JP4701988B2 - Image processing apparatus and image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a printer or a copying machine that forms an image using powder toner, and multi-tone image data is converted into image data of a predetermined gradation for image formation in the image forming apparatus. The present invention relates to an image processing apparatus.

Electrophotographic color printers and color copiers generally have three colors of cyan (C), magenta (M), and yellow (Y), or four colors of these three colors plus black (K). Is used to form a full-color image as a whole by forming dots of each color toner on the paper. On the other hand, color image data (for example, color image data created by an application of a host computer) that is the basis of an image to be printed generally represents each color component in multiple gradations (for example, 256 gradations for full color). Data. Therefore, how to reflect the image data expressed in multiple gradations (hereinafter referred to as “multi-gradation image data”) on the dots of the toners of respective colors becomes a problem.
In this case, there are a density gradation method and an area gradation method. The former expresses the density of each color, for example, by controlling the amount of ink transferred onto the paper by the energization time. However, due to the need for special paper and complicated control mechanisms, there are few density gradation method machines in the electrophotographic method, and the latter method is used to express light and shade. Is common. This area gradation method is a method in which the density of a pixel is replaced with a dot area ratio in the pixel. Therefore, it is necessary to perform binarization processing for converting multi-gradation image data into two-gradation data representing dot on / off for each color component. Further, even when a predetermined number of gradations other than binary values can be expressed as in some types of thermal transfer printers, it is necessary to convert multi-tone image data into the predetermined number of gradation data. is there.

Now, the method of gradation number conversion is roughly divided into AM screening (Amplitude Modulation Screening) that expresses shading with dot size and FM screening (Frequency Modulation Screening) that expresses shading with dot density. Is done.
As the former method, a dither method is known (see, for example, Patent Document 1). As the latter technique, an error diffusion method is known (see, for example, Patent Document 2). Further, as the latter method, a blue noise mask method can be cited (for example, see Patent Document 3).

Among them, the dither method is an example in which 256 gradations are binarized. For example, a dither matrix in which threshold values having various gradation values from 0 to 255 are arranged in a matrix form is used in advance. Prepare and apply the dither matrix in order to a pixel matrix of the same size in a multi-tone image, compare the threshold value of each location of the dither matrix with the tone value of the corresponding pixel, and select the pixel according to the comparison result This is a technique of binarizing the original gradation value of the.
Here, as the dither matrix, a so-called systematic dither matrix in which threshold values are arranged according to an artificially determined arrangement is generally used. This systematic dither matrix consists of a centralized dither matrix in which dots increase in a concentrated manner in the matrix as the tone of the image to be converted increases, and dots are dispersed in the matrix. It can be roughly divided into a distributed dither matrix that increases as a result. A Bayer dither matrix is widely known as an example of a distributed dither matrix.

The error diffusion method is an example in which 256 gradations are binarized, and the gradation value of each pixel is compared with a predetermined threshold while scanning a multi-tone image in a raster shape. The original gradation value is binarized to 0 or 255 according to the comparison result, and the quantization error (= the original gradation value−the gradation value after binarization) caused by the binarization is This is a method of diffusively adding to the gradation values of pixels that have not yet been binarized.
Further, the average error minimum method is a method for correcting the gradation value of the next pixel to be binarized using a weighted average value of quantization errors generated in the surrounding binarized pixels. This method is logically equivalent to the error diffusion method. In the following, this method including the average error minimum method is referred to as “error diffusion method”.

  Furthermore, the blue noise mask method is a method in which a mask (blue noise mask) in which a threshold value is set so that the frequency characteristic is as high as possible in a certain size matrix is stored in a storage device. At the time of toning, as in the dither method, the blue noise mask is used to cover the original image in a tile shape, and the pixel value of the original image and the threshold value of the blue noise mask are compared point by point to turn on / off the output dots. It is determined to be off.

On the other hand, various methods for forming an image using a transparent toner have been proposed (for example, see Patent Documents 4 and 5). In Patent Document 4, transparent toner is used to improve the surface property of an image, and in Patent Document 5, transparent toner is used to improve transfer efficiency and improve image quality.
In recent years, with the popularization of digital cameras and the like, photographic print output using an image forming apparatus has been actively performed. Thus, transparent toner is used to obtain a photographic print having a smooth image surface and high gloss (see, for example, Patent Documents 6 and 7). Among these, Patent Document 6 proposes a technique of changing the transparent toner adhesion amount according to a color image and smoothing the total toner amount. Patent Document 7 proposes a technique for developing transparent toner between lines of colored toner.

JP 2004-135317 A JP 2003-348347 A JP 2000-261669 A JP-A 63-143563 Japanese Patent Laid-Open No. 2-176777 JP-A-4-362960 JP 11-202583 A

As described above, conventionally, the conversion of the number of gradations of color multi-gradation image data has been widely performed, and it has also been known to perform image formation using transparent toner.
However, when an image signal that forms the basis of image formation using transparent toner is input as multi-tone image data, what method is used to convert the number of gray levels of the multi-tone image data? Until now, not much has been proposed.

