JP2011077993A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of preventing retransfer dirt or strike-through of a solid portion or bold letter portion and capable of obtaining gradation reproducibility of a photograph when printing the photograph including the solid portion or the bold letter. <P>SOLUTION: The image forming apparatus having a printing unit for performing printing on a printing medium includes: an image input unit 1 for inputting image data; and a control unit in which, on the basis of image data inputted by the image input unit 1, a gamma thinning-out processing part 21 performs thinning-out processing based on a predetermined density conversion curve, on a region having less than a predetermined density, a pattern thinning-out processing part 33 performs thinning-out processing based on a checkered pattern, on a region having the predetermined density or more, and image data after thinning-out processing are output from an image output unit 4a. The printing unit prints an image obtained based on image data outputted by the control unit at least on one side of the printing medium. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that performs single-sided printing or double-sided printing.

  In recent years, double-sided printing for printing on the front and back surfaces of paper has been promoted from the viewpoint of effective use of resources. In the case where the image forming apparatus performs double-sided printing or the like, there is a problem that if the paper transport roller or the like comes into direct contact with the paper after surface printing, ink retransfer occurs and the printed matter is significantly soiled. This problem will be described using a stencil printing apparatus as an example with reference to the drawings. FIG. 10 is a diagram for explaining the flow of double-sided printing operation in a conventional stencil printing apparatus. As shown in FIG. 10, the stencil printing apparatus presses and conveys the fed paper 10 between the first drum 11 and the press roller 12, and the press roller 12 moves to the first drum 11 during the press and transport process. The ink image is transferred onto the upper surface (one surface) of the paper 10 by pressing against the stencil paper. The first drum-printed paper 13 printed on the upper surface is peeled off from the outer peripheral surface of the first drum 11 and guided to the duplex unit 14 by a belt conveyance mechanism or the like. Thereafter, the first drum-printed paper 13 taken out from the duplex unit 14 is conveyed by a paper conveying roller 15 or a belt and is fed to the second drum 16. The fed first drum printed paper 13 is pressed against the stencil paper of the second drum 16 by the press roller 17, whereby the ink image is transferred to the lower surface (the other surface) of the first drum printed paper 13. Then, the second drum-printed paper 18 printed on both sides is discharged.

  In the stencil printing apparatus for double-sided printing described above, the first drum-printed paper 13 printed only on the upper surface side is fed to the paper transport roller 15 and the second drum 16 in a state where the ink is not dried and fixed. In some cases, a part of the unfixed ink may adhere to the paper transport roller 15 or the press roller 17 that has come into contact with the paper transport roller 15 or the press roller 17 to be transferred. As a result, the paper transport roller 15 or the press roller 17 to which the ink has been transferred has a problem that the ink is re-transferred to a printed material fed thereafter and becomes dirty. This problem is not limited to the stencil printing apparatus, but also in an inkjet printing apparatus or the like when there is a member that is present on the downstream side of the printing unit and that contacts the ink image printed on the paper by the printing unit. appear.

  Also, in image forming apparatuses such as stencil printing apparatuses, ink jet printing apparatuses, and copying machines, depending on the type and amount of printing agent such as ink and toner used, an image printed on one side can be seen through the other side. So-called see-through occurs. When the show-through occurs, it becomes difficult for the user to see the printed material in double-sided printing, and the problem arises that confidentiality is lacking in single-sided printing.

  Patent Document 1 describes a stencil printing apparatus that has a simple configuration and can prevent contamination of a print medium with almost no reduction in print density of the print medium. This stencil printing apparatus is provided between a downstream plate cylinder on which a stencil sheet can be mounted on the outer peripheral wall and freely rotatable, and a pressing position for pressing the outer peripheral wall of the plate cylinder and a spaced position separated from the outer peripheral wall of the plate cylinder. And the press rotation member that is displaceable and rotatable, and the fed printing paper is pressed and conveyed between the plate cylinder and the press rotation member that rotate together, and ink is transferred to the printing paper in this press conveyance process. This is a device that performs printing, and is provided with minute irregularities on the outer peripheral surface of the pressing rotation member.

  According to this stencil printing apparatus, it is only necessary to provide minute irregularities on the outer peripheral surface of the press rotating member. Even when the press rotating member directly presses the stencil sheet, the contact area with the ink is small, or the press rotating member. The contact area between the outer peripheral surface of the printing medium and the unfixed ink surface side of the printing medium is small, and the pressing rotating member comes into contact when the pressing rotating member is separated from the stencil paper or when the pressing rotating member is separated from the printing medium or the like. Since the ink or unfixed ink in the part corresponding to the part that did not adhere does not adhere to the pressing rotation member side, the unfixed ink does not adhere much to the pressing rotation member side. Contamination of the print medium can be prevented without lowering.

  Patent Document 2 describes a stencil printing apparatus and a printing paper transport apparatus that prevent paper from being smeared by ink retransfer of a transport member or the like with a simple structure. This stencil printing apparatus includes a rotatable printing cylinder on which a stencil sheet is mounted, an ink supply mechanism for supplying ink to the printing cylinder, and a printing sheet by sandwiching and conveying the printing sheet between the printing cylinder. A pressure roller for transferring the ink to the ink is provided. In the vicinity of the image forming unit formed between the plate cylinder and the pressing roller, a timing roller and a guide roller that are driven in synchronization with the driving timing of the plate cylinder to sandwich the printing paper and send it to the image forming unit are disposed opposite to each other. On the timing roller, an outer peripheral layer made of fine particles or an outer peripheral layer made of a solid material having ink repellency is formed.

  According to this stencil printing apparatus, an outer peripheral layer of a solid material having ink repellency is formed on an outer peripheral surface of a timing roller, a paper feeding belt, or a conveying roller for feeding, feeding or conveying a sheet on which at least one side is printed. Or by forming an outer peripheral layer of fine granular material, it is possible to prevent contamination of the printing surface or the printing paper itself due to retransfer of ink in the process of paper feeding or conveyance, etc. Can be improved.

