JP2000115540A - Image processing unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reproduce more smoothly a character edge section by identifying the edge direction of a pixel of interest based on the difference of a density level among the pixel of interest and its surrounding pixels and setting the density level to a plurality of sub areas in the pixel of interest so as to change density distribution in each pixel of interest. SOLUTION: An edge discrimination section 12 uses gradation data outputted from an image data output device 10 obtain the gradation difference between a pixel of interest and its surrounding pixels so as to identify an edge direction in the main scanning direction. A density level control section 14 generates a parameter signal to control density gravity center in each pixel in the unit of sub pixels resulting from dividing the pixel of interest in the main scanning direction. Then a density level setting section 18 controls a density level as to data corrected by a gamma correction section 16 by using a density control parameter signal generated by the density level control section 14 to change the density gravity center in the pixel. Thus, the density is controlled in the unit of sub pixels to convert the density into a density level for print.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to processing of digital image data, and more particularly to edge processing of a character image.

[0002]

2. Description of the Related Art An image processing apparatus processes digital image data obtained by reading a document and outputs digital print data for printing. The image is reproduced based on the digital print data. The image processing apparatus performs various processes on digital image data obtained by reading the image of the document in order to better reproduce the image of the document. For a character document, it is desirable to emphasize the edges of the character in order to reproduce the character image. For this reason, various edge determination methods and data enhancement methods based on edge determination results have been proposed.

[0003]

When a character image contains an edge which is particularly oblique, if such an edge is emphasized, the obtained image becomes jagged. For this reason, the quality of the reproduced image is lower than that of the original image. This is because the image is read and reproduced in pixel units. However, it is desirable that the edge portion of the character be reproduced smoothly.

An object of the present invention is to provide an image processing apparatus capable of reproducing a character edge portion more smoothly.

[0005]

An image processing apparatus according to the present invention divides a pixel into a plurality of sub-regions in the main scanning direction based on multi-valued image data and forms an image in units of sub-regions. An image processing device that outputs digital image data. The image processing apparatus includes: an edge determining unit that identifies an edge direction of the target pixel from a difference in density level between the target pixel and peripheral pixels based on the multi-valued image data; Density control means for setting a density level in a plurality of sub-regions in the target pixel in accordance with the edge direction to change a density distribution in each target pixel. Here, in order to change the center of the density distribution of the target pixel, one pixel is divided into a plurality of sub-pixels in the main scanning direction, and the density distribution of the target pixel is controlled in sub-pixel units. Based on the edge direction identified by the edge determination unit, the density control unit can change the density given to the target pixel in sub-pixel units, and change the density distribution (including the density centroid) given to the target pixel. Thereby, the resolution of the pixel of interest is improved, and the character edge portion is smoothly reproduced. Further, the density control means includes density level control means for setting a density level setting parameter for each sub-region in the pixel of interest according to the edge direction of the pixel of interest determined by the edge determination means; Density level setting means for setting a density level for each of a plurality of sub-regions in the target pixel based on the density level of the target pixel using the density level setting parameters set by the means. The density level setting parameter is used to convert input multi-valued image data into image data for printing, and is set for each sub-pixel in a pixel. Further, the edge direction identified by the edge determination means includes an edge in any one of the main scanning directions and a thin line edge. In addition, the edge determination means includes:
If the density level of a pixel adjacent to the target pixel in the edge direction is higher than the threshold value, the determined edge direction is canceled.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An image processing apparatus according to an embodiment of the present invention will be described below with reference to the accompanying drawings. In addition,
In the drawings, identical reference symbols indicate identical or equivalent items. FIG. 1 shows an embodiment of the image processing apparatus of the present invention. This image processing apparatus, based on digital image data input from a document reading device, a computer, etc.,
By exposing the photoconductor, digital image data for forming an image on the photoconductor is generated and output to a printer. Here, a gradation difference between the target pixel and its surrounding pixels is obtained, and an edge direction in the main scanning direction is identified by a combination of the gradation differences. Then, the center of the density distribution in the target pixel is changed according to the identified edge direction, and the edge is smoothly reproduced. Here, in order to change the center of gravity of the density distribution of the pixel of interest, one pixel, which is a unit of image reading, is divided into a plurality of sub-pixels in the main scanning direction in image formation, and the density level given to the pixel of interest is To control the density distribution in the pixel of interest.

