JPH0681256B2 - Method of setting highlight point and shadow point in gradation conversion device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a gradation conversion device used in the field of image processing, and a highlight point and a shadow point are determined from a specific position of a cumulative density histogram of an original image. By doing so, the present invention relates to a method for making the setting process of the highlight point and the shadow point common to all the original images.

[Conventional technology]

As is well known, a plate-making scanner or the like is provided with a gradation conversion device for performing gradation conversion of image data obtained by reading a reproduction target original image having gradation. In this gradation conversion, the operator extracts highlight points and shadow points on the original image, and sets a gradation conversion curve based on the density value of these points and the desired finished halftone dot% (generally set up. It is carried out through the process of

[Problems to be Solved by the Invention]

However, in such a conventional method, the selection of the highlight point and the shadow point depends on the skill of the operator. For this reason, if an operator with a low degree of skill sets up, the gradation of the highlight part and the shadow part of the finish (reproduced image) becomes unsuitable, and there is a problem that the gradation expression property deteriorates.

To deal with this, it is conceivable to try to automate the setup (auto setup). However,
Unless there is a systematic technology to develop unique image data for each original picture, for example, to accurately and objectively find the characteristics of the density distribution and to reflect it in highlight points and shadow points,
Increasing the effectiveness of auto setup is difficult.

[Object of the Invention]

The present invention has been made in response to the above-mentioned problems, and it is possible to set the highlight point and the shadow point to any original picture accurately and commonly, and to automatically set up the gradation conversion device. Especially effective in
It is an object to provide a method for setting a highlight point and a shadow point.

[Means for Solving the Problems]

According to the present invention, a plurality of sample original images belonging to a predetermined type are prepared when setting up a gradation conversion device for performing gradation conversion on image data obtained by reading an image of an original image to be reproduced having gradation. Then, for each of the plurality of sample original images, an empirical curve showing the relationship between the cumulative density appearance rate of the image of the sample original image and the halftone dot value or the duplicate density value that has been empirically obtained, is obtained. With multiple empirical curves,
With respect to at least one of the highlight portion and the shadow portion of the sample original image, a value of the cumulative density appearance rate of the area where the plurality of empirical curves are concentrated is found as a reference cumulative density appearance rate, and a reference copy for the appearance rate. The halftone dot value or the reference copy density value is found, the cumulative density histogram of the image of the copy target original image for which the gradation conversion is performed is obtained, and the density value corresponding to the reference cumulative density appearance rate is calculated in the cumulative density histogram. At least one of the highlight point and the shadow point of the original image to be copied is determined so as to set the reference copy halftone dot% value or the reference copy density value to the reference density value. Set.

It should be noted that the term "density" in the present invention is a general term for not only the optical density but also a quantity representing the optical density such as a signal level obtained by photoelectrically reading the optical density or a Munsell value.

[Action]

In the present invention, the value of the reference cumulative density appearance rate as the information reflecting the commonality of the gradation conversion curve obtained empirically,
A reference copy halftone value or a reference copy density value for this is specified, and the reference density value used for setting the highlight point and the shadow point is a cumulative density histogram that quantitatively shows the density distribution state of the original image to be copied. Therefore, the highlight point and the shadow point are commonly set while reflecting both the rule of thumb and the characteristics of the original image.

〔Example〕

A. Overall Configuration FIG. 2 is a schematic block diagram of a plate-making scanner to which the embodiment of the present invention is applied, which is based on FIG. 1 of JP-A-63-42572. With this scanner,
The image of the original image 1 having gradation is read by the scanning reading device 2 pixel by pixel, and the image data obtained thereby becomes digital image data in the A / D converter 3. The digital image data is subjected to shading correction in the shading correction circuit 4 and then given to the selector 5.

