JPH07170391A - Picture element density converter - Google Patents

Abstract

PURPOSE:To improve resolution and gray level properties by performing a picture element density transformation by performing a filter processing with a filter coefficient suitably used based on a character or picture decision. CONSTITUTION:After and A/D conversion 2 is performed for the 200dpi analog image data read by a scanner 1 and a gamma correction 3 is performed for the data as 8-bit digital image data, the data is transformed into 300dpi and 8-bit multivalued image data in a resolution transformation part 5 and the data is inputted in a filter processing part 7. At the same time, the result as to whether the gamma corrected image is the picture area or the character area of noted picture elements is decided in a character/picture decision part 6 is supplied to the filter processing part 7 and a binarization processing part 8. After the calculations of the weighing coefficients of an edge emphasis and a smoothing are performed the coefficient of a filter is switched by an objectiveness in the filter processing part 7, an optimum binarization processing is performed based on the result of the character/picture decision in the threshold processing part 8. Thus, a proper picture element density transformation which is suitable to each characteristic of character and picture images can be performed for the character and picture images.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pixel density conversion device for converting the pixel density of input multivalued image data.

[0002]

2. Description of the Related Art In recent years, resolution and gradation of image input / output devices have been increased, and 200 dpi, 300 dpi, 400 d have been developed.
Images of various resolutions such as pi and 600 dpi are handled. For example, an image handled by a facsimile has a standard of 200 dpi and 400 dpi, whereas a laser printer or the like often has a standard of 300 dpi.

Therefore, when inputting / outputting an image between apparatuses having different resolutions, resolution conversion is performed.

For example, when image data read by a scanner of 200 dpi is printed out by a laser printer of 300 dpi, multi-valued image data read by the scanner is resolution-converted to 300 dpi and binarized and output to the laser printer. Control is performed.

As a method for performing resolution conversion of such multi-valued image data, the closest pixel replacement method (SP) has hitherto been used.
C method), linear interpolation method, cubic function convolution method and the like are known.

The closest pixel replacement method is the simplest to process, but when applied to a gradation image such as a photograph, the gradation is lost and the image quality is deteriorated. There is a problem that jaggies (jagged) occur in the shaded areas such as, and the linear interpolation method has good storage stability for gradation images, but the edge parts of images such as characters and line drawings are rounded and resolved. However, the cubic function convolution method is superior in image quality to the above two methods and has good resolution and gradation, but the processing is complicated. Therefore, there is a problem that the scale of hardware becomes very large.

As described above, all the resolution conversion methods have advantages and disadvantages under the present circumstances.

Conventionally, as a pixel density conversion device using such a resolution conversion method, that disclosed in Japanese Patent Laid-Open No. 4-170868 has been known.

This is because the image data from the image reading unit is γ-corrected so as to match the recording characteristics, then stored in a line memory, and the image data stored in the line memory is replaced with the nearest pixel by a pixel density conversion unit. The pixel density is converted by the method and written in the area memory. Then, the converted image data in the area memory is filtered using a smoothing filter to be binarized.

[0010]

However, this publication has a problem that edge information is lost for a character image because all images are uniformly smoothed.

Therefore, according to the present invention, it is possible to perform an appropriate pixel density conversion suitable for the characteristics of a character image and a photographic image, and to perform pixel density conversion excellent in resolution and gradation. Provide a device.

[0012]

According to the present invention, there is provided a resolution conversion means for converting input multi-valued image data into a desired resolution, a target pixel for each input pixel of the input multi-valued image data, and a peripheral pixel of the target pixel. Character / photo determination means for determining characters / photos based on the pixel level of, and image data whose resolution has been converted by the resolution conversion means are switched by filtering the filter coefficient based on the characters / photo determination of the character / photo determination means. And a filter processing means for performing the same.

[0013]

In the present invention having such a structure, the input multi-valued image data is a character for each pixel by the character / photograph judging means,
A photograph is determined, and the image data subjected to the resolution conversion by the resolution conversion means is filtered by the filter processing means with different filter coefficients suitable for the character or the photograph determination.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. In this embodiment, the present invention is applied to an apparatus which prints out image data read by a 200 dpi scanner with a 300 dpi laser printer.

