JPH11185017A - Color image monochromating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide images for which optimum gradation conversion is applied to respective hues almost without eliminating the gradation of a specified hue, yellow for instance. SOLUTION: This monochromating device reads the L*a*b value of one picture element of a color image by a scanner 3 (step 501). In a computer, saturation C and a hue angle H are calculated from the L*a*b value (step 502). Then, it is retrieved to determine between which hue angles of a lookup table 19 for indicating the correspondence relation of a hue angle and a correction coefficient, the hue angle H of the color image is present (step 503). The hue angles before and after the hue angle H of the color image are read the lookup table 19 and a correction amount corresponding to the hue angle H of the color image is calculated by using arithmetic interpolation (step 504). By using the obtained correction amount, one picture element of the color image is appropriately monochromated (step 505).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for converting a color image into a monochrome image and converting the color image into a monochrome image.

[0002]

2. Description of the Related Art Conventionally, when a color image is converted to monochrome, brightness information of the image is mainly used. One of the methods for expressing brightness information is an L * a * b * color space.
In the L * a * b * color space, L * represents lightness,
Taking a value of 0, the larger the value, the higher the brightness. That is, it is a bright image. A * indicates the intensity in the red to green direction, and generally takes a value of -100 to 100 (this range may be exceeded). Red for positive values,
A negative value indicates green. B * represents the intensity of the color in the yellow to blue direction, and generally takes a value of -100 to 100 (may exceed this range). Yellow for positive values,
A negative value indicates blue.

When a monochrome image is expressed in an L * a * b * color space, a * = b * = 0 for all pixels.
Only L * has a value. On the other hand, color images are L *, a *
, B *. Therefore, in order to convert a color image into a monochrome image, the operation may be performed according to the equation (1). The lightness value of the color image is L *, and the L * a * b * values of the monochrome image are L * ', a *', b * '
Then, L * 'is expressed as a function of L *. A * 'and b *' are always 0 irrespective of the values of a * and b * of the color image. L * '= f (L *) a *' = 0 b * '= 0 (1)

[0004]

By the way, in a general color image, in a region having a yellow hue, the range of L * is about 70 to 90, which is considerably narrower than the range of other colors. . Therefore, when L * 'is converted to monochrome as a function of only L *, if it is converted so as to make use of the gradation of achromatic or other color portions in a color image,
There is a problem that the yellow gradation almost disappears. Further, if the conversion is performed so as to make use of the yellow gradation, the gradation of an achromatic color or another color will be lost.

The present invention has been made in view of such a problem, and an object of the present invention is to prevent a specific hue, for example, a yellow tone from almost disappearing. An object of the present invention is to provide a color image monochromizing device capable of obtaining an image subjected to optimal gradation conversion.

[0006]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention provides a color image characterized by converting a color image into a monochrome image by using a correction coefficient corresponding to a hue angle and saturation. Device.

In the present invention, first, a *,
The saturation C and the hue angle H are determined from b *. C = (a * 2 + b
* 2) 1/2 When a *> 0, b * ≧ 0 H = atan (b * / a *) When a * = 0, b * ≧ 0 H = π / 2 a * <H = Π + atan (b * / a *) a * = 0, b * <0 H = 3π / 2 a *> 0, b * <0 H = 2π + atan (b * / a *) 2) Monochrome conversion of a color image is performed by equation (3) using L * and C and H determined by equation (2). L * '= L * + g (H) C a *' = 0 b * '= 0 (3)

[0008] g (H) is a function of the hue angle H, and indicates the correction amount when the saturation C at a certain hue angle H is "1". g (H) can be determined by the user so that optimum correction can be applied. By using the expression (3), monochrome conversion can be performed while correcting each hue.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram of a color image monochrome conversion apparatus 1 according to one embodiment of the present invention. As shown in FIG. 1, the monochrome conversion device 1 includes a scanner 3 and a computer 5. The scanner 3 reads an original color image 11 and a color image. The computer 5 performs appropriate processing on the image read by the scanner 3.

In the monochrome converting apparatus 1, when a color image is converted to a monochrome image, the color image is read by the scanner 3, the computer 5 performs the conversion of the expression (3), and the color image is expressed by the expression (3). Monochrome conversion is performed using the correction coefficient according to the hue angle and the saturation.

