JP2001103328A - Image processing method and image processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the reproducibility of an image by preventing an undesirable color pattern from being produced. SOLUTION: In this image processing method, applies a color error spread processing is applied to input color data of each pixel of an image to select and output a proper output enabled color by each pixel from an output enabled color group. This method selects a specific function among a plurality of functions on the basis of each color element of a received color and the output enabled color group and selects the output enable color by using the selected specific function.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image processing method and apparatus for selecting an appropriate output color for each pixel by applying a color error diffusion process and outputting image data with good color reproducibility.

[0002]

2. Description of the Related Art Conventionally, when a halftone color image is output to a binary printer, or when the data capacity is reduced to store or transfer image data,
A binarization process represented by an error diffusion method is widely used.

In other words, as shown in FIG. 5, in a conventional image processing apparatus 80 using the error diffusion method, a peripheral error adding section 81
In, the correction is performed by distributing the error generated in the processing up to that time to the input image data DI. In accordance with the corrected image data, the output color selection processing unit 82 selects an appropriate outputable color and outputs it as output image data DO. The error calculation / storage unit 83 calculates and stores an error between the image data input to the output color selection processing unit 82 and the available output color selected by the output color selection processing unit 82. The colors that can be output are halftone data.

[0004] A peripheral error giving section 81 calculates an error calculated from an error generated in a pixel already processed and a weight matrix (also called a diffusion matrix or the like) based on the position of the pixel of interest as input image data. Assigned to DI. The output color selection processing unit 82 performs a binarization process using a threshold value.

In a general error diffusion method, input image data DI is continuous tone data (halftone data), and output image data DO is binary data. By applying the error diffusion method, the color and density of the input image data DI, which is the original image, are maintained, and a relatively faithful image can be reproduced. Further, the image data is compressed by the binarization, and the memory capacity for storing the output image data DO is reduced. Such an image processing apparatus is known as JP-A-9-307776.

In the conventional binarization processing, for a full-color image, a signal depending on a device (output device or display device) such as a printer or a display, for example, each signal represented by CMYK or RGB is usually used. , Independently perform the binarization processing. Then, at the time of output (or at the time of display), these signals or outputs are combined to reproduce a pseudo full-color image.

[0007] However, the colorimetric value of each output color depends on the device. Also, when expressing the halftone using the same output device, even if the dot generation rates of the CMYK colors are the same, the colors will look different if the degree of overlap is different. In general color error diffusion processing, an input signal depending on a device is used, and furthermore, the problem of dot overlap in the same pixel is not controlled, so that it is difficult to obtain a desired color reproduction.

On the other hand, there has been proposed a method of performing halftoning by error diffusion processing using color vectors. This method is characterized in that input image data is handled as a multi-dimensional quantity as compared with the conventional technique which handles input image data as a one-dimensional quantity. As a color signal to be handled as a vector, it is naturally possible to use a device-dependent color signal. However, an input image expressed in a device-independent color space such as XYZ or CIELAB and an output device known in advance in an output device are used. Possible colors (Cyan, Magenta, Yellow,
Using the XYZ values of the eight colors of red, green, blue, white, and black or the CIELAB value (colorimetric values of the paper itself are often used as white), halftone processing is performed as follows. Thus, it is theoretically expected that the input and output colors are colorimetrically matched. 1) The colors that can be output are determined by comparing the input color (vector) with the output candidate color (vector) and determining the difference (vector) between the two.
Is selected by selecting a color that minimizes. That is, the output possible color closest to the input color in the color space is selected. 2) Calculate the error between the input color and the output color generated by the color selection. 3) With respect to the next unprocessed pixel, the input color is corrected by weighting and adding the error generated in the peripheral processed pixels to the input color, and the process of 1) is performed again.

This technique is also called vector error diffusion. In the conventional vector error diffusion, in the processing of 1), “the color that minimizes the sum of squares of the differences between the elements of the color vector” is a selection criterion. A selection is made.

[0010]

As described above, in the prior art, output possible colors are selected based on minimizing the sum of squares of the difference as the only selection criterion. When used in a color binary printer, color reproduction differs for each color region, and it is difficult to achieve accurate color reproduction. Specifically, there is a problem that the saturation of the low-saturation color or the achromatic color becomes higher than the target value of the output.

In the above-described conventional method, only the color is considered when determining the color of each pixel. Therefore, regardless of the quality of the color reproduction, dots that increase the visual granularity are generated. There was a case.