In FIG. 1, reference numeral 1 denotes a scanner having a resolution of 200 dpi, which optically reads an image from a document and outputs it as analog data.

After the analog data of 200 dpi from the scanner 1 is converted into, for example, 8-bit digital data by the A / D conversion unit 2, the γ correction unit 3 corrects the input gradation characteristic of the scanner 1. The gradation may be corrected according to the human visual characteristics.

The multivalued image data corrected by the γ correction unit 3 is input to a pixel density conversion unit 4 which constitutes a main part.

The pixel density conversion unit 4 comprises a resolution conversion unit 5, a character / photograph determination unit 6 and a filter processing unit 7. The multi-valued image data corrected by the γ correction unit 3 is converted into the resolution conversion unit 5 and the character / photo image data. Inputs are made to the photo determination unit 6, respectively.

The resolution conversion unit 5 has a resolution of 8 at 200 dpi.
The resolution of the bit multi-valued image data is converted into 8-bit multi-valued image data of 300 dpi, that is, the pixel density is converted, and the resolution is converted by, for example, the nearest pixel replacement method or the linear interpolation method.

Here, the nearest pixel replacement method and the linear interpolation method will be described by a one-dimensional conversion for the sake of simplicity. The nearest pixel replacement method, as shown in FIG. The value of the sampling point of the original pixel closest to the point is used. Further, in the linear interpolation method, as shown in FIG. 3, the value of the conversion pixel is determined according to the distance between the sampling points of the original pixels adjacent to the sampling point of the conversion pixel.

The resolution conversion is carried out by an algorithm, but it may be carried out by using a conversion table when the conversion rate is specified in several types.

The character / photo discriminating unit 6 discriminates whether the pixel of interest is a photo region or a character region from image data within a predetermined range including the pixel of interest in the input multi-valued image data,
By performing this for one page, the character / photo determination result for each pixel is obtained. Then, the determination result is output to the filter processing unit 7 and the subsequent binarization processing unit 8 as a character / photograph determination signal.

The character / photo discriminating unit 6 is, specifically,
Divide the input multi-valued image data into blocks of 3 × 3 pixels,
The central pixel is set as the pixel of interest, and the eight surrounding pixels are set as peripheral pixels. Among these pixels, the level difference between the pixel with the maximum pixel level and the pixel with the minimum pixel level is obtained, and the level difference is determined in advance. The first parameter P1 is obtained by dividing by a constant and adding the level value of the minimum pixel to the division result. Further, the average of the absolute values of the respective density differences between the target pixel and the peripheral pixels is set as the second parameter P2. And P1 <P
If it is 2, the pixel of interest at that time is a character (binary image area)
, And if P1 ≥P2, the pixel of interest at that time is the pixel of the photograph (halftone area). The filter processing unit 7 switches the filter coefficient of the resolution-converted image data based on the determination result of the character / photograph to perform the filtering process, and performs the filter calculation of 3 × 3 on the resolution-converted image data. Is supposed to do.

That is, the filter processing unit 7 is configured as shown in FIG.
As shown in, the input image data is sequentially stored in the flip-flops 7a, 7b, 7c, 7d. The output of the flip-flop 7b is input to the first FIFO (first in, first out) memory 71. This first FIFO
The memory 71 can store image data for one line.

The output of the first FIFO memory 71 is sequentially stored in the flip-flops 7e, 7f, 7g and 7h. Then, the output of the flip-flop 7e is set to the second FI.
It is input to the FO memory 72. Like the first FIFO memory 71, the second FIFO memory 72 can also store image data for one line.

The output of the second FIFO memory 72 is sequentially stored in the flip-flops 7i and 7j.

The flip-flops 7d and 7f
Is input to the first adder 73, and the outputs of the flip-flops 7h and 7j are input to the second adder 74.

The output of the first adder 73 is stored in the flip-flop 7k, and the output of the second adder 74 is stored in the flip-flop 7l. The outputs of the flip-flops 7k and 7l are input to the third adder 75.

