JP3197299B2 - Image processing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus for processing halftone image data, and more particularly to an image processing apparatus for converting multi-tone image data into low-tone image data by an error diffusion method.

[0002]

2. Description of the Related Art In a digital copying machine or the like, when a grayscale image (halftone image) is reproduced by a binary recording method, an organized dither method is generally used. However, the systematic dither method has a drawback in that it is not possible to represent a portion where the density change is large, and the edges of characters and line drawings are blurred. Further, when a halftone image used for a printed matter is reproduced, moire occurs due to interference between a periodic pattern of the halftone dots and a dither pattern.

On the other hand, the gradation to be recorded by a printer capable of converting multi-gradation image data into low-gradation image data and capable of recording low-gradation images, that is, a printer capable of half-tone recording of three or more values. The recording has a characteristic that moire hardly occurs.
As a method of converting image data from multiple gradations to small gradations, there is an error diffusion method (minimum average error method). The error diffusion method has a feature that moire is less likely to occur in a halftone dot image. However, even when the error diffusion method is used, a striking stripe pattern may occur in a region where the density change is small.

[0004]

That is, in the error diffusion method, if the conversion of image data from multi-gradation to low-gradation, that is, conversion to reduce the number of data bits of the image data, is performed, the reproduced image becomes pseudo. Contours are more likely to occur. The pseudo contour is generated when a single matrix is used for weighting processing for error diffusion, because a periodic structure is likely to be formed in image data, and the reproduced image is deteriorated.

In the case where thinning processing is performed on image data such that two pixels are regarded as one pixel, conventionally, only image data of one of an odd-numbered pixel and an even-numbered pixel has been extracted. An accurate image could not be reproduced for the data.

An object of the present invention is to prevent an output image from deteriorating in an image processing apparatus using an error diffusion method.

[0007]

An image processing apparatus according to a first aspect of the present invention.
The unit replaces image data with fewer bits than its data bits.
Requantization means (201) for converting the image data into
Error data generated when the quantization means (201) re-quantizes
Storage means (202) for temporarily storing data, and the storage means
The error data stored in (202) is
First correction means (201, 203) dispersed to side pixels,
In an image processing device using multi-level error diffusion, the main scanning direction
Into two groups that are adjacent to each other in the sub-scanning direction
Between main scans, the grouping is performed by one pixel in the main scan direction.
Shifting grouping means (501, 502, 503, 504, 505, 506);
And within the same groupLeading, trailingThe first complement of two pixels
Corrected by the correct means (201, 203) and requantized by the requantizer
Each image data(ID1, ID2)ToThe density from the following pixel
A part (ID2 / 2) of (ID2) is subtracted and the density (ID
As in 1)Second correction means (503) for converting the In addition,To make it easier to understand,In parenthesesTo
IsShown in the drawingCorresponding elements of the embodiment described laterOr corresponding items
The signAdded for reference.The same applies to the following.

According to a second aspect of the present invention, there is provided an image processing apparatus comprising: a requantizing means for converting image data into image data having a bit number smaller than the number of data bits; Storage means (202) for temporarily storing error data generated when the error occurs, and a first means for dispersing the error data stored in the storage means (202) to peripheral pixels in a predetermined matrix.
In an image processing apparatus based on multi-level error diffusion, comprising correction means (201, 203), two pixels continuous in the main scanning direction
Grouping means (501a, 502a) as a group; comparing means (505a) for comparing the image data of two pixels in each group before the requantization and determining whether the following pixel has a higher density than the preceding pixel. 506a, 507a); and image data (ID1, I2 ) of the preceding and succeeding two pixels in the same group, which are corrected by the first correction means (201, 203) and requantized by the requantization means.
D2) , a part of the density (ID2) from the subsequent pixel (ID2 / 2)
Is subtracted and added to the density (ID1) of the preceding pixel, and when the comparing means determines that the succeeding pixel has high density, the conversion data destined for the two pixels are replaced with each other and the same group is replaced. Second correction means (504a, 503a, 508a) for making image data of two pixels among them.