  In addition, by forming a fine granular layer on the outer peripheral surfaces of the timing roller, the paper feed belt, and the transport roller, slip between the print paper and the print paper can be reliably sent out or transported.

  In particular, in a stencil double-sided printing device that has two plate cylinders and prints the front and back surfaces of paper continuously, it occurs at the time of refeeding, transporting, and refeeding to the second plate cylinder after surface printing. Easy ink transfer can be effectively prevented and the utility value is increased.

JP 2002-219849 A JP 2005-66900 A

  However, the countermeasures by the stencil printing apparatuses described in Patent Documents 1 and 2 are not necessarily sufficient as a stain prevention method, and stains and show-through may still occur due to ink retransfer. Therefore, when performing single-sided printing or double-sided printing, it is conceivable to perform a thinning process on an image printed on the first printing side of the printing paper. FIG. 11 is a block diagram showing a configuration of a portion that performs a thinning process of a conventional stencil printing apparatus. As shown in FIG. 11, the conventional stencil printing apparatus includes an image input unit 1, a photo processing unit 2, a character processing unit 3, and an image output unit 4. Here, the photograph processing unit 2 includes a gamma decimation processing unit 21 and a first binarization processing unit 23. The character processing unit 3 includes a second binarization processing unit 31 and a pattern thinning processing unit 33.

  The image input unit 1 reads an original image with an input device such as a scanner and outputs image data of the read original image to the photo processing unit 2 and the character processing unit 3. The image data output from the image input unit 1 is composed of, for example, a plurality of pixels arranged in a two-dimensional matrix, and has density information corresponding to the gradation of each pixel.

  The photo processing unit 2 performs a necessary thinning process on the assumption that the image data output from the image input unit 1 is “photograph”, and outputs the processed image data.

  Specifically, the gamma thinning processing unit 21 determines the output density based on the input density and the gamma curve based on the density information of the image data. FIG. 12 is a diagram showing a gamma curve when performing gamma thinning processing in the stencil printing apparatus. For example, even when the input density is 255, the gamma thinning processing unit 21 can suppress the output density to 128 by performing density conversion using the “gamma curve with thinning” in FIG. Density 255 is the maximum density that can be read by the scanner, and black indicates white, while 0 indicates white.

  The first binarization processing unit 23 converts the image data after density conversion by the gamma decimation processing unit 21 into binary image data composed of binary values of white and black, and outputs the binary image data to the image output unit 4. The first binarization processing unit 23 uses an error diffusion method when performing binarization processing. According to this error diffusion method, the gradation of a photograph can be reproduced in a pseudo manner by the area gradation, and at the same time, a fine line such as a character can be reproduced to some extent.

  On the other hand, the character processing unit 3 performs a necessary thinning process on the assumption that the image data output from the image input unit 1 is “character”, and outputs the processed image data.

  Specifically, the second binarization processing unit 31 converts the image data output from the image input unit 1 into binary image data composed of white and black binary values with a single threshold value.

  Based on the image data output from the second binarization processing unit 31, the pattern thinning processing unit 33 thins out a checkered pattern as shown in FIG. 13 according to the coordinate position of the pixel. In other words, the pattern thinning processing unit 33 superimposes the checkered pattern on the image data, and forcibly converts it to a white pixel when the portion corresponding to “0” in FIG. 13 is a black pixel. Thereafter, the pattern thinning processing unit 33 outputs the image data after the thinning processing to the image output unit 4.

  The image output unit 4 selects either the image data processed by the photo processing unit 2 or the image data processed by the character processing unit 3 based on the “image mode switching signal” by the user, and selects the selected image Output an image based on the data. That is, the user can switch between the “character mode” and the “photo mode” by operating a button, a panel, or the like, and the image output unit 4 selects “photo” according to the mode selected by the user. In the case of “mode”, an image subjected to error diffusion processing is output, and in the case of “character mode”, an image subjected to single threshold processing is output.

  Specifically, the image output unit 4 makes binary image data on the master and winds it. That is, each stencil sheet is thermally perforated by the thermal head based on the image data output by the image output unit 4. The stencil sheet thus made is wound around the first drum 11 and printed on the first drum 11 with a set press pressure.

  The problems in the thinning process described above are listed. First, there is no particular problem when the user selects “character mode” and prints an image of only characters. The pattern thinning process using the checkered pattern is repeated because the thinning corresponding to the checkered pattern is performed in any part, so there is no part where the image rate increases locally and the ink does not concentrate in one part. This is because there is a low possibility of transfer or set-off. The single threshold processing has a disadvantage that gradation cannot be reproduced, but it is not necessary to reproduce gradation in the case of an image of only characters.

  However, when the user selects the “photo mode” and prints an image of only a photo or an image in which a photo and a character are mixed, there is a possibility that the above-described problems of retransfer and set-off occur. The method of performing error diffusion processing after performing gamma decimation has the advantage that gradation can be reproduced, but there are minute areas in which the image rate increases locally (ie, ink concentrates) locally. It is.

  In the binarization method using the error diffusion method, a stripe pattern called “texture” or “worm” peculiar to the error diffusion process may occur and cause deterioration of image quality. There are also stencil printing apparatuses that intentionally superimpose image data on image data to prevent the occurrence of worms and the like. FIG. 14 is a block diagram showing a configuration in the case where a noise addition processing unit is provided in a part for performing a thinning process of a conventional stencil printing apparatus. The difference from the stencil printing apparatus shown in FIG. 11 is that a noise addition processing unit 53 is provided after the gamma decimation processing unit 51.

  The noise addition processing unit 53 superimposes noise generated by random numbers on the density-converted image data output by the gamma decimation processing unit 51 and outputs it. This stencil printing apparatus can improve the highlight worm by providing the noise addition processing unit 53.

