JP2844480B2 - Image gradation conversion method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a method for producing halftone dots of continuous tone black and white or multicolor original images through a process such as color separation by electronic plate making. The present invention relates to a gradation conversion method of an image used when creating a duplicate image such as a monochrome or multicolor print image. In particular, the present invention relates to an image gradation conversion method capable of arbitrarily and rationally adjusting the image quality of a halftone dot image such as a printed image through adjustment (including correction and change of gradation) of the image. It is.

(Prior Art and its Problems) The present invention is used not only as a print image but also as a general technology and a basic technology of a reproduced image such as a digitally reproduced printer image, a copy image, and a CRT image as described later. Things. Here, for the sake of convenience of explanation, the related art and its problems will be described by taking printing, which is the most typical industrial field, as an example in creating a duplicate image.

As is well known, in producing a print image from a document image, a core technique thereof is a color separation (color separation) technique.

In the above-mentioned color separation technology, a scanner (monochrome, including color; the same applies hereinafter) manufactured using modern technology such as electronics and computers is currently used as a working tool, but essentially,
It is still an unscientific and irrational technology based on cloudy experience and intuition. In the industry, even today, the debate over whether color separation work is industrial or craft demonstrates this fact.

To explain this further, the conventional color separation technology has been developed mainly on a color photographic processing technology and a photographic masking method. That is, in the color separation technology, the basic concept of the formation of the print image is too inclined to the photographic image processing technology, and the color separation technology lacks understanding, examination, and analysis of the essence of the print image and the production process of the print image creation. For this reason,
In the conventional color separation technology, only the color separation technology from the viewpoint of photo processing technology is considered and improved, and it is located in the first step of print image formation and has the decisive effect on the quality of the printed image. Techniques such as tone conversion and subsequent adjustments, corrections, and changes to the tone and hue of the printed image, as well as to the colors, result in being outside the scope of research, consideration, and improvement, and therefore, those techniques are still He has made the fundamental mistake of remaining based on human experience and intuition.

Currently, while using a scanner, which is a modern work equipment, in color separation work, the halftone dot characteristic curve (tone curve for monochrome scanner, color separation curve for color scanner, color separation curve) It is said that the setting technique is still the most suitable among the several characteristic curves created by the experience of the scanner manufacturer and stored in the storage device of the scanner. Work out of it (although this is not guaranteed to be satisfactory), or each scanner user may have experience with the same characteristic curve that appears to be most suitable for their working conditions. Make it based on intuition and store it in the storage device of the scanner, call it when necessary and work It has been done to say.

This is a good example of the fact that current color separation techniques, while using a scanner, are essentially based on human experience and intuition.

In this way, in electronic plate making technology systems such as monochrome scanners and color scanners, although advanced mechanisms are adopted, the halftone dot characteristic curve itself, which is the core of scanner operation, is scientifically and rationalized. At present, it is completely indifferent to making it. The fact that the dot characteristic curve, which is the core of the operation of this scanner, is made scientifically and rationally,
This is an essential prerequisite for scientifically and rationally performing conversion of image gradation.

(Problems to be Solved by the Invention) It is an object of the present invention to solve and overcome the disadvantages and limitations of the above-described conventional color separation techniques, and to establish a scientific and rational color separation technique.

The present inventors scientifically set a halftone dot characteristic curve which is the core of the work when forming a print image which is a halftone halftone first, and the halftone dot characteristic thus scientifically set. Japanese Patent Application No. 62-148912 (US Pat. No. 4,811,108) as a technique for performing gradation conversion of an image using a curve, and Japanese Patent Application No. 62-1652 as a plate making technique applying the gradation conversion technique of the image.
No. 31 (US Pat. No. 4,833,546), Japanese Patent Application No. 62-19 as a method for controlling the dot percentage by applying the gradation conversion technology of the image.
No. 8302 has been proposed.

In addition, the present inventors have adopted the technology for setting the dot characteristic curve described above, in addition to a standard original whose original image has a standard image quality,
Japanese Patent Application No. 63-63 as a sublimation technology that can handle non-standard originals such as over and under exposure
No. 114599, No. 63-207326, and a technology that uses image information values relating to the amount of light (exposure) from a subject constituting a document image from a technology system that uses density information of a document image when setting a dot characteristic curve. Patent application Hei 1
It was at the time when No. 135825 was proposed.

The present invention relates to these proposals and relates to tone adjustment (including tone correction and change) performed simultaneously with or subsequent to tone conversion of a document image. That is, an object of the present invention is to provide a technique for rationally converting the gradation of a print image produced by the initially set halftone dot characteristic curve into a desired gradation. In other words, an object of the present invention is to replace the conventional gradation conversion and adjustment technology based on irrational experience and intuition with a scientific and rational gradation conversion and adjustment technology.

