JPH09326941A - Image processing unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the image processing unit in which high quality and adaptive color processing is attained by providing a correction means conducting color balance correction taking notice of balance of a specific color area where color unbalance is remarkable. SOLUTION: At first a reference color for color balance adjustment with respect to an image is set by a reference color setting section 101. A color by which color unbalance is easily remarkable is adopted for the reference color. Then a color extract section 102 extracts colors in the vicinity of the reference color from color image data BGR received by a preliminary scanning of a scanner or the like and a representative value arithmetic section 103 sets a representative color from the extracted color and a condition setting section 104 sets a correction condition of an image to a correction section 105 as to each color based on the reference color and the representative color. Thus, color image data RGB received by main scanning are subject to correction processing in response to a feature of an original image and adaptive image processing with high quality is attained.

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 that reproduces a color image with high quality, and more particularly to an image processing apparatus that executes color balance correction processing according to the characteristics of the image to be processed.

[0002]

2. Description of the Related Art In recent years, advances in image processing apparatuses for processing digital images, such as facsimiles, digital copying machines, image scanners, printers, and their interfaces.
The spread is remarkable. Along with this, there is a demand for higher image quality provided by the image processing apparatus. For example, a digital copying machine has conventionally been required to reproduce an image (copy) as faithful as possible to a document. However, when the original document itself is of low quality, it is preferable that the original image is appropriately corrected and output by performing signal processing unique to digital rather than faithfully reproducing the original document. The appropriate correction mentioned here includes, for example, a process of giving a good contrast by gradation correction, a process of emphasizing a character or a line to make it easy to read.

In some cases, various interfaces are connected to the digital copying machine to be used as a facsimile machine or a printer. For example, when used as a facsimile, generally, the output image and the original image to be compared are not at hand. Further, when used as a printer, since the image displayed on the monitor is output as an electronic file, it is extremely difficult to obtain a printed image that matches the quality of the displayed image. On the other hand, for these images, it is possible to obtain a more preferable reproduced image by detecting the characteristic of the image and adaptively performing the processing such as noise removal or range conversion according to the characteristic.

Such image processing is performed by contrast,
There are several items to be corrected such as saturation, exposure, and sharpness, and among them, there is image processing for correcting the hue and color balance. As a conventional technique for correcting the hue and color balance, for example, a technique is widely known in which attention is paid to the distribution state of RGB color signals, and the distribution range is unified to automatically correct the color balance of an image.

[0005]

However, the conventional system described above has the following problems. First, there is a problem in that it is difficult to obtain the correction effect when the image to be processed has a sufficient dynamic range. For example, even if an image has an apparently unbalanced color balance, the distribution of each primary color has a wide range from light to dark, or (mainly occurs during image input). When it is detected that the distribution of each primary color is sufficiently wide due to noise, gradation saturation, or the like, sufficient correction processing is not performed on the image, and the effect cannot be obtained. Secondly, the colors where the color balance deviation is detected the most are observing the colors that people should remember, such as human skin, the blue of the sky, and the green of trees, and what kind of color should be in the image. The color is known to the person. Therefore, even if the range is corrected based on the feature amount that is not directly associated with those colors, a good processing result is not always obtained.

The present invention has been made in view of the above problems, and an object of the present invention is to execute a color balance correction focusing on the balance of a specific color region in which the deviation of the color balance is apt to be noticeable to obtain high quality. An object of the present invention is to provide an image processing device capable of various adaptive color processing.

[0007]

In order to solve the above-mentioned problems, the present invention provides a reference color setting means for setting a reference color and a color within a predetermined range from the input image data. Representative color calculation means for extracting and calculating a representative color of the color, condition setting means for setting a correction condition to be applied to the input image data based on the reference color and the representative color, and setting It is characterized by comprising a correction means for performing correction on the input image data according to the specified conditions. According to the present invention, a reference color that is a reference for correction is set in advance, a color within a predetermined range of the reference color is extracted from the input image data, and a representative color of the extracted colors is calculated. The input image data is corrected such that the representative color approaches the reference color. Here, by setting the reference color as a color in which the deviation of the color balance becomes noticeable, it is possible to obtain an output having no color deviation with respect to the original image.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the arrangement of an image processing apparatus according to the first embodiment of the present invention. This image processing device has red (R) and green (G)
-A red (R ')-green (G')-blue (B 'is corrected by performing automatic image correction on a color image represented by a blue (B) digital signal based on the characteristics of the image. ) A system that generates a signal. In FIG. 1, reference numeral 101 denotes a reference color setting unit that sets a color that serves as a reference for image correction. A color extraction unit 102 extracts a color in the vicinity of the set reference color from the color image data supplied by the pre-scan performed before the main scan in a scanner (not shown) or the like. A representative value calculation unit 103 calculates the value (representative value) of a representative color (representative color) from the colors extracted by the color extraction unit 102. A condition setting unit 104 sets an image correction condition based on the representative value calculated by the representative value calculation unit 103 and the reference color set by the reference color setting unit 101. Reference numeral 105 denotes a correction unit that performs a correction process on the color image data supplied in the main scan according to the correction conditions set by the condition setting unit 104.