In general, in order to obtain a high-quality print such as a silver salt photograph in which the screen structure is not visible, it is necessary to increase the number of screen lines of the color toner, but the number of lines of the binarized color toner image is 1 inch. If the number of hits exceeds 200, the dot interval formed by the color toner will be 120 μm or less. For this reason, it is necessary to set the positional deviation of the color toner image and the transparent toner image on the paper to several tens of μm or less. However, it is actually very difficult to prevent such a positional deviation with high accuracy.
In Patent Document 6, the toner amount is assumed to be uniform regardless of the position on the paper. However, in practice, there is a problem in that the printed image does not become smooth because a deviation occurs in the toner adhesion position. It was. Patent Document 6 discloses that the layer thickness of the transparent toner is set to the maximum value of the total amount of the color toner, and that the transparent toner is attached to the entire surface. However, since this method consumes a large amount of transparent toner, it is costly and a large toner bottle must be prepared, which is disadvantageous in terms of apparatus size.
On the other hand, Patent Document 7 only mentions the tone number conversion of transparent toner. However, the method of developing the transparent toner between the lines of the colored toner has a problem that the positional deviation is conspicuous because it is weak against the positional deviation of the toner.

  The present invention has been made to solve the technical problems as described above, and the object thereof is to provide a smooth and good image surface even when the transparent toner image is misaligned with respect to the color toner image. Therefore, it is possible to print a simple color image at a low cost.

  For this purpose, in the present invention, the number of gradations of all image data including those for transparent toner is converted by a dither method using a dot concentration type dither matrix. That is, the image processing apparatus of the present invention uses the first multi-tone image data that is a source of image formation with a specific color toner and the second multi-tone image data that is a source of image formation with a transparent toner. An input means for inputting, the first multi-tone image data is converted by the dither method using the first dot-concentrated dither matrix, and the second multi-tone image data is converted into the second dot-concentrated dither matrix. Conversion means using the dither method, the first halftone image data that is the conversion result of the first multi-tone image data, and the second intermediate that is the conversion result of the second multi-tone image data Output means for outputting tone image data.

Here, the number of screen lines for transparent toner is preferably higher than the number of screen lines for color toner. In that case, in the image processing apparatus of the present invention, the second dot-concentrated dither matrix is determined so that the number of screen lines resulting therefrom is higher than the number of screen lines formed by the first dot-concentrated dither matrix. The
The number and angle of the screen for the transparent toner may be equal to the number and angle of the screen for the yellow toner that is not easily affected by appearance. In that case, in the image processing apparatus of the present invention, the specific color toner is yellow toner, and the second dot concentration type dither matrix has a screen line number and an angle according to the first dot concentration type dither matrix. It is determined to be equivalent to the number of lines and the angle of the screen.

  Furthermore, in the present invention, it is also preferable to convert the number of gradations of image data of at least one color including for transparent toner by a dither method using a dot dispersion type dither matrix. In this case, the image processing apparatus of the present invention includes first multi-tone image data that is a source of image formation using specific colored toner, and second multi-tone image data that is a source of image formation using transparent toner. The first multi-tone image data is converted by a dither method using a dot-concentrated dither matrix, and the second multi-tone image data is converted by a dither method using a dot-dispersed dither matrix Conversion means for converting, first halftone image data that is the conversion result of the first multi-tone image data, and second halftone image data that is the conversion result of the second multi-tone image data Output means for outputting.

  Furthermore, in the present invention, the number of gradations of image data of at least one color including for transparent toner may be converted by an error diffusion method. Here, it is preferable that only the gradation number conversion of the image data for transparent toner is performed by the error diffusion method, and the gradation number conversion of the image data for colored toner is performed by the dither method. In this case, the image processing apparatus of the present invention includes first multi-tone image data that is a source of image formation using specific colored toner, and second multi-tone image data that is a source of image formation using transparent toner. , Input means for converting the first multi-tone image data by the dither method, conversion means for converting the second multi-tone image data by the error diffusion method, and the first multi-tone image data Output means for outputting the first halftone image data as the conversion result and the second halftone image data as the conversion result of the second multi-tone image data.

  In the present invention, the number of gradations of image data of at least one color including for transparent toner may be converted using a threshold matrix generated by a blue noise mask creation algorithm. In this case, the image processing apparatus of the present invention includes first multi-tone image data that is a source of image formation using specific colored toner, and second multi-tone image data that is a source of image formation using transparent toner. And input means for inputting the first multi-tone image data by a dither method, and conversion means for converting the second multi-tone image data using a threshold matrix created by a blue noise mask creation algorithm; Output means for outputting first halftone image data that is the conversion result of the first multi-tone image data and second halftone image data that is the conversion result of the second multi-tone image data; It has.

On the other hand, the present invention can also be understood as an image forming apparatus that forms an image based on binary gradation data binarized through such gradation number conversion.
Here, the image forming apparatus uses a cooling peeling fixing method (after cooling the recording medium to which the toner image has been transferred in contact with the fixing belt and then peeling the toner image from the fixing belt to fix the toner image on the recording medium. It is desirable to provide fixing means by a fixing method) and the recording medium is resin-coated paper.

