 104. The gradation correction is correction for correcting the density gradation characteristic of the image forming apparatus so that the density gradation of the final printed matter becomes a desired characteristic.

  The dither processing is performed by the dither processing unit 105 on the gradation corrected data. Here, the dither processing is processing that performs area gradation suitable for the image forming apparatus, and area gradation processing using systematic dither or an error diffusion method is performed as conventionally known. Currently, in order to reproduce high-definition images such as small-size characters and thin lines, the basic resolution of the image forming apparatus is 600 dpi or more. On the other hand, in an image forming apparatus using an electrophotographic method, a stable density gradation characteristic cannot be obtained at a resolution of 600 dpi or higher. Therefore, the minimum unit for expressing density gradation by area gradation processing is set to a resolution lower than the basic resolution. Conversion is achieved to obtain stable density gradation characteristics.

  The dithered data is sent to a laser 106, which is a light emitting member of the image forming apparatus, and is subjected to image exposure.

A plurality of color conversion units 103, gradation correction units 104, and dither processing units 105 described above may be provided if necessary. Specifically, color conversion may have two types, color conversion suitable for natural images and having a small color difference from the monitor, and color conversion suitable for graphics that maximizes the color reproduction range. Regarding gradation correction, there may be a plurality of gradation corrections according to the correction of gradation characteristics of an input device such as an optical scanner or a digital camera or according to user preference. As for dither processing, the image uses a low line number dither, the character uses a high line number, and the like.
JP 2003-54042 A

  However, in the above-described conventional example, there is a problem that small characters, particularly color characters, are poorly reproducible and cannot be recognized as human characters.

  In order to improve this problem, Japanese Patent Application Laid-Open No. 2003-54042 discloses that the number of dither lines is changed depending on the character size, and that the number of lines is increased for small size characters.

  The above Japanese Patent Application Laid-Open No. 2003-54042 discloses a result of using two types of dither lines, 85 lines and 141 lines, and using 24 points and 48 points as character sizes. However, as a result of examination by the present inventor, it has been found that there is no problem with the above-described characters of 24 points or more, but with characters of 10 points or less, the reproducibility of the characters is insufficient and identification is difficult.

  Furthermore, since color characters are used in the color image forming apparatus, the density contrast between the character portion and the non-character portion is lower than that of the black character, and there is a problem that the character recognizability is further deteriorated.

  An object of the present invention is to realize good character reproducibility regardless of the size of characters and even in the case of color characters.

  In order to achieve the above object, a first invention according to the present application converts character code information into bitmap information of basic resolution including multi-value density information, and area gradation means having a resolution lower than the basic resolution. In the image forming method and the image forming apparatus, the bit map information is converted by the method to switch between the case where the density is expressed and the case where the density is expressed at the basic resolution depending on the character attribute included in the character code. is there.

  According to a second aspect of the present invention, the character attribute is a character size. When the character size is a certain threshold value or more, the bitmap information is converted by area gradation means having a resolution lower than the basic resolution, and a character less than the threshold value is obtained. In the case of size, the image forming method and the image forming apparatus are characterized in that the density is expressed with a basic resolution.

  According to a third aspect of the present invention, the character attribute is at least one of a font type and a deformation and a character size, and the threshold is varied according to the type or deformation of the font. A forming method and an image forming apparatus.

  A fourth invention is an image forming method and an image forming apparatus characterized in that a user can modify and add a threshold value correlated with the character attribute.

  A fifth invention is an image forming method and apparatus characterized in that the threshold character size is 4 to 10 points.

  According to a sixth aspect of the present invention, there is provided an image forming apparatus comprising density gradation expression means in basic resolution units.

  A seventh invention is an image forming apparatus that forms a latent image by performing image exposure on an electrostatic latent image carrier, converts the latent image into a toner visible image, and transfers the latent image to a recording material. The density gradation expression means in the basic resolution unit is an exposure amount modulation means for image exposure.

  As described above, according to the present invention, in the first invention according to the present application, the character code information is converted into bitmap information of basic resolution including multi-value density information, and the resolution is lower than the basic resolution. By converting the bitmap information by the area gradation means and expressing the density at the basic resolution and switching the density at the basic resolution according to the character attribute included in the character code, On the other hand, it is possible to provide an image forming method and an image forming apparatus capable of obtaining good reproducibility.

