Image forming apparatus and method for making density correction in a low resolution image based on edge determination

The image forming apparatus comprises a resolution converting unit configured to convert a high-resolution image data into a low-resolution image data, an edge judgment unit configured to judge a shape of an edge in the high-resolution image data, and a density correcting unit configured to make a density correction in the low-resolution image data in accordance with the shape of the edge judged by the edge judgment unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus, an image forming method and a program, which make a density correction for realizing a stable reproducibility of an image in pseudo high-resolution processing.

2. Description of the Related Art

In recent years, progress of a high-quality image in a printer is remarkable, and a high-resolution processing of an engine, a high speed of the processing unit accompanying the high-resolution processing and an increase in memory capacity have rapidly been progressed. However, since enormous costs are required for meeting all of them, several methods are at present proposed for achieving the high-quality image or the high speed and the low costs all together.

An example of a method conventionally performed in a printer of an electronic photo system includes a method where in a low-resolution printer engine, low-resolution image data are exposed on a photo conductor so as to overlap between dot pitches of each pixel (for example, refer to Japanese Patent Laid-Open No. H04-336859(1992). In consequence, latent images are formed so that the overlapped portions between the pixels become also effective pixels. This is called spot multiplexing for reproducing an image with a pseudo higher resolution than an actual resolution.

In the above conventional technology (for example, refer to Japanese Patent Laid-Open No. H04-336859(1992), rendering is made to a high-resolution image data, various types of image processing are performed to the high-resolution image data as it is, and thereafter, it is required to generate and print the low-resolution image data for printing. Therefore, an example of a method of realizing the spot multiplexing at low costs includes a method in which a high-resolution image data is converted into a low-resolution image data and thereafter, various types of image processing are performed to the low-resolution image data, and a pseudo high-resolution image data is reproduced using the spot multiplexing (for example, refer to Japanese Patent Laid-Open No. 2004-201283).

However, since the above spot multiplexing forms the latent image from the overlapping of the two adjacent exposure portions for reproducing one dot, there is a problem that the dot reproduction is unstable and is difficult to control as compared to the usual processing. In particular, it is difficult to reproduce a small character or a thin line, which may be reproduced in such a manner as to be blurred with a light density at best. Therefore, the reproduction stabilization has been achieved by making the density correction so as to increase the density of the character or the whole line, but the spot multiplexing has also an issue that the color appearance of the character or the line largely changes.

SUMMARY OF THE INVENTION

For solving the above problems, an image forming apparatus according to the present invention comprises a resolution converting unit configured to convert a high-resolution image data into a low-resolution image data, an edge judgment unit configured to judge a shape of an edge in the high-resolution image data, and a density correcting unit configured to make a density correction in the low-resolution image data in accordance with the shape of the edge judged by the edge judgment unit.

According to the present invention, since the density can be controlled locally to the edge of the low-resolution image, the density can be stably reproduced to the edge of the low-resolution image converted from the high-resolution image.

Further, since only the edge is defined as an object for the density correction as compared to the conventional method of making the density correction in such a manner as to increase the density of the character or the whole line, the density correction can be made to the edge which possibly disappears due to the resolution conversion without changing the color appearance of the character or the whole line.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, best modes for carrying out the present invention will be explained with reference to the accompanying drawings.

FIG. 1is a schematic block diagram showing an image forming apparatus, and a block diagram showing a digital complex machine having general functions of copying, printing, faxing, and so on.

An image forming apparatus10in a first embodiment shown inFIG. 1includes a scanner unit101for performing manuscript reading processing and a controller102.

Here, an image processing unit301for performing image processing to an image data read from the scanner unit101and a memory105for storing data are stored in the controller102.

The image forming apparatus10further includes an operation unit104for setting various printing conditions to the image data read from the scanner unit101.

The image forming apparatus10includes a printing unit103for performing image formation of visualizing the image data read from the memory105on a print sheet according to a print setting condition set by the operation unit104.

The image forming apparatus10is connected through a network106to a server107for managing the image data and a personal computer (PC)108for instructing execution of printing to the image forming apparatus10.

Here, apparatuses other than the image forming apparatus10, the server107and the PC108may be connected to the network106.

InFIG. 1, the scanner unit101, the printing unit103, the operation unit104and the network106are connected to the controller102.

FIG. 2is a cross section showing the image forming apparatus10. By referring toFIG. 2, the image forming apparatus10inFIG. 1will be in more detail explained.

The image forming apparatus10has the functions of copying, printing and faxing. As shown inFIG. 2, the image forming apparatus10in the first embodiment includes the scanner unit101, a document feeding (DF) unit202and the printing unit103.

