JP3249341B2 - 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 performing digital color image processing, such as a full-color copying machine or a color printer.

[0002]

2. Description of the Related Art In an image processing apparatus for performing digital color image processing, as shown in FIG. 4, an original image 51 of a document is read by an input device 52 such as a scanner, and the original image 51 is corrected by decomposing it into three additive primary color signals. Input to the device 53. The correction device 53 corrects the input color signal into a corrected color signal, and supplies the corrected color signal to an output device 54 such as a printer to form a duplicated image 55. The correction device 53 performs a color correction process of converting the color signals of the three primary colors of the additive color mixture input from the input device 52 into the three primary colors of the subtractive color mixture by calculating a polynomial expression including a linear term or a nonlinear term. For example, using a color correction matrix (MTX) of a polynomial obtained by a multiple regression model, a color correction process is performed by the following first expression (reference documents: Kodera,
"Color reproduction in digital prints", Journal of the Institute of Image Electronics Engineers of Japan, 14, 5, 1985).

[0003]

(Equation 1)

Generally, the BIT width of a color signal input from an input device and a BIT width of a color signal reproducible in an output device are as follows.
For example, it is limited to 8 BIT, and the color of each pixel is 0 to 2
Expressed in the range of 55 values. On the other hand, in the color correction processing in the correction device, a larger BIT such as 32 BIT is used.
When the correction color signal output from the correction device has a value of 0 or less or 255 or more, the correction color signal takes a value exceeding the reproducible color signal value range. is there.

[0005] In such a case, in the conventional image processing apparatus, a value of 0 or less of the correction color signal is set to 0 and a value of 255 or more of the correction color signal is set to 255 in the output device. The same applies to the case where the corrected color signal is binarized into multi-tone image data in order to improve the color reproducibility (for example, JP-A-5-56265). The binarization process is performed using the set threshold value, and only color signals having values within a reproducible range are handled.

[0006]

However, in the processing in the conventional image processing apparatus, when the value of the corrected color signal exceeds the range of the reproducible color signal, the correction processing result of the color signal in the correction apparatus is duplicated. There was a problem that the image could not be faithfully reproduced in the image, and the color correction processing in the correction device was wasted.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image processing method in which, when the value of a corrected color signal is smaller than the minimum value of a reproducible color signal or larger than the maximum value, an error in the range of a reproducible color signal causes an error in the color of peripheral pixels. An object of the present invention is to provide an image processing device that can effectively utilize a result of a color signal correction process performed by a correction device on a replicated image by reflecting the result on a signal.

[0008]

According to the present invention, the three primary colors of the additive color mixture for each pixel read from the original image are provided.
Larger quantization bit width due to color signal color correction means
In an image processing apparatus that performs image reproduction processing using a corrected color signal corrected to three subtractive primary colors, an error between a corrected color signal for each target pixel and a range of a quantization bit width of the color signal is determined by using a peripheral pixel of each pixel. And a signal correcting means for diffusing the corrected color signal.

According to a second aspect of the present invention, the signal correcting means includes a correction color signal for each pixel and a quantization bit width of the color signal.
A difference calculating means for calculating an error from the range, an error storing means for storing the error calculated by the difference calculating means for each pixel, and adding the errors of peripheral pixels read from the error storing means at a predetermined ratio. It is characterized by comprising an error calculating means for calculating the cumulative error for the target pixel, and an error adding means for adding the cumulative error calculated by the error calculating means to the correction color signal.

According to a third aspect of the present invention, the signal correction means determines whether or not the target pixel is a pixel at the edge of the image, and the target pixel is an edge of the image. A correction color signal switching unit for invalidating the output of the signal correction unit in such a case.

[0011]

According to the present invention, the error between the corrected color signal for each target pixel and the range of the reproducible color signal is:
The signals are diffused into correction color signals of peripheral pixels of each pixel by the signal correction means. Therefore, even if a corrected color signal out of the range of reproducible color signals is obtained by the color signal correction processing, the color signal correction result can be effectively reflected on the duplicated image.

