JPH0366872B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image sharpness enhancement method used in color scanners for plate making, laser color printers, facsimiles, and the like.

When emphasizing the sharpness of an image, the difference between the sharp signal and the unsharp signal obtained from the image is multiplied by a constant that determines the degree of sharpness emphasis to form a sharpness emphasis signal, and this sharpness emphasis signal is applied to the image. A method of adding the signal to the signal is generally used. By the way, when the image is a color image, it is separated by a three-color filter, and the above-mentioned sharpening process is applied to each of the obtained three-color image signals. Now, the input three color image signals are represented by Ii (i = 1, 2, 3), the sharp signal is represented by Si (i = 1, 2, 3), and the unsharp signal is represented by Ui (i = 1, 2, 3). Then, the image signal Ii' (i=1, 2, 3) after the sharpening process is stabilized as Ii'=Ii+Ki·(Si−Ui) (1). Note that Ki is a constant parameter that determines the degree of sharpness enhancement.

However, in the sharpness enhancement method of equation (1) above, it is necessary to form sharpness enhancement signals Ki·(Si−Ui) for three colors, which has the disadvantage of complicating the processing device. Therefore, as a conventional method for simplifying the enhancement method expressed by equation (1) above, the sharpness enhancement signal is
A method in which (Si−Ui) of Ki·(Si−Ui) is represented by one of three colors has been widely used. That is, if the sharpness enhancement method for one selected color is K _i (S ₀ - U ₀ ), then in this method, the image signal Ii' after sharpening processing is Ii' = Ii + K _i (S ₀ - U 0 ). ₀ ) ...(2).

However, with such a conventional simple method, a problem may occur unless the setting of the constant parameter (Ki) that determines the degree of sharpness enhancement is devised. This will be explained below using an example.

For example, consider a field in which three color image signals I ₁ , I ₂ , and I ₃ have waveforms such as A and C in FIG. When the image signal _I1 is used as the sharp signal _S0 , the unsharp signal _U0 has a waveform as shown in Figure 1B, so the difference between the sharpened signal _S0 and the unsharp signal _U0 is as shown in Figure 1D. If the constant parameter K _i that determines the degree of sharpness emphasis is made constant for each color, the waveforms after the sharpening process will be E and F in Figure 1.
As a result, an inappropriate amount of sharpening is applied to the image signals I ₂ and I ₃ , and the contours (see Fig. 1) are sharpened to an inappropriate extent.
CN), the intensity ratios of the three color image signals I ₁ ′, I ₂ ′, and I ₃ ′ differ from those of the original image signals I ₁ , I ₂ , and I ₃ . Generally, the hue of an image is determined by the intensity ratio of the three color image signals, so the hue changes when the intensity ratio of the three color image signals changes. As a result, color changes and color turbidity occur in the contours. In order to prevent this, it is necessary to make the constant parameter K _i smaller than I ₁ for colors other than I ₁ . On the other hand, the waveforms of the three color image signals I ₁ , I ₂ , I ₃ are shown in FIG.
In a case like C, in order to prevent color change and color turbidity at the contour (CN in Figure 2), it is necessary to make the constant parameter K _i larger than I ₁ for colors other than I ₁ . There is. Therefore, in the conventional simple method of sharpness enhancement, in order to prevent color changes and color turbidity at the edge of the image, the constant parameter K _i must be set in accordance with the intensity ratio of the three color image signals. Must be. Moreover, since the intensity ratio of the image signal differs depending on the position of the image, there is a drawback that the constant parameter K _i must be changed depending on the position.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image sharpness enhancement method that does not have the above-mentioned drawbacks.

This object of the present invention can be achieved by changing the degree of sharpness emphasis of each color according to the intensity ratio of input image signals of three colors. That is, in the present invention, the value obtained by dividing the image signals I ₁ , _{I 2 ,} I ₃ of each color by the image signal I 0 of the selected one color is calculated as the sharpness emphasis signal K _i (S) for the selected one color. ₀ −U ₀ ) to automatically prevent color changes and color turbidity at the outline of the image. Therefore, according to the present invention, the image signal Ii' of each color after the sharpening process can be expressed by the following equation.