  According to the present invention, even if the transparent toner image is misaligned with respect to the color toner image, the image surface is smooth and a good color image can be printed at low cost.

The best mode for carrying out the present invention (hereinafter referred to as “embodiment”) will be described below in detail with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating a configuration of an image processing apparatus 50 according to the present embodiment.
As shown in FIG. 1, the image processing apparatus 50 includes an address generator 51, a dither matrix memory 52, and a comparator 53.
Among these, the address generator 51 corresponds based on pixel position information indicating the position of each pixel in the image data and matrix information indicating the size and angle of the dither matrix defined for each color of the image data. Address information indicating the corresponding position of each pixel in the dither matrix is generated. Further, the dither matrix memory 52 stores a dither matrix in which threshold information corresponding to all positions in the image data is stored, and receives address information, whereby the threshold at positions in the dither matrix corresponding to the address information is stored. A memory that outputs information. The comparator 53 compares the threshold information output from the dither matrix memory 52 with the corresponding multi-tone image data (hereinafter simply referred to as “image data”), and outputs a comparison result. . The comparator 53 is understood to include input means for inputting image data, conversion means for converting the image data into halftone image data having a predetermined number of gradations, and output means for outputting the conversion result. Can do.

Next, the operation of the image processing apparatus 50 will be described in detail. Here, it is assumed that a dither matrix having a resolution of 1200 dpi shown in FIGS. 2 to 5 is stored in the dither matrix memory 52. Specifically, the dither matrix composed of the basic dither matrix of 0 degrees 240 lines shown in FIG. 2 is used for yellow, and the dither matrix composed of the basic dither matrix of 26.5 degrees 268 lines shown in FIG. 3 is used for magenta. A dither matrix composed of a basic dither matrix of 64.4 degrees 268 lines shown in FIG. 4 is stored for cyan, and a dither matrix composed of a basic dither matrix of 0 degrees 300 lines shown in FIG. 5 is stored for transparent toner. To do. Here, regarding the color toner, in order to actually increase the number of gradations, 8 to 16 basic dither matrices are combined to ensure a sufficient number of gradations. In this embodiment, the basic dither matrix as shown in FIG. 5 is used for the transparent toner, so that the screen line for the transparent toner is 300 lines per inch. That is, since the screen line interval is 85 μm, it is almost the same as the beam diameter of laser exposure and is hardly resolved. By using a dither matrix that is smaller than the color toner in this way for the transparent toner, the gradation is impaired, but the transparent toner is difficult to see, so this is not a problem, and the dither matrix is stored. The effect that the amount of memory required for the operation can be reduced is obtained.
In the present specification, “transparent toner” is not used as a concept that means only a completely transparent toner that does not absorb visible light at all, but as a concept that includes a toner that slightly absorbs visible light. “Color toner” is synonymous with color toner (color toner), but this color toner does not exclude achromatic colors such as black and white. It can also be understood as meaning.

In this manner, the image processing apparatus 50 operates as follows with each dither matrix stored in the dither matrix memory 52.
That is, first, pixel position information for the pixel of interest in the image data and matrix information indicating the size and angle of the dither matrix defined for each color of the image data are input to the address generator 51. Thereby, the address generator 51 generates address information indicating a position corresponding to the pixel position information in the dither matrix for each color.
For example, for the dither matrix for transparent toner shown in FIG. 5, information “4 rows, 4 columns, 0 °” is input to the address generator 51 as matrix information. On the other hand, as the pixel position information, if the pixel position is expressed in the form of (row number, column number), (1, 1), (1, 2), ..., (2, 1), ( 2, 2), ..., (3, 1), (3, 2), ..., (4, 1), (4, 2), ..., (5, 1), (5, 2), ... Are sequentially input. Thereby, address information indicating which position in the dither matrix specified by each piece of pixel position information is output to the dither matrix memory 52.

As described above, when the address information output from the address generator 51 is input to the dither matrix memory 52, threshold information is output from the dither matrix memory 52.
For example, with respect to the dither matrix for transparent toner shown in FIG. 5, if the position in the dither matrix is expressed in the form of (row number, column number), threshold information “ 13 ”is the threshold information“ 10 ”for the address information (1, 2), the threshold information“ 6 ”is for the address information (1, 3), and the threshold information is for the address information (1, 4). “14” is output. Further, the threshold information “5” for the address information (2, 1), the threshold information “1” for the address information (2, 2), and the threshold information “2” for the address information (2, 3). ”Is output as threshold information“ 11 ”for the address information (2, 4). Similarly, the threshold information at the position of the dither matrix corresponding to the address information is output for the third and fourth rows.