Next, the flow of image processing in the image processing apparatus having the above configuration will be described. In this image processing apparatus, a color image is read in two times, prescanning and main scanning, to perform adaptive color balance adjustment processing. That is, in this image processing apparatus, first, a color near the color set by the reference color setting unit 101 is extracted from the color image supplied in the first pre-scan, and based on the characteristics of the image. The correction condition is set. Then, the color image supplied in the next second main scan is subjected to the color adjustment processing of the correction condition set in the pre-scan.

The details of this operation will be described below. In this image processing apparatus, first, a reference color setting unit 101 sets a color that serves as a reference for color balance adjustment for an image. In FIG. 2, the point (r,
g and b) are reference colors set by the reference color setting unit 101. The reference color here is, for example, a memory color such as human skin or sky blue, or a color such as a highlight gray area in which a color shift is easily noticeable.

Next, a prescan is performed, and the color image data at that time is supplied to the color extraction unit 102. The color extraction unit 102 extracts the colors included in the range of the near region (r ± Δr, g ± Δg, b ± Δb) of the reference color set by the reference color setting unit 101. The shaded rectangular parallelepiped region in FIG. 2 is the reference color neighborhood region defined by (r ± Δr, g ± Δg, b ± Δb).

The representative value calculation unit 103, as described above,
The representative value of the extracted color is set, for example,
The average value (r _ave , g _{a ve} , b _ave ) of the colors extracted by the color extracting unit 102 is set as a representative value. The representative value is not limited to the average value, and may be the median value of the extracted colors, for example. Condition setting part 1
04 is the reference color (r,
g, b) and a representative value (r _ave , g _ave , b _ave ) of the color obtained by the representative value calculation unit 103, a one-dimensional LUT (look-up table) is created for each color. Then, the correction condition of the image is set in the correction unit 105.

Details of image correction conditions will be described below. FIG. 3A shows the representative colors (r _ave , r _ave , extracted from the original image) as the reference colors (r, g, b) and their neighboring colors.
g _ave, shows the positional relationship between the b _{av e).} In this relationship, the correction condition set in the condition setting unit 104 is the representative color (r _ave , g _ave ,
For b _{av e),} the reference color (r, g, be one that in b), for the color in the vicinity of the representative colors, as close to only reference color amount corresponding to the difference between the color and the representative color It is something to do. An example of the input / output characteristics of the LUT created in this case is shown in FIG. In this example, in the input / output characteristics of each color, when the input is the representative color (r _ave , g _ave , b _ave ), the coordinates whose output is the reference color (r, g, b) are the vertices. Although it is a polygonal line, this characteristic is not particularly limited to this, and for example, when the input is a representative color (r _ave , g _ave , b _ave ), it is a curve approximated by a polynomial or a spline function. It goes without saying that it is also possible to use a curve that passes through the same coordinates or neighboring coordinates.

When the correction condition is set in the pre-scan, the main scan is performed next. In the main scan, the data of the color image supplied thereby is referred to the LUT, that is, the BGR as shown in FIG.
The gradation correction is executed according to the input / output characteristics for each color. By such processing, the correction processing according to the characteristics of the original image is executed based on the set reference color.
High quality adaptive color processing becomes possible.

Next, an image processing apparatus according to the second embodiment of the present invention will be described. The image processing apparatus according to the present embodiment also corrects brightness and range in accordance with color balance correction. FIG. 4 is a block diagram showing the configuration of this image processing apparatus. In this figure, reference numeral 401 denotes an image memory, which stores image data to which color balance correction is applied. A monitor 402 displays an image stored in the image memory 401 via the video interface 403. A database 404 stores a reference image, color data, and the like used for color adjustment. An input unit 405 includes a keyboard, a pointing device, and the like, and inputs / sets necessary information via the I / O controller 406. An image editing unit 407 includes an image analysis unit 407.
The image data stored in the image memory 401 is processed according to the information input by the input unit 405. An internal bus 408 is used to transfer data, instructions, etc. between the above-mentioned components.