Ii′=Ii+Ii/I ₀・Ki・(S ₀ −U ₀ )……(3) FIG. 3 shows an example of a device to which this invention method is applied, in which the sharp signal S ₀ and the unsharp signal U ₀ and is input to subtracter 1 and the difference (S ₀ − U ₀ )
This difference (S ₀ - U ₀ ) is input to multiplier 2 to obtain the product Ki・(S ₀ - U ₀ ) with constant Ki, and this multiplication result is input to multiplier 3. There is. Then, the image signal _I0 used to form the sharpness emphasis signal is input to the divider 4, and the value Ii/ _I0 obtained by dividing the image signal Ii by this is input to the multiplier 3, and the multiplication result is Ii/I ₀ ·Ki · (S ₀ −U ₀ ) is input to the adder 5. The adder 5 adds this multiplication result to the image signal Ii and outputs the sharpened image signal Ii′ (=Ii+Ii/I ₀ ·Ki · (S ₀ −U ₀ )). . In this way, in this invention, the image signal Ii (i=1, 2, 3) of each color is converted into a sharpness emphasis signal.
By multiplying the sharpness emphasis signal by the value (Ii/I ₀ ) divided by the color image signal I ₀ used to form Ki・{S ₀ −U ₀ ), the intensity ratio of the three color image signals can be calculated. Since the differences are taken into consideration, there will be no muddiness or change in color even in contours or intricate details. Figures 4 A to D and Figures 5 A to D are
The state of sharpening processing is shown in correspondence with FIG. 1 and FIG. 2, respectively. In the case of FIG. 4D, the sharpness-enhanced color image signals I ₂ ' and I ₃ ' are the color image signals I 2 and I ₂ with respect to the image signal I ₁ of the selected color.
The degree of sharpness enhancement is smaller than that for I _{1 ' in proportion to the intensity ratio of I 3} , that is, in proportion to the magnitudes of I ₂ /I ₁ and I ₃ / _{I 1} . On the _other hand _, in _{the case of} FIG. In proportion to the magnitudes of ₂ /I ₁ and I ₃ /I ₁ , the degree of sharpness enhancement increases compared to I ₁ '. As a result, for example, when compared with FIG. 1, the ratio of the three color image signals in FIG. 4 is I ₁ ′:I ₂ ′:
I ₃ ' is close to the original image signal ratio I ₁ :I ₂ :I ₃ even at the edge portions of the image, and such color muddiness and color change at the edge portions can be prevented.

As described above, the sharpness enhancement method of the present invention has a function of changing the degree of sharpness enhancement according to the intensity of the input image signal by introducing the parameter (I _i /I ₀ ). , it is possible to prevent color turbidity and color change in contour areas and image fine areas due to inappropriate sharpness enhancement, and it is possible to approach the ideal sharpening process (Equation (1)). .

[Brief explanation of drawings]

1 A to F and FIGS. 2 A to F are diagrams showing the state of the conventional sharpness enhancement method, respectively, and FIG. 3 is a block diagram showing an example of a device to which the method of the present invention is applied.
FIGS. 4A to 4D and 5A to 5D are diagrams showing the sharpness enhancement method according to the present invention. 1...subtractor, 2, 3...multiplier, 4...divider, 5...adder.

Claims (1)

[Claims] 1 Color-separated three-color image signal I _i (i=1, 2,
A sharp signal S ₀ and an unsharp signal U ₀ are formed by the image signal I ₀ of one color selected from 3), each color constant parameter is K _i , and I _i ′=I _i +I _i /
The image signals of the three colors are expressed by I ₀ · K _i · (S ₀ - U ₀ ).
Image signal I _{i ′} (i=1, 2,
3) An image sharpness enhancement method characterized by outputting the following.