That is, in the conventional vector error diffusion method,
Since the output possible colors that minimize the color error are selected in the simple order, depending on the positional relationship between the target color and the output possible colors, an undesired low-frequency color pattern is generated, and the granularity is visually deteriorated.

If there are many types of colors that can be output,
Since it takes time to select a target outputable color from among them, the selection process has to be executed easily. The present invention has been made in view of the above-described problem, and has as its object to prevent the generation of an undesired color pattern and improve the reproducibility of an image.

[0014]

According to a first aspect of the present invention, a color error diffusion process is applied to input color data of each pixel of an image, and an appropriate color for each pixel is selected from a group of outputable colors. An image processing method for selecting and outputting an outputable color, wherein a specific function value is selected from a plurality of function values based on each color element of the input color and the outputable color group, and the selected specific value is selected. The output possible colors are selected using the function values.

According to a second aspect of the present invention, there is provided an output color selection processing unit for selecting an appropriate outputable color from a group of outputable colors for each pixel of an image, and the output color selection unit generates the output color selection processing unit. An image processing apparatus that has an error diffusion processing unit that diffuses an error to pixels around the pixel, and that applies color error diffusion processing to input color data of each pixel to select and output an appropriate outputable color Wherein the output color selection processing unit selects a specific function value from a plurality of function values based on each color element of the input color and the outputable color group, and uses the selected specific function value. Select the possible output colors.

According to a third aspect of the present invention, the output color selection processing section sets a specific output available color in the output available color group as a selection prohibition color, and sets the remaining output available colors as candidate colors. The output possible colors are selected from the candidate colors.

An apparatus according to a fourth aspect of the present invention has a look-up table storing data of an outputable color group corresponding to the input color, and the output color selection processing section uses the look-up table to perform the processing. Select a color that can be output.

[0018]

FIG. 1 is a block diagram showing an example of the configuration of an image processing apparatus 1 according to the present invention. In FIG.
The image processing apparatus 1 includes a peripheral error providing unit 11, an output color selection processing unit 12, an error calculation / storage unit 13, an attribute determination unit 14, a selection condition determination unit 15, and the like. The peripheral error giving section 11 and the error calculating / recording section 13 have the same functions as the functions described in FIG. That is, these correspond to the error diffusion processing unit in the present invention. The error diffusion processing itself by the error diffusion processing unit is the same as the conventional one, and the above-mentioned Japanese Patent Application Laid-Open No. 9-307776 can be referred to.

The output color selection processing section 12 selects an outputtable color based on the selection condition output from the selection condition determination section 15. That is, the output color selection processing unit 12 compares the image data corrected by the peripheral error providing unit 11 with the output available color output from the selection condition determination unit 15,
The available colors are selected based on the selection criteria output from the selection condition determination unit 15.

The selection condition determining section 15 provides the following selection criteria. A specific function value is selected from a plurality of function values based on each color element of the input color and outputable color group, and an outputable color is selected using the selected specific function value. In other words, an output-capable color is selected based on a function value different from the conventional function value that minimizes the sum of squares of the difference between the elements. A specific output available color in the output available color group is set as a selection prohibition color, and the remaining output available colors are set as candidate colors, and an output available color is selected from the candidate colors.

The attribute determining unit 14 acquires three attributes of hue, lightness, and saturation from the input image data DI as the target color, and determines the attribute of the input image data DI based on the three attributes. Note that, instead of the input image data DI, the attribute may be acquired from the corrected image data output from the peripheral error applying unit 11.

In the selection condition determining unit 15, the attribute determining unit 14
A specific selection criterion function (function value) is determined from a plurality of selection criterion functions (function values) in accordance with the attribute determined in (1). The selection condition determination unit 15 prohibits the selection of a specific outputable color in advance, and sets a set of candidate colors.

It should be noted that the possible output colors to be selected are different due to the change of the candidate colors and the selection criterion. However, the error in the target pixel is calculated as described above, and is determined by the effect of the error diffusion processing. It is possible to satisfy the condition within the area.

FIG. 2 is a diagram showing an example of the selection condition SJ, and FIG. 3 is a diagram showing an example of a function expression serving as a selection criterion. In FIG. 2, conditions 1 to n and other conditions are set as the selection condition SJ. In each of the conditions 1 to n, a set 1 to n is set as an outputable color group or a set of candidate colors extracted therefrom. And each set 1 to n
It is set that the function formulas F1 to Fn are minimized as selection criteria for.