[0009]

According to the first aspect, the grouping means (501, 50)
2, 503, 504, 505, 506) make two pixels continuous in the main scanning direction into one group, and shift the grouping by one pixel in the main scanning direction between main scannings adjacent in the sub-scanning direction. Also, the second
The correction means (503) compares each image data (ID1, ID2) of the preceding and succeeding two pixels in the same group, which has been corrected by the first correction means (201, 203) and re-quantized by the re-quantization means, to the subsequent Line drawing
Part of the concentration (ID2) (ID2 / 2)
It is converted to be added to the pixel density (ID1) . This density shift causes a density difference between the two pixels in the main scanning direction.
At the even-numbered scan lines and the odd-numbered scan lines in the main scanning direction, the ends of the grouped pixel blocks are shifted in the main scanning direction. Therefore, even if error diffusion is performed using a single matrix, the periodicity is lost, and the occurrence of a false contour on the reproduced image is prevented, and the image quality of the formed image is improved. Breaking the periodicity is also effective for moiré components included in the input data.

Further, according to the second invention, the grouping means (501a, 502a) groups two pixels continuous in the main scanning direction into one group, and the comparing means (505a, 506a, 507a) sets The image data of the two pixels before the requantization is compared to determine whether the following pixel has a higher density than the preceding pixel. Also, the second correction means (504a, 503a, 508a)
Each image data (ID1, ID2) of the preceding and succeeding two pixels, which is corrected by the first correction means (201, 203) and requantized by the requantization means.
2) , a part (ID2 / 2) of the density (ID2) from the subsequent pixel
The conversion is performed so as to be added to the density (ID1) of the preceding pixel, and when the comparing means determines that the subsequent pixel has high density, the conversion data destined for the two pixels are replaced with each other and within the same group. It is assumed that the image data is two pixels. Therefore, the pixel reproduction of the output image with respect to the input data becomes more accurate, and the image quality of the output image can be improved.

Other objects and features of the present invention will become apparent from the following description of embodiments with reference to the drawings.

[0012]

Embodiment 1 FIG. 1 shows a schematic configuration of an image forming apparatus including an image processing apparatus of the present invention. The image data read by the scanner unit 101 is subjected to γ correction processing in a γ correction processing unit 102, and further subjected to error diffusion processing in a multi-level error diffusion processing unit 103. Further, the image data on which the error diffusion processing has been performed is subjected to predetermined processing by a two-pixel unit processing unit 104 based on a space between two consecutive pixels. The processed data is sent to the multi-value printer 105 to form an image.

FIG. 2 shows a multi-level error diffusion processor 1 shown in FIG.
03 shows a schematic configuration. The multi-level error diffusion processing unit 103
This is a circuit for converting 64-value image data into 8-value image data by an error diffusion method. The 64-value signal a is represented by 6 bits, where 0 in the 64 values is (00000) and 63 is (1111).
1) respectively. Signals a0 to a5 shown in FIG.
Corresponds to the signal a shown in FIG. 1, for example, 16 out of 64 values.
Is a4 = 1, a5 = a3 = a2 = a1 = 0.

The adder 201 determines that S (S0 to S5) = A
An S that is (A0 to A5) + B (B0 to B5) is output.
The upper three bits S6, S5 and S4 of the output S are the signals b2 and b
1 and b0 are output as a 3-bit 8-value quantized signal. These b2, b1, and b0 correspond to the signal b in FIG. The lower 4 bits S3, S2, S1 and S0 of the output S are quantization errors at the time of this quantization and are temporarily stored in the memory 202.

The arithmetic unit 203 calculates the average of the errors temporarily stored in the error memory 202 by weighting the error amount around the pixel to be quantized according to the error position. A representation of the weighting coefficients corresponding to the pixel positions in a pixel matrix is called a weight matrix, and in this embodiment, a weight matrix as shown in FIG. 3 is used.