In the setup process described later, the selector 5 selects the histogram calculation circuit 6 side, but selects the gradation conversion circuit 7 side in actual operation. The gradation conversion circuit 7 has a RAM 8 and performs gradation conversion on the input image data based on the gradation conversion table in the RAM 8. The image data after the gradation conversion is subjected to processing such as unsharp masking and magnification conversion in the image processing circuit 9, and then given to the halftone dot signal generating circuit 10.

Then, the halftone dot signal generation circuit 10 converts the input image data into a halftone dot signal and outputs it to the scanning exposure device 11. The scanning exposure device 11 exposes and records a halftone image on the photosensitive film 12 based on the halftone signal.

Each of these circuits operates under the control of the computer 13.
The computer 13 has a CPU 14 and a memory 15,
Various data processing described later is also performed. The exchange of signals (data) between each circuit and the computer 13 is performed via the signal line group 16. Also, the keyboard 17 is the computer 13
It is for performing a command operation to.

B. Setup Preparation Step As a setup preparation step in the gradation conversion circuit 7, first, in step S1 in FIG. 3, a reference gradation conversion curve F (x) (x is a density value) obtained empirically is prepared. Then, the data expressing it is stored in the memory 15.

In the next step group S2, the parameter values for auto setup are determined as follows (step S1
The execution order between the step group S2 and the step group S2 may be reversed. ). First, the type of original image is specified in relation to its use (for example, "monochrome photograph for commercial printing"). Then, a plurality of (preferably many, for example, 50 to 100) sample original images belonging to this type are prepared.

For each of these sample original images, a cumulative density histogram (corresponding to FIG. 1 (b) described later) of the image is obtained, and the gradation conversion curve empirically obtained for each sample original image is examined. By the former,
Density value x and cumulative density histogram value (cumulative density appearance rate)
Since the relationship between the density value x and the halftone dot% can be known by the latter, an empirical curve showing the relationship between the cumulative density appearance rate and the value of the halftone dot% can be obtained by combining these. . The method for obtaining the cumulative density histogram is disclosed in the above-mentioned publication and will not be described in detail.

Figure 4A shows the highlights of the empirical curves thus obtained, taking a negative recording as an example.
In addition, the horizontal axis and the vertical axis of FIG. 4A are divided into 0.1 steps, whereby a matrix arrangement of unit cells UC is assumed.

The result of counting how many empirical curves pass through each unit cell UC in FIG. 4A is shown in FIG. 4B. As can be seen in Figure 4B, in this example the cumulative density histogram value is
The maximum number (6) of empirical curves passes through the unit cell in which the percentage is 0.1% and the dot percentage is 100.0%. In other words, the point with the two-dimensional coordinates (0.1,100.0) in Figure 4A is
This is the most frequently used point. Further, although not shown, the most frequently used points in the shadow portion are specified by the same procedure (hereinafter, these are referred to as “representative points”). When there are multiple unit cells UC where the empirical curves are most concentrated, the unit cell that best fits the average curve (not shown) obtained by averaging the empirical curves is the representative point. And

When the cumulative density histogram value is Y and the halftone dot% is Q, the representative point (Y ₁ , Q on the Y / Q plane (Fig. 4C) in each of the highlight part and the shadow part is calculated by the above procedure. ₁ ) and (Y ₂ , Q ₂ ) can be obtained. According to various measurements by the inventor, the representative point in the general printing original image is (Y ₁ , Q ₁ ) = (0.1,100.0) (1a) (Y ₂ , Q ₂ ) = (99.5 , 5.0)… (1b) is the most common, and the value that Y ₁ and Y ₂ (%) can have in negative plate making is also in consideration of the change in the image content of the original image and the printing conditions. It is within the range shown in equations (2a) and (2b).