Next, the processing for obtaining the function g (H) of the equation (3) and performing monochrome conversion will be specifically described. FIG.
FIG. 3 is a flowchart and an explanatory diagram showing a procedure for creating the lookup table 19. As shown in FIGS. 2 and 3, the original color image 11 is read, and L * a
A * b * value (L * 1, a * 1, b * 1) is obtained (step 201). As the original color image 11 in FIG. 3, an image containing color patches in which the colors are uniformly distributed for all hues is prepared. For example, an IT8.7 / 1-1993 color target specified in ISO 12641 is optimal. This is a color patch made of a transparent reversal film.
a * b * values are specified.

The original color image 11 is read by the scanner 3, and the computer 5 generates the teacher monochrome image 15 by using the image processing software 13. The teacher monochrome image 15 is an image obtained by optimally converting the original color image 11 to monochrome. That is, L * a * of the teacher monochrome image 15
The b * value (L * 2, 0, 0) is obtained (step 202).
The teacher monochrome image 15 can also be obtained by performing correction when scanning a color target using the color correction function of the plate-making scanner.

L * a * b * of original color image 11
The values (L * 1, a * 1, b * 1) are converted to the teacher monochrome image 1
5 with the L * a * b * value (L * 2, 0, 0)
The lookup table 19 is created by obtaining the interpolation coefficient T (H) (step 203). That is, the values obtained by converting the L * a * b * values (L * 1, a * 1, b * 1) of the original color image 11 into the saturation and the hue angle using the equation (2) are expressed as (L * 1
, C1, H1) and L * a * of the teacher monochrome image 15
Let the b * value be (L * 2, 0, 0). The look-up table 19 is created by obtaining the correction coefficient T (H1) at the hue angle H1 by equation (4). T (H1) = (L * 2-L * 1) / C1 (4) FIG. 4 shows a lookup table 19 to be created.
A correction coefficient T (H) is determined for the hue angle H. IT8.7 / 1-
When a teacher monochrome image is created using the color correction function of a plate-making scanner using a 1993 color target, when correction coefficients are obtained for a total of 12 color patches of A to L rows and 7 columns, almost all hue angles are obtained. A look-up table 19 having 12 values distributed evenly can be obtained.

Next, a process for converting a color image into a monochrome image using the look-up table 19 obtained as described above will be described with reference to FIG. First, a color image is read by the scanner 3 (step 501).
Converts the value of L * a * b * to equation (2) and calculates the saturation.
The hue angle is calculated (step 502). Next, a search is performed to determine which hue angle of the calculated hue angle H (for example, hue angle H3) is in the look-up table 19 (step 5).
03). That is, hue angles H2 and H2 satisfying H2 <H3 <H4
4 is obtained from the lookup table 19. For example, H3
= 1 / 4π, H2 = 1 / 6π, H4 = 1 / 3π
Is calculated.

Next, a correction coefficient for the hue angle H3 is obtained from the lookup table 19 by interpolation (step 504). That is, H2 and H4 are linearly interpolated using the equation (5), and the correction amount g (H3) at H3 is calculated as follows: g (H3) = {(H4-H3) T (H2) + (H3-H2) T ( H4)} / (H4-H2) (5) FIG. 6 is an explanatory diagram of the processing in step 504. As shown in FIG. 6, the value of g (H3) is determined from the values of T (H2) and T (H4) using linear interpolation.

This correction amount g (H 3) is calculated as g
By substituting into (H), an L * a * b * value (L * ', 0, 0) appropriately monochrome for one pixel in the color image is obtained (step 505). The above operation is performed on all the pixels of the color image to obtain an optimal monochrome image.

The form of the color image to be read is L * a *
Other than b * value, for example, in the case of RGB, RG
Perform B → L * a * b * conversion and then convert to monochrome.
Similarly, if the form of the monochrome image to be output is other than the L * a * b * value, for example, CMYK, the L * a *
Output after b * → CMYK conversion.

[0018]

As described above in detail, according to the present invention, a specific hue, for example, a yellow gradation is hardly lost, and an optimum gradation conversion is applied to each hue. The obtained image can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a color image monochrome converting apparatus 1.

FIG. 2 is a flowchart for creating a lookup table 19;

FIG. 3 is a diagram illustrating a procedure for creating a lookup table 19;

FIG. 4 is a diagram showing a lookup table 19;

FIG. 5 is a flowchart showing a procedure for converting one pixel in a color image into a monochrome image;

FIG. 6 is an explanatory diagram showing processing of step 504.

[Explanation of symbols]

 1. Monochrome conversion device 3. Scanner 5. Computer

Claims (2)

[Claims] 1. Using a correction coefficient according to a hue angle and saturation,
An apparatus for converting a color image into a monochrome image. 2. An apparatus for converting a color image into a monochrome image, wherein a correction coefficient used for the conversion is calculated according to a hue angle and a saturation.