In FIG. 3, a hatched portion indicates a pixel to be quantized, that is, a signal a (a0 to a) at the present time.
This corresponds to 5). A pixel adjacent to this pixel and already quantized is weighted according to the distance from the target pixel in accordance with the matrix of FIG.

FIG. 4 shows each error amount of the peripheral pixels temporarily stored in the error memory 202. 3 and FIG. 4, the output e (e0 to e3) of the calculation unit 203 is e = (1 × E1 + 3 × E2 + 1 × E3 + 3 × E4) / 8. Data S temporarily stored in the memory 202
The above operation is performed on 3, S2, S1, and S0, and the four bits obtained by the operation are output as e3, e2, e1, and e0. The outputs e3, e2, e1, and e0 are the lower 4 bits (B3, B2, B) of the B input terminal of the adder 201.
1, B0). 3 and 4, x indicates the input image signal and the main scanning direction of printing on the printer 105, and y indicates the sub-scanning direction.

The memory 202 includes a one-line buffer memory for temporarily storing an error amount (for example, E1, E2, E3) one line before the pixel to be quantized, and a pixel one line before the same line as the pixel to be quantized. And a shift register for temporarily storing the minute (for example, E4).

The operation (calculation) of the multi-level error diffusion processing unit 103 shown in FIG. 2 described above is summarized as follows. 1) S = A + B = a + e = a + (1 × E1 + 3 × E2 + 1 × E3 + 3 × E4) / 8, and a is corrected by a quantization error of peripheral pixels. 2) Quantizing to 8 values by extracting the upper 3 bits from S. 3) The lower 4 bits in S are temporarily stored in a memory as a quantization error of this pixel, and are used for correction of peripheral pixels to be quantized in the future.

In this manner, quantization is performed from 64 values to 8 values while maintaining the gradation of the signal a.

FIG. 5 shows a two-pixel unit processing unit 10 shown in FIG.
4 shows an outline of the configuration. The two-pixel unit processing unit 104 is a circuit that performs arithmetic processing based on two consecutive pixels.
The data b after the multi-level error diffusion processing is input to the latch 501 and the latch 502. The latch 501 operates with the pixel clock CK1, and latches input pixel data for each pixel. On the other hand, the latch 502 outputs the pixel clock C
Since it operates with a clock CK2 having half the frequency of K1,
The input pixel data is latched every other pixel. The output of the latch 501 is input to the latch 502. As a result,
The preceding and succeeding pixels of the continuous two-pixel data are simultaneously output from the latch 502. At this time, the latch 50
2, Q0, Q1, and Q2 are output to the succeeding pixel,
3, Q4 and Q5 respectively correspond to the preceding pixels. Less than,
Q3, Q4, and Q5 are represented by ID1, and Q0, Q1, and Q2 are represented by ID2.

In the data conversion RAM 503, data is moved between two consecutive pixels ID1 and ID2 by a table conversion method. The counter 504 is a counter in the main scanning direction operated by the pixel clock CK1, and the LSB (least significant bit) is input to the data conversion RAM 503. The data conversion RAM 503 uses the LSB signal of the main scanning counter to convert continuous two-pixel data after data conversion into different values. Hereinafter, of the two consecutive pixels after data conversion, the preceding pixel is represented by ID1 'and the succeeding pixel is represented by ID2'. The counter 504 has a line synchronization signal LSYNC.
And is always 0 at the beginning of each line.

A conversion table as shown in FIG. 6 is written in the data conversion RAM 503 in advance. The conversion shown in FIG. 6 can be expressed by using the following formula: ID1 '= ID1 + INT (ID2 / 2) ID2' = ID2-INT (ID2 / 2) However, INT is a function that rounds down decimal places.