  FIG. 15 is a diagram illustrating output images according to the presence or absence of worm processing by the noise addition processing unit 53. As shown in FIG. 15A, when the highlight worm process is not performed by the noise addition processing unit 53, the output image includes a worm in which each pixel is connected in a chain without being uniformly distributed. It will be a thing. On the other hand, when noise is added by the noise addition processing unit 53 and the highlight worm process is performed, as shown in FIG. 15B, the occurrence of the worm and the accompanying deterioration in image quality are prevented. can do.

  However, the stencil printing apparatus shown in FIG. 14 still has the same problems as the stencil printing apparatus of FIG. That is, when the user selects the “photo mode” and prints an image of only a photo or an image in which a photo and a character are mixed, there is a possibility that the above-described problems of retransfer and set-off occur. In the method of performing error diffusion processing after performing thinning by gamma, even when worm processing is performed by adding noise, there is a small area where the image rate increases locally (that is, ink concentrates) continuously. Because.

  The present invention solves the above-described problems of the prior art. When printing a photo including a solid portion or bold characters, the solid portion or the bold portion is prevented from being retransferred and smeared and show-through, It is an object of the present invention to provide an image forming apparatus capable of obtaining tone reproducibility.

  In order to solve the above problems, an image forming apparatus according to the present invention includes an image input unit that inputs image data, and an image input unit that includes a printing unit that prints one side of a print medium. Based on the input image data, a thinning process based on a predetermined density conversion curve is performed on an area having a density lower than a predetermined density, and a thinning process using a checkered pattern is performed on an area having the predetermined density or higher. A control unit that outputs the processed image data, and the printing unit prints an image based on the image data output by the control unit on at least one surface of a print medium.

  In order to achieve the above object, a second feature of the image forming apparatus according to the present invention is that the control unit, based on the image data input by the image input unit, and the predetermined density area and the predetermined A density specifying unit that specifies a region of density or higher, and a first thinning processing unit that performs a thinning process and a halftone binarization process based on a predetermined density conversion curve based on the image data input by the image input unit A second thinning processing unit that performs binarization processing using a single threshold and thinning processing using a checkered pattern based on the image data input by the image input unit, and the predetermined density by the density specifying unit. The image data processed by the first thinning-out processing unit is applied to the area specified as being less than the specified level, and the density specifying unit specifies that the density is equal to or higher than the predetermined density It is provided with an image output unit that outputs the image data to be printed in the printing unit by applying the image data processed by the second thinning section with respect to the region.

  In order to achieve the above object, a third feature of the image forming apparatus according to the present invention is that the first thinning processing unit uses an error diffusion method when performing halftone binarization processing.

  In order to achieve the above object, a fourth feature of the image forming apparatus according to the present invention is that the control unit is configured to perform predetermined processing on an area less than the predetermined density based on image data input by the image input unit. A density conversion unit that performs a thinning process based on the density conversion curve, and performs density conversion so that the output density in the region of the predetermined density or higher is uniformly half the maximum density, and the density conversion unit A binarization processing unit that performs halftone binarization processing on the image data that has been subjected to density conversion by the method, and image data that has undergone halftone binarization processing by the binarization processing unit is printed by the printing unit. An image output unit that outputs the image data as image data.

  In order to achieve the above object, according to a fifth feature of the image forming apparatus of the present invention, the control unit is configured to control the region below the predetermined density based on the image data input by the image input unit and the predetermined Based on the result of the specification by the density specifying unit and the density specifying unit for specifying the area of the density or higher, noise is applied to the image data that has undergone density conversion by the density conversion unit in the area below the predetermined density. And a noise addition processing unit that does not add noise to the image data that has been subjected to density conversion by the density conversion unit in the region of the predetermined density or higher.

  In order to achieve the above object, a sixth feature of the image forming apparatus according to the present invention is that the binarization processing unit uses an error diffusion method when performing halftone binarization processing.

  According to the first aspect of the present invention, when printing a photograph including a solid portion or a bold character, the retransfer stain or back-through of the solid portion or the bold character portion is prevented, and the gradation reproduction of the photograph is obtained. Can do.

  According to the second aspect of the present invention, pattern thinning processing is adopted for a portion having a high input density such as a solid portion or bold type, and gamma thinning processing is adopted for other portions having a low input density, thereby performing both thinning processing. As a result, it is possible to prevent re-transfer stains and back-through of the solid portion and the bold portion, and to obtain gradation reproduction of a photograph.

  According to the third aspect of the present invention, since the first thinning-out processing unit uses the error diffusion method when performing the halftone binarization processing, the gradation of the halftone photograph is reproduced in a pseudo manner using the area gradation. At the same time, it has the advantage of good reproducibility for fine lines such as characters.

  According to the invention of claim 4, the density conversion unit and the binarization process are both used for the checkered pattern thinning process in a portion where the input density is high, such as a solid part or bold type, and the gamma thinning process in a part where the other input density is low. This can be realized by providing the unit, and a simple configuration can be obtained.

  According to the fifth aspect of the present invention, it is possible to improve the highlight worm by adding noise from the noise addition processing unit, and noise addition by the noise addition processing unit is performed on an area having a predetermined density or more. Therefore, the regularization is not disturbed, and the binarization processing unit can generate a clean checkered pattern by performing error diffusion processing on the region.

  According to the sixth aspect of the invention, since the binarization processing unit uses the error diffusion method when performing the halftone binarization process, the halftone photograph gradation can be reproduced in a pseudo manner using the area gradation. At the same time, it has the advantage of good reproducibility for fine lines such as characters.