More specifically, the image data output device 10 outputs digital image data input from a document reading device, a computer, or the like (not shown). Here, the image data is output as 8-bit gradation data per pixel. The edge determination unit 12 uses the grayscale data output from the image data output device 10 to determine the grayscale difference between the target pixel and the peripheral pixels, and determines the grayscale difference in the main scanning direction by combining the grayscale differences. Identify the edge direction. Then, based on the determination result of the edge determination unit 12, the density level control unit 14 generates a parameter signal for controlling the density centroid in the pixel in units of sub-pixels obtained by dividing the target pixel in the main scanning direction. . On the other hand, the gamma correction unit 16 performs a non-linear conversion of the gradation data, and
2 is corrected. Next, the density level setting unit 18 controls the density level of the data corrected by the gamma correction unit 12 using the density control parameter signal generated by the density level control unit 16 to set the density centroid in the pixel. Change. In this way, the density is controlled in units of sub-pixels by the density level control unit 14 and the density level setting unit 18, and the data corrected by the gamma correction unit 16 is converted into a print density level (digital gradation data) in sub-pixel units. Is done. D / A converter 2
“0” converts the digital gradation data obtained by the density level setting unit 18 into an analog signal and outputs the analog signal to the laser drive circuit of the printing unit 22. The printing unit 22 modulates the intensity of the laser beam in units of sub-pixels based on the input signal, and forms a halftone image on a recording medium by raster scanning.

The edge judging section 12 judges the edge direction of the pixel of interest in the main scanning direction in the following four cases. Based on the result of the determination, it is determined in which direction the edge is to be shifted. Here, the main scanning direction is the left-right direction. The “right edge” refers to an edge on the right side of a character, that is, an edge when a character portion is on the left side of the pixel of interest. The “left edge” refers to an edge on the left side of a character, that is, an edge in the case where a character portion is on the right side of a target pixel. The “thin line edge” refers to an edge in the case where a character portion is located at the center of the pixel of interest, that is, a right edge and a left edge in one pixel of interest. If none of the above applies, it is a “non-edge portion”.

FIG. 2 is a block diagram of the edge determination unit 12. The edge is determined using, for example, a 3 × 3 pixel matrix. First, the tone difference between the target pixel and the eight surrounding pixels is calculated, and the target pixel is divided into pixels with higher density and pixels with lower density. As shown in FIG. 3, in the 3 × 3 pixel matrix, V33 represents the gradation data of the target pixel, and V22, V23, V24, V32, V34, V4
2, V43 and V44 represent gradation data of eight pixels adjacent to the target pixel. In the edge determination unit 12 shown in FIG. 2, the eight gradation difference signal generation circuits 120 include the gradation data V33 of the target pixel and the gradation data V2 of the eight surrounding pixels.
2, V23, V24, V32, V34, V42, V4
3, V44, and the difference (gradation difference signal) between the gradation data of the peripheral pixel and the target pixel is obtained. Combination determination circuit 122
Inputs a gradation difference between a target pixel and a peripheral pixel, and determines an edge direction based on a combination of the gradation differences. That is, the tone difference between the target pixel and the eight surrounding pixels is calculated,
The pixel is divided into a pixel having a higher density and a pixel having a lower density than the target pixel. Then, the edge direction is identified from the relationship between the density value of the target pixel and the surrounding pixels. Specifically, the combination determination circuit 1
Reference numeral 22 denotes an edge direction (right edge, left edge, etc.) in the main scanning direction based on a combination of these eight gradation difference signals.
Is determined, and the right-aligned signal MARKR and the left-aligned signal MARK are determined.
Generate L. The right alignment signal MARKR indicates that a right edge exists, and the left alignment signal MARKL indicates that a left edge exists. The logic circuit 124 including the NAND gate, the two AND gates, and the three selectors (selecting A when S = L) determines the edge direction from the right alignment signal MARKR and the left alignment signal MARKL, and determines the edge direction. The signal EDG is output. That is, M
If both ARKR and MARKL are output, ED
If G = “01” (fine line edge) is output and MARKR or MARKL is output, EDG = “03” (right edge) or EDG = “02” (left edge) is output, and MARKR and MARKL are output. If neither is output, EDG = "00" (non-edge portion) is output.