[Configuration of the invention]

(Means for Solving the Problems) In summary of the present invention, the present invention performs tone conversion of a continuous tone image for producing an arbitrary desired halftone image into a halftone image. In the gradation conversion method of an image, (i) Image information value and halftone dot area% from an H part (lightest part) to an S part (darkest part) of a document image for performing gradation conversion
Setting a basic halftone characteristic curve that governs the correlation by operating the following <gradation conversion formula> based on the initial setting conditions; (ii) replacing the grayscale conversion by the basic halftone characteristic curve with
At least one control point for setting an area for performing tone adjustment in order to perform tone conversion so as to have a desired halftone dot tone is set in the image information values ranging from H to S. (Iii) For each area divided by the control point, the following <gradation conversion formula> is operated as follows to determine an individual halftone dot characteristic curve for each area; (b) Each control point desired y _S values for determining the one of the adjacent individual dots characteristic curve the value of dot area percent of, be y _H values for determining the other individual dots characteristic curve, (b) A desired value is employed as a γ value for determining an adjacent individual halftone dot characteristic curve. (C) In a predetermined gradation adjustment region, a desired halftone dot area% set for each control point is set. At least one of the numerical value and the γ value for determining the individual halftone dot characteristic curve is determined based on the basic halftone dot characteristic curve. It is different from the halftone dot area% value at the control point and the γ value used to determine the basic halftone dot characteristic curve. (Iv) The individual halftone dot characteristic curves determined as described above are integrated. A halftone dot characteristic curve for gradation adjustment from H to S, and a halftone dot image is subjected to gradation conversion of a continuous tone image based on the halftone dot characteristic curve. And a method for converting the gradation of an image.

<Tone conversion formula> Hereinafter, the configuration of the present invention will be described in detail. In addition,
Subsequently, the gradation conversion method of the image of the present invention will be described by taking the production of a printed image as an example.

The gradation conversion method for an image according to the present invention is largely divided into two halftone characteristic curves and set.

The first setting of the halftone dot characteristic curve is the process (i) for scientifically and rationally converting a continuous tone original image such as a color photograph into a halftone print image. The halftone dot characteristic curve (hereinafter referred to as the basic tone adjustment)
This characteristic curve is called a basic halftone dot characteristic curve. ) Is set. The basic halftone characteristic curve from the H portion to the S portion of the document image is obtained by setting the <gradation conversion formula> (1) of the present invention to the initial setting conditions (y _H , y _S , γ) of each parameter. Value).

The second halftone dot characteristic curve is based on the first basic halftone dot characteristic curve, and is based on the image information value related to the density information of any original image (this is a density measured by a densitometer as described later). Value, or an image information value related to the amount of exposure obtained from the density value using the photographic density characteristic curve.) (Hereinafter, this characteristic curve is referred to as a tone adjustment dot characteristic curve.) That is, the second tone adjustment dot characteristic curve is described in (ii) to (ii). It is set by the process of (iii).

The first basic halftone characteristic curve setting technique of the present invention,
An earlier application proposed by the present inventors (Japanese Patent Application No. 62-55831,
No. 62-148912), so only this point will be mentioned here.

The <gradation conversion formula> (1) used in setting the first basic halftone dot characteristic curve of the present invention is rationally derived as described below. In other words, in the production of a halftone print image, the relational expression (equation) that defines the tone conversion is based on the values of the reflection density of the printing paper and printing ink, and the H (most bright portion, Highlight section)
And S (darkest portion, shadow portion), while arbitrarily selecting the size of a desired halftone dot, from the basic density value (x) of an arbitrary control sample point (X) on the original image on the print image. It is desirable to formulate such that a numerical value (y) of the halftone dot area percentage of the halftone dot at the corresponding management sample point (Y) is obtained.

The relational expression for obtaining the numerical value (y) of the halftone dot area percentage is a generally accepted density formula (photo density, optical density), that is, D = log ^Io / I = log ^l / T Is derived from

Applying this general formula for density D to plate making and printing gives: here, It is.

And Concentration formula (D ') on this plate-making and printing, the brightest portion minimum halftone dot area percentage (y _H) and darkest maximum dot area percent of the halftone image created as described above (y _S) Is set arbitrarily, the basic density value (x) at an arbitrary management sample point (X) on the original image which is continuous tone, and the management on the print image which is a halftone dot corresponding thereto. A request to associate the numerical value (y) of the halftone dot area percentage of the halftone dot at the sample point (Y) is incorporated, and a relational expression is derived so that the theoretical value and the measured value approximately match. As a result, the following relational expression that determines the numerical value (y) of the dot area% is obtained.