According to the second aspect of the present invention, the difference between the corrected color signal for each pixel and the range of the reproducible color signal is calculated by the difference calculation means, and the error calculated by the difference calculation means is stored in the error storage means. Is stored for each pixel. The error calculating means calculates a cumulative error for the target pixel by adding the errors of the peripheral pixels read from the error storing means at a predetermined ratio, and calculates the cumulative error calculated by the error calculating means into a correction color signal in the error adding means. to add. Therefore, an error between the value of the correction color signal for each pixel and the range of the reproducible color signal is added to the correction color signal of the target pixel at a predetermined ratio.

According to the third aspect of the present invention, the edge portion detecting means determines whether or not the target pixel is a pixel at an edge portion of the image. If the target pixel is an edge portion of the image, a signal is output. Disable the output of the correction means. Therefore,
In the edge portion of the image, the error between the value of the corrected color signal and the range of the reproducible color signal is not diffused to peripheral pixels, and the image does not become unclear.

[0014]

FIG. 1 is a block diagram showing the configuration of an image processing apparatus according to an embodiment of the present invention. The input unit 1 reads an original image and outputs a color signal for each pixel. The color correction unit 2 corrects the color signal by, for example, a method using the above-described color correction matrix, and outputs a corrected color signal. The correction color signal output from the color correction means 2 is input to the error correction means 3 and the edge detection means 4. Error correction means 3
Corrects a 32-bit correction color signal to an 8-bit reproduction color signal. As in the conventional processing, the edge detecting means 4 sets the corrected color signal having a value of 0 or less to 0 among the corrected color signals of 32 BITs to 0, and outputs the corrected color signal having a value of 255 or more to 255.
Is converted into an 8-bit correction color signal and input.

The edge detecting means 4 detects whether or not the target pixel is an edge part of the original image by a process described later, and if the target pixel is an edge part of the image, converts the edge detection signal into an error correcting means. Output to 3. Error correction means 3
Does not execute the correction process of the correction color signal when the edge detection signal is input. The reproduced color signal is input to the output unit 5 from the error correction unit 3. The output means 5 creates a duplicate image based on the input reproduced color signal.

FIG. 2 is a diagram showing an example of a method of detecting an edge portion of an image by the edge detecting means. The edge detection means 4 creates a histogram of pixels around the target pixel. As shown in FIG. 2A, the edge detection unit 4 determines that the target pixel is near the edge of the image when the frequency at both ends is higher than the frequency of the intermediate density as shown in FIG. to decide. On the other hand, if the density distribution is substantially uniform as shown in FIG. 7B, it is determined that the target pixel is not near the edge of the image.

FIG. 3 is a diagram showing the configuration of the error correcting means in the image processing apparatus. The error correction unit 3 includes an error adding unit 31, a difference calculating unit 32, an error calculating unit 33, an error storing unit 34, and a signal converting unit 35. Assuming that the coordinates of the target pixel of the error correction process are (x, y), the error addition unit 31 calculates the 32 BIT correction color signal Pxy input from the color correction unit 2 and the accumulated error Sxy read from the error storage unit 34. And outputs a corrected color signal dxy. The difference calculation unit 32 compares the value of the corrected color signal dxy with a range of values of 0 to 255 that can be taken by the 8-bit color signal reproducible in the output unit 5, and when dxy> 255 or dxy <0, An error signal Exy representing the difference is output. Error signal E
xy is stored in the error storage unit 34.

The error calculator 33 calculates the accumulated error Sxy by the following equations (2) and (3). Where i and j
Represents the coordinates in the error distribution matrix Zij, and the error distribution matrix Zij is a coefficient in which the value of the difference signal Exy of each pixel is weighted when the value is distributed to neighboring pixels. The values of the coefficients in the third expression show one example.

[0019]

(Equation 2)

Therefore, when determining the cumulative error Sxy of the target pixel at the coordinates (x, y), the error calculation unit 33 reads out the errors E of the peripheral pixels 34a to 34d from the error storage unit 34, and A value obtained by multiplying the error distribution matrix Zij of the third expression by a coefficient determined is added. The signal conversion unit 35 converts the 32-bit corrected color signal dxy into an 8-bit reproduced color signal Dxy. This conversion processing can be performed in the same manner as the conventional processing. A value of 0 or less is set to 0, and a value of 255 or more is set to 255.