It is a block diagram which shows the structure of the stencil printing apparatus of the form of Example 1 of this invention. It is a figure which shows the gamma curve used in the gamma thinning process part of the stencil printing apparatus of the form of Example 1 of this invention. It is a figure which shows the example of an output image at the time of performing an error diffusion process by the photographic processing part of the stencil printing apparatus of the form of Example 1 of this invention. It is a figure which shows the example of an output image at the time of performing a pattern thinning process by the character processing part of the stencil printing apparatus of the form of Example 1 of this invention. It is a figure which shows the effect by the stencil printing apparatus of the form of Example 1 of this invention. It is a block diagram which shows the structure of the stencil printing apparatus of the form of Example 2 of this invention. It is a figure which shows the gamma curve used in the gamma thinning-out process part of the stencil printing apparatus of the form of Example 2 of this invention. It is a figure which shows the error diffusion process of the stencil printing apparatus of the form of Example 2 of this invention. It is a figure which shows the example of an output image at the time of performing an error diffusion process by a binarization process part, when input density is 204 or more in the stencil printing apparatus of the form of Example 2 of this invention. It is a figure explaining the flow of the double-sided printing operation in the conventional stencil printing apparatus. It is a block diagram which shows the structure of the part which performs the thinning process of the conventional stencil printing apparatus. It is a figure which shows the gamma curve used in the gamma thinning process part of the conventional stencil printing apparatus. It is a figure explaining the checkered pattern used in the pattern thinning process part of the conventional stencil printing apparatus. It is a block diagram which shows the structure at the time of providing the noise addition process part in the part which performs the thinning process of the conventional stencil printing apparatus. It is a figure which shows the output image according to the presence or absence of the worm process by the noise addition process part of the conventional stencil printing apparatus.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings, taking a stencil printing apparatus as an example.

  Embodiments of the present invention will be described below with reference to the drawings. First, the configuration of the present embodiment will be described. The stencil printing apparatus of the present invention has a mechanism similar to that of the conventional stencil printing apparatus shown in FIG. That is, the stencil printing apparatus of the present invention has a printing unit that prints one side of a printing paper that is a printing medium. Here, the first drum 11 shown in FIG. 10 corresponds to the printing unit of the present invention. The second drum 16 prints the other side of the printing paper on which one side is printed by the first drum 11.

  FIG. 1 is a block diagram illustrating a configuration of a portion that performs a thinning process in the stencil printing apparatus according to the first embodiment of the present invention. As shown in FIG. 1, the stencil printing apparatus includes an image input unit 1, a photo processing unit 2a, a character processing unit 3, and an image output unit 4a. Here, the photographic processing unit 2a includes a gamma decimation processing unit 21, a density comparison unit 22, and a first binarization processing unit 23. The character processing unit 3 includes a second binarization processing unit 31 and a pattern thinning processing unit 33.

  The image input unit 1 inputs image data. Specifically, the image input unit 1 reads an original image with an input device such as a scanner, and outputs image data of the read original image to the photo processing unit 2 a and the character processing unit 3. As described in the conventional example, the image data output by the image input unit 1 is composed of, for example, a plurality of pixels arranged in a two-dimensional matrix, and has density information corresponding to the gradation of each pixel.

  The photo processing unit 2a, the character processing unit 3, and the image output unit 4a correspond to the control unit of the present invention. Based on the image data input by the image input unit 1, the photo processing unit 2a, the character processing unit 3, and the image output unit 4a A thinning process based on the density conversion curve is performed, a thinning process using a checkered pattern is performed on an area having a predetermined density or more, and image data after the thinning process is output. The photo processing unit 2a, the character processing unit 3, and the image output unit 4a constituting the control unit will be described in detail.

  Similar to the conventional photo processing unit 2, the photo processing unit 2 a performs a necessary thinning process on the assumption that the image data output from the image input unit 1 is “photograph”, and outputs the processed image data.

  However, the photographic processing unit 2 a includes a density comparison unit 22. The density comparison unit 22 corresponds to the density specifying unit of the present invention, and specifies a region having a density lower than a predetermined density and a region having a density higher than a predetermined density based on the image data input by the image input unit 1. In the present embodiment, it is assumed that the density comparison unit 22 specifies an area having an input density of 204 or more and an area having an input density of less than 204 for an input density in the range from 0 to 255. Thereby, the density | concentration comparison part 22 can identify the solid part and bold part with high input density, for example. The region determination by the density comparison unit 22 may be performed in units of pixels, or may be performed by regarding a predetermined plurality of pixels as one unit.

  The density comparison unit 22 in the present embodiment is configured in the photo processing unit 2a, but is not necessarily configured in the photo processing unit 2a. It may be configured.

  The gamma decimation processing unit 21 and the first binarization processing unit 23 correspond to the first decimation processing unit of the present invention and are based on image data input by the image input unit 1 and based on a predetermined density conversion curve. Thinning processing and halftone binarization processing are performed.

  Specifically, the gamma thinning processing unit 21 determines the output density based on the input density based on the density information of the image data and the density conversion curve (gamma curve). FIG. 2 is a diagram showing a gamma curve when performing a gamma thinning process in the stencil printing apparatus of the present embodiment. The gamma decimation processing unit 21 performs density conversion using the “gamma curve with decimation” in FIG.

  The first binarization processing unit 23 is a binary image composed of white and black binaries by gradation number conversion of halftone binarization processing based on the image data after density conversion by the gamma decimation processing unit 21. Data is converted and output to the image output unit 4a. The first binarization processing unit 23 uses an error diffusion method when performing the halftone binarization process.

  On the other hand, similarly to the conventional stencil printing apparatus, the character processing unit 3 performs a necessary thinning process on the assumption that the image data output from the image input unit 1 is “character”, and outputs the processed image data. Specifically, the character processing unit 3 corresponds to the second thinning processing unit of the present invention, and based on the image data input by the image input unit 1, binarization processing by a single threshold value and checkered pattern Perform thinning processing.

  The second binarization processing unit 31 converts the image data output from the image input unit 1 into binary image data including white and black binary values with a single threshold value. Based on the image data output from the second binarization processing unit 31, the pattern thinning processing unit 33 thins out the checkered pattern according to the coordinate position of the pixel, and the image data after the thinning processing is sent to the image output unit 4 a. Output.