<Relational expression> In the above relational expression, among the parameters,
By using β, k as β = 10− ^γ k = γ / (difference in image information value related to density information between S part and H part of original image) γ = arbitrarily high versatility It will be. In addition, this point is (the problem which the invention is going to solve)
The present invention is disclosed in detail in the patent application proposed earlier by the present inventors described in the section.

Therefore, in the <relational expression> derived as described above, when α = 1.0, the dot area% value (y) as planned (as determined in advance) in the brightest portion and the darkest portion is always used. Can be set.

This is clear from the following points. That is,
In the brightest part (H), by definition, x (basal density value)
= 0, and x = (image information value related to the density information of the original image) in the darkest part. It is quite reasonable to operate the <relational expression> (operating with α = 1) as described above. This is because in the art, when obtaining density information of an image using a densitometer, the zero (0) point of the densitometer is always adjusted based on the whiteness of the printing paper. is there.

The gradation conversion method for an image according to the present invention is intended to be performed rationally under the <gradation conversion formula> (1) derived when α = 1. <Tone conversion formula> of the present invention
In (1), since the above-mentioned relationship of α = 1 is built in, when setting the halftone characteristic curve for gradation adjustment, each adjacent individual halftone characteristic curve is continuously connected without causing a tone jump. be able to.

Further, the halftone gradation curve set by the <gradation conversion formula> (1) of the present invention is apparent from the characteristics of the relational expression. The shape of the curve can be changed to a downward convex shape by setting the γ value to a negative value while the γ value to a negative number. That is,
The desired gradation can be adjusted by the γ value.

In the present invention, it is possible to produce a print image having a desired desired gradation characteristic while utilizing the characteristics of the above <gradation conversion formula> (1). That is, following the process of setting the basic halftone dot characteristic curve of (i), it is determined which range of gradation is to be adjusted (including correction and change) based on an organic relationship therewith. (Ii) An individual halftone characteristic curve is obtained by the process (iii), a halftone characteristic curve for gradation adjustment is set, and finally, the halftone characteristic curve for tone adjustment is obtained by the process (iv). It is only necessary to convert the tone of the continuous tone image (original image) into a halftone tone image (print image).

The manner of adjusting the gradation according to the present invention is not particularly limited. For example, the control point for managing the gradation adjustment area in the process (ii) is set as follows: (1). The highlight part of the document image is set to be emphasized or the highlight part is suppressed.

(2). The setting is made so that the shadow part of the document image is emphasized or the shadow part is suppressed.

(3). The setting is made so that only the density region from the brightest part to the middle part of the document image is emphasized or suppressed.

(4). The setting is made so that only the darkest part density area from the middle part of the document image is emphasized or suppressed.

(5). The setting is made so that the middle area density region is emphasized from the brightest part of the document image and the darkest part density area is also emphasized from the middle part.

(6). The setting is made so as to suppress the density region from the brightest portion to the middle portion of the document image and also suppress the density region from the middle portion to the darkest portion.

(7). Highlight or suppress highlights, highlight or suppress shadows, highlight or suppress the middle density range from the brightest, enhance or suppress the darkest density range from the middle, brightest From the part that emphasizes the middle part density area and also emphasizes the darkest part density area from the middle part, and the one that suppresses the middle part density area from the brightest part and also suppresses the darkest part density area from the middle part Set to combine.

As described above, any number of control points can be set as desired. In the present invention, the control point is arbitrarily set within the image information value of the document image from H to S, and the control point controls the gradation adjustment area. When the gradation adjustment region is determined in this manner, a desired halftone dot area% value is set for each control point, and an individual halftone characteristic curve is obtained by the above-mentioned process (iii) under a desired γ value. Is required. And the curve is the process (iv)
Is integrated into the halftone characteristic curve for gradation adjustment.

Next, in the gradation conversion method of the image developed under the <gradation conversion formula> (1) of the present invention, x obtained in the <gradation conversion formula> (1) is obtained from the original image. Specific contents of the image information value related to the density information used as the basis for calculating the value (basic density value) will be described.

The present invention is to convert the density information of an original image into appropriate halftone information in the development of a scientific and rational tone conversion technology which is the core for converting continuous tone to halftone tone. In the first place. Color correction (color correction) is also important in the production of various types of reproduced images, including printed images, but the present invention is based on the idea that adjustment of gradation (gradation control) is primarily considered as a primary consideration. I have. Needless to say, this concept is reflected in the above-mentioned <gradation conversion formula> (1).