First, a reading operation of an image performed mainly by the scanner unit101will be explained. In a case of setting a manuscript on a manuscript table207inFIG. 2to read the manuscript, the manuscript is set on the manuscript table207and the DF202is closed. Thereafter, an opening/closing sensor224detects that the manuscript table207has been closed, and optical reflection type manuscript-size detecting sensors226to230accommodated in a casing of the scanner unit101detect a size of the manuscript set on the manuscript table207. A light source210starts to illuminate the manuscript based upon the size detection and a CCD (charge-coupled device)231receives reflected light through a reflecting plate211and a lens212from the manuscript to read an image. The controller102in the image forming apparatus10converts the image data read by the CCD231into a digital signal, performing image processing for scanning, and stores the image as image data for printing in the memory105in the controller102.

In a case of setting a manuscript on the DF202to read the manuscript, the manuscript is so arranged as to face up on a tray of a manuscript setting unit203in the DF202. Thereafter, a manuscript detecting sensor204detects that the manuscript is set, and a sheet feeding roller205and a carrier belt206rotate in response to this detection to carry the manuscript, which is thereafter set at a predetermined position on the manuscript table207. Subsequently, the image data is read in the same way as in a case where the image is read on the manuscript table207and the obtained image data for printing is stored in the memory105in the controller102.

Completion of the image reading causes the carrier belt206to rotate once more and feed the manuscript to a right side in the cross section of the image forming apparatus inFIG. 2, and the manuscript is discharged to a manuscript discharging tray209via a carrier roller208in the discharged side. In a case where a plurality of manuscripts exist, the manuscript is discharged and carried to the right side in the cross section of the image forming apparatus from the manuscript table207and at the same time, the next manuscript is fed from the left side in the cross section of the image forming apparatus via the sheet feeding roller205, thus continuously performing the reading of the next manuscript. The operation of the scanner unit101is performed as described above.

Next, a printing operation performed mainly at the printing unit103will be explained. The image data for printing once stored in the memory105in the controller102is once more subject to image processing for printing, which will be described later, in the controller102and thereafter, is transferred to the printing unit103. In the printing unit103, the image data is converted into a pulse signal by PWM control in the printing unit103and at a laser printing unit, is converted into print laser light of four colors composed of yellow, magenta, cyan, and black. The print laser light illuminates a photosensitive element214of each color to form an electrostatic latent image on each photosensitive element214. The printing unit103performs a toner phenomenon to each photosensitive element by toner supplied from a toner cartridge215and a toner image visualized on each photosensitive element is primarily transferred on an intermediate transfer belt219. The intermediate transfer belt219rotates in a clockwise orientation inFIG. 2. When a print paper fed from a sheet cassette216through a sheet feeding carrier path217arrives at a secondary transfer position218, the toner image is transferred from the intermediate transfer belt219onto the print paper.

At a fixing container220, toner is fixed by pressurizing and heating on the print paper on which the image is transferred, and the print paper is carried to the sheet delivery carrier path. Thereafter, the print paper is discharged to a center tray221at a face downward or a side tray222at a face upward. A flapper223changes the carrier path for changing these sheet discharging openings. In a case of double-faced printing, after the print paper passes through the fixing container220, the flapper223changes the carrier path and thereafter, the print paper is switched back in a downward orientation, and the print paper is fed through a paper carrier path225for double-faced printing at the secondary transfer position218once more, thus performing the double-faced printing.

Next, the aforementioned image processing for printing will be explained with reference toFIG. 3.

FIG. 3is a block diagram showing the image processing unit301.

The image processing unit301inFIG. 3performs image processing for printing in the controller102inFIG. 1. Here, the image data for printing once printed in the memory105in the controller102is data of eight bits and has the color number of 256 gradations per one pixel. Pseudo high-resolution converting processing to be described later is performed to the image data for printing input from the memory105at a pseudo high-resolution converting processing unit302, thereby converting the image data with a resolution of 1200 dpi into that with resolution of 600 dpi. Thereafter, a gamma correction is made in a gamma correcting unit303, and in the half tone processing unit304, the image data for printing of eight bits is converted into that of four bits printable in the printing unit103, which is thereafter delivered to the printing unit103.

A CPU306inFIG. 3controls an operation of the entire image processing unit301based upon a control program stored in a ROM305. A RAM307is used as an operational region of the CPU306. In the RAM307, besides, a multiply accumulation calculating coefficient to be described later, edge patterns for edge judgment and edge correction tables used for a density correction of the edge are recorded.

Next, the processing of the pseudo high-resolution converting processing unit302inFIG. 3will be in detail explained with reference toFIGS. 3 to 6.

FIG. 4is a block diagram showing an edge correcting unit310inFIG. 3.FIG. 5is a flow chart showing the pseudo high-resolution converting processing in the pseudo high-resolution converting processing unit302inFIG. 3.FIG. 6is a diagram showing a relation between an image data for printing and a processing rectangle in the pseudo high-resolution converting processing.