  In the second invention, the character attribute is a character size. When the character size is a certain threshold value or more, the bitmap information is converted by an area gradation unit having a resolution lower than the basic resolution, and a character less than the threshold value is obtained. In the case of size, by expressing the density at the basic resolution, image forming method and image forming that can obtain good reproducibility for characters with different character sizes that contribute most to reproducibility among character attributes Equipment can be provided.

  In the third invention, the character attribute is a character size and at least one of font types and deformations, and various characters can be obtained by varying the threshold value according to the font type or deformation. An image forming method and an image forming apparatus capable of obtaining good reproducibility can be provided even when using.

  In the fourth invention, the user can modify and add the threshold value correlated with the character attribute, so that the user can adjust the reproducibility of the character, and the newly added character is also good. An image forming method and an image forming apparatus capable of obtaining reproducibility can be provided.

  In the fifth aspect of the present invention, the character size serving as the threshold is set from 4 points to 10 points, whereby an image forming method and an image forming apparatus that can obtain the most effective good character reproducibility can be provided.

  According to the sixth aspect of the present invention, it is possible to provide an image forming apparatus that ensures density gradation even when the density is expressed by the basic resolution by having the density gradation expressing means in the basic resolution unit.

  According to a seventh aspect of the present invention, there is provided an image forming apparatus for forming a latent image by performing image exposure on an electrostatic latent image carrier, converting the latent image into a toner visible image, and then transferring the latent image to a recording material. The density gradation expression means in the basic resolution unit can be an exposure amount modulation means for image exposure, thereby providing an image forming apparatus with excellent density gradation.

Example 1
FIG. 1 shows a flow of an image forming method according to the first embodiment of the present invention. This will be described below with reference to the drawings.

  The numbers from 100 to 103 are the same as those of the conventional example, and the same numbers are given and the description is omitted. As described above, the RGB signal is converted into a YMCK signal adapted to the image forming apparatus by the color conversion unit 103. Further, this color conversion limits the amount of toner to be used. More specifically, logically, when the solid image of each color of YMCK is 100%, it is possible to print a 400% image, which is a 100% solid image of all colors. In addition, a limit of 400% or less is necessary from the viewpoint of scattering of characters in transfer. This limitation depends on the characteristics of the toner used, the configuration of the transfer member, the configuration of the fixing device, and the like. However, in an image forming apparatus using electrophotography, good results are obtained when the ratio is 250% to 180%.

  The character attribute of the color-converted image data is determined by the character attribute determination unit 107. Here, the attribute of the character is the size of the character. For character data having a character size less than a predetermined character size N, image data is sent to the gradation correction unit 104a. Image data other than characters and character data of character size N or larger are sent to the gradation correction unit 104b. Note that the image data sent to the gradation correction unit 104b may be further subdivided into character data, graphic data, and image data, and gradation correction and dither processing suitable for each may be performed.

  The small-size character image data sent to the tone correction unit 104a is tone-corrected according to the tone characteristics of an image forming apparatus, which will be described later, and sent to the laser modulator 108. Here, a specific modulation method in the laser modulation unit 108 will be described. FIG. 2 is a diagram showing an example of a laser modulation method.

  The image data is stored in the buffer memory 1081, the signal is latched by the reference clock by the clock generator 1083, and sent to the DA converter 1082. The DA converter 1082 converts the signal into an analog voltage signal corresponding to the digital data. The analog signal output from the DA converter 1082 is sent to the comparator 1085. Reference numeral 1084 denotes a triangular wave generator, which sends a triangular wave having the same frequency as the reference clock frequency from the clock generator 1083 to the comparator 1085. Therefore, the analog image signal and the triangular wave are synchronized because the reference clock from the same clock generator 1083 is used. The comparator 1085 compares the magnitude relationship between the input analog image signal and the triangular wave. When the image signal is smaller than the triangular wave, the comparator 1085 turns on, and when the image signal is larger, the comparator 1085 outputs a time-modulated output corresponding to the level of the analog image signal. Send to laser 106. Specifically, the analog image signal, the triangular wave, and the output of the comparator are shown in FIG.

  As a laser modulation method, the image signal may be divided by a clock having a frequency higher than that of the image clock, and the laser light amount may be directly modulated from the digital data of the image. Instead of time-modulating the laser light quantity, conventionally known luminance modulation may be performed. In this way, by providing the laser light amount modulation unit 108 in the basic resolution unit of the image forming apparatus, density gradation expression can be performed even in the case of 600 dpi direct without performing dither processing that is area gradation, and dither processing is used. In this case, the product of the number of pixels included in the dither matrix and the modulation number of the laser light quantity modulation means becomes the density gradation expression that can be expressed, and a smooth density gradation characteristic can be obtained.