The threshold information output from the dither matrix memory 52 in this way is input to the comparator 53. The comparator 53 also receives image data.
In the image data, yellow, magenta, and cyan are represented by, for example, 256 gradations from 0 to 255.
On the other hand, for transparent toner, an image signal for transparent toner obtained as follows is input as image data. That is, the total value is calculated for the three-color color toner multi-value input signals. If the total value is 100% or less, the transparent toner image signal is 100%. If the total value is 200% or more, The transparent toner image signal is 0%. If the total value is 100% or more and 200% or less, the transparent toner image signal is obtained by linearly decreasing from 100% to 0%. According to such a method, when an average color image is input, the consumption can be reduced to half compared to the case where 100% of the transparent toner adheres to the entire surface.

Thereby, the comparator 53 compares the image data with the threshold information, and outputs “1” as the comparison result when the image data is equal to or greater than the threshold, and as the comparison result when the image data is less than the threshold. “0” is output. This is performed for each color component for all pixels in the image data.
For example, if the transparent toner image signal is 0%, the image data is 0, so that the comparison result “0” is output for all positions in the dither matrix. As a result, the output result is as shown in FIG.
If the image signal for transparent toner is 10%, the image data is 1.6 (= 16 × 10%). Therefore, the comparison result “1” is obtained for the position corresponding to the threshold “1” of the dither matrix. And the comparison result “0” is output for the positions corresponding to the threshold values “2” to “16” of the dither matrix. As a result, the output result is as shown in FIG.
Further, assuming that the image signal for transparent toner is 30%, the image data is 4.8 (= 16 × 30%), so the positions corresponding to the dither matrix thresholds “1” to “4” are compared. The result “1” is output, and the comparison result “0” is output for the positions corresponding to the threshold values “5” to “16” of the dither matrix. As a result, the output result is as shown in FIG.
Furthermore, if the image signal for transparent toner is 50%, the image data is 8 (= 16 × 50%), and the comparison results are obtained for the positions corresponding to the dither matrix thresholds “1” to “8”. “1” is output, and the comparison result “0” is output for the positions corresponding to the threshold values “9” to “16” of the dither matrix. As a result, the output result is as shown in FIG.

Next, the image forming apparatus 100 that performs image formation based on the image data output in this way will be described.
FIG. 7 is a schematic configuration diagram illustrating the image forming apparatus 100. The image forming apparatus 100 is an intermediate transfer type image forming apparatus generally called a tandem type, and includes a plurality of image forming units 1T that form toner images of respective color components of transparent, yellow, magenta, and cyan by electrophotography. , 1Y, 1M, 1C, a primary transfer portion for sequentially transferring (primary transfer) the color component toner images formed by the image forming units 1T, 1Y, 1M, 1C to the intermediate transfer body 9, and the intermediate transfer body 9 A secondary transfer unit that collectively transfers (secondary transfer) the superimposed toner image transferred to the recording paper 10, and a primary fixing device 23 that fixes the secondary transferred image on the recording paper 10.

  In the present embodiment, the image forming units 1T, 1Y, 1M, and 1C are uniformly charged around the electrostatic latent image carriers 2T, 2Y, 2M, and 2C that rotate counterclockwise. Chargers 3T, 3Y, 3M, and 3C, laser exposure devices 4T, 4Y, 4M, and 4C that write electrostatic latent images thereon, each color component toner is contained, and the electrostatic latent images are visible with toner. Developing devices 5T, 5Y, 5M, and 5C for imaging are sequentially arranged. Further, around the electrostatic latent image carriers 2T, 2Y, 2M, and 2C, primary transfer rolls 6T, 6Y, 6M, and 6C for transferring the color component toner images to the intermediate transfer member 9, and electrostatic latent image carriers The electrostatic latent image carrier cleaning devices 7T, 7Y, 7M, and 7C for removing residual toner on the bodies 2T, 2Y, 2M, and 2C and the surfaces of the electrostatic latent image carriers 2T, 2Y, 2M, and 2C are neutralized. Static elimination devices 8T, 8Y, 8M, and 8C are provided. These image forming units 1T, 1Y, 1M, and 1C are arranged substantially linearly in the order of transparency (T), yellow (Y), magenta (M), and cyan (C) from the upstream side of the intermediate transfer member 9. Has been.

  The primary transfer portion is composed of primary transfer rolls 6T, 6Y, 6M, and 6C that are disposed to face the electrostatic latent image carriers 2T, 2Y, 2M, and 2C, respectively, with the intermediate transfer body 9 interposed therebetween. The primary transfer rolls 6T, 6Y, 6M, and 6C are arranged in pressure contact with the electrostatic latent image carriers 2T, 2Y, 2M, and 2C with the intermediate transfer body 9 interposed therebetween, and further, are attached to the primary transfer rolls 6T, 6Y, 6M, and 6C. Is applied with a voltage (primary transfer bias) having a polarity opposite to the charging polarity of the toner (negative polarity; the same applies hereinafter). As a result, the toner images on the electrostatic latent image carriers 2T, 2Y, 2M, and 2C are sequentially electrostatically attracted to the intermediate transfer body 9, and a superimposed toner image is formed on the intermediate transfer body 9. It has become.