First, at step S501inFIG. 5, a multiply accumulation calculating processing unit309inFIG. 3performs the multiply accumulation calculating processing to be described later. A processing rectangle having nine pixels with a sum of a width of 3 pixels and a height of 3 pixels used in the multiply accumulation calculating processing is input from the image data for printing of 1200 dpi corresponding to three lines delayed by two lines at a FIFO memory308to the multiply accumulation calculating processing unit309. The image data for printing of 600 dpi is outputted by one pixel from the multiply accumulation calculating processing unit309by the multiply accumulation calculating processing.

FIG. 6shows a relation between an image data601for printing of 1200 dpi and a processing rectangle604composed of nine pixels around a pixel of interest603. Since the pseudo high-resolution converting processing in the present embodiment is the converting processing from 1200 dpi into 600 dpi, the processing rectangle604is sequentially generated to the image data601for printing of 1200 dpi in such a manner that the pixel of interest603corresponds to positions602which are spaced by an interval of one pixel longitudinally or laterally.

Next, at step S502inFIG. 5, a binarization processing unit401in the edge correcting unit310shown inFIG. 4processes the processing rectangle604of 1200 dpi to be binary. The binarization processing converts all values of the nine pixels in the processing rectangle604inFIG. 6into numeral 1 (black pixel) when the value is larger than a binarization threshold value and into numeral 0 (white pixel) when the value is equal to or smaller than the binarization threshold value. In the present embodiment, the binarization threshold value is made to numeral 0 as one example.

Next, at step S503inFIG. 5, the edge judgment unit402in the edge correcting unit310shown inFIG. 4performs edge judgment processing to be described later. The edge judgment unit402judges whether or not the result of the binarization of the rectangle region604binarized by the binarization processing at step S502corresponds to an edge pattern to be described later.

Next, in a case where the edge judgment processing at step S503judges that the edge pattern corresponding to the binarization result exists, at step S504it is judged that the processing rectangle604inFIG. 6is an edge, and the process goes to step S505. In a case where the edge judgment processing at step S503judges that the edge pattern corresponding to the binarization result does not exist, at step S504it is judged that the processing rectangle604is not the edge, and one pixel of 600 dpi found at step S501is outputted without performing density correcting processing to be described later.

Finally, at step S506, the density correcting unit403in the edge correcting unit310performs the density correcting processing to the one pixel of 600 dpi found at step S501by using the edge correction table determined at step S505, and thereafter, outputs the image data. A detail of the density correcting processing and the edge correction table will be explained later.

Next, by referring toFIGS. 6 to 8, a detail of the multiply accumulation calculating processing performed at the multiply accumulation calculating processing unit309ofFIG. 3will be explained.

FIG. 7is a diagram showing a relation between a processing rectangle and a multiply accumulation calculating coefficient in the multiply accumulation calculating processing. As described above, the processing rectangle604inFIG. 6input to the multiply accumulation calculating processing unit309inFIG. 3is constructed of a sum of nine pixels around the pixel of interest603. The multiply accumulation calculating coefficient701inFIG. 7has nine values a to i corresponding to the respective nine pixels contained in the processing rectangle604. When coordinates of the pixel of interest603inFIG. 7are made of (j, i) and a value of a pixel is I (j, i), the result OUT of the multiply accumulation calculating processing is found according to the following formula.
OUT=(I(j−1,i−1)×a+I(j−1,i)×b+I(j−1,i+1)×c+I(j, i−1)×d+I(j, i)×e+I(j, i+1)×f+I(j+1,i−1)×g+I(j+1,i)×h+I(j+1,i+1)×i)>>6

In this calculation, a product of each pixel in the processing rectangle604inFIG. 7and a value of the multiply accumulation calculating coefficient701corresponding to the coordinates of the pixel is found, and the products corresponding to nine pixels are summed up, which are shifted right by six bits. This bit shift means that the summed value of the nine pixels is divided by 64. A sum of a to i of the multiply accumulation calculating coefficient701inFIG. 7is set so as to be 64. A multiply accumulation calculating coefficient801inFIG. 8is an example in regard to values a to i of the multiply accumulation calculating coefficient701in the present embodiment, and a sum of the multiply accumulation calculating coefficient801is 64 as described above.

Here, in the above multiply accumulation calculating processing, a sum of the products is divided by 64, but not limited thereto and for example, a sum of the products by the processing rectangle604may be divided by a sum of a to i of the multiply accumulation calculating coefficient701inFIG. 7. a to i of the multiply accumulation calculating coefficient701inFIG. 7are made to a decimal figure, and the sum is made to one. In consequence, it is required only to find a sum of the products by the processing rectangle604.