  Next, tone correction will be described.

  FIG. 4 shows image data and density in a case where two dithers having different numbers of lines are used in an image forming apparatus having a basic resolution of 600 dpi and in a case of 600 dpi direct in which gradation is directly expressed by laser modulation means without using dither. FIG. Dithers having a screen line number of 141 and a screen angle of 45 ° and dithers having a screen line number of 212 and a screen angle of 45 ° were used. Note that the image data is 8-bit data and has a level of 0 to 255. As can be seen from the figure, the number of dither lines and the characteristics of image data versus density differ depending on the case where dither is not used. This difference in characteristics is due to the spatial frequency characteristics of the image pattern in the electrophotographic system. In other words, the spatial frequency in the case of 600 dpi direct is about 24 cycles / mm, but such a high spatial frequency is used to develop a latent image formed on the photosensitive drum or image data when developing the formed latent image. Cannot be reproduced faithfully, and changes rapidly from a solid white state of 0 data to a solid black state of 255 data (indicated by A in the figure). For this reason, when an area image of an intermediate tone is printed, unevenness in the surface potential of the photosensitive drum and unevenness in the toner charging state of the developing device are picked up, resulting in non-uniform density and chromaticity. On the other hand, with a 212-line dither, the latent image and development can be performed faithfully to the image data as the spatial frequency decreases, approximately 8 cycles / mm for the 141-line dither, and approximately 6 cycles / mm for the 141-line dither, and gentle gradation characteristics are obtained. The density and chromaticity uniformity of the area image of the intermediate gradation are good (the 212 line is indicated by B and the 141 line is indicated by C).

  Since the gradation characteristics are different as described above, in the gradation correction, the image data is converted and corrected to a desired gradation characteristic. FIGS. 5 (1) and 5 (2) show the gradation correction in the case of 141 dpi and the case of 141 line dither (the same applies to the case of 212 line dither, and the description is omitted). The graphs indicated by T in FIGS. 5 (1) and 5 (2) are desired gradation characteristics, and here, the density is proportional to the image data. With respect to the graph T which is the desired characteristic, the graph A (FIG. 5 (1)) which is the gradation characteristic at 600 dpi direct of the image forming apparatus and the gradation characteristic graph C (FIG. 5 (2)) at the 141-line dither. ) Are indicated by dotted lines in the figure, and graph A ′ (FIG. 5 (1)) and graph C ′ (FIG. 5 (2)) are tone correction characteristics. That is, the image data input to the 600 dpi direct tone correction unit 104a is converted into image data using the graph A ', and the image data is input into the 141 line dither tone correction unit 104b using the graph C'. , All are corrected to the same gradation characteristics.

  Next, reproducibility of lowercase letters, which is an effect of the present invention, will be described.

  FIG. 6 is a diagram in which image data created under the following conditions is enlarged about 80 times.

Font type: Mincho type Text: "Amazing"
Resolution: Bitmap with 600 dpi Gradation level: 50%
Size: 2 points, 3 points, 5 points, 6 points, 8 points, 12 points Dither: 141 lines, 212 lines, 600 dpi direct from the left Regarding character reproducibility, it must be at least recognizable as characters. It is necessary that the shape unique to the font can be recognized. Here, the font-specific shape is the difference in font shape design between Gothic and Mincho. Furthermore, for example, in the Kanji characters, the font designer's intention is the shape of words such as “Tome, Splash, Pay” that imitates the brushes found in the Mincho style, and the shape of the serif that is a character decoration that is seen in Times New Roman in the alphabet. It is important to be able to reproduce the street.

  FIG. 7 shows the result of evaluating the characters shown in FIG. 6 from this viewpoint.

  On the 141 line, there is no problem in the recognizability of whether or not a character can be discriminated at 8 points or more (indicated by a circle in the figure), but it becomes worse when it is 6 points or less. Specifically, at 6 points and 5 points, the character is barely recognizable (indicated by Δ in the figure), and at 3 points and 2 points, the character cannot be recognized (indicated by x in the figure). Further, whether or not the characteristics of the Mincho style can be recognized, that is, the recognizability of the font shape (font shape column in the figure) can be recognized at 12 points (indicated by ○ in the figure), but at 8 points the Mincho The shape of the body, “stop, splash, pay” is not clearly understood (indicated by Δ in the figure). Furthermore, below 6 points, the shape of “Motome, Hane, Paid”, which is a characteristic of Mincho, is at a level that cannot be recognized (indicated by x in the figure). At line 212, the level of recognition and font shape improved, and the level of recognition was 6 points or more for a ◯ level and 5 points for a △ level. X level with 3 points or less. In the font shape, it is ◯ level at 8 points or more, and △ level at 5 and 6 points. X level with 3 points or less.