Under other conditions, the above-described eight output possible colors in the case of binary are set as the candidate color set, and the sum of squares of the differences between the elements is minimized as a selection criterion. Conventional standards are set as standards. The selection condition determination unit 15 selects one condition from these selection conditions SJ.

In this example, both the candidate color set and the selection criterion are individually set for each condition.
A configuration in which only one of them is changed may be adopted. The output color selection processing unit 12 assigns values (DATA1 to DATAm) obtained from the selected specific function formula Fi to the target color and each of the determined candidate color sets (candidate colors 1 to m). Compare.

That is, DATA1 to DATAm are represented as follows. Fn (Target color, Candidate color 1) = DATA1 Fn (Target color, Candidate color 2) = DATA2... Fn (Target color, Candidate color m) = DATAm Candidate color which is the smallest of these function values DATA1 to DATAm That is, a candidate color k for which DATAk = Min is selected as an outputable color.

The function formula of FIG. 3A is shown as a sum of weights of α, β, and γ for the differences between the XYZ elements. The function expression of FIG.
The values obtained by subtracting the rate of change of the difference between the XYZ elements from 1 are weighted with α, β, and γ, respectively, and are shown as the sum total.

The function equation of FIG. 3C is expressed as XYZ-L * a *
After b * conversion, | ΔL | (brightness difference), | △ C |
(Saturation difference) and | △ H | (hue difference)
It is shown as the sum of the weighted values of α, β, and γ.

Generally, CIE1976L * a * b or L * u * v, which is widely used as a uniform color space for calculating color differences, is designed so that the color differences correspond to visual differences. However, it is well known that, in fact, different colors have different feelings even with the same color difference.

For example, a color having a low saturation such as gray may be more sensitive to a color difference than a color having a high saturation.
Also, even if the colors have the same color difference for the same color,
The permissible level differs depending on the direction of the deviation. Furthermore, complicated factors such as chromatic adaptation and observation conditions are also involved. Further, depending on the setting of the function formula, it is possible to further reduce the calculation load in the selection processing. In consideration of these various factors, the above-described method of the present embodiment is effective for designing an outputtable color or a candidate color.

Restricting the candidate color set is effective in the following cases. Specifically, for a color determined to be both high in lightness and high saturation in a hue near blue, if the color reproduction range of blue output by the printer is particularly narrow, blue is excluded from candidate colors and cyan is excluded. , Magenta, and white as candidate colors. This makes it easy to generate dots of juxtaposed color mixture, and enables reproduction of relatively high saturation.

Further, even when the colorimetric color reproducibility is satisfied to some extent, a pattern in which the perceived granularity is not preferable may occur. For example, when reproducing skin color, if dots such as black and blue are scattered, the subjective image quality deteriorates. It is also possible to limit the occurrence of a color that deteriorates the graininess as described above.

Next, another embodiment will be described. FIG. 4 is a block diagram illustrating an example of a configuration of an image processing apparatus 1B according to another embodiment of the present invention.

In the image processing apparatus 1B shown in FIG. 4, an LUT processing section 12B replaces the output color selection processing section 12 of the image processing apparatus 1 described in FIG. 15B are provided respectively.
This makes it possible to more quickly and easily select the colors that can be output.

That is, the LUT selection determining unit 15B includes:
A look-up table (LUT) storing data of outputtable colors corresponding to input colors is provided for each selection condition. The LUT selection determination unit 15B determines a specific lookup table from the plurality of lookup tables according to the attribute determined by the attribute determination unit 14. In addition, the LUT selection determining unit 15B
A selection criterion similar to that of the output color selection processing unit 12 is provided to the processing unit 12B.

The LUT processing unit 12B determines a color that can be output directly from the corrected image data (input color) based on the selection criterion provided by the LUT selection determination unit 15B using the selected lookup table. Output.

According to the above embodiment, a plurality of function values that can be calculated from each element of the vector are prepared without fixing the selection criterion to one, and a range of selection is provided. Further, among all the prepared output possible colors, the remaining output possible colors excluding the selection prohibition colors are set as candidate colors, and the output possible colors are selected from the candidate colors.

