JPH07250253A - Image signal converting device - Google Patents

Abstract

PURPOSE:To suppress the generation of moire by removing specified cyclicity from an image signal in the case of reading a dot original with smoothing processing, and converting that image signal to a standard chrominance signal. CONSTITUTION:An image signal input part 1 is composed of photoelectric converting elements such as a color scanner and a CCD camera and gets the input image signal of respective eight bits of R, G and B, for example, by conducting A/D conversion while reading the original image and getting color separation signals in three colors of red, green and blue. At a smoothing processing part 2, smoothing filter processing is conducted to the input image signal. This smoothing filter processing is conducted to the RGB image signals of linear reflectance. A purpose to conduct the smoothing filter processing is to remove the moire of the printed dot original, so that in this case, it is most effective to conduct the processing to the signals of linear reflectance. Further, at a color converting circuit 3, the chrominance signals are converted from the smoothing filter processed RGB signals into L*a*b* image signals corresponding to lightness, hue and saturation spaces.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image signal converting apparatus for image processing and image quality improving processing for handling a color image signal, and is particularly applied to an image processing apparatus such as a color scanner, a color copying machine and a color FAX. Is suitable.

[0002]

2. Description of the Related Art Conventionally, in an image processing apparatus such as a color copying machine, an image output unit such as a printer receives an image signal of RGB with three substantially linear reflectances from an image signal input unit such as a scanner. In general, image processing is performed by converting to a density linear CMYK signal corresponding to the ink amount in.

However, in the recent image processing apparatus, in consideration of the ease of color processing by the user and the interface with external equipment, the RGB signal whose reflectance is approximately linear with respect to the original image is generated inside the image processing apparatus. , A standard color signal defined in consideration of human visual characteristics, for example, brightness (luminance)
And L * a * b * signals, which are represented by chromaticity (hue and saturation), can be temporarily converted and used directly for color processing or output to the outside. Has become.

Further, a standard color signal that can be mutually handled by the transmitting side and the receiving side is required as a color signal for color image communication by color facsimile, but at the present time, the above-mentioned L * a * is required. It is effective that the b * signal is adopted as a standard color signal in color image communication.

By the way, in order to maintain the resolution of a reproduced image and to suppress the generation of moire when a halftone image is used as a document, a spatial filter process for image data is widely known as an effective means. For example, in FIG. 3 of Japanese Patent Application No. 5-239559 of the present applicant, edge enhancement is performed on a density linear CMYK signal generated by table conversion after smoothing processing on a reflectance linear RGB signal. We propose a technology that improves the image quality by applying processing.

However, in order to obtain the standard color signal by utilizing the characteristics of this technique, the density linear signal after the edge enhancement processing is converted again into the standard color signal such as the L * a * b * signal. Therefore, the number of signal conversion circuits is increased, which is not preferable from the viewpoint of maintaining the gradation of the input image signal, and there is a problem that the hardware scale becomes large.

Further, Japanese Patent Application Laid-Open No. 4-236 by the present applicant.
No. 571 discloses a technique of generating a hue and a saturation signal from a smoothed RGB signal, generating a lightness signal from an unprocessed RGB signal, and then synthesizing the two.
In this technique, since the smoothing process is not performed on the brightness signal, the effect of the filtering process for improving the image quality of the output image, that is, the process effect for achieving both the image resolution and the gradation, and the print document The treatment effect for suppressing moire could not be obtained sufficiently.

[0008]

In view of the above problems, the present invention is an image signal conversion for obtaining a standard color signal (for example, L * a * b * signal) from an input image signal (for example, RGB signal). In the apparatus, an image signal conversion apparatus capable of obtaining a sufficient effect of filtering processing for improving the image quality of an output image, achieving both image resolution and gradation, and obtaining a standard color signal in which moire of a printed document is suppressed The purpose is to provide.

It is another object of the present invention to provide an image signal conversion device in the above image signal conversion device, which maintains the gradation of the input image signal and suppresses the hardware scale.

[0010]

In order to achieve the above object, according to the invention of claim 1, in an image signal conversion device for converting an input image signal into a standard image signal, an image signal for inputting a color image signal is inputted. The present invention is characterized by input means, signal processing means for performing smoothing processing on the color image signal, and color space conversion means for converting the output of the signal processing means into a standard image signal corresponding to the brightness and chromaticity space.

According to a second aspect of the present invention, in an image signal conversion device for converting an input image signal into a standard image signal, image signal input means for inputting a color image signal and smoothing processing for the color image signal. A first signal processing means, a color space conversion means for converting the output of the signal processing means into a standard image signal corresponding to the brightness and chromaticity space; and an edge enhancement processing for only the image signal corresponding to the brightness. It is characterized by two signal processing means.

According to a third aspect of the present invention, in an image signal conversion device for converting an input image signal into a standard image signal, image signal input means for inputting a color image signal and smoothing processing for the color image signal are performed. First signal processing means for applying, color space conversion means for converting the output of the signal processing means into a standard image signal corresponding to lightness and chromaticity space, image signal corresponding to lightness and image signal corresponding to chromaticity Is characterized by a second signal processing means for performing edge enhancement processing with different intensities.