Therefore, the basic information to be obtained from the document image in the present invention is the density and the image information value related thereto.

In the operation of the <gradation conversion formula> (1) of the present invention, the image information value related to the density information value of the original image used for calculating the x value (basic density value) should be interpreted in the broadest sense. It is. For example, there is a density value itself measured from a document image by a densitometer. When the original image is a color or monochrome photograph, it goes without saying that the transmission density or the reflection density may be used. In addition to the density value from the densitometer, any physical quantity related to the density value of the original image, such as the current value and the voltage value after photoelectric conversion, can be used.

In particular, in the present invention, as an image information value related to the density information value of the original image, an original original image forming the original image, which will be described below, that is, a light amount incident on a recording medium from a subject (real scene) is described. Physical quantities have a very important meaning.

As is well known, a document image is a medium image recorded on various recording media. For example, in the case of a photographic original, in a recording system called a photographic photosensitive agent, the photographic photosensitive agent is blackened by an amount of light from a subject (actual scene), that is, an exposure amount, to form an image density. In addition, as a recording medium, there is a photoconductor such as a CCD, a photocathode, a light or a magnetic disk, and the density of an image is formed by the photosensitive characteristics of each recording system. However, it should be noted that the density of the medium image depends on the photosensitive characteristics of the recording system.

The present inventors previously obtained an information value relating to the amount of light which is not affected by the photosensitive characteristics of the recording system from the density information value of the original image as a gradation conversion technique of an image of a medium image recorded on various recording media. Have been proposed (hereinafter referred to as <gradation conversion formula>) (1) (Japanese Patent Application Nos. 1-135825 and 1-212118). This new image tone conversion technology is superior to the conventional image tone conversion technology based on density information obtained directly from an original image in the reproducibility of the original image.

From the density information value of the medium image, the information value regarding the light amount can be easily obtained from the characteristic curve regarding the light amount and the density of the recording system. For example, for a photo manuscript,
The vertical axis (hereinafter referred to as the D axis) represents the density value (D value = log ^Io /
I), a photographic characteristic curve (phot) represented by a coordinate system in which the horizontal axis (hereinafter referred to as X axis) is a logarithmic value (logE) of the exposure amount (E).
ographic characteristic curve), D
A physical quantity (hereinafter, referred to as an X value) relating to an exposure amount can be easily obtained from the value. In the case of a medium image recorded on another recording medium, the physical quantity related to the light amount may be similarly obtained. In the present invention, these physical quantities relating to the light quantity are positioned as image information values related to the density information values.

A specific example of obtaining the X value from the D value of the photographic image will be described below. Since the gradation conversion technique of the image based on the light amount value is adopted in an embodiment described later, the following description will help the understanding.

As the photographic characteristic curve described above, Ektachrome manufactured by EK
64. A professional film (Daylight) was adopted and its mathematical formula was created. Note that the method of formulating is not particularly limited, and it is only necessary to faithfully formulate the photographic characteristic curve. The results are shown in Table 1.

Table 2 shows the results of formulating Fujichrome manufactured by Company F.

In formulating Ektachrome 64 manufactured by EK, a photographic characteristic curve was formed for each emulsion layer, that is, for each of the R, G, and B emulsion layers. Further, a color original concentration on the D-axis in each table (D _n), when converted to a value on the X-axis (X _n), have been shown formulas determining the X _n for each segment of the D _n .

The formulas of these pictures characteristic curves, photo lateral density value to the characteristics of the emulsion from (D _n values), to obtain more physical amount related to the exposure amount close to the subject a (X _n value) easily Can be.

Using the X _n value relating to the exposure amount obtained as described above, the y value (halftone dot area% value) is obtained by <gradation conversion formula> (1).
Can be determined. That is, a halftone dot characteristic curve and a color separation curve showing the correlation between the y value and the _Xn value (however, the base density value) are obtained. The inventor of the present invention has determined that the image obtained based on the density information value on the D axis is a color separation curve on the D axis, and the image obtained based on the image information value on the exposure amount (light amount value) on the X axis is the color on the X axis. The two are distinguished from each other as a separation curve, and the latter is a color separation technique having more excellent surfaces than the former.