The output of the third adder 75 is input to the flip-flop 7m, and the output of the flip-flop 7h is input to the flip-flop 7n. The outputs of the flip-flops 7m and 7n are read by a ROM (read
It is input to the (only memory) 76.

The ROM 76 also receives the character / photo judgment signal from the character / photo judgment unit 6.

The flip-flops 7c, 7d, 7f,
Reference numerals 7g, 7h, 7i, and 7j constitute a reference pixel group, and a pixel of interest that is a pixel to be filtered is flip-flop 7g.
, And the reference pixels are flip-flops 7d, 7f,
The pixels are 7h and 7j.

With this reference pixel, the calculation with the weighting coefficient for edge enhancement shown in FIG. 5 and the calculation with the weighting coefficient for smoothing shown in FIG. 6 can be performed. In the circuit of FIG. 4, the coefficient of the filter calculation is performed by adding the peripheral pixels first by the first, second, and third adders 73, 74, and 75 according to the symmetry centered on the pixel of interest, and finally In addition, the calculation is performed with the pixel of interest.

The final weighting is performed by the ROM 76. The ROM 76 has a weighted pattern table for two patterns, one for a character area and the other for a photograph area, and the pattern table to be used is switched according to a character / photo judgment signal from the character / photo judgment unit 6. .

Since the character / photo judgment by the character / photo judgment unit 6 is performed on the image data before the resolution conversion, the character / photo judgment signal input to the ROM 76 has a single magnification for the resolution conversion. Must be expanded in order.

The output from the ROM 76, that is, the multi-valued image data after the resolution conversion from the pixel density conversion unit 4 is input to the binarization processing unit 8, and the binarization processing unit 8 outputs the character / photograph determination result. The optimum binarization process is performed based on

That is, in the case of a character area, it is binarized by using a simple binarization process which is a binarizing method suitable for the character portion, and in the case of a photograph area, it is a binarizing method suitable for the photograph portion. Binarization is performed using the systematic dither method or the error diffusion method.

The image data binarized by the binarization processing unit 8 is supplied to the laser printer 9. The laser printer 9 is designed to print the input image data on a sheet by performing an image forming process by an electrophotographic method.

In the embodiment having such a configuration, the analog image data of 200 dpi obtained by reading the image with the scanner 1 is converted into 8-bit digital image data by the A / D conversion unit 2 and then the γ correction unit. Γ correction is performed in 3.
Then, the gamma-corrected 200-dpi, 8-bit multivalued image data is input to the resolution conversion unit 5 and the character / photo determination unit 6, respectively.

The resolution converting section 5 converts 200 dpi, 8-bit multivalued image data into 300 dpi, 8-bit multivalued image data by the nearest pixel replacement method or linear interpolation method.

Further, the character / photograph determination unit 6 divides the input multi-valued image data into blocks of 3 × 3 pixels, the pixel at the center thereof is set as the pixel of interest, and the surrounding 8 pixels are set as peripheral pixels. Finds the level difference between the maximum pixel and the minimum pixel, divides the level difference by a fixed constant,
The minimum pixel level value is added to the division result to obtain the first parameter P1. In addition, the second parameter P2
Ask for. Then, the parameters P1 and P2 are compared and P
If 1 <P2, the pixel of interest at that time is the pixel of the character, and if P1 ≧ P2, the pixel of interest at that time is the pixel of the photograph.

The image data whose resolution has been converted to 300 dpi by the resolution conversion unit 5 is input to the filter processing unit 7.

The filter processing unit 7 sequentially shifts the input image data into the flip-flops 7a to 7d and fetches it, and fetches the output of the flip-flop 7a into the first FIFO memory 71. Then, after the image data for one line is stored in the first FIFO memory 71, it is sequentially shifted into the flip-flops 7e to 7h, 7n and taken in, and the output of the flip-flop 7e is taken into the second FIFO memory 72. After the image data for one line is stored in the second FIFO memory 72, the flip-flops 7i, 7
sequentially shift to j.