Therefore, according to the present embodiment, the frequency of occurrence of necessary colors can be increased or unnecessary colors can be reduced even if the image quality is deteriorated by processing based on uniform selection criteria as in the prior art. It is possible to control the frequency of occurrence and control the pattern of color and dot generation. This makes it possible to control the characteristics of the image according to the type or use of the image, prevent an undesirable color pattern from occurring, and improve the reproducibility of the image.

Further, by processing the selected outputtable colors into a look-up table and referring to the selected look-up table to obtain the output colors according to a desired selection criterion, the processing can be performed quickly and easily.

In the above-described embodiment, the selection of the output possible color group corresponding to the input color has been described. However, the edge / non-edge obtained from the input image data by using the spatial filter processing or the like is described. It is also possible to adopt a configuration in which the discrimination information of a copy is used as an attribute and the selection criterion is switched based on the result.

In the above embodiment, the selection of the outputtable color or the determination of the attribute is performed with one pixel as a unit. However, a block composed of m × n pixels (m and n are positive integers) is set as a unit. In other words, one block may be regarded as one pixel to select an outputtable color or determine an attribute. The determination of the attribute may be performed in a binary manner, or a parameter indicating a selection criterion, for example, a parameter represented by a function of lightness may be set and continuously set.

Further, a signal representing a photographic area, a graphic area, a character area, or the like is included in the input image data DI in advance, and these signals are sequentially referred to when performing the error diffusion processing. Good. Further, the various methods or configurations described above may be appropriately combined.

In the above-described embodiment, the configuration, circuit, processing contents,
The processing timing and the like can be appropriately changed in accordance with the gist of the present invention.

[0045]

According to the present invention, an undesirable color pattern can be prevented from occurring, and the reproducibility of an image can be improved.

According to the fourth aspect of the present invention, the processing is performed more quickly and easily.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating an example of a selection condition.

FIG. 3 is a diagram illustrating an example of a function expression serving as a selection criterion.

FIG. 4 is a block diagram illustrating an example of a configuration of an image processing apparatus according to another embodiment of the present invention.

FIG. 5 is a diagram illustrating an image processing apparatus using a conventional error diffusion method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Image processing apparatus 11 Peripheral error provision part (error diffusion processing part) 12 Output color selection processing part 13 Error calculation / storage part (error diffusion processing part) 14 Attribute determination part 15 Selection condition determination part 1B Image processing apparatus 12B LUT processing part (Output color selection processing unit) 15B LUT selection determination unit LUT lookup table

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5B057 BA23 CA01 CA07 CA12 CA16 CB01 CB07 CB12 CB16 CC01 CE13 CE17 CE18 CH11 5C077 LL03 LL19 MP01 MP08 NN11 PP31 PP35 PP36 PP37 PP43 PP47 PP68 PQ08 PQ12 PQ18 HQ05H08B08H08B08 HB11 LA31 LB01 LC09 MA04 MA11 NA02 NA03 NA11 NA27

Claims (4)

[Claims] 1. An image processing method for applying color error diffusion processing to input color data of each pixel of an image, selecting an appropriate outputable color for each pixel from an outputable color group, and outputting the selected color. Selecting a specific function value from a plurality of function values based on each color element of the input color and the outputable color group, and selecting the outputable color using the selected specific function value. An image processing method characterized by the following. 2. An output color selection processing section for selecting an appropriate output possible color from a group of output possible colors for each pixel of an image, and an error generated by the selection by the output color selection section is set to a pixel around the pixel. An image processing apparatus comprising: an error diffusion processing unit configured to apply a color error diffusion process to input color data of each pixel to select and output an appropriate outputable color. The processing unit selects a specific function value from a plurality of function values based on each color element of the input color and the outputable color group, and selects the outputable color using the selected specific function value. An image processing apparatus characterized in that: 3. An output color selection processing section for selecting an appropriate output possible color from a group of output possible colors for each pixel of an image, and an error generated by the selection by the output color selection section is set to a pixel around the pixel. An image processing apparatus comprising: an error diffusion processing unit configured to apply a color error diffusion process to input color data of each pixel to select and output an appropriate outputable color. The processing unit selects a specific output available color in the output available color group as a selection prohibition color, sets the remaining output available colors as candidate colors, and selects the output available color from the candidate colors. Image processing apparatus. 4. A look-up table storing data of an output possible color group corresponding to the input color, wherein the output color selection processing unit selects the output possible color using the look-up table. The image processing device according to claim 2 or 3.