The secondary transfer unit includes a secondary transfer roll 21 and a backup roll 20 that are disposed on the toner image carrying surface side of the intermediate transfer member 9. The secondary transfer roll 21 is placed in pressure contact with the backup roll 20 with the intermediate transfer body 9 interposed therebetween. Further, the secondary transfer roll 21 is grounded, and a secondary transfer bias is formed between the secondary transfer roll 21 and the backup roll 20. The toner image is secondarily transferred onto the recording paper 10 conveyed to the transfer unit.
Further, on the downstream side of the secondary transfer portion of the intermediate transfer member 9, residual toner and paper dust on the intermediate transfer member 9 after the secondary transfer are removed, and the surface of the intermediate transfer member 9 is cleaned. The apparatus 22 is provided so that contact / separation is possible.

Further, the image forming apparatus 100 takes out the paper feeding cassettes 16a and 16b that store the recording paper 10 and the recording paper 10 accumulated in the paper feeding cassettes 16a and 16b at a predetermined timing as a paper transport system. Paper feed rolls 17a and 17b for transporting the paper, transport roll 18 for transporting the recording paper 10 fed by the paper feed rolls 17a and 17b, and the position of the recording paper 10 and the position of the toner image are aligned. A resist roll 19 and a transport direction switching gate 24 for switching the transport direction of the recording paper 10 that has been subjected to secondary transfer by the secondary transfer roll 21 and fixed by the primary fixing device 23 are provided.
Further, when the transport direction is switched to the left in FIG. 7 by the transport direction switching gate 24, the transport roll 26 that guides the recording paper 10 to the first recording paper discharge port 25 and the first recording paper discharge port 25. And a first paper discharge tray 27 for collecting the discharged recording paper 10.
Further, the image forming apparatus 100 includes a belt fixing device 101 that performs cooling peeling and fixing on the recording paper 10 as will be described in detail later. A cutter mechanism 110 for cutting the recording paper 10 is also provided.
The image forming apparatus 100 further includes a second paper discharge tray 29 for collecting the recording paper 10 discharged from the second recording paper discharge port 28.

Next, a basic image forming process of the image forming apparatus 100 will be described.
Note that this image forming process uses color image data sent from a personal computer (not shown), color image data of a color original read by the image reading device 102, and the like by the image processing device 50 in FIG. It starts after the processing is done.
Here, the image forming operation on the intermediate transfer member 9 will be described by taking the image forming unit 1Y for forming a yellow toner image as an example.
First, the surface of the electrostatic latent image carrier 2Y is uniformly charged by the uniform charger 3Y. Next, image exposure corresponding to the yellow image is performed by the laser exposure device 4Y based on a digital image signal input from an image processing unit (not shown), and the surface of the electrostatic latent image carrier 2Y corresponds to the yellow image. An electrostatic latent image is formed.
The electrostatic latent image corresponding to the yellow image is converted into a yellow toner image by the developing device 5Y, and is transferred onto the intermediate transfer member 9 by the pressing force and electrostatic attraction of the primary transfer roll 6Y constituting a part of the primary transfer portion. . The yellow toner remaining on the electrostatic latent image carrier 2Y after the transfer is scraped off by the electrostatic latent image carrier cleaning device 7Y. The surface of the electrostatic latent image carrier 2Y is discharged by the discharging device 8Y and then charged again by the uniform charger 3Y for the next image forming cycle.
In the image forming apparatus 100 that performs multicolor image formation, the image forming process similar to the above is performed at the timing in consideration of the relative position difference between the image forming units 1T, 1Y, 1M, and 1C. , 1Y, 1M, and 1C, a full-color toner image is formed on the intermediate transfer member 9. Here, the transfer toner weight TMA per unit area of the intermediate transfer member 9 is 4.5 g / m ² for the image portion with an area ratio of 100% for the color toner, and 4.0 g / m ² for the transparent toner. ² . Further, the positional accuracy of each color toner image on the intermediate transfer member 9 is about 50 to 100 μm.

In the present embodiment, the recording paper 10 for high-gloss printing is accommodated in one of the paper feed cassettes 16 a and 16 b provided in the lower part of the image forming apparatus 100. Here, it is assumed that such a recording sheet 10 is stored in the sheet feeding cassette 16a. On the other hand, it is assumed that the recording paper 10 that does not require high gloss is stored in the paper feed cassette 16b.
In this state, the recording paper 10 is fed from the paper feed cassette 16a by the paper feed roll 17a according to the mode specified in the image output instruction. Then, the fed recording paper 10 is conveyed to a secondary transfer position of the intermediate transfer body 9 by a plurality of conveyance rolls 18 and registration rolls 19 at a predetermined timing. A full color toner image is transferred onto the recording paper 10 from the intermediate transfer member 9 by the backup roll 20 and the secondary transfer roll 21.
At this time, the residual toner on the intermediate transfer member 9 that could not be transferred onto the recording paper 10 by the secondary transfer unit is conveyed as it is attached to the intermediate transfer member 9 and is transferred by the intermediate transfer member cleaning device 22. The intermediate transfer member 9 is removed to prepare for the next image formation.
Further, the recording paper 10 onto which the full color toner image has been transferred from the intermediate transfer member 9 is separated from the intermediate transfer member 9 and then conveyed to the primary fixing device 23 disposed on the downstream side of the secondary transfer unit. The primary fixing device 23 fixes the toner image onto the recording paper 10 with heat and pressure.