  The image output unit 4a outputs an image based on appropriate image data based on the “image mode switching signal” by the user. Here, when the user selects the “character mode”, the image output unit 4a outputs an image on which the single threshold processing has been performed by the character processing unit 3, as in the conventional stencil printing apparatus. .

  When the user selects the “photo mode”, the image output unit 4a displays the image data processed by the first thinning processing unit for the area specified by the density comparison unit 22 to be less than the predetermined density. The image data to be printed on the first drum 11 by applying the image data processed by the second thinning processing unit to the area specified by the density comparison unit 22 to be equal to or higher than the predetermined density. Output.

  That is, in this embodiment, the image output unit 4a applies the image data output by the first binarization processing unit 23 to an area having a density less than 204, such as a normal photograph. Further, the image output unit 4a applies the image data output by the pattern thinning processing unit 33 to a region having a density of 204 or more, such as bold and solid portions.

  The first drum 11 prints an image based on the image data output by the control unit (image output unit 4a) on one side of the printing paper as a print medium.

<Operation of stencil printing machine>
Next, the operation of the present embodiment configured as described above will be described using stencil printing as an example. The user selects one of the “character mode” and the “photo mode” by operating a button, a panel, or the like. Since the “photo mode” is characteristic in the present embodiment, the description here assumes that the user has selected “photo mode”.

  First, the image input unit 1 reads an original image with an input device such as a scanner, and outputs image data of the read original image to the photo processing unit 2 a and the character processing unit 3.

  Based on the image data input by the image input unit 1, the density comparison unit 22 in the photographic processing unit 2a specifies an area having a density lower than the predetermined density 204 and an area having a density higher than the predetermined density 204. Note that the value of density 204 means that the density of the input image is 80% of the maximum density, and for a solid portion or a bold portion of a document that requires pattern thinning to prevent retransfer stains. The value obtained experimentally from the reading level of the scanner. Therefore, the density threshold value by the density comparison unit 22 may be set to an appropriate value according to the stencil printing apparatus.

  The gamma decimation processing unit 21 in the photographic processing unit 2a determines the output density based on the input density based on the density information of the image data and a predetermined density conversion curve (gamma curve). The gamma decimation processing unit 21 can reproduce gradation while performing density decimation by performing density conversion using the “gamma curve with decimation” in FIG.

  The first binarization processing unit 23 is a binary image composed of white and black binaries by gradation number conversion of halftone binarization processing based on the image data after density conversion by the gamma decimation processing unit 21. Data is converted and output to the image output unit 4a.

  FIG. 3 is a diagram illustrating an output image example when the error diffusion processing is performed by the photographic processing unit 2a of the stencil printing apparatus of the present embodiment. That is, the first binarization processing unit 23 outputs image data as shown in FIG. 3 in the solid part and the bold part. The output image shown in FIG. 3 has an image rate of 50% as a whole, but there is a possibility of retransfer stains or set-off because there are locally minute areas with a high image rate.

  On the other hand, the second binarization processing unit 31 in the character processing unit 3 converts the image data output from the image input unit 1 into binary image data composed of white and black binary values with a single threshold. Convert. Based on the image data output from the second binarization processing unit 31, the pattern thinning processing unit 33 thins out the checkered pattern according to the coordinate position of the pixel, and the image data after the thinning processing is sent to the image output unit 4 a. Output.

  FIG. 4 is a diagram illustrating an output image example when the pattern thinning process is performed by the character processing unit 3 of the stencil printing apparatus of the present embodiment. That is, the pattern thinning processing unit 33 outputs image data as shown in FIG. 4 at the solid portion or the bold portion. The output image shown in FIG. 4 has an image rate of 50% as in the case of FIG. 3, but has a disadvantage that gradation cannot be reproduced by adopting a regular checkered pattern, but locally black pixels. Is not continuous, and there is little possibility of retransfer stains and set-off.

  The image output unit 4a applies the image data processed by the first decimation processing unit to the area specified by the density comparison unit 22 as being less than the predetermined density. That is, the image output unit 4a applies the image data output by the first binarization processing unit 23 to an area having a density less than 204, such as a normal photograph. In this image data, gradation is reproduced by gamma decimation processing and halftone processing by the error diffusion method, and since the input density is originally low, black pixels are not concentrated so that the image data can be reversed and reproduced. The possibility of transfer contamination is low.

  In addition, the image output unit 4a applies the image data processed by the second thinning processing unit to the area specified by the density comparison unit 22 as being equal to or higher than the predetermined density. That is, the image output unit 4a applies the image data output by the pattern thinning processing unit 33 to areas having a density of 204 or more such as bold and solid portions. Since this image data is solid or bold, it is not necessary to reproduce gradation, and black pixels do not continue locally by thinning out a checkered pattern, and there is a possibility of retransfer stains and setbacks. Is low.

  As described above, the image output unit 4a combines a part of the image data output from the photo processing unit 2a with a part of the image data output from the character processing unit 3, thereby reducing the density less than a predetermined density. It is possible to generate and output image data obtained by performing a thinning process based on a predetermined density conversion curve for an area and performing a thinning process using a checkered pattern for an area having a predetermined density or higher.

  That is, the control unit configured by the photo processing unit 2a, the character processing unit 3, and the image output unit 4a is based on the image data input by the image input unit 1 and has a predetermined density for an area less than the predetermined density. It can be said that thinning processing based on the conversion curve is performed, thinning processing using a checkered pattern is performed on a region having a predetermined density or more, and image data after the thinning processing is output.

  The first drum 11 prints an image based on the image data output by the control unit (image output unit 4a) on one surface of the printing paper 10 as a print medium. The plate making / discarding process performed in the first drum 11 is general. That is, the stencil printing apparatus checks whether or not the stencil sheet is wound around the first drum 11, and if it is wound, removes the stencil sheet from the first drum 11 and discharges the stencil sheet.

  When the plate removal process is completed, the stencil sheet is thermally perforated by the thermal head based on the image data output by the image output unit 4a. The stencil sheet thus made is wound around the outer peripheral wall of the first drum 11.