  Compared to the case of 212 line dither processing having a high spatial frequency, in the case of 600 dpi direct, the recognizability is ◯ level even at 2 points, and the font shape is also ◯ level at 5 points or more. Points will be △ level, and 2 points will be × level, which will improve greatly.

  On the other hand, for characters of 12 points or more, the area of the image constituting the characters increased, and when printed with 600 dpi direct, uneven density and uneven chromaticity were recognized. On the other hand, when dithering was performed, uniform density and chromaticity could be obtained. This result was more remarkable as the character size was larger, such as 24 points or 48 points. On the other hand, when printing with 600 dpi direct printing with characters of 8 points or less, density unevenness and chromaticity unevenness are generated microscopically, but since the image area is small, the density unevenness and chromaticity unevenness are visually recognized. Can not.

  Therefore, according to the present invention, the character size N in the character attribute determination in the character attribute determination unit 107 in FIG. 1 is set to 8, dither processing is performed for characters of 8 points or more, and for characters less than 8 points, By selecting 600 dpi direct, it is possible to ensure good character reproducibility, that is, good recognizability in lowercase letters and uniform density and chromaticity in uppercase letters, regardless of the character size.

  An example of an image forming apparatus suitable for the image forming method described above is shown in FIG.

  Referring to the drawing, 10a, 10b, 10c, and 10d are first, second, third, and fourth image forming units, respectively, and photosensitive drums 1a, 1b, 1c, and 1d, charging rollers 2a, 2b, 2c, 2d, exposure devices 3a, 3b, 3c, 3d having lasers 106a, 106b, 106c, 106d as light emitting members, developing devices 4a, 4b, 4c, 4d, and cleaners 5a, 5b, 5c, 5d, respectively. . Each operation will be described. Photosensitive drums 1a, 1b, 1c, and 1d configured by applying a photoconductor made of an organic photoconductor (OPC) or A-Si, CdS, Se, or the like on a conductive substrate, respectively. Each is uniformly charged by charging rollers 2a, 2b, 2c and 2d to which a bias is applied from a high voltage power supply (not shown). On the charged photosensitive drums 1a, 1b, 1c, 1d, laser exposure is performed by exposure devices 3a, 3b, 3c, 3d, respectively, and electrostatic latent images are formed. Reference numeral 99 denotes a printer controller which performs the image processing shown in FIG. 1 and sends image data corresponding to each color to the lasers 106a, 106b, 106c and 106d, respectively. The electrostatic latent images on the photosensitive drums 1a, 1b, 1c, and 1d are developed by the developing devices 4a, 4b, 4c, and 4d, respectively, and toner visible images are formed. The visible toner images on the photosensitive drums 1a, 1b, 1c, and 1d are transferred onto the transfer belt 61 by transfer rollers 62a, 62b, 63c, and 63d, respectively, to which a bias is applied from a high-voltage power source (not shown). Transferred onto P. The transfer process will be described in detail.

  61 is an endless transfer belt made of a resin such as EPDM, NBR, urethane, silicone rubber, or a resin such as PVdF, polyimide, polycarbonate, polyamide, polyethylene, and the tension roller 63, which is two tension rollers, It is stretched on the driving roller 64. As the driving roller 64 rotates in the direction of the arrow in the figure, the transfer belt 61 moves in the direction of the arrow in the figure. The transfer material P is attracted and conveyed to the transfer belt 61 by electrostatic force. This electrostatic force is generated when electric charges are supplied to the transfer material P during transfer by the transfer rollers 62a, 62b, 63c, and 63d. In order to further stabilize the adsorption force of the transfer material P, the first transfer roller is used. Prior to 62a, the transfer material P may be charged by the suction roller 65 to which a bias is applied. The transfer material P is moved from the first image forming unit 10 a to the fourth image forming unit 10 d by the movement of the transfer belt 61, separated from the transfer belt 61 at the position of the driving roller 64, and fixed by the fixing device 7. The The processes for forming, developing, and transferring the latent image on the photosensitive drums 1a, 1b, 1c, and 1d described above are synchronized with the movement of the transfer belt 61, and the so-called registration of the toner image positions of the respective colors is adjusted.