0 ≦ Y ₁ ≦ 0.2 (2a) 99.0 ≦ Y ₂ ≦ 100 (2b) These Y ₁ and Y ₂ values are used as the “reference cumulative appearance rate” value in the auto setup process described later. . However, when Y ₁ = 0% or Y ₂ = 100% (that is, when there is a representative point at the end of the range of 0% to 100% of the full range where halftone dot% can be taken), the corresponding representative point The value of is shifted inward by a predetermined minute amount within the full range to obtain the corrected reference density appearance rate (the reason will be described later). Therefore, the value of the cumulative density appearance ratio after correction in the highlight part and the shadow part
When written as Y ₁ and Y ₂ , a preferable range is Y ₁ : 0 <Y ₁ ≦ 0.2 (3a) Y ₂ : 99.0 ≦ Y ₂ <100 (3b). Specifically, the predetermined minute positive amounts δ ₁ , δ ₂
Is determined in advance, and the values of Y ₁ and Y ₂ are in the range of Y ₁ : δ ₁ ≦ Y ₁ ≦ 0.2 (4a) Y ₂ : 99.0 ≦ Y ₂ <(100−δ ₂ )… (4b) Correction is performed so that the value becomes.

Further, according to various analyzes of the inventor, it is more preferable that Y ₁ : 0.1 ≦ Y ₁ ≦ 0.2 (5a) Y ₂ : 99.0 ≦ Y ₂ ≦ 99.6 (5b) (this is (4a ) And (4b)
It is equivalent to setting ₁ = 0.1 and δ ₂ = 0.4). In many cases, the values of the reference duplicated dots% Q ₁ and Q ₂ are (6
The values are within the range shown in a) and (6b).

Q ₁ : 99.0 ≤ Q ₁ ≤ 100 (6a) Q ₂ : 4.5 ≤ Q ₂ ≤ 6.5 (6b) From the above, Table 1 shows the preferred range of parameter values used in the subsequent auto setup process. The range shown is obtained.

Similarly, in positive plate making, (Y ₁ , Q ₁ ) = (0.1,0.0) (7a) (Y ₂ , Q ₂ ) = (99.5,95.0) (7b) is obtained as the optimum value.

The value of the representative point (hence the reference cumulative density value) thus obtained is stored in the memory 15. The above-described representative point specifying step may be performed using the scanner itself shown in FIG. 2 or may be performed using a system other than this.

C. Setup execution process When actually performing the auto setup of the gradation conversion circuit 7 for the original image 1, the following routine is performed.
First, the selector 5 in FIG. 2 is connected to the histogram coefficient circuit 6 side. Next, the original image 1 to be reproduced is prescanned by the scanning reading device 2, and the cumulative density histogram of the reading target area is obtained by the histogram counting circuit 6. The cumulative density histogram h (x) in the case where the density histogram r (x) of the original image 1 is as shown in FIG. 1 (a) is shown in FIG. However, xmin and xmax are the minimum density value and the maximum density value in the image of the original image 1, respectively. Further, y is the appearance rate on the density histogram r (x).

Next, the values of the reference cumulative density appearance rates Y ₁ and Y ₂ are read out from the memory 15, and the values Y ₁ and Y are calculated in the cumulative density histogram h (x).
_The density values x ₁ and x ₂ (FIG. 1 (b)) corresponding to ₂ are obtained. (The density values x ₁ and x ₂ correspond to the “reference density value” in the present invention.) Furthermore, the values of the reference copy halftone dots% Q ₁ and Q ₂ are read from the memory 15 and shown in FIG. 1 (c). Two points on the gradation conversion plane (x / Q plane) shown in: P ₁ (x ₁ , Q ₁ ) ... (8) P ₂ (x ₂ , Q ₂ ) ... (9) Set as a point. However, FIG. 1 (c) shows an example of positive plate making. Specifically, the standard gradation conversion curve F (x) stored in advance in the memory 15 is corrected so as to pass through the two points P ₁ and P ₂ , whereby the desired gradation conversion curve G (x ) Get. This correction is performed by, for example, G (x) = {ΔQ · F (x) + Δ (QF)} / ΔF (1
0) ΔQ ≡ Q ₂ − Q ₁ (11) Δ (QF) ≡ Q ₁ · F (x ₂ ) −Q ₂ · F (x ₁ )… (12) ΔF ≡ F (x ₂ ) −F (x ₁ )… (13) can be done. Then, the data expressing the gradation conversion curve G (x) is stored in the RAM 8 in the look-up table format, and the setup of the gradation conversion circuit 7 is completed. The above routine is also shown in the flowchart of FIG. The actual plate making process thereafter is as described above, and detailed description thereof will be omitted.