Next, by referring toFIGS. 9 to 11, a detail of the edge judgment processing performed at the edge judgment unit402in the edge correcting unit310shown inFIG. 4, the edge patterns used in the edge judgment processing, and the edge correction tables used in the density correcting unit403will be explained.

FIG. 9is an example showing edge patterns in the present embodiment.FIG. 10is an example showing edge correction tables associated with the edge patterns according to the present embodiment.FIG. 11is a diagram showing an input/output characteristic in the edge correction table with a graph of two axes.

An edge pattern901inFIG. 9is a pattern called the edge number of 0 in which only one pixel of nine pixels in the upper right corner has numeral 1 and each pixel other than that has numeral 0. The edge pattern901inFIG. 9is made associated with an edge correction table1001inFIG. 10and is stored in the RAM307inFIG. 3. Likewise, an edge pattern902inFIG. 9called the edge number of 1 is made associated with an edge correction table1002inFIG. 10and is stored in the RAM307inFIG. 3. An edge pattern903inFIG. 9called the edge number of 2 is made associated with an edge correction table1003inFIG. 10and is stored in the RAM307inFIG. 3. Likewise, in the present embodiment, the edge patterns901to916corresponding to the edge numbers0to15inFIG. 9and the edge correction tables1001to1016inFIG. 10are made associated with each other and are stored in the RAM307inFIG. 3.

In the edge judgment processing at step S503inFIG. 5, the processing rectangle604which is binarized to numeral 1 or 0 by the binarization processing unit401inFIG. 4and all of the edge patterns901to916inFIG. 9are compared. It is judged whether or not each of the edge patterns901to916inFIG. 9corresponds to the binarized processing rectangle604. When any of the edge patterns corresponding to the processing rectangle exists, the edge number of the corresponding edge pattern is outputted to the density correcting unit403inFIG. 4.

FIG. 11is a graph showing input/output characteristics in the edge correction tables1001to1016inFIG. 10.

The input/output characteristic in the edge correction table1001inFIG. 10is shown in a graph1101inFIG. 11. The input/output characteristic in the edge correction table1002inFIG. 10is shown in a graph1102inFIG. 11. The input/output characteristic in the edge correction table1003inFIG. 10is shown in a graph1103inFIG. 11. The input/output characteristic in the edge correction table1004inFIG. 10is shown in a graph1104inFIG. 11.

For example, in the graph1101inFIG. 11showing the input/output characteristic of the edge correction table1001inFIG. 10, an output value minus an input value is larger than in any of the graphs1102to1104showing the other input/output characteristics. Since this relation allows one pixel of 600 dpi to which the edge correction table1001inFIG. 10is applied to largely increase the value in the density correcting processing performed at the density correcting unit403inFIG. 4, the edge density is strongly corrected, thus enabling the stable density reproduction.

On the other hand, in the graph1104inFIG. 11showing the input/output characteristic in the edge correction table1004inFIG. 10made associated with the edge pattern904inFIG. 9, the input value is nearly equal to the output value. Therefore, it can be said that the density correction is not made in the density correcting unit403inFIG. 4.

In the graphs1102and1103inFIG. 11showing the input/output characteristics in the edge correction tables1002and1003inFIG. 10made associated with the edge patterns902and903inFIG. 9, an intermediate density correction between the two edge correction tables is made. That is, in the edge pattern in which the numbers of numeral 1 are small (the numbers of black pixel are small), the pixel is converted into one pixel of 600 dpi having a very thin density by the multiply accumulation calculating processing to make the density reproduction unstable, therefore correcting the edge density strongly. In reverse, in the edge pattern in which the numbers of numeral 1 are the larger (the numbers of black pixel are larger), the pixel is converted into one pixel of 600 dpi having the stronger density by the multiply accumulation calculating processing, therefore correcting the edge density lightly.

In the edge correction tables1001to1016inFIG. 10, the density correction is made in consideration of characteristics of orientation in the edge patterns901to916inFIG. 9.

Since numeral 1 concentrates on the right side in the edge patterns901to904inFIG. 9, the edge patterns901to904form a pattern group for detecting edges in a specific orientation mainly at the left side.

Likewise, the edge patterns905to908ofFIG. 9form a pattern group for detecting edges in a specific orientation mainly at the lower side, the edge patterns909to912form a pattern group for detecting edges in a specific orientation mainly at the right side, and the edge patterns913to916form a pattern group for detecting edges in a specific orientation mainly at the upper side.

InFIG. 10, the edge correction tables1013to1016in the pattern group for detecting the edges at the upper side controls the density correction in accordance with the numbers of numeral 1 (the numbers of black pixel) in the same way as the edge correction tables1001to1004in the pattern group for detecting the edges at the left side.