The output signal C of the data conversion RAM 503 (C
0, C1, C2, C3) represent ID1 'when the LSB signal of the counter 504 is at "L" level, and represent ID2' when it is at "H" level.

The sub-scanning method counter 505 is cleared by the sub-scanning synchronization signal FGATE, and counts the number of lines in the main scanning direction. With the LSB signal of this counter, the processing of the even-numbered lines and the processing of the odd-numbered lines are performed separately. EX-OR (exclusive OR) gates 506 and 507 change the unit of actual pixelization for each line. More specifically, as shown by the hatched portion in FIG. 7, the configuration of the two-pixel conversion of the even lines and the odd lines is changed.

The image data formed by the two-pixel unit processing 104 is input to the multi-value printer 105. The multi-value printer modulates the pulse width of the 4-bit 11-value image data into 11 types and performs laser exposure. By controlling the time, gradation recording of 11 values per pixel is performed.

Embodiment 2 FIG. 8 shows a schematic configuration of an image forming apparatus including the image processing apparatus of the present invention. The configuration illustrated in FIG. 8 partially differs from the configuration illustrated in FIG. 1 (Example 1) in the configuration of the two-pixel unit processing unit. Further, a delay memory 106 is provided for forming input data of the two-pixel unit processing unit. Other configurations are the same as those of the first embodiment.

FIG. 9 shows a schematic configuration of the two-pixel unit processing unit 104a. The latches 501a and 502a and the data conversion RAM 503a correspond to the latch 5 shown in the first embodiment (FIG. 5).
01 and 502 and the data conversion RAM 503 have the same configuration, and a circuit for forming a drive signal for the data RAM 503a is different. A circuit for forming a drive signal for the data RAM 503a includes a counter 504a, latches 505a and 606a, a comparator 507a, and an EX-OR gate 508a. Note that the output signal C (C0, C1,
C2, C3) are the A input to the data RAM 503a.
When six signals are at "L" level, they represent ID1 ', and when they are at "H" level, they represent ID2'.

The data from the scanner 1 is subjected to gamma correction processing,
Delay memory 1 by the amount delayed by multi-level error diffusion processing
06 and input to the latch 505a.
Here, the latch 506a holds data of two consecutive pixels in the same manner as the latch 502a (or the latch 502 of the first embodiment). The input terminal P of the comparator 507a receives the preceding pixel data, and the input terminal Q receives the following pixel data. Comparator 507a outputs “H” level when P <Q. Therefore, when the preceding pixel data is large, the preceding pixel data is output with priority, and when the following pixel data is large, the succeeding pixel data is output with priority.

That is, when the succeeding pixel data is larger, the comparator 507a outputs "H" level,
Since the A6 signal of the RAM 503a becomes H,
3a assigns ID2 ′ to the preceding pixel (LSB output of 504a:
L), and ID1 ′ is output to the succeeding pixel (L of 504a).
SB output: output to H). That is, the conversion data ID1 'and ID2' destined for two pixels are exchanged with each other to obtain image data of two pixels in the same group.

When the preceding pixel data is larger, the comparator 507a outputs an "L" level,
Since the A6 signal of the signal 03a goes low, the RAM 503a
D1 ′ is output to the preceding pixel (LSB output of 504a: L), and ID2 ′ is output to the following pixel (LSB output of 504a: L).
H). That is, the conversion data ID1 'and ID2' destined for the two pixels are not exchanged,
It is assumed that each pixel is image data.

The above is summarized as follows.

Preceding pixel Subsequent pixel 507a A6 signal Replacement of output of density P: ID1 of density Q: ID2 of A6 signal conversion data Small Large H H: ID2 '/ L: ID1' Yes Large Small L L: ID1 '/ H: ID2 'None LSB of the main scanning counter 504a is L for the preceding pixel and H for the succeeding pixel.