The invention according to claim 4 is characterized in that the color signals corresponding to the lightness, hue, and saturation spaces are L * a * b * signals.

[0014]

A standard color signal is obtained in which moire of a print original is removed while achieving both image resolution and gradation by performing smoothing processing on the input linear RGB signal of the reflectance and performing color conversion. By reducing the number of conversion blocks of an image signal and suppressing the hardware scale, a standard color signal faithful to the gradation of an input image can be obtained.

[0015]

1 is a block diagram of an image signal converting apparatus according to a first embodiment of the present invention. Reference numeral 1 is an image signal input unit, 2 is a smoothing processing unit, 3 is a color conversion circuit, and 4 is an image signal output unit.
The details of each block are described below.

The image signal input unit 1 is a color scanner, C
It is composed of a photoelectric conversion element such as a CD camera, reads a document image, and obtains color separation signals of three colors of red, green, and blue.
By performing / D conversion, for example, an input image signal of 8 bits for each RGB is obtained. In the image signal input unit 1, the input image signal is obtained as a reflectance linear signal that is linear with respect to the reflectance.

The smoothing processing section 2 performs a smoothing filter process on the input image signal. The smoothing filter processing according to the present invention is performed on the RGB image signal having the linear reflectance, and the standard color signal after conversion (Lab image signal in this embodiment) is not subjected to the smoothing processing. The purpose of performing the smoothing filter process is to remove moire from a printed halftone dot original document, and the most effective way to do this is to process a linear reflectance signal. The size and coefficient of the smoothing filter in the smoothing processing unit 2 are matters to be selected at the time of designing, and for example, filter coefficients as shown in FIG. 2 are used.

In the color conversion circuit 3, the color signal is converted from the RGB image signal after the smoothing filter processing to the L * a * b * image signal which is a standard color signal corresponding to the brightness, hue and saturation space. To do. As a color conversion method, conversion by a memory map method that obtains a converted image signal data corresponding to an input image signal by referring to a table is suitable.

For example, Japanese Patent Application Laid-Open No. 5-758 by the present applicant
The conversion by the memory map method of the triangular prism interpolation method described in No. 48 will be described with reference to FIG. In the triangular prism interpolation method, XY that defines a three-dimensional memory address of converted image data
The Z-axis is the RBG axis, and the input RGB image signal is divided into upper 2 bits and lower 6 bits for handling.
As shown in FIG. 3, the RG of the input image signal is plotted on the RGB coordinate axes.
The upper 2 bits of B are divided into four, and the corresponding cube (including the memory address of the input image signal) of the input image signal is determined. Next, the triangular prism corresponding to the input image signal (including the memory address of the input image signal) is determined by the RB lower 6 bits of the input image signal. Further, the conversion image data corresponding to the input image data is obtained by performing an interpolation operation using each interpolation coefficient set at each vertex of the determined triangular prism.

Here, the converted L * a * b * signals are each 8
It is output in bits, but the range of the value of each signal is
In the L * a * b * color space, it is linearly assigned as follows. Value of converted output signal L * signal; 0-255 0-100 a * signal; 0-255-80-80 b * signal; 0-255-60-100

Here, each interpolation coefficient used when obtaining the converted image data (a in Japanese Patent Laid-Open No. 5-75848)
i, bi) may be experimentally determined from the following relational expression (1). That is, an appropriate number (for example, 512) of color patches are prepared, and RGB values read by the target scanner and L * a * b * read by an appropriate colorimeter.
By determining the coefficient from the value data pair group using the least squares method, the values k11 to k34 (constants) of the respective lattice points of the triangular prism in the equation (1) are obtained.

L * = k11 × R + k12 × G + k13 × B + k14 a * = k21 × R + k22 × G + k23 × B + k24 b * = k31 × R + k32 × G + k33 × B + k34 Formula (1)

In the present embodiment, the color conversion circuit has been described as a memory map system, but another system, for example, a system for performing a masking calculation performed on the input image signal based on the above formula (1). May be used.

The image signal output section 4 is an image output means such as a color printer, and L * a * b * from the color conversion circuit 3.
A density linear CMYK signal is generated from the signal and an image is output. The image signal output unit 4 may be configured as an interface unit for outputting the L * a * b * signal converted into the standard signal to an external device for color processing or communication.

A second embodiment of the present invention including the edge enhancement processing section 5 will be described with reference to FIG. The image signal input unit 1, the smoothing processing unit 2, and the color conversion circuit 3 are the same as those in the first embodiment, and therefore their explanations are omitted. The edge enhancement processing unit 5 performs edge enhancement processing for adding a filter output to the image signal input from the color conversion circuit 3. An example of this filter coefficient is shown in FIG.

What is important in the flow of processing of this apparatus is that the smoothing process is applied to all the RGB color signals before conversion to standard color signals, and the edge enhancement process is converted to standard color signals, and then the brightness is changed. This is to be applied only to a signal representing (luminance) information, that is, an L * signal. The reason why edge enhancement processing is performed only on the L * signal is that controlling the contrast of light and dark is the most effective in view of human visual characteristics, and the filtering processing is performed only on the signal component representing the luminance information. It is possible to obtain the desired effect of the filtering process and to suppress the circuit scale of the filtering process.