Thereafter, the recording paper 10 is guided to the conveyance direction switching gate 24 and conveyed to the belt fixing device 101. The belt fixing device 101 is a device that heats and presses the recording paper 10 in close contact with the belt surface, and then cools and peels off the recording paper 10. Thereby, it is possible to obtain a print having a uniform high-gloss surface with a silver salt photographic tone. The recording paper 10 is fixed again by the belt fixing device 101 so that the recording paper 10 is glossy, and after the periphery is cut by the cutter mechanism 110, the recording paper 10 is discharged from the second recording paper discharge port 28 to the second discharge tray 29. To be discharged.
On the other hand, the recording paper 10 accommodated in the paper feed cassette 16b is guided by the transport direction switching gate 24 after being transferred to the full color toner image and primary fixed, and is inserted between the transport rolls 26 to discharge the first recording paper. The paper is discharged from the outlet 25 to the first paper discharge tray 27.
Note that the switching of the transport direction switching gate 24 described above can be realized by, for example, ascertaining which paper feed cassette is instructed to feed the recording paper 10.

Here, the belt fixing device 101 will be described in detail.
As shown in FIG. 8, the belt fixing device 101 includes a heat fixing roll 40 that also has a heat source and also serves as a driving roll, a peeling roll 44, a steering roll 45, a fixing belt 47 stretched over these, and a fixing belt. A pressure roll 42 that forms a nip by pressing against the heat-fixing roll 40 via 47, and a cooler 46 that cools the fixing belt 47 downstream of the nip in the rotation direction of the fixing belt 47. With such a configuration, the recording paper 10 carrying the toner is conveyed to the nip portion so that the toner image is in contact with the fixing belt 47, and is heated and pressurized and fixed by the heating and fixing roll 40 and the pressure roll 42. After the fixing belt 47 and the recording paper 10 are cooled, the fixing belt 47 and the recording paper 10 are peeled off by the peeling roll 44. Then, the peeled recording paper 10 is inserted between the discharge rolls 48 and discharged.

The heat fixing roll 40 includes a release layer 40b made of a fluororesin layer such as a PFA tube on the surface of a metal core 40a having high thermal conductivity, and a heating source 41 such as a halogen lamp provided in the core 40a. ing. Then, the heating source 41 heats the surface temperature of the heat fixing roll 40 to a predetermined temperature, and heats the fixing belt 47 and the recording paper 10 onto which the toner image is transferred.
The pressure roll 42 is formed with an elastic layer 42b made of silicone rubber or the like having a rubber hardness (JIS-A) of about 40 ° around a metal core 42a having a high thermal conductivity, and a PFA on the surface thereof. A release layer 42c made of a fluororesin layer such as a tube is formed, and a heating source 43 such as a halogen lamp is provided in the core 42a. Then, the heating source 43 heats the pressure roll 42 so that the surface temperature becomes a predetermined temperature, applies pressure to the recording paper 10 at the time of fixing, and simultaneously heats the recording paper 10 from the back surface.
The configurations of the heat fixing roll 40 and the pressure roll 42 are not limited to this, and the toner image formed on the recording paper 10 can be fixed on the recording paper 10 via the fixing belt 47. Any configuration may be used.

The peeling roll 44 peels the recording paper 10 from the fixing belt 47 due to the rigidity of the recording paper 10. The outer diameter shape (dimension) of the peeling roll 44 is the adhesion between the fixing belt 47 and the recording paper 10 and the fixing belt 47. It is determined by the winding angle around the peeling roll 44.
The steering roll 45 is for preventing the belt end from being damaged due to a deviation generated by rotating the fixing belt 47. One shaft is fixed, and the other shaft is heated and fixed by a driving device (not shown). By tilting with respect to the roll 40, when the fixing belt 47 is offset, it plays the role of changing the belt traveling direction to the opposite direction.

The cooler 46 has fins inside, and cools the recording paper 10 in close contact with the fixing belt 47 by sending air around the fins by a fan (not shown). The cooler 46 is disposed on the inner peripheral surface of the fixing belt 47, on the downstream side of the heat fixing roll 40 and on the upstream side of the peeling roll 44, and contacts the inner peripheral surface of the fixing belt 47 to absorb the heat. To do. The cooler 46 cools the toner image of the recording paper 10 melted by the heat fixing roll 40 and the pressure roll 42, and solidifies the entire image surface in a smooth state following the surface of the fixing belt 47. Enables glossy printing. For this reason, the surface of the fixing belt 47 is finished with high gloss.
Here, as the fixing belt 47, an endless film made of thermosetting polyimide is covered with a silicone rubber layer having a smooth surface and a thickness of 35 μm. From the viewpoint of power consumption, it is desirable that the belt be thin, but from the viewpoint of strength, the polyimide base material needs to be 75 μm or more, and from the viewpoint of fixing to the toner image on the recording paper 10, the silicone rubber layer should be 30 μm or more. .