  The fed printing paper 10 is pressed against the stencil paper of the first drum 11 by the press roller 12, whereby an image based on the image data output by the image output unit 4a is printed.

  The first drum-printed paper 13 printed on the upper surface is peeled off from the outer peripheral surface of the first drum 11 and guided to the duplex unit 14 by a belt conveyance mechanism or the like. Thereafter, the first drum-printed paper 13 taken out from the duplex unit 14 is conveyed by a paper conveying roller 15 or a belt and is fed to the second drum 16. The fed first drum printed paper 13 is pressed against the stencil paper of the second drum 16 by the press roller 17, whereby the ink image is transferred to the lower surface (the other surface) of the first drum printed paper 13. Then, the second drum-printed paper 18 printed on both sides is discharged. Note that the image printed on the second drum 16 does not have to worry about retransfer stains, and therefore, it is not necessary to perform the thinning process unlike the printed image on the first drum 11.

  As described above, according to the stencil printing apparatus according to the first embodiment of the present invention, when printing a photograph including a solid portion or bold characters, re-transfer stains or back-through of the solid portions or bold characters are prevented. At the same time, the gradation reproducibility of the photograph can be obtained. That is, it can be said that the image forming apparatus of the present invention is effective not only for double-sided printing but also for single-sided printing in terms of preventing back-through.

  FIG. 5 is a diagram showing the effect of the stencil printing apparatus of this embodiment. As shown in FIG. 5, the pattern thinning process using the checkered pattern cannot reproduce the photographic gradation, but the thin characters are easy to read, and thinning corresponding to the checkered pattern is performed at any point. There is no part where the image rate is locally increased, and the ink does not concentrate in one place, so the possibility of retransfer or show-through is low.

  On the other hand, the gamma decimation process has the advantage of being able to reproduce photographic gradation, but on the other hand, there are small areas where the image rate increases locally (that is, the ink concentrates), so solid retransfer stains and It is unsuitable for readability of fine characters.

  Therefore, the stencil printing apparatus according to the present embodiment employs pattern thinning processing for portions with high input density such as solid portions and bold characters, and adopts gamma thinning processing for other portions with low input density. As a result of taking advantage of the processing, it is possible to prevent the re-transfer stains of the solid portion and the bold portion, and to obtain the gradation reproducibility of the photograph.

  Further, since the stencil printing apparatus of this embodiment uses the error diffusion method when the first binarization processing unit 23 performs the halftone binarization process, the gradation of the halftone photograph is simulated by the area gradation. At the same time, it also has the advantage of good reproducibility for fine lines such as characters.

  That is, the stencil printing apparatus of the present invention stabilizes a constant image rate even in a minute area by performing pattern thinning processing in a solid portion or a bold portion having a specific density or more while reproducing gradation by error diffusion processing. Therefore, good prevention of retransfer stains can be realized.

  The present invention is applicable not only to stencil printing apparatuses but also to image forming apparatuses such as inkjet printing apparatuses and copying machines.

  First, the configuration of the present embodiment will be described with stencil printing. The stencil printing apparatus of the present invention has a mechanism similar to that of the conventional stencil printing apparatus shown in FIG. That is, the stencil printing apparatus of the present invention has a printing unit that prints one side of a printing paper that is a printing medium. Here, the first drum 11 shown in FIG. 10 corresponds to the printing unit of the present invention. The second drum 16 prints the other side of the printing paper on which one side is printed by the first drum 11.

  FIG. 6 is a block diagram illustrating a configuration of a portion that performs a thinning process in the stencil printing apparatus according to the second embodiment of the present invention. As shown in FIG. 6, the stencil printing apparatus includes an image input unit 1, a photographic processing unit 5 a, and an image output unit 7. Here, the photographic processing unit 5a includes a gamma decimation processing unit 21, a density comparison unit 52, a noise addition processing unit 53a, and a third binarization processing unit 54.

  In the present embodiment, the character processing unit 3 described in the first embodiment is not described in the configuration of FIG. 6, but it is assumed that it actually exists. However, unlike the first embodiment, the character processing unit 3 is used only when the user selects the “character mode” and is not used when the user selects the “photo mode”. In this embodiment, the stencil printing apparatus can solve the problem of retransfer and set-off without using the character processing unit 3 even when a solid portion or a bold portion exists in the photograph. In describing the configuration of this embodiment, the description of the character processing unit 3 is omitted.

  The image input unit 1 is the same as that of the first embodiment, and a duplicate description is omitted.

  The photo processing unit 5a and the image output unit 7 correspond to the control unit of the present invention, and are based on a predetermined density conversion curve for an area less than a predetermined density based on the image data input by the image input unit 1. A thinning process is performed, a thinning process using a checkered pattern is performed on an area having a predetermined density or more, and image data after the thinning process is output. The photographic processing unit 5a and the image output unit 7 constituting the control unit will be described in detail.

  Similar to the conventional photo processing unit 5 described with reference to FIG. 14, the photo processing unit 5a adds noise to prevent the occurrence of worms and the like, and the image data output by the image input unit 1 is “photograph”. If necessary, a necessary thinning process is performed, and the processed image data is output.

  However, the photographic processing unit 5 a includes a density comparison unit 52. The density comparison unit 52 corresponds to the density specifying unit of the present invention, and specifies a region having a density lower than a predetermined density and a region having a density higher than a predetermined density based on the image data input by the image input unit 1. In this embodiment, it is assumed that the density comparison unit 52 specifies an area having an input density of 204 or more and an area having an input density of less than 204 for an input density in the range from 0 to 255. Thereby, the density comparison unit 52 can specify, for example, a solid part or a bold part having a high input density. The region determination by the density comparison unit 52 may be performed in units of pixels, or may be performed by regarding a predetermined plurality of pixels as one unit.

  The gamma decimation processing unit 51 corresponds to the density conversion unit of the present invention, and performs decimation processing based on a predetermined density conversion curve for an area less than a predetermined density based on the image data input by the image input unit 1. At the same time, density conversion is performed so that the output density in a region of a predetermined density or higher is uniformly about half the maximum density.