Then, the outputs of the flip-flops 7d and 7f are added by the first adder 73, the result is stored in the flip-flop 7k, and the flip-flops 7h and 7f are also stored.
The output of j is added by the second adder 74, and the result is stored in the flip-flop 7l. Further, the outputs of the flip-flops 7k and 7l are added by the third adder 75, and the result is stored in the flip-flop 7m.

Then, the outputs of the flip-flops 7m and 7n are supplied to the ROM 76. The ROM 76 performs weighting based on the character / photograph determination signal from the character / photograph determination unit 6 based on the pattern table for the character region if the pixel of interest is in the character region. If the pixel of interest is in the photograph area, weighting is performed based on the pattern table for the photograph area.

In this case, when the resolution conversion in the resolution conversion unit 5 is performed by the closest pixel replacement method, the edge information is retained but the gradation is lost by this method, so the ROM 76 is stored in the ROM 76. When the pixel of interest is determined to be a character region, weighting is performed using the pattern table having the weighting coefficient of the through mode filter shown in FIG. 7, and when it is determined that the pixel of interest is a photographic region, the smoothing filter shown in FIG. Weighting is performed using a pattern table having weighting factors.

By performing such filter processing, it is possible to obtain a resolution-converted image having excellent resolution and gradation even for an image in which characters and photographs are mixed.

Further, when the resolution conversion in the resolution conversion section 5 is performed by the linear interpolation method, the gradation characteristic of a photograph or the like is maintained but the edge portion of the character is blunted by this method. Therefore, the ROM 76 performs weighting using the pattern table having the weighting coefficient of the edge emphasis filter shown in FIG. 5 when the pixel of interest is determined to be the character region, and the ROM 76 performs drawing when the pixel of interest is determined to be the photographic region. Weighting is performed using the pattern table having the weighting coefficient of the through mode filter shown in FIG.

By performing such filter processing, it is possible to obtain a resolution-converted image having excellent resolution and gradation even for an image in which characters and photographs are mixed.

In this way, the filter processing unit 7 outputs multivalued image data in which the character region and the photograph region are weighted differently, and the multivalued image data is further binarized by the binarization processing unit 8. It At this time, the character area is binarized suitable for the character portion, and the photograph area is binarized suitable for the photograph portion.

The image data binarized by the binarization processing unit 8 is supplied to a laser printer 9, and the laser printer 9 prints at a resolution of 300 dpi.

In this way, even for an image in which characters and photographs are mixed, weighting can be performed by performing filter processing suitable for each, and resolution conversion excellent in both resolution and gradation can be performed. Therefore, the quality of the image printed out by the laser printer 9 can be improved.

In the above embodiment, the resolution conversion of 200 dpi image data into 300 dpi image data is described, but the present invention is not limited to this and can be applied to resolution conversion of other conversion rates. .

[0053]

As described above, according to the present invention, it is possible to perform an appropriate pixel density conversion suitable for the characteristics of a character image and a photographic image, respectively, and to obtain a pixel density conversion excellent in resolution and gradation. It is possible to provide a pixel density conversion device capable of performing

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a diagram for explaining resolution conversion by the nearest pixel replacement method.

FIG. 3 is a diagram for explaining resolution conversion by a linear interpolation method.

FIG. 4 is a circuit diagram showing a configuration of a filter processing unit of the embodiment.

FIG. 5 is a diagram showing a configuration of an edge enhancement filter used in the same embodiment.

FIG. 6 is a diagram showing a configuration of a smoothing filter used in the example.

FIG. 7 is a diagram showing a configuration of a through mode filter used in the same embodiment.

[Explanation of symbols]

 5 ... Resolution conversion unit 6 ... Character / photograph determination unit 7 ... Filter processing unit 76 ... ROM (read only memory)

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Claims (1)

[Claims] 1. A resolution conversion means for converting input multi-valued image data into a desired resolution, and input multi-valued image data for each target pixel based on a pixel level of the target pixel and peripheral pixels of the target pixel, A character / photograph judging means for judging a photograph and a filter processing means for switching the filter coefficient of the image data whose resolution is converted by the resolution converting means based on the character or photograph judgment of the character / photograph judging means and performing a filtering process. A pixel density conversion device characterized by being provided.