Next, a recording paper 10 suitable for use in the present embodiment will be described.
In the present embodiment, as shown in FIG. 9 (a), the surface of the base material 10b or both the front and back surfaces are mainly composed of a thermoplastic resin made of polyester or the like, with a thickness in the range of 5 to 20 μm, For example, it is preferable to use as the recording paper 10 a resin-coated paper provided with a transparent image receiving layer (transparent resin layer) 10a coated with a thickness of 10 μm. Thereby, a uniform glossy feeling can be obtained on the entire surface of the paper. In the case of plain paper, gloss other than the toner image portion decreases.
As other recording paper 10, a resin layer for embedding a toner image on the image forming surface side is applied after applying a waterproof resin on both sides in order to prevent cracking and curling of the image surface due to cellulose expansion due to moisture absorption after printing. You may use what was apply | coated. As shown in FIG. 9B, a receiving layer for carrying a toner image on a support provided with a polyolefin resin coating layer 10c such as polyethylene, polypropylene, polyethylene terephthalate, polystyrene on the surface or both surfaces of the base 10b. Cover. The polyolefin resin coating layer 10c has a thickness of 10 to 30 [mu] m, and the receiving layer carrying the toner image is a resin-coated paper coated with a thermoplastic resin made of polyester or the like as a main component in a thickness range of 5 to 20 [mu] m. Used. For example, a thermoplastic resin receiving layer provided with a transparent image receiving layer (transparent resin layer) 10a coated with a thickness of 10 μm can be used. Thereby, a uniform glossy feeling can be obtained on the entire surface of the paper.

Hereinafter, the present embodiment will be described in detail based on examples and comparative examples.
[Example 1]
In this embodiment, the dither matrix referred to in the above description is used. That is, the dot concentration type dither matrix of FIG. 2 for yellow, the dot concentration type dither matrix of FIG. 3 for magenta, the dot concentration type dither matrix of FIG. 4 for cyan, and the dot concentration of FIG. 5 for transparent toner. Each type dither matrix was used.
[Example 2]
In this embodiment, for the yellow, magenta, and cyan, the dot-concentrated dither matrix shown in FIGS. 2 to 4 is used as in the first embodiment, but for the transparent toner, the dot-concentrated dither matrix shown in FIG. 2 is used. It was. That is, the dither matrix for the transparent toner was changed to that of 0 degrees 240 lines used for yellow in Example 1.
[Example 3]
In the present embodiment, the dot concentration type dither matrix shown in FIGS. 2 to 4 was used for yellow, magenta, and cyan, respectively, as in the first embodiment. However, for transparent toner, the dot dispersion type dither matrix (Bayer) shown in FIG. Matrix).
[Example 4]
In this embodiment, for yellow, magenta, and cyan, binarization processing was performed by the dither method using the dither matrix shown in FIGS. 2 to 4 as in the first embodiment, but for transparent toner, an error of 1200 dpi was performed. Binarization was performed by a diffusion method.
[Example 5]
In this embodiment, for yellow, magenta, and cyan, binarization processing was performed by the dither method using the dither matrix shown in FIGS. 2 to 4 as in the first embodiment. A binarization process was performed using the threshold value matrix generated by the blue noise mask creation algorithm of Example 1 of -261669.

[Comparative Example 1]
In this comparative example, the dot-concentrated dither matrix shown in FIGS. 2 to 4 was used for yellow, magenta, and cyan, respectively, as in Example 1, but the dot-concentrated dither matrix shown in FIG. 3 was used for transparent toner. It was. That is, the dither matrix for transparent toner was changed to that of 26.5 degrees 268 lines used for magenta in Example 1.
[Comparative Example 2]
In this comparative example, the dot concentration type dither matrix shown in FIGS. 2 to 4 was used for yellow, magenta, and cyan, respectively, as in Example 1. However, for transparent toner, the dot concentration type dither matrix (0) shown in FIG. It was changed to 200 degrees).
[Comparative Example 3]
In this comparative example, the amounts of yellow, magenta, and cyan were the same as in Example 1, but transparent toner was always applied to the 100% image.

The image quality of the images created in the above examples and comparative examples was visually evaluated. Evaluation items are a step difference in image, a graininess, and in-plane unevenness. FIG. 12 shows the evaluation results.
As can be seen from FIG. 12, in each of the above examples, graininess and uniformity comparable to those of Comparative Example 3 in which transparent toner was applied to the entire surface were obtained while reducing the consumption of transparent toner. In Comparative Example 1, the dots of the transparent toner and the magenta toner interfered, and the low frequency density unevenness was conspicuous. In Comparative Example 2, it was visually confirmed that the color toner was disturbed in the dot cycle of the transparent toner, and the graininess was poor. In each example, since the total amount of toner was made uniform as compared with Comparative Example 3, a sense of level difference was improved.