When producing printed images, most of them (generally C
The four editions of the edition, the M edition, the Y edition, and the K (ink) edition are considered as one set. ). To operate the <gradation conversion formula> (1) of the present invention and set the respective D-axis or X-axis color separation curves to perform gradation conversion of an image, the following method may be used. The <gradation conversion formula> (1) is derived from the viewpoint of rationally determining the most important plate C among multicolor plate making. Therefore, it is the D-axis or X-axis color separation curve for the C plate that is derived by the operation of the <gradation conversion formula> (1), and the other D and D colors for the M plate and the Y plate.
The axis or X-axis color separation curve may be determined by multiplying by an appropriate adjustment value known in the art to maintain gray balance and color balance. In addition, K (ink)
The D-axis or X-axis decomposition curve for the plate may be determined according to a conventional method in the art from the viewpoint of reducing the consumption of C, M, and Y inks.

The image tone conversion method of the present invention is not applied only to the production of a color print image as described above,
As will be described later, its application range is extremely wide. That is, light,
The present invention can be applied to all fields in which image information of a real image is photographed or converted by photographing using an information transmission medium such as an electromagnetic wave to obtain output information of a reproduced image by processing.

Therefore, it goes without saying that the tone conversion method of the image must be operated so as to be compatible with the system for producing each duplicate image.

Although the application aspect of the image tone conversion method of the present invention has been described particularly in relation to the production of a print image, the application aspect is not limited to the production of a print image.

That is, examples of applications of the image tone conversion method of the present invention include: (a) letterpress, planographic, halftone gravure,
The tone and color tone of the duplicated image based on the size of the halftone dot (dot) seen in printed images such as silk screens, or in the melt transfer type thermal transfer images where the dot size can be changed (multi-valued) (B) Sublimation transfer type thermal transfer image, (using silver salt) thermal development transfer image, conventional gravure image, etc. When a gradation or a color tone is to be expressed by shading of a pigment such as a printing ink or a dye (dye) attached per pixel (for example, per dot) (this is also called a density gradation method). ) Digital copiers (color copy, etc.), printers (ink jet, bubble jet, etc.),
Alternatively, when the gradation is to be expressed by changing the recording density per fixed area such as a facsimile, for example, the number of dots, the number and size of ink droplets, and the size of the ink (this is the case of the area gradation in (a) above). (D) A CRT image that expresses an image by adjusting the intensity of luminance per unit area from an electric signal related to image information such as a video signal, a television signal, a high-definition signal, and the like. When trying to obtain a certain printed matter, hard copy, or the like, (e) not only the case of the image conversion processing between the original image and the duplicate image in the above-mentioned substantially equivalent density (luminance, illuminance) area, but also the spatial Imaging in the invisible range of luminance, wavelength and time, for example, in the low illuminance region where the contrast of the original image is extremely low so that the density range difference between the original image and the duplicate image is small. (E.g., emphasis conversion of image contrast rather than image gradation conversion in such a case) (f) X-ray photography, etc. In order to perform more accurate diagnosis by arbitrarily changing the contrast of the subject (affected part, lesion, etc.) in the medical precision image for examination of (3), the contrast in the detection of fingerprints and fingerprints for ID cards If it is desired to perform the detection accurately by changing arbitrarily, (h) In addition, a densitometer with a density / gradation conversion mechanism for displaying the dot area% as well as the density display, It can be applied to printing related equipment such as inspection (for example, color proofing for proofing) and simulator for color separation education.

In order to apply the image gradation conversion method of the present invention to the various application fields described above, in each field, the density information value of the original image or the image information value related to the density information of the subject (substantial image) ( For example, the light amount value and the exposure amount value are processed by the image conversion processing unit (tone conversion unit) of the equipment in various application fields under the <tone conversion formula>, and the processed value, ie, the y value (tone) The current value and the voltage value of the recording unit (recording head) of the device or the printing time thereof are controlled in correspondence with the intensity value), and the dot area, the number of dots per fixed area (1 pixel),
It is only necessary to change the density per fixed area (for example, one dot) or the like to output a duplicated image such as a halftone gradation in which the gradation is arbitrarily adjusted.

For example, using the image tone conversion method of the present invention to produce an original of a print image that is a halftone image, that is, an original for printing, an existing system known in the art may be used. This can be achieved by incorporating the image tone conversion method of the present invention into a color separation / shading mechanism of a commercially available electronic color field device (color scanner, total scanner) or the like.

(Embodiment) Hereinafter, the gradation conversion method of an image of the present invention will be described in more detail with reference to an embodiment for producing a color print image from a color original image, but the present invention is not limited to the embodiment.

(Example 1) In Example 1, based on the basic halftone dot characteristic curve (predetermined X-axis color separation curve), a gradation capable of producing a print image having an arbitrary predetermined gradation (gradation) A method of setting the halftone characteristic curve for adjustment will be described.

(I) Setting of basic halftone dot characteristic curve (predetermined X-axis color separation curve) X value on X-axis (hereinafter also referred to as light amount value) Range = 0