Further, as shown in FIG. 6, edge enhancement processing is not performed on the linear reflectance RGB signal, but edge enhancement processing is performed on the L * signal obtained by multiplying the RGB signal by [1/3]. As a result, the change rate of the L * signal increases in the dark portion where the density of the image signal to be processed is high, so that the effect of contrast enhancement in the dark portion increases. As a result of this edge enhancement processing, even thin lines such as characters can be reproduced with a fine and clear resolution.

FIG. 7 shows the third embodiment in the case where the emphasis processing is required up to the color edge in the input image. That is, the brightness L
This is an embodiment corresponding to the case where edge emphasis is required at a portion where the hue and the saturation change with a constant value. However, Example 2
However, as described above, it is effective to apply edge enhancement processing to the lightness signal in order to effectively enhance the edge in terms of visual characteristics. Therefore, the hue and saturation (specifically, a * b *) are Since the edge emphasis filter to be applied may be weaker than that of lightness, for example, if the L * signal which is a lightness signal is multiplied by the filter coefficient of the second embodiment shown in FIG. 2, it is a hue and saturation signal a. The * b * signal may be multiplied by a filter coefficient as shown in FIG.

Further, as an application example of the present invention, it is possible to determine the color component of the input image data and switch the edge enhancement processing in the second and third embodiments according to the input image data. Is.

In the embodiment of the present invention described above, the brightness,
L * a * as a standard color signal corresponding to space in hue and saturation
Although the b * signal has been described as an example, the present invention can be configured by using a YIQ signal as a standard color signal.

Further, although the edge emphasis processing in the second and third embodiments is explained as the linear edge emphasis processing, it is not limited to the linear edge emphasis processing, and is shown in, for example, Japanese Patent Laid-Open No. 61-261966. It goes without saying that the present invention also includes a configuration for performing non-linear edge enhancement processing corresponding to the edge degree of the target pixel as described above.

[0032]

As described above, according to the first aspect of the invention, the image signal (R
After the periodicity which causes moire from GB) is effectively removed by smoothing processing, the standard color signal L * a * b
In order to convert to *, it is possible to effectively suppress the occurrence of moire when an image is reproduced or transmitted by the standard color signals L * a * b *.

According to the second aspect of the invention, in addition to the effect of the first aspect of the invention, it is possible to perform edge enhancement processing that matches human visual characteristics, and it is possible to achieve both resolution and gradation. Therefore, the hardware scale can be suppressed.

According to the third aspect of the present invention, it is possible to further perform the edge emphasis processing of the color edge portion such that the brightness is constant and the hue or the saturation is changed.

[0035]

[Brief description of drawings]

FIG. 1 is a block configuration diagram of a first embodiment of the present invention.

FIG. 2 is an example of smoothing filter coefficients of the present invention.

FIG. 3 is a diagram illustrating a memory mapping method of the present invention.

FIG. 4 is a block configuration diagram of a second embodiment of the present invention.

FIG. 5 is an example of edge enhancement filter coefficients for a lightness signal of the present invention.

FIG. 6 is a diagram illustrating edge enhancement processing for a brightness signal of the present invention.

FIG. 7 is a block configuration diagram of a third embodiment of the present invention.

FIG. 8 is an example of edge enhancement filter coefficients for chromaticity and saturation signals of the present invention.

[Explanation of symbols]

 1 image signal input unit 2 smoothing processing unit 3 color conversion circuit 4 edge enhancement processing unit 5 image signal output unit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location H04N 1/46 G06F 15/68 310 A H04N 1/46 Z

Claims (4)

[Claims] 1. An image signal conversion device for converting an input image signal into a standard image signal; image signal input means for inputting a color image signal; and signal processing means for smoothing the color image signal. An image signal conversion device characterized by color space conversion means for converting the output of the signal processing means into a standard image signal corresponding to a lightness and chromaticity space. 2. An image signal conversion device for converting an input image signal into a standard image signal, and image signal input means for inputting a color image signal, and first signal processing for smoothing the color image signal. Means, a color space conversion means for converting the output of the signal processing means into a standard image signal corresponding to the brightness and chromaticity space, and a second signal processing means for performing edge enhancement processing only on the image signal corresponding to the brightness. A characteristic image signal conversion device. 3. An image signal conversion device for converting an input image signal into a standard image signal, and image signal input means for inputting a color image signal, and first signal processing for smoothing the color image signal. Means, a color space converting means for converting the output of the signal processing means into a standard image signal corresponding to the brightness and chromaticity space, and an edge of different strength for the image signal corresponding to the brightness and the image signal corresponding to the chromaticity. An image signal conversion device characterized by a second signal processing means for performing an emphasis process. 4. The image signal conversion apparatus according to claim 1, wherein the color signals corresponding to the lightness and chromaticity space are L * a * b * signals.