  Specifically, the gamma thinning processing unit 51 determines the output density based on the input density based on the density information of the image data and the density conversion curve (gamma curve). FIG. 7 is a diagram showing a gamma curve when performing the gamma thinning process in the stencil printing apparatus of the present embodiment. The gamma decimation processing unit 51 performs density conversion using the “gamma curve with decimation” in FIG.

  As shown in FIG. 7, the gamma decimation processing unit 51 in this embodiment performs decimation based on a predetermined density conversion curve for an area less than a predetermined density 204 based on the image data input by the image input unit 1. In addition to processing, density conversion is performed so that the output density in a region of a predetermined density 204 or higher is uniformly half the maximum density 255 (ie, 128). Since the gamma decimation processing unit 51 specifies a region according to the density when performing the above processing, it may be considered that the gamma decimation processing unit 51 has the same function as the density comparison unit 52.

  The noise addition processing unit 53a adds noise to the image data that has been subjected to density conversion by the gamma decimation processing unit 51 in an area less than the predetermined density 204 based on the identification result by the density comparison unit 52. In an area of a predetermined density 204 or higher, no noise is added to the image data whose density has been converted by the gamma decimation processing unit 51. That is, the noise addition processing unit 53a does not add noise to the area whose density has been converted so that the output density is uniformly 128 as shown in FIG. Add noise.

  The third binarization processing unit 54 corresponds to the binarization processing unit of the present invention, and performs halftone binarization processing based on the image data subjected to density conversion by the gamma decimation processing unit 51. Strictly speaking, the third binarization processing unit 54 is based on the image data after the noise addition processing unit 53a performs the noise addition processing on the image data whose density has been converted by the gamma decimation processing unit 51. By converting the number of gradations in the halftone binarization process, the image data is converted into binary image data composed of binary values of white and black and output to the image output unit 7. As shown in FIG. 8, the third binarization processing unit 54 uses an error diffusion method when performing the halftone binarization process.

  The image output unit 7 outputs the image data subjected to the halftone binarization processing by the third binarization processing unit 54 as image data for printing on the first drum 11. The first drum 11 prints an image based on the image data output by the control unit (image output unit 7) on at least one surface of a printing paper as a print medium.

<Operation of stencil printing machine>
Next, the operation of the present embodiment configured as described above will be described. The user selects one of the “character mode” and the “photo mode” by operating a button, a panel, or the like. Since the “photo mode” is characteristic in the present embodiment, the description here assumes that the user has selected “photo mode”.

  First, the image input unit 1 reads an original image with an input device such as a scanner, and outputs image data of the read original image to the photo processing unit 5a.

  Based on the image data input by the image input unit 1, the density comparison unit 52 in the photographic processing unit 5 a identifies an area having a density lower than the predetermined density 204 and an area having a density higher than the predetermined density 204.

  The gamma decimation processing unit 51 in the photographic processing unit 5a determines the output density based on the input density and density conversion curve (gamma curve) based on the density information of the image data. At that time, the gamma decimation processing unit 51 performs density conversion using the “gamma curve with decimation” in FIG. Specifically, the gamma decimation processing unit 51 performs a decimation process based on a predetermined density conversion curve for an area less than a predetermined density 204 based on the image data input by the image input unit 1, and also performs a predetermined process. Density conversion is performed so that the output density in a region of density 204 or higher is uniformly half the maximum density 255 (ie, 128).

  The noise addition processing unit 53a adds noise to the image data that has been subjected to density conversion by the gamma decimation processing unit 51 in an area less than the predetermined density 204 based on the identification result by the density comparison unit 52. In an area of a predetermined density 204 or higher, no noise is added to the image data whose density has been converted by the gamma decimation processing unit 51. That is, the noise addition processing unit 53a intentionally superimposes noise generated by random numbers on the image data in an area below a predetermined input density 204 to prevent the occurrence of a worm or the like in highlights.

  The third binarization processing unit 54 performs halftone binarization based on image data after the noise addition processing unit 53a performs noise addition processing on the image data whose density has been converted by the gamma decimation processing unit 51. By the conversion of the number of gradations of processing, the image data is converted into binary image data consisting of binary values of white and black and output to the image output unit 7.

  FIG. 9 is a diagram illustrating an example of an output image when error diffusion processing is performed by the third binarization processing unit 54 when the input density is 204 or more in the stencil printing apparatus of the present embodiment. That is, since the density conversion is performed by the gamma decimation processing unit 51 so that the output density in the region where the input density is 204 or higher is uniformly fixed to 128, the third binarization processing unit 54 is substantially half of the maximum density. By performing error diffusion processing on the (50%) area, image data on which the checkered pattern thinning processing as shown in FIG. 9 has been performed is output. As a result, a solid portion or a bold portion having an input density of 204 (80%) or more becomes a substantially checkered binary output with an image rate of 50%.

  The stencil printing apparatus of the present embodiment eliminates the need for the image data after the checkered pattern thinning processing by the pattern thinning processing unit 33 as in the first embodiment when a solid portion or a bold portion is included in the photograph. It is characterized in that thinning-out processing of checkered patterns can be performed by performing error diffusion processing with the output density in a high density area, such as the bold and bold parts, fixed at 128, which is approximately half of the maximum density. .

  Further, as described above, since the noise addition processing unit 53a does not perform noise addition on the region where the density conversion is performed so that the output density is uniformly 128, the regularity is not disturbed. 3 binarization processing unit 54 can generate a beautiful checkered pattern by performing error diffusion processing on the region.

  In the present embodiment, when the gamma decimation processing unit 51 is less than the input density 204 (80%), the density conversion according to the gamma curve is performed without fixing the output density to 128 only to reproduce the gradation. In other words, the checkered pattern collapses when it is less than 80%, so that the error diffusion of less than 80% and the settling performance in the checkered pattern of 80% or more are equalized.