As described above, according to the present embodiment, the image signal for transparent toner is binarized by a specific method, so that a smooth image can be obtained even if there is a slight misalignment between the color toner image and the transparent toner image. In addition, it is possible to obtain a high-quality print in which the transparent toner screen does not disturb the image structure of the color toner. Although the binarized transparent toner image may disturb the color toner image, at least the transparent toner image is made higher than the color toner image by using the error diffusion method or the high linear number dither method. In terms of the number of lines, the deterioration of the color toner image quality can be reduced to such an extent that it does not actually cause a problem. In general, it is said that when the number of lines is increased, the dots are disturbed and the density stability and graininess are deteriorated. However, the disturbance of the dots of the transparent toner hardly affects the visual characteristics. For transparent toner images, even if some errors and unnaturalness are present in the output image, the apparent image quality is not significantly affected.
However, when the number of lines of the transparent toner image is lower than the number of lines of the color toner image, the presence of large dots of transparent toner disturbs the color toner dots, and screen moire occurs. For these reasons, it is desirable to use a binarization process in a specific manner only for a transparent toner image and to use a dot concentration type dither method for a color toner image in order to minimize image quality degradation. The transparent toner image is preferably processed by an FM screen called error diffusion or blue noise mask, or dot concentration type dither or dot dispersion type dither having a higher number of lines than color toner. In addition, when the number of lines is not higher than that of the color toner, it is preferable to have the same number of lines and the same angle as that of the yellow toner image in which the dot disturbance is less visible.

It is the figure which showed the function structure of the image processing apparatus in embodiment of this invention. It is the figure which showed an example of the dither matrix used for yellow in embodiment of this invention. It is the figure which showed an example of the dither matrix used for magenta in embodiment of this invention. It is the figure which showed an example of the dither matrix used for cyan in embodiment of this invention. FIG. 3 is a diagram illustrating an example of a dither matrix used for transparent toner in the embodiment of the present invention. It is the figure which showed the example of the output result by the image processing apparatus of embodiment of this invention. 1 is a diagram illustrating an overall configuration of an image forming apparatus according to an embodiment of the present invention. 1 is a diagram illustrating a configuration of a belt fixing device in an image forming apparatus according to an embodiment of the present invention. FIG. 3 is a diagram showing a structure of a recording paper suitable for use in the embodiment of the present invention. FIG. 6 is a diagram showing a dither matrix used in Example 3. 6 is a diagram showing a dither matrix used in Comparative Example 2. FIG. It is the figure which showed the evaluation result with respect to an Example and a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 50 ... Image processing apparatus 51 ... Address generator 52 ... Dither matrix memory 53 ... Comparator 100 ... Image forming apparatus

Claims (6)

An image processing apparatus for converting multi-tone image data into halftone image data having a predetermined number of gradations,
Input means for inputting first multi-tone image data that is a source of image formation with specific colored toner and second multi-tone image data that is a source of image formation with transparent toner;
The first multi-tone image data is converted by a dither method using first threshold information, and the second multi-tone image data is converted to a screen line more than when the first threshold information is used. Conversion means for converting by the dither method using the second threshold information that increases in number ;
Output means for outputting first halftone image data that is the conversion result of the first multi-tone image data and second halftone image data that is the conversion result of the second multi-tone image data An image processing apparatus comprising: The first threshold information is threshold information stored in the first dot concentration dither matrix.
The second threshold information is threshold information stored in a second dot-concentrated dither matrix having a higher number of screen lines than the first dot-concentrated dither matrix. Image processing device. The first threshold information is threshold information stored in a dot concentration type dither matrix,
The image processing apparatus according to claim 1, wherein the second threshold information is threshold information stored in a dot-dispersed dither matrix having a higher number of screen lines than the dot-concentrated dither matrix. The first multi-tone image data that is the basis of image formation with specific colored toner is binarized to generate first two-tone data, and the second multi-tone that is the source of image formation with transparent toner Binarization means for binarizing image data to generate second binary gradation data;
Latent image forming means for forming on the image carrier a first latent image based on the first two-tone data and a second latent image based on the second two-tone data;
Development that forms a colored toner image by developing the first latent image using the specific colored toner, and forms a transparent toner image by developing the second latent image using the transparent toner. Means,
A transfer means for transferring the colored toner image and the transparent toner image formed by the developing means onto a recording medium,
The binarization means binarizes the first multi-tone image data by the dither method using the first threshold information, and the number of lines of the screen is higher than when the first threshold information is used. An image forming apparatus characterized in that the second multi-tone image data is binarized by a dither method using the second threshold information . The recording medium on which the color toner image and the transparent toner image are transferred by the transfer unit is cooled while being in contact with the fixing belt, and then peeled off from the fixing belt, whereby the color toner image and the transparent toner image are 5. The image forming apparatus according to claim 4 , further comprising fixing means for fixing on the recording medium. The image forming apparatus according to claim 4 , wherein the recording medium is resin-coated paper.

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