Next, the flow of image processing in the image processing apparatus having the above-mentioned configuration will be described. FIG. 5 is a flowchart showing the operation of the image processing apparatus according to this embodiment. In this image processing device, first, in step S50.
1, the RGB stored in the image memory 401
The image data that is the image data to be processed is
The image memory 4 is loaded into the image processing unit 407b and converted into an L ^* HC space of lightness, hue, and saturation, and then the image memory 4
It is stored again in 01. The color space conversion process performed here will be described with reference to FIG. 6A. RGB image data in the image memory 401 is gamma-converted by referring to the one-dimensional LUT 601 and then the matrix operation formula shown in the figure is used. Approximately transformed into L ^* a ^* b ^* space by the executing operation unit 602, of which a ^* b ^* is transformed from the Cartesian coordinate system to the polar coordinate system of HC by referring to the two-dimensional LUT 603, and the L ^* HC space is transformed. Is converted to image data.

Returning to FIG. 5 again, after the spatial transformation in step S501, in the next step S502, lightness correction for adjusting the overall brightness of the image with respect to the lightness L ^* is executed. Here, the brightness correction will be described.
In the brightness correction, first, the image analysis unit 407a (see FIG. 4) uses the brightness L ^* as shown in FIGS.
The histogram of is generated and it is determined whether or not the brightness correction is performed. This determination is performed by comparing the minimum value of the histogram distribution with the dark area threshold L1, and further comparing with the maximum value of the distribution and the bright area threshold L2. As a result of this comparison, as shown in FIG. 7A, the minimum value of the histogram distribution is smaller than the dark area threshold L1, and
If the maximum value of the distribution is larger than the bright area threshold L2,
Since the brightness of the image is widely spread over the entire image, it is determined that no correction is performed. Further, as shown in FIG. 7B, when the minimum value of the histogram distribution is smaller than the dark area threshold L1 and the maximum value of the distribution is also smaller than the bright area threshold L2, the brightness of the image is Since it is biased toward the dark direction, it is decided to perform correction in the bright direction. On the other hand, FIG.
As shown in (c), if the minimum value of the histogram distribution is larger than the dark area threshold L1 and the maximum value of the distribution is also larger than the bright area threshold L2, the brightness of the image is biased toward the bright side. Therefore, it is decided to correct the darkening direction. Then, as shown in FIG. 7D, if the minimum value of the histogram distribution is larger than the dark area threshold L1 and the maximum value of the distribution is smaller than the light area threshold L2, the brightness of the image is reduced. Has an appropriate brightness, so it is determined that no correction is performed. When the image analysis unit 407a determines to perform the correction, the image processing unit 407b determines that L ^* _out = L.
Brightness correction is performed by calculating ^* _in γ. Where γ is L ^*
Is determined from the bias of the distribution of, and when correcting brightly, γ
The brightness correction parameter is <1 and γ> 1 for darkening. The brightness correction is executed by such processing, and the processed image is stored in the image memory 401.

Returning to FIG. 5, after the image determination / brightness correction in step S502, range correction is performed on the lightness L ^* in the next step S503. This correction is to improve the contrast by expanding the distribution of lightness L ^* to its full quantization range. The range correction will be described. As with the brightness correction, the image analysis unit 407a first generates a histogram of the brightness L ^* and detects the highlight and shadow representative points, respectively. The detection of the representative point is performed by detecting a point showing a frequency of 5% of the whole from the brightest part and the darkest part. After that, the image processing unit 407b
_Performs range correction by calculating L ^* _out = α · L ^* _in + β. Here, α and β are range correction parameters, respectively, which are both determined from the highlight and shadow representative points. With such processing, range correction is executed, and the processed image is stored in the image memory 401.

Next, in step S504, color balance correction conditions are set for the image subjected to the brightness correction / range correction. In setting the color balance correction condition, first, the input unit 405 sets a color that serves as a reference for color balance adjustment for an image. At this time, as the reference color, a specific color may be designated from the color data stored in the database 404,
It may also be designated by selecting a specific area of the reference image also stored in the database 404. The reference color designated here is calculated and converted into a value (l, h, c) belonging to the same L ^* HC space as the image data. Second
Further, in the image analysis unit 407a, similar to the first embodiment, the neighboring regions (l ± Δl, h ± Δh, c ± Δ of the reference color are used.
The colors included in the range of c) are extracted, and the average value (l
_ave , h _ave , c _ave ) are set as representative values.

By the way, the actual color balance correction is
It is performed in the RGB color space, not the L ^* HC space. Therefore, thirdly, the image processing unit 407b sets the reference colors (l, h, c) and the representative values (l _ave , h _ave , c _ave ) set by the input unit 405 to RGB colors by a method described later. The values (r, g, b) and (r in space
_ave , g _ave , b _ave ). Followed by a fourth image analysis unit 407a, the reference color (r, g, b) the maximum value Max _base of the representative value _{(r ave, g ave, b} av e) the maximum value Max _ave of And, respectively. That is, Max _base = Max (r, g, b), Max _ave = Max (r _ave , g _ave , b _ave ).