In such an auto setup process, it is important to use the cumulative density histogram h (x) first. That is, the density values x ₁ and x ₂ of the highlight point P ₁ and the shadow point P ₂ with respect to the predetermined values of Y ₁ and Y ₂ are in a form that reflects the density state of the image of the original image 1. Will be set automatically. For example, the increase rate (h
When (gradient of (x)) is small, highlight density value x ₁
Is automatically set to a relatively large value. Therefore, the original picture 1
The smaller the total area of the low-density areas of the image, the closer the highlight point P ₁ is to the high-density side, which enhances the gradation expression in the relatively high-density area. (X) will be obtained. Similarly, shadow points
Also in P ₂ , a desirable setting is made according to the cumulative density histogram h (x).

The second feature of this embodiment is that Y ₁ , Y ₂ , Q ₁ and Q obtained in the preparatory process.
_It means to use the value of ₂ . That is, the values of Y ₁ and Y ₂ can be generally determined arbitrarily, but the above-mentioned embodiment has a feature that the values of Y ₁ and Y ₂ are determined through analysis of an empirical curve. ing.

This means that by utilizing the highlight point density value and the shadow point density value, which are empirically desired, for the auto setup, automation utilizing the experience of a skilled operator can be realized.

Empirical data are also cleverly incorporated into the values of Q ₁ and Q ₂ .

By the way, the concentrated part of the empirical curve is Y ₁ = 0% and Y ₂ ＝ 10
The reason why the above-mentioned correction is applied to these values of Y ₁ and Y ₂ when it is 0% is as follows. First, due to a trimming error or noise in the original image 1, a rising point (minimum density value xmin) from 0% of the cumulative density histogram h (x) or
Note that an error may occur at the 100% arrival point (maximum density value xmax). At this time, Y ₁ = 0% and Y ₂ = 100
If the value of% is taken as the reference cumulative density appearance rate as it is, the highlight point P ₁ and the shadow point P ₂ are (as a value having an error)
It is decided to include xmin and xmax. As a result, in the vicinity of the true minimum density value and the maximum density value, there is a possibility that the gradation reproducibility of the finished image is lost.

On the other hand, if the above correction is performed, the highlight point P ₁ and the shadow point P ₂ are determined with a certain margin of error, and the above situation can be dealt with.

D. Modified Example Normally, the highlight point and the shadow point are uniquely determined through the above steps, but when the flat portions FL ₁ and FL ₂ are present in the cumulative density histogram h (x) as shown in FIG. 6A. Has ₁ highlight density and a shadow density
The value of x ₂ is not uniquely determined. At this time, as shown in FIG. 6A, x = 0 for the highlight point density x _1.
%, And for the shadow point density x ₂ , a value closest to x = 100% may be selected.

Further, as described above, the values of Y ₁ , Y ₂ , Q ₁ , and Q ₂ can be set for each type and use of the original image 1. For example, when a photograph for newspaper printing is used as the original image, A smooth gradation conversion curve G _N (x) as shown in the figure is obtained. However, Q _1N and Q _2N are respectively the highlight dot mesh% value and the shadow dot mesh% value for newspapers, which are closer to the center of the vertical axis than the values of Q ₁ and Q ₂ for general use.

Further, as can be understood from the above-mentioned embodiment, the method of the present invention can be applied to both highlight points and shadow points, but it is always applied to both highlight points and shadow points at the same time. There is no reason to do it.