(Example 2)
FIG. 8 shows a flow of an image forming method according to the second embodiment of the present invention. Components having the same functions as those in the previous embodiment are denoted by the same reference numerals and description thereof is omitted. The feature of the present invention is that it is possible to ensure good character reproducibility even when the user uses various characters as a character attribute by changing the threshold value not only according to the size but also according to the font type and thickness.
The character attribute determination unit 107 is a process of determining the attribute of the character and selecting the gradation correction unit 104a or the gradation correction unit 104b, which is a subsequent process. Here, the determination is made based on the table shown in FIG.

  In FIG. 9, in order to deal with the case where the font type and the transformation with the same font are performed, as a transformation for one font, it is usually subdivided into italic and bold, and each tone correction unit 104a performs tone correction. The table stores character size threshold values for selecting the portion 104b. The gradation correction unit 104a is selected for characters having a size smaller than the threshold, and step 104b is selected for characters having a size larger than the threshold.

  Specifically, in order to determine the character attribute by the character attribute determination unit 107, the character data color-converted by the color conversion unit 103 includes information about the font type, size, and deformation along with the bitmap information. It is necessary to have together. The table information corresponding to the font type and deformation may be read as a threshold value, and the threshold value and the font size may be compared.

  Next, the font type, deformation, and threshold will be described. First, as a modification, italic forms a character with a finer line by performing oblique character deformation as compared with a normal case, and character recognition is reduced. Therefore, it is better to perform 600 dpi direct printing from a larger character size compared to normal characters (it is 10 points for italics in the case of Mincho in Fig. 9, which is usually 8 points). On the other hand, since bold characters are formed with thicker lines than normal, it is better to perform 600 dpi direct printing from a smaller character size compared to normal characters (normal in the case of Mincho in Fig. 9). Is 8 points, bold is 6 points).

  As typical font types used in Chinese characters, Mincho and Gothic fonts were given. Compared to the Mincho font, the Gothic font is wider than the Mincho font, and the lines that form the characters are thicker, and there are fewer expressions such as “stop, splash, pay” etc. There is little decline in recognition even when performing (the normal Mincho type has 8 points, the normal Gothic type has 7 points). In Gothic style, the deformation is similar to that of Mincho style (usually 7 points, italic 9 points, bold 5 points).

  Further, regarding the alphabet, a threshold table may be provided separately from the Chinese characters, and an example is shown in FIG. As a font type, a serif style similar to the Mincho style of the Chinese character and a sans serif style equivalent to the Gothic style are used, and the threshold value table in the case of normal, italic, and bold type is shown as the character deformation. In the case of the alphabet, the threshold tends to change with respect to the type and deformation of the font, but the overall structure is simpler than that of kanji, and the recognizability is higher for the same character size. May be smaller than kanji (for example, normal Mincho type is 8 points, normal serif type is 7 points, normal Gothic type is 7 points, normal sans serif type is 6 points).

  Needless to say, the type of font is not limited to the font described above, and a table may be provided according to the number of fonts used. Further, regarding the deformation, not only italic and bold, but other deformations can be applied in the same manner as long as it is a process for deforming a normal font bitmap.

  As described above, even when various characters are used by changing the threshold of whether to perform dither processing or 600 dpi direct between 4 points and 10 points according to the type and deformation of the font. Good character reproducibility could be secured. Further, the present invention can be applied to the image forming apparatus shown in FIG.

(Example 3)
A feature of the present invention is that the user can modify and add the threshold table in the previous embodiment.

  In the previous embodiment, it has been described that threshold values for selecting character processing are tabulated in order to correspond to a plurality of character attributes. The most preferable threshold value is set in advance. For example, the character size is equal to or larger than the threshold value, and even when dither processing is performed, there is a case where importance is placed on character recognition rather than density uniformity. In addition, new fonts may be added to the host computer or printer.

  Such a case can be dealt with by allowing the user to modify and add the threshold value table.

  FIG. 11 shows an example of the threshold table in the present embodiment.

  A region indicated by α in the figure is a threshold value set as a default in advance. Here, the threshold value can be modified by the user. An area indicated by β in the figure is a user-defined area, and a threshold value of a newly added font is set in the areas indicated by A, B, and C in the figure. When this threshold value table is modified or added, it may be displayed on the display unit of the printer and operated by the user, or may be operated on the display screen of the printer driver software of the host computer. Data that converts character code information into bitmap information (hereinafter referred to as character bitmap information) may be in the printer or in the host computer, but there is an undefined font in the threshold table. In order to avoid this, it is preferable to create a threshold value table by sharing the font information each printer has and the host computer.