FIG. 4 is a block diagram of the density level control unit 14. An edge direction signal EDG output from the edge determination unit 12 is input as an address signal, and eight density control parameter signals A1, A2, A3, A4, B1, and B are obtained from a table stored in eight parameter RAMs 140.
2, B3 and B4 are obtained. The obtained density control parameters are sent to the density level setting unit 18.

FIG. 5 is a block diagram of the density level setting section 18. The four density level calculators 180 apply the gradation data VG nonlinearly converted by the gamma corrector 12 to the grayscale data VG.
Four combinations of density control parameter signals A1, B1, and A
2 and B2, A3 and B3, and A4 and B4, respectively, the primary operation as shown in the block (VH = A *
(VG-B)). As a result, four gradation signals VH1, VH2, VH3, and VH4 are obtained from VG. Next, the selector 182 uses the pixel clock CLK and the double-speed clock XCLK having a frequency twice as high as the pixel clock to output the four gradation signals VH1, VH2, and VH3 obtained by the primary operation in the density level operation unit 180. , VH4 to 1
The signal VD is generated by switching every sub-pixel within the pixel. Thus, one pixel is divided into four sub-pixels, and digital gradation data VD is output for each sub-pixel.

FIGS. 6 to 9 show how the density within one pixel changes for each of the edge types determined by the edge determination unit 12. FIG. This indicates whether the digital gradation data given to the four sub-pixels in one pixel changes as the gradation of the gradation data nonlinearly converted by the gamma correction unit 16 increases. In the figure, the height of the black portion in each sub-pixel indicates the density level. FIG. 6 shows the change for the right edge. Here, the density control parameter signal is as follows. A1 = A2 = A3 = A4 = 4. B1 = 0. B2
= 64. B3 = 128. B4 = 192. In the figure, the gradation levels are 0, 32, 64, 96, 128, 160, 19
2, 224 and 255 are shown. As is clear from the figure, the density is increased in order from the left sub-pixel because of the right edge. In this way, the center of gravity of the pixel density moves sequentially from the left to the center.

FIG. 7 shows a change in the case of the left edge. This is symmetric with the right edge in FIG. Here, the density control parameter signal is as follows. A1 = A
2 = A3 = A4 = 4. B1 = 192. B2 = 128. B
3 = 64. B4 = 0. The figure shows that the gradation levels are 0, 32, 6
4, 96, 128, 160, 192, 224, 255 are shown. As is clear from the figure, the density is increased in order from the right sub-pixel because of the left edge. In this way, the center of gravity of the density of the pixel sequentially moves from the right to the center.

FIG. 8 shows the change in the case of non-edge. Here, the density control parameter signal is as follows. A1
= A2 = A3 = A4 = 1. B1 = B2 = B3 = B4 =
0. In the figure, the gradation levels are 0, 32, 64, 96, 12
8, 160, 192, 224 and 255 are shown. As is apparent from the figure, since no edge exists, 4
Each of the sub-pixels has the same density, and thus the center of gravity of the pixel density is always at the center. The density increases corresponding to the gradation level.

FIG. 9 shows a change in the case of a thin line edge.
Here, the density control parameter signal is as follows.
A1 = A2 = A3 = A4 = 2. B1 = 128, B2 = B
3 = 0. B4 = 128. The figure shows that the gradation levels are 0, 32,
64, 96, 128, 160, 192, 224, 255
The case of is shown. Since the right edge and the left edge are thin line edges that exist at the same time, as is apparent from the figure, first, the density of the two central sub-pixels increases in accordance with the gradation level. The center of gravity of the pixel density is always at the center. next,
The densities of the two sub-pixels on both sides increase according to the gradation level. The center of gravity of the density of the pixel is always at the center, but the density distribution gradually expands to the left and right when the gradation level exceeds 128. 6 to 9, mainly the change in the center of gravity of the density has been described. However, as shown in FIG. 9, the density does not always increase from the end in the target pixel. Also,
What is changed is not only the center of gravity but also the concentration distribution.

FIG. 10 schematically shows what effect can be obtained when applied to an actual image. The image data read from the image reading device includes a shaded portion at an oblique line edge as shown on the left side. This edge is the left edge. Using this density as a density level, the edge direction is detected, and the density is controlled in units of sub-pixels to change the density centroid of the pixels in the edge portion. As a result, as shown on the right side, the resolution of the edge is increased, and an image in which the edge changes smoothly is obtained.