[0034]

According to the first aspect of the present invention, the configuration of two pixels is changed between the even scan line and the odd scan line in the main scanning direction. Thus, the occurrence of a false contour on the reproduced image can be prevented, and the image quality can be improved. Breaking the periodicity is also effective for moiré components included in the input data.

According to the second aspect of the present invention, in the two-pixel unit processing, image data is formed corresponding to data values of two consecutive pixels, so that pixel reproduction of an output image with respect to input data becomes more accurate, and Image quality can be improved.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a schematic configuration of an image forming apparatus including an image processing apparatus of the present invention.

FIG. 2 is a block diagram schematically showing a configuration of a multi-level error diffusion processing unit 103 shown in FIG.

FIG. 3 is a weight matrix used when the arithmetic unit 203 weights an error temporarily stored in an error memory 202 to an error amount around a quantization target pixel according to an error position.

FIG. 4 is a matrix showing error amounts of peripheral pixels temporarily stored in an error memory 202;

FIG. 5 is a block diagram illustrating a schematic configuration of a two-pixel unit processing unit 104 illustrated in FIG. 1;

FIG. 6 is a data conversion table of a data conversion RAM 503.

FIG. 7 is a plan view showing the position of a pixel unit, showing how the two-pixel unit is changed between an even line and an odd line in the two-pixel unit processing unit 104;

FIG. 8 is a block diagram illustrating a schematic configuration of an image forming apparatus including the image processing apparatus of the present invention.

FIG. 9 is a block diagram illustrating a schematic configuration of a two-pixel unit processing unit 104a.

[Explanation of symbols]

101: scanner 102: γ correction processing unit 103: multi-value error diffusion processing unit 201: adder (requantization unit) 202: memory (storage unit) 203: arithmetic unit (201, 203: first correction unit) 104 : Two pixel unit processing units 501, 502: latch 503: data conversion RAM (second correction means) 504, 505: counter 506: exclusive OR gate (501, 502, 503, 504, 505, 506:
Grouping unit) 105: Printer 104a: 2-pixel unit processing unit 501a, 502a, 505a, 506a: Latch (501a, 502a: Grouping unit) 503a: Data conversion RAM 504a: Counter 507a: Comparator (505a, 506a, 507a: Comparison means) 508a: Exclusive OR gate (503a, 504a, 508a: Second correction means) 106: Delay memory

Claims (2)

(57) [Claims] 1. A re-quantizing means for converting image data into image data having a bit number smaller than the data bit number, a storing means for temporarily storing error data generated when the re-quantizing means re-quantizes, and A first correction means for dispersing the error data stored in the storage means to peripheral pixels in a predetermined matrix, wherein two pixels continuous in the main scanning direction are grouped as one group. Grouping means for shifting the grouping by one pixel in the main scanning direction between main scans adjacent in the sub-scanning direction; and the first correction means of the preceding and succeeding two pixels in the same group which are corrected and requantized. Each image data requantized by the
A part of the density is subtracted from the succeeding pixel to obtain the preceding image.
An image processing apparatus comprising: a second correction unit that performs conversion so as to add to the elemental density . 2. A requantization means for converting image data into image data having a smaller number of bits than the number of data bits, a storage means for temporarily storing error data generated when the requantization means requantizes, and A first correction means for dispersing the error data stored in the storage means to peripheral pixels in a predetermined matrix, wherein two pixels continuous in the main scanning direction are grouped into one group. grouping means; two pixels in each group, the determined comparison means comparison Kogyo pixels previous image data re-quantization or a higher concentration than the preceding pixel; and, in the same group prior, after Each image data of two pixels in a row, which is corrected by the first correction means and requantized by the requantization means,
A part of the density is subtracted from the succeeding pixel to obtain the preceding image.
If the comparison means determines that the subsequent pixel has a high density, the conversion data addressed to the two pixels are exchanged with each other to replace the two pixels in the same group.
An image processing apparatus comprising: a second correction unit configured to generate image data of pixels.