Therefore, it is apparent from the above description of the embodiment that the method of the present invention may be applied to either the highlight point or the shadow point.

That is, the present invention can be applied to at least one of the highlight point and the shadow point.

The present invention can be applied not only to the plate-making scanner, but also to a copying machine or a facsimile having gradation reproducibility. The empirical curve in this case shows the relationship between the cumulative density appearance rate and the empirically obtained duplication density, and the same processing as in the above-described embodiment may be performed based on this empirical curve.

Further, once the image information of the original image 1 is once stored in the large-capacity memory, prescanning for obtaining the cumulative density histogram is unnecessary, and the image data may be read from the memory and used.

〔The invention's effect〕

As described above, according to the present invention, the empirical curve concentration state for a plurality of sample original images analyzed in advance (that is, the reference cumulative density appearance rate and the reference duplicate dot% value or the reference duplicate density value for this) Relationship with
Since the highlight point and shadow point are set based on the cumulative density histogram of the original image to be gradation-converted, these settings can be made exactly and commonly for any original image. Thus, it is possible to obtain a method of setting highlight points and shadow points, which is particularly effective for automatic setup of the gradation conversion apparatus and can be executed by simple processing.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing the concept of an embodiment of the present invention, FIG. 2 is an overall block diagram of a plate-making scanner to which the embodiment is applied, FIG. 3 is a flow chart showing a preparation process, and FIG. Is an explanatory diagram of a process for obtaining a value of a representative point from a set of empirical curves, FIG. 5 is a flowchart showing an auto-setup process, and FIG. 6 is a diagram showing a modified example of the present invention. 1 ...... original, 6 ...... histogram counting circuit, 7 ...... gradation conversion circuit, h (x) ...... cumulative density histogram, P ₁ ...... highlight point, P ₂ ...... shadow point, G (x) ... … Gradation conversion curve

Front Page Continuation (72) Inventor Akihiro Nomura 1 in 1 Nihon Screen Manufacturing Co., Ltd., 1-chome Tenjin Kita-cho, 4-chome, Horikawa-dori Teranouchi, Kamigyo-ku, Kyoto-shi, Kyoto (72) Inventor Kunio Tomohisa Kyoto-shi, Kyoto 4-chome, Tenjin Kitamachi, 1-chome, Horikawa-dōji, Kamigyo-ku, 1st in Japan Screen Manufacturing Co., Ltd. (72) Inventor, Sakamoto Takashi 4-chome, Tenjin-kita-cho, 4-chome, Horikawa-duji, Kyoto, Kyoto 1 Dainippon Screen Mfg. Co., Ltd. (72) Inventor Susumu Takita 1-5-9 Tenmabashi, Kita-ku, Osaka-shi, Osaka Dainippon Screen Mfg. Co., Ltd. Osaka Technical Center

Claims (1)

[Claims] 1. A plurality of sample original images belonging to a predetermined type in setting up a tone converting device for performing the tone conversion on image data obtained by reading an image of an original image to be reproduced having gradation. For each of the plurality of sample original images, an empirical curve showing the relationship between the cumulative density appearance rate of the image of the sample original image and the copy halftone dot value or the copy density value which is empirically obtained is obtained. , Thereby obtaining a plurality of empirical curves, and with respect to at least one of the highlight portion and the shadow portion of the sample original image, the value of the cumulative density appearance rate of the portion where the plurality of empirical curves are concentrated is defined as the reference cumulative density. In addition to finding out as an appearance rate, a reference duplication halftone dot value or a reference duplication density value for the appearance rate is found, and duplication for performing the gradation conversion is performed. A cumulative density histogram of the image of the target original image is obtained, and in the cumulative density histogram, a density value corresponding to the reference cumulative density appearance rate is found and used as a reference density value. At least one of a highlight point and a shadow point of the original image to be duplicated is set so as to set a point% value or a reference duplication density value. Setting method.