Fifthly, the image analysis unit 407a causes the ratio (rateR _base , rateG _base , rateB _base ) of the reference color (r, g, b) to the maximum value Max _base .
Is calculated by the following equation. That is, it is calculated by rateR _base = r / Max _base , rateG _base = g / Max _base , rateB _base = b / Max _base .

And sixthly, the image analysis unit 407a
Using the obtained ratio, the input / output characteristics of the LUT referred to in the correction are set to those shown in FIG. 8, for example. That is, the input / output characteristic of the LUT is set to be a polygonal line having a vertex whose coordinates are Max _ave as an input among the straight lines determined by the respective ratios. As a result, each primary color ratio of the representative color is brought close to each primary color ratio of the reference color. In addition,
The input / output characteristics shown in this figure are set under the following conditions. That is, such input / output characteristics are settings when Max _base > Max _ave , r _ave > g _ave > b _ave , rateR _base > rateG _base > rateB _base .

In this way, when the color balance correction conditions are set in the first to sixth steps, the next step S50
In 5, the image data to be processed is reconverted into the color space of the RGB space which is the processing space. In detail,
The L ^* HC image data stored in the image memory 401 is loaded into the image processing unit 407b, where the brightness /
The L ^* HC space of hue / saturation is converted to the RGB3 primary color space and is stored again in the image memory 401. The color space conversion process performed here is shown in FIG.
Explaining with reference to (b), L ^{* of the} image memory 401
The HC image data is converted from the polar coordinate system of HC to the Cartesian coordinate system of a ^* b ^* by referring to the two-dimensional LUT 604, and then the calculation unit 605 that executes the matrix calculation formula shown in the figure.
Approximate conversion to RGB space by, and one-dimensional LU
Gamma conversion is performed in T606, and the image data is converted into RGB space image data.

In step S506, the image processing unit 407b refers to the set LUT and corrects the color balance of the converted image data in the RGB space. With the above, a series of image correction processing is completed. In this way, by storing the maximum value of the input signal and correcting each primary color component ratio, it is possible to correct only the color balance while maintaining the overall color depth of the original image. become. As a result, it is possible to correct the color balance without impairing the effects of the lightness correction and range correction performed prior to the color balance correction.

In each of the above embodiments, the RG
Although color balance correction in B space has been shown, it is clear that the invention is not so limited. Further, it is easily inferred that it is also possible to give a plurality of reference colors to the image processing system and set the correction curve.

[0026]

As described above, according to the present invention,
It is possible to perform color balance correction focusing on the balance of a specific color area where the color balance deviation is noticeable, and high-quality adaptive color processing is possible.

[Brief description of drawings]

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

FIG. 2 is a diagram for explaining the concept of the positional relationship between a reference color and its neighboring region in the same embodiment.

FIG. 3A is a diagram for explaining the concept of the positional relationship between the reference color and the representative color, and FIG. 3B is a diagram showing the input / output characteristics of the LUT.

FIG. 4 is a block diagram showing a configuration of an image processing apparatus according to a second embodiment of the present invention.

FIG. 5 is a flowchart showing an operation of the image processing apparatus according to the same embodiment.

FIG. 6A is a block diagram showing a configuration for performing color conversion from RGB space to L ^* HC space,
(B) is a block diagram showing a configuration for performing color conversion from the L ^* HC space to the RGB space.

7A to 7D are diagrams showing a relationship between a histogram of brightness and a threshold value in image data.

FIG. 8 is a diagram showing input / output characteristics of an LUT.

[Explanation of symbols]

101 ... Standard color setting unit (standard color setting means), 102 ...
... Color extraction unit, 103 ... Representative value calculation unit (102 and 1
A representative color calculation means by 03, 104 ... Condition setting section (condition setting means), 105 ... Correction section (correction means)

Claims (5)

[Claims] 1. A reference color setting means for setting a reference color, a representative color calculating means for extracting a color in the vicinity of the reference color from input image data, and calculating a representative color of the reference color, the reference color Condition setting means for setting a correction condition to be applied to the input image data based on the color and the representative color, and correction means for performing the correction according to the set condition on the input image data. An image processing apparatus comprising: 2. The image processing apparatus according to claim 1, wherein the condition setting unit sets a condition for bringing the representative color closer to the reference color. 3. The image processing apparatus according to claim 1, wherein the condition setting unit sets a condition for bringing each primary color gradation of the representative color closer to each primary color gradation of the reference color. 4. The image processing apparatus according to claim 1, wherein the correction unit applies the condition set by the condition setting unit to the entire input image data. 5. The image processing apparatus according to claim 1, wherein the condition setting unit sets a condition for bringing each primary color ratio of the representative color closer to each primary color ratio of the reference color.