  The image output unit 7 outputs the image data subjected to the halftone binarization processing by the third binarization processing unit 54 as image data for printing on the first drum 11. The image data output by the image output unit 7 is obtained by performing density conversion that reproduces a halftone for an area having an input density of less than 204, such as a normal photograph. As described above, for the region having the input density 204 or higher, the thinning process is performed on the checkered pattern.

  That is, the control unit configured by the photo processing unit 5a (gamma decimation processing unit 51, density comparison unit 52, noise addition processing unit 53a, third binarization processing unit 54) and image output unit 7 is an image input unit. Based on the image data input by 1, a thinning process based on a predetermined density conversion curve is performed on an area below a predetermined density, and a thinning process using a checkered pattern is performed on an area above a predetermined density, It can be said that the image data after the thinning process is output.

  Note that when the noise addition processing unit 53a is not present, the density comparison unit 52 is also unnecessary, so that the control unit of the stencil printing apparatus of the present invention has a gamma decimation processing unit 51 and a third binarization processing unit 54. And the image output unit 7.

  The first drum 11 prints an image based on the image data output by the control unit (image output unit 7) on at least one surface of the printing paper 10 as a print medium. Since the subsequent operation is the same as that of the first embodiment, a duplicate description is omitted.

  As described above, according to the stencil printing apparatus according to the second embodiment of the present invention, the same effect as that of the first embodiment can be obtained, and checkerboard pattern thinning is performed in a portion having a high input density such as a solid portion or a bold font. Both the processing and the gamma decimation processing in other portions where the input density is low can be performed in the photo processing unit 5a, and the pattern decimation processing unit 33 as in the first embodiment is unnecessary. In the stencil printing apparatus of this embodiment, the photographic processing unit 5a performs error diffusion processing with the output density in a high density area, such as a solid part or a bold part, fixed at 128, which is substantially half of the maximum density, so that the error diffusion process is performed. There is an advantage that the pattern thinning process can be automatically performed.

  Further, highlighting worms can be improved by adding noise from the noise addition processing unit 53a, and noise addition by the noise addition processing unit 53a is performed on a region whose density has been converted so that the output density is uniformly 128. Therefore, the regularity is not disturbed, and the third binarization processing unit 54 can generate a beautiful checkered pattern by performing error diffusion processing on the region.

  Similarly to the case of the first embodiment, the stencil printing apparatus of the present embodiment uses an error diffusion method when the third binarization processing unit 54 performs the halftone binarization process, so that a halftone dot photograph is used. Can be reproduced in a pseudo manner using the area gradation, and at the same time, it has the advantage of good reproducibility for fine lines such as characters.

  The present invention is applicable not only to stencil printing apparatuses but also to image forming apparatuses such as inkjet printing apparatuses and copying machines.

  The image forming apparatus according to the present invention can be used for an image forming apparatus that performs single-sided printing or double-sided printing.

DESCRIPTION OF SYMBOLS 1 Image input part 2, 2a Photo process part 3 Character process part 4, 4a Image output part 5, 5a Photo process part 7 Image output part 10 Paper 11 1st drum 12 Press roller 13 1st drum printed paper 14 Duplex unit 15 Paper transport roller 16 Second drum 17 Press roller 18 Second drum printed paper 21 Gamma thinning processing unit 22 Density comparison unit 23 First binarization processing unit 31 Second binarization processing unit 33 Pattern thinning processing unit 51 Gamma decimation processing unit 52 Density comparison unit 53a Noise addition processing unit 54 Third binarization processing unit

Claims (6)

In an image forming apparatus having a printing unit for printing on a print medium,
An image input unit for inputting image data;
Based on the image data input by the image input unit, a thinning process based on a predetermined density conversion curve is performed on an area less than a predetermined density, and thinning by a checkered pattern is performed on an area above the predetermined density. A control unit that performs processing and outputs the image data after the thinning processing,
The image forming apparatus, wherein the printing unit prints an image based on the image data output from the control unit on at least one surface of a print medium. The controller is
A density specifying unit that specifies an area below the predetermined density and an area above the predetermined density based on the image data input by the image input unit;
A first decimation processing unit that performs decimation processing and halftone binarization processing based on a predetermined density conversion curve based on image data input by the image input unit;
A second thinning processing unit that performs binarization processing using a single threshold value and thinning processing using a checkered pattern based on the image data input by the image input unit;
Applying the image data processed by the first thinning-out processing unit to the region specified by the density specifying unit as being less than the predetermined density, and having the density specified by the density specifying unit equal to or higher than the predetermined density An image output unit that outputs image data to be printed in the printing unit by applying the image data processed by the second thinning processing unit to the specified region;
The image forming apparatus according to claim 1, further comprising:   The image forming apparatus according to claim 2, wherein the first thinning processing unit uses an error diffusion method when performing the halftone binarization processing. The controller is
Based on the image data input by the image input unit, a thinning process based on a predetermined density conversion curve is performed on the area below the predetermined density, and the output density in the area above the predetermined density is uniform. A density conversion unit that performs density conversion so that the value is approximately half of the maximum density;
A binarization processing unit that performs halftone binarization processing on the image data density-converted by the density conversion unit;
An image output unit that outputs the image data subjected to the halftone binarization processing by the binarization processing unit as image data for printing in the printing unit;
The image forming apparatus according to claim 1, further comprising: The controller is
Based on the image data input by the image input unit, a density specifying unit that specifies an area below the predetermined density and an area above the predetermined density;
Based on the identification result by the density identification unit, noise is added to the image data subjected to density conversion by the density conversion unit in the area below the predetermined density, and in the area of the predetermined density or more. Is a noise addition processing unit that does not add noise to the image data subjected to density conversion by the density conversion unit;
The image forming apparatus according to claim 4, further comprising:   6. The image forming apparatus according to claim 4, wherein the binarization processing unit uses an error diffusion method when performing the halftone binarization processing.