On the other hand, the edge correction tables1005to1012inFIG. 10in the pattern group in the edge patterns905to912inFIG. 9for detecting the edges in the other orientation do not make the density correction in accordance with the numbers of numeral 1 (the numbers of black pixel) in the edge pattern, as understood from the graphs1105to1112inFIG. 11showing the input/output characteristics.

Making the density correction substantially equally in the edges in all orientations including every specific orientation allows stable density reproduction and in reverse, prevents a color appearance of a thin line or a thin character from largely changing after printing or a thin line or a thin character from becoming a thick line or a thick character after printing.

FIG. 23is a diagram showing an example of an image data601for printing of 1200 dpi. A line having a width of three pixels is drawn in a center of the image data for printing601inFIG. 23.

FIGS. 24A,24B and24C are examples showing the result of a pseudo high-resolution converting processing to the image data for printing inFIG. 23.FIG. 24Ashows an image data of 600 dpi immediately after the multiply accumulation calculating processing is performed.FIG. 24Bshows an image data of 600 dpi after the edge correcting processing is performed using the edge pattern and the edge correction table in the present embodiment.FIG. 24Cis an image data of 600 dpi in a case where the density correction is made equally in strength to the edges in all orientations not considering the characteristic of orientation of the edge. It is to be understood that inFIG. 24C, thick lines are reproduced as each having a thicker density than inFIG. 24AorFIG. 24B.

It should be noted that in the explanation of the present embodiment, the density correction is made only to the edges at the upper side and at the left side, but the present invention is not limited thereto, and a lighter density correction may be made to the edges, for example, at the lower side and at the right side than at the upper side and at the left side. Further, for example, the edges in the other orientation, such as the edges only at the lower side and at the right side may be corrected strongly in density. The edge pattern also is not limited to the pattern described in the present embodiment.

As explained above, according to the first embodiment, the edge alone is set as an object for control, and it is possible to locally control the edge density in accordance with a shape of the edge (edge pattern). Therefore, it is possible to stabilize the spot multiplexing to reproduce the image without changing the color appearance of the character or the entire line.

In the first embodiment, the pseudo high-resolution converting processing by the processing rectangle having a width of three pixels and a height of three pixels is explained in the multiply accumulation calculating processing unit309and the edge correcting unit310inFIG. 3.

In the present embodiment, pseudo high-resolution converting processing by a processing rectangle having a width of two pixels and a height of two pixels will be in detail explained with reference to the following drawings. It should be noted that since components other than the pseudo high-resolution converting processing unit302are identical to those in the first embodiment, an explanation for the components is omitted.

By referring toFIG. 5, andFIGS. 12 to 14, the processing of the pseudo high-resolution converting processing unit302inFIG. 12will be in detail explained.

FIG. 12is a block diagram showing an image processing unit301in the second embodiment.

FIG. 13is a block diagram showing an edge correcting unit1203.

FIG. 14is a diagram showing a relation between an image data for printing and a processing rectangle in the pseudo high-resolution converting processing.

First, in the second embodiment, at step S501inFIG. 5, the multiply accumulation calculating processing unit1202inFIG. 12performs multiply accumulation calculating processing to be described later. The processing rectangle formed of four pixels with a sum of a width of two pixels and a height of two pixels in use for the multiply accumulation calculating processing is input to the multiply accumulation calculating processing unit1202inFIG. 12from the image data for printing of 1200 dpi corresponding to a two-line amount delayed by one-line amount at a FIFO memory1201. According to the multiply accumulation calculating processing, the image data for printing of 600 dpi is outputted by one pixel from the multiply accumulation calculating processing unit1202.

FIG. 14shows a relation between an image data601for printing of 1200 dpi and a processing rectangle1404composed of four pixels around a pixel of interest1403. The pseudo high-resolution converting processing in the present embodiment is converting processing from the image data of 1200 dpi into the image data of 600dpi. Therefore, the processing rectangle1404inFIG. 14is sequentially generated to the image data601for printing of 1200 dpi in such a manner that the pixel of interest1403corresponds to positions602which are spaced by an interval of one pixel longitudinally or laterally.

Next, at step S502inFIG. 5, a binarization processing unit1301in the edge correcting unit1203shown inFIG. 13processes the processing rectangle1404of 1200 dpi inFIG. 14to be binary. The binarization processing converts all values of four pixels in the processing rectangle1404inFIG. 14into numeral 1 when the value is larger than a predetermined binarization threshold value and into numeral 0 when the value is equal to or smaller than the binarization threshold value. In the present embodiment, the binarization threshold value is made to numeral 0 as one example.

Next, at step S503inFIG. 5, an edge judgment unit1302in the edge correcting unit1203shown inFIG. 13performs edge judgment processing to be described later. The edge judgment unit1302inFIG. 13judges whether or not the result of the binarization of the rectangle region1404inFIG. 14binarized by the binarization processing corresponds to an edge pattern to be described later.

Next, in a case where the edge judgment processing at step S503judges that the edge pattern corresponding to the binarization result exists, at step S504it is judged that the processing rectangle1404inFIG. 14is an edge, and the process goes to step S505.

In a case where the edge judgment processing at step S503judges that the edge pattern corresponding to the binarization result does not exist, at step S504it is judged that the processing rectangle1404is not the edge, and one pixel of 600 dpi found at step S501is outputted without performing the density correcting processing to be described later.

Finally, at step S506, the density correcting unit403in the edge correcting unit1203inFIG. 12performs the density correcting processing to the one pixel of 600 dpi found at step S501by using the edge correction table determined at step S505, and thereafter, outputs the image data. A detail of the density correcting processing and the edge correction table will be explained later.

Next, by referring toFIGS. 14 to 16, the multiply accumulation calculating processing performed at the multiply accumulation calculating processing unit1202inFIG. 12will be explained.

FIG. 15is a diagram showing a relation between a processing rectangle and a multiply accumulation calculating coefficient in the multiply accumulation calculating processing. As described above, the processing rectangle1404inFIG. 15input to the multiply accumulation calculating processing unit1202inFIG. 12is constructed of a sum of four pixels around the pixel of interest1403. The multiply accumulation calculating coefficient1501inFIG. 15has four values a to d corresponding to the respective four pixels constituting the processing rectangle1404. Coordinates of the pixel of interest1403inFIG. 15are made of (j, i). When a value of a pixel is I (j, i), the result OUT of the multiply accumulation calculating processing is found according to the following formula.
OUT=(I(j, i)×a+I(j, i+1)×b+I(j+1,i)×c+I(j+1,i+1)×d)>>6

In this calculation, a product of each pixel in the processing rectangle1404inFIG. 15and a value of the multiply accumulation calculating coefficient1501corresponding to the coordinates of the pixel is found, and the products corresponding to four pixels are summed up, which are shifted right by six bits. This hit shift means that a sum of four pixels is divided by 64. A sum of a to d of the multiply accumulation calculating coefficient1501inFIG. 15is set so as to be 64. A multiply accumulation calculating coefficient1601inFIG. 16is an example of values a to d of the multiply accumulation calculating coefficient1501in the present embodiment, and a sum of the multiply accumulation calculating coefficient1601is 64 as described above.

Here, in the above multiply accumulation calculating processing, a sum of the products is divided by 64, but the present invention is not limited thereto. For example, a sum of the products by the processing rectangle1404may be divided by a sum of a to d of the multiply accumulation calculating coefficient1501inFIG. 15. a to d of the multiply accumulation calculating coefficient1501inFIG. 15are made to a decimal figure, and the sum is made to numeral 1. In consequence, it is required only to find a sum of the products by the processing rectangle1404.

Next, by referring toFIGS. 17 to 19, a detail of the edge judgment processing performed at the edge judgment unit1302of the edge correcting unit1203shown inFIG. 13, the edge pattern used in the edge correcting processing, and the edge correction table used in the density correcting unit403will be explained.

FIG. 17is an example showing edge patterns in the present embodiment.FIG. 18is an example showing edge correction tables made associated with the edge patterns according to the present embodiment.

FIG. 19is a diagram showing an input/output characteristic in the edge correction table with a graph of two axes.

An edge pattern1701inFIG. 17is a pattern called the edge number of 0 in which only one pixel in the upper right corner in four pixels has numeral 1 and each pixel other than that has numeral 0. The edge pattern1701inFIG. 17is made associated with an edge correction table1801inFIG. 18and is stored in the RAM307inFIG. 12.

Likewise, an edge pattern1702inFIG. 17called the edge number of 1 is made associated with an edge correction table1802inFIG. 18and is stored in the RAM307inFIG. 12. Likewise, an edge pattern1703called the edge number of 2 inFIG. 17is made associated with an edge correction table1803inFIG. 18and is stored in the RAM307inFIG. 12.

In this way, in the present embodiment, the edge patterns1701to1712corresponding to the edge numbers0to11inFIG. 17and the edge correction tables1801to1812inFIG. 18inFIG. 18are made associated with each other and are stored in the RAM307inFIG. 12.

In the edge judgment processing at step S503inFIG. 5, the processing rectangle1404inFIG. 14which is binarized to numeral 1 or 0 and all of the edge patterns1701to1712inFIG. 17are compared by the binarization processing unit1301inFIG. 13. The binarization processing unit1301inFIG. 13judges whether or not each of the edge patterns1701to1712inFIG. 17corresponds to the binarized processing rectangle1404inFIG. 14. When any of the edge patterns corresponding to the processing rectangle exists, the edge number of the corresponding edge pattern is outputted to the density correcting unit403.

FIG. 19is a graph showing input/output characteristics in the edge correction tables1801to1812inFIG. 18.

The input/output characteristic in the edge correction table1801inFIG. 18is shown in a graph1901inFIG. 19. The input/output characteristic in the edge correction table1802inFIG. 18is shown in a graph1902inFIG. 19. The input/output characteristic in the edge correction table1803inFIG. 18is shown in a graph1903inFIG. 19.

For example, in the graph1901showing the input/output characteristic of the edge correction table1801inFIG. 18, an output value minus an input value is larger than in each of the graphs1902and1903showing the other input/output characteristics.

Thereby, the following effect is produced. In the same way as in the first embodiment, in the density correcting processing performed at the density correcting unit403inFIG. 13, one pixel of 600 dpi to which the edge correction table1801inFIG. 18is applied results in a large increase in value. Therefore, one pixel of 600 dpi is strongly corrected in density, thus enabling stable density reproduction.

When the edge correction tables1802and1803inFIG. 18made associated with the edge patterns1702and1703inFIG. 17are applied to one pixel, the pixel is corrected more lightly in density than in a case of the edge correction table1801. That is, in the edge pattern in which the numbers of numeral 1 are small, the one pixel is converted into one pixel of 600 dpi having a very thin density by the multiply accumulation calculating processing to make the density reproduction unstable, therefore correcting the density strongly. In reverse, in the edge pattern in which the numbers of numeral 1 are the larger, one pixel is converted into one pixel of 600 dpi having the stronger density by the multiply accumulation calculating processing, therefore correcting the density lightly.

In the edge correction tables1801to1812inFIG. 18, the density correction is made in consideration of characteristics of orientation in the edge patterns1701to1712inFIG. 17.

Since numeral 1 concentrates on the right side in the edge patterns1701to1703inFIG. 17, the edge patterns1701to1703form a pattern group for detecting edges in a specific orientation mainly at the left side.

Likewise, the edge patterns1704to1706inFIG. 17form a pattern group for detecting edges in a specific orientation mainly at the lower side, the edge patterns1707to1709form a pattern group for detecting edges in a specific orientation mainly at the right side and the edge patterns1710to1712form a pattern group for detecting edges in a specific orientation mainly at the upper side.

InFIG. 18, the edge correction tables1810to1812in the pattern group for detecting the edges at the upper side controls the density correction in accordance with the numbers of numeral 1 in the same way as the edge correction tables1801to1803in the pattern group for detecting the edges at the left side.

On the other hand, the edge correction tables1804to1809inFIG. 18in the pattern group in the edge patterns1704to1709inFIG. 17for detecting the edges in the other orientation do not make the density correction in accordance with the numbers of numeral 1 in the edge pattern as understood from the graphs1904to1909showing the input/output characteristics.

Thereby, in the same way as in the first embodiment, making the density correction substantially equal in the edges in all orientations including every specific orientation prevents a color appearance of a thin line or a thin character from largely changing after printing or a thin line or a thin character from becoming a thick line or a thick character after printing.

It should be noted that in the explanation of the present embodiment, the density correction is made only to the edges at the upper side and at the left side, but the present invention is not limited thereto, and a lighter-density correction may be made to the edges, for example, at the lower side and at the right side than at the upper side and at the left side. Further, for example, the edges in the other orientation, such as the edges only at the lower side and at the right side may be corrected strongly in density. The edge pattern also is not limited to the pattern described in the present embodiment.

As explained above, according to the second embodiment, also in a case where the pseudo high-resolution converting processing is performed in the processing rectangle having a height of two pixels and a width of two pixels, it is possible to stabilize the spot multiplexing for reproducing the image.

In the first embodiment and the second embodiment, the edge patterns used in the edge correcting unit310inFIG. 3and in the edge correcting unit1203inFIG. 12are made associated with the edge correction tables used in the density correcting units403inFIGS. 4 and 13with a relation of one to one. In addition, the edge pattern and the edge correction table are stored at the associated state in the RAMs307inFIGS. 3 and 12and are used.

In the present embodiment, a method of, for cutting down on each storage region of the RAMs307inFIGS. 3 and 12, associating the same edge correction table with a plurality of edge patterns to share the edge correction table will be hereinafter explained based on the first embodiment with reference to the drawings in detail.

It should be noted that since components other than the density correcting unit403in the present embodiment are identical to those in the first embodiment, an explanation of the identical components is omitted.

Next, by referring toFIG. 9, andFIGS. 20 to 22, the edge judgment processing performed at the edge judgment unit402of the edge correcting unit310shown inFIG. 4and the density correcting processing performed at the density correcting unit403inFIG. 4will be explained. The edge pattern used at the edge judgment unit402inFIG. 4and the edge correction table used in the density correcting unit403will be in detail explained.