The edge detection signal output from the edge detection means 4 shown in FIG.
When the edge detection signal is input, the error adding unit 31 does not perform the process of adding the cumulative error Sxy to the correction color signal Pxy. Therefore, the correction color signal Pxy is directly input to the signal converter 35.

With the above configuration, according to this embodiment,
The value of the corrected color signal d obtained by adding the accumulated error S to the corrected color signal P is compared with a reproducible color signal range, and the error E is stored in the error storage unit 34 for each pixel. The accumulated error S is calculated based on the content stored in the error storage unit 34 and added to the corrected color signal P. Therefore, the error of the target pixel at this time is distributed at a predetermined ratio and added to the correction color signal when correcting the correction color signal of the peripheral pixel, and as a result, the error of each pixel becomes the color of the peripheral pixel. The signal will be spread at a predetermined rate.

If the target pixel is a pixel near the edge of the image, the edge detecting means 4 detects this, and
Since the edge detection signal is output to the error addition unit 31 of the color correction unit 3, correction processing of the correction color signal is not performed on the pixels near the edge of the image, thereby preventing a line image or a character image portion from becoming unclear. it can.

In an image processing apparatus which does not perform image processing on a line drawing or a character image, the edge detecting means can be omitted. Further, the configuration of the error correction means for diffusing the error between the value of the corrected color signal and the range of the reproducible color signal is not limited to the configuration shown in FIG.

[0025]

According to the first aspect of the present invention, the error between the corrected color signal of each target pixel and the range of the reproducible color signal is converted into the corrected color signal of the peripheral pixel of each pixel by the signal correcting means. By the diffusion, even if a corrected color signal out of the range of the reproducible color signal is obtained by the color signal correction processing, the color signal correction result can be effectively reflected on the duplicated image.

According to the second aspect of the present invention, the error of the peripheral pixels stored in the error storage means is added to the correction color signal of each target pixel at a predetermined ratio, so that the correction color for each pixel is added. An error between the signal value and the range of the reproducible color signal can be diffused to a correction color signal of a peripheral pixel, and the color signal correction result can be effectively reflected on a duplicate image.

According to the third aspect of the present invention, it is determined by the edge detecting means whether or not the target pixel is a pixel at the edge of the image. If the target pixel is an edge of the image, a signal is output. By disabling the output of the correction means, the error between the value of the corrected color signal and the range of the reproducible color signal is not diffused to peripheral pixels at the edge portion of the image, thereby preventing the image from becoming unclear. it can.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of an image processing apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing an edge detection method in edge detection means of the image processing apparatus.

FIG. 3 is a block diagram illustrating a configuration of an error correction unit of the image processing apparatus.

FIG. 4 is a diagram illustrating a configuration of a conventional image processing apparatus.

[Explanation of symbols]

 1-input means 2-color correction means 3-error correction means 4-edge detection means 5-output means 31-error addition unit 32-difference calculation unit 33-error calculation unit 34-error storage unit 35-signal conversion unit

Claims (3)

(57) [Claims] 1. An additive color mixture for each pixel read from an original image .
The color correction means for the color signals of the three primary colors reduces the quantization bit width.
In an image processing apparatus that performs image reproduction processing using a corrected color signal corrected to three subtractive primary colors, the corrected color signal for each target pixel and the range of the quantization bit width of the color signal are compared. An image processing apparatus, comprising: signal correction means for diffusing an error into a correction color signal of a peripheral pixel of each pixel. 2. The apparatus according to claim 1, wherein said signal correcting means includes a correction color signal for each pixel.
Calculating means for calculating an error between the image data and the range of the quantization bit width of the color signal, an error storing means for storing the error calculated by the difference calculating means for each pixel, and an error of a peripheral pixel read from the error storing means. 2. An error calculating means for calculating a cumulative error for a target pixel by adding the cumulative error at a predetermined ratio, and an error adding means for adding the cumulative error calculated by the error calculating means to the correction color signal. Image processing device. 3. An image processing apparatus according to claim 1, wherein said signal correcting means determines whether or not the target pixel is a pixel at an edge of the image. 3. The image processing apparatus according to claim 1, further comprising: a correction color signal switching unit that invalidates an output.