FIG. 11 shows a modification of the edge judging section.
If the density level of the pixel adjacent to the target pixel on the opposite side to the edge direction is higher than the threshold value, the density centroid is shifted to one side in the target pixel, and white spots may be conspicuous on the side without the density centroid. is there. The edge determination unit cancels the edge determination result in such a case, so that the density center of gravity in the target pixel is not changed. Here, the determination is made for each of the left and right. FIG. 11 is obtained by adding two comparators 146 and 148 and two AND gates 1410 and 1412 to the configuration shown in the left part of FIG. At the left edge, the comparator 146 compares the signal V32 of the left pixel of the target pixel with a threshold value, and the comparison result signal is input to the AND gate 1410 together with the left edge signal MARKL. Accordingly, when the signal V32 of the left pixel of the target pixel is larger than the threshold value, the left edge signal MARKL is not output, and the edge determination result is canceled. Also, the comparator 148 outputs
The signal V34 of the right pixel of the target pixel is compared with a threshold value,
The signal of the comparison result is input to the AND gate 1412 together with the right edge signal MARKR. Accordingly, when the signal V34 of the pixel on the right side of the target pixel is larger than the threshold value, the right edge signal MARKR is not output, and the edge determination result is canceled. Thus, if the density level of a pixel adjacent to the target pixel on the same side as the edge direction is higher than the threshold value, the determined edge direction is canceled. In this case, the density is controlled as the non-edge portion described above. In the example shown in FIG. 12, as shown on the left side, even if the central pixel is a pixel on the right edge, if the density level of the pixel on the right is higher than the threshold value, The edge determination result is canceled, and white spots in the center pixel are prevented.

[0018]

In the image processing apparatus, the character edge can be reproduced more smoothly by changing the density distribution in the target pixel.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of an image processing apparatus.

FIG. 2 is a block diagram of an edge determination unit.

FIG. 3 is a diagram showing a distribution of signals of nine pixels for edge determination.

FIG. 4 is a block diagram of a density level control unit.

FIG. 5 is a block diagram of a density level setting unit.

FIG. 6 is a diagram showing a change in density with respect to a gradation level in the case of a right edge.

FIG. 7 is a diagram showing a change in density with respect to a gradation level in the case of a left edge.

FIG. 8 is a diagram showing a change in density with respect to a gradation level in the case of a non-edge portion.

FIG. 9 is a diagram showing a change in density with respect to a gradation level in the case of a thin line edge.

FIG. 10 is a view showing actual image data after processing.

FIG. 11 is a block diagram of a part of an edge determination unit according to a modified example.

FIG. 12 is a view showing an example of white spot prevention.

[Explanation of symbols]

10 image data output device, 12 gamma correction unit,
14 edge determination unit, 16 density level control unit,
18 Density level setting section.

Continued on the front page F term (reference) 5B057 CA08 CA12 CA16 CB08 CB12 CB16 CC03 CE03 CE11 DA08 DA17 DB02 DB09 DC16 DC22 5C077 LL19 NN07 PP15 PP47 PP54 PQ12

Claims (4)

[Claims] 1. An image processing apparatus which divides a pixel into a plurality of sub-regions in a main scanning direction based on multi-valued image data and outputs digital image data for forming an image in units of sub-regions. Edge determining means for identifying the edge direction of the pixel of interest from the difference between the density levels of the pixel of interest and its surrounding pixels based on the value image data; and a pixel of interest according to the edge direction of the pixel of interest determined by the edge determining means. And a density control means for setting a density level in a plurality of sub-regions within the pixel and changing a density distribution in each pixel of interest. 2. A density level control unit for setting a density level setting parameter for each sub-region in a pixel of interest according to an edge direction of the pixel of interest determined by the edge determination unit. Using the density level setting parameter set by the level control means, based on the density level of the pixel of interest,
2. The image processing apparatus according to claim 1, further comprising density level setting means for setting a density level for each of the plurality of sub-regions in the pixel of interest. 3. The edge direction identified by the edge determination means includes an edge in any one of a main scanning direction and a thin line edge.
An image processing apparatus according to claim 1. 4. The apparatus according to claim 1, wherein the edge determining means cancels the determined edge direction when the density level of a pixel adjacent to the target pixel in the edge direction is higher than a threshold value. An image processing apparatus according to any one of claims 1 to 3.