FIG. 20is a diagram showing an example of edge correction tables according to the third embodiment.

FIG. 21is a diagram showing an example of an input/output characteristic in each of edge correction tables with a graph of two axes.

FIG. 22is a diagram showing an example of an edge table for associating edge patterns with edge correction tables.

In the present embodiment, only four types of the edge correction tables2001to2004inFIG. 20to 16 types of the edge patterns901to916inFIG. 9are stored in the RAM307inFIG. 3.

The respective input/output characteristics of the edge correction tables2001to2004inFIG. 20are shown in the graphs2101to2104.

The edge patterns901to916inFIG. 9respectively have the edge numbers of 0 to 15. On the other hand, the edge correction tables2001to2004inFIG. 20respectively have the edge correction table numbers of 0 to 3. This respect differs from the first embodiment and the second embodiment.

The edge patterns901to916inFIG. 9are made associated with the edge correction tables2001to2004inFIG. 2010by an edge table2201inFIG. 22stored in the RAM307inFIG. 3in the same way as the edge patterns and the edge correction tables.

The edge judgment unit402inFIG. 4compares the processing rectangle604which is binarized to numeral 1 or 0 by the binarization processing unit401with all of the edge patterns901to916inFIG. 9to judge whether or not each of the edge patterns901to916inFIG. 9corresponds to the binarized processing rectangle604inFIG. 6. When any of the edge patterns corresponding to the processing rectangle exists, the edge number of the edge pattern is outputted to the density correcting unit403inFIG. 4.

The density correcting unit403inFIG. 4obtains an edge correction table number from an edge table2201inFIG. 22based upon an edge number outputted from the edge judgment unit402. Further, the density correcting unit403inFIG. 4makes a density correction of the image data of one pixel of 600 dpi outputted from the multiply accumulation calculating processing unit309inFIG. 3using the edge correction table corresponding to the obtained edge correction table number.

As explained above, the third embodiment uses the edge table for associating the edge pattern with the edge correction table. Thereby, the third embodiment can obtain the same effect as the first embodiment and the second embodiment while cutting down on a memory amount for storing the edge density table.

The present invention may adopt an embodiment of a system, a apparatus, a method, a program, a memory medium or the like. Specially the present invention may be applied to a system constructed of a plurality of units or a device constructed of a single unit.

The present invention supplies a program of software realizing the function in the aforementioned embodiment (program corresponding to the flow chart shown in the figure in the embodiment) to the system or the device directly or from a remote location. Further, the present invention also includes a case of realizing the function of the aforementioned embodiment by reading and performing the supplied program code by a computer equipped in the system or the device.

Accordingly, the computer program itself installed in the computer for realizing the functional processing in the present invention with the computer also realizes the present invention. That is, the present invention includes also the computer program itself for realizing the functional processing in the present invention.

In this case, if equipped with the function of the program, a form such as an object code, a program performed by an interpreter, and a script data supplied to an OS may be adopted.

The print medium supplying the program includes, for example, a floppy (trade mark) disc, a hard disc, and an optical disc. Further, the print medium includes an optical magnetic disc, a MO, a CD-ROM, a CD-R, a CD-RW, a magnetic tape, an involatile memory card, a ROM, a DVD (DVD-ROM, or DVR-R) and the like.

Besides, a supply method of the program includes connecting the program to a home page in the Internet using a browser of a client computer. The program may be also supplied by downloading the computer program in the present invention from the home page to be connected or a file which is compressed and includes an automatic installation function to a print medium such as a hard disc. The supply method may be realized by dividing the program code constituting the program in the present invention into a plurality of tiles and downloading each file from different home pages. That is, the present invention includes also a WWW server for downloading the program file for realizing the functional processing in the present invention with a computer to a plurality of users.

The program in the present invention is encrypted, which is stored in a memory medium such as a CD-ROM, and the memory medium is distributed to a user. Key information for breaking the encryption is downloaded to a user meeting a predetermined condition from a home page through the Internet. The key information is used to perform the encrypted program, which is installed in the computer, realizing the function of the aforementioned embodiment.

The function in the aforementioned embodiment can be realized by performing the program read by the computer. The function in the aforementioned embodiment can be realized by performing all or a part of the actual processing with an OS working on a computer based upon an instruction of the computer program.

Further, the computer program read from the print medium is written in a memory equipped with a function expansion board inserted into the computer or a function expansion unit connected to the computer. Therefore, the function of the aforementioned embodiment can be realized also by performing all or a part of the actual processing with a CPU equipped in the function expansion board or the function expansion unit based upon an instruction of the computer program.

This application claims the benefit of Japanese Patent Application No. 2008-208597, filed Aug. 13, 2008, which is hereby incorporated by reference herein in its entirety.