  As described above, according to this embodiment, it is possible to select density uniformity and character recognizability according to user's preference, and good character reproducibility of characters even when a new font is added. Was able to be secured.

Further, the present invention can be applied to the image forming apparatus shown in FIG.
In the three embodiments described above, the dithering process is used as the area gradation process, but the present invention is not limited to this, and a conventionally known area gradation process can be used. Further, although 600 dpi has been described as the basic resolution of the printer, the present invention can also be applied to a resolution of 600 dpi or higher such as 1200 dpi.

  Furthermore, the effect of the present invention can be obtained even in combination with a so-called edge smoothing process in which additional data is added to the edge portion of the font bitmap data in order to smooth the edges of the conventionally known characters. .

First embodiment of the present invention The figure which showed the laser modulation method which concerns on 1st Example of this invention The figure which showed the laser modulation output based on 1st Example of this invention The figure which showed the gradation characteristic based on 1st Example of this invention The figure which showed the gradation correction method based on 1st Example of this invention The figure which showed the reproducibility of the character which concerns on 1st Example of this invention The figure which showed the evaluation result of the reproducibility of the character which concerns on 1st Example of this invention Second embodiment of the present invention Threshold table according to the second embodiment of the present invention Threshold table according to the second embodiment of the present invention Threshold table according to the third embodiment of the present invention The figure which showed the image forming apparatus suitable for this invention Figure showing a conventional example

Explanation of symbols

P transfer material 1a, 1b, 1c, 1d photosensitive drum 2a, 2b, 2c, 2d charger 3a, 3b, 3c, 3d exposure device 4a, 4b, 4c, 4d developer 5a, 5b, 5c, 5d cleaner 61 transfer belt 62a, 62b, 62c, 62d Transfer roller 63 Tension roller 64 Drive roller 7 Fixing device 10a, 10b, 10c, 10d Image forming unit 99 Printer controller 100 Application software 101 Printer driver 102 Bitmap processing unit 103 Color conversion unit 104 Tone correction Part 105 dither processing part 106 laser 107 character attribute judgment part

Claims (12)

  When character code information is converted into bitmap information of basic resolution including multi-value density information, and the bitmap information is converted by area gradation means having a resolution lower than the basic resolution to express density, and the basic resolution A method of forming an image by switching between a case where density is expressed by a character attribute included in the character code.   2. The character attribute according to claim 1, wherein the character attribute is a character size, and when the character size is a certain threshold value or more, the bitmap information is converted by an area gradation means having a resolution lower than the basic resolution, and the character attribute is less than the threshold value. In the case of character size, the image forming method is characterized by expressing the density at a basic resolution.   3. The image according to claim 2, wherein the character attribute is at least one of a font type and a deformation and a character size, and the threshold value is varied according to the type or deformation of the font. Forming method.   The image forming method according to claim 3, wherein the user can modify and add the threshold value correlated with the character attribute according to claim 3.   5. The image forming method according to claim 2, wherein the character size serving as the threshold is 4 to 10 points.   When character code information is converted into bitmap information of basic resolution including multi-value density information, and the bitmap information is converted by area gradation means having a resolution lower than the basic resolution to express density, and the basic resolution The image forming apparatus is characterized in that the density is expressed by using a character attribute included in the character code.   7. The image forming apparatus according to claim 6, wherein the image forming apparatus has density gradation expression means in a basic resolution unit.   9. The image forming apparatus according to claim 7, wherein the image forming apparatus forms a latent image by performing image exposure on the electrostatic latent image carrier, converts the latent image into a toner visible image, and transfers the latent image to a recording material. An image forming apparatus according to claim 1, wherein the density gradation expression means in the basic resolution unit is an exposure amount modulation means for image exposure.   9. The character attribute according to claim 6, wherein the character attribute is a character size. If the character size is a certain threshold value or more, the bitmap information is converted by an area gradation unit having a resolution lower than the basic resolution. In the case of a character size less than a threshold, the image forming apparatus is characterized in that the density is expressed at a basic resolution.   9. The character attribute according to claim 9, wherein the character attribute is at least one of font type and deformation and character size, and the threshold value is made different according to the font type or deformation. Image forming apparatus.   The image forming apparatus according to claim 10, wherein the user can modify and add the threshold value correlated with the character attribute according to claim 10.   12. The image forming apparatus according to claim 9, wherein the threshold character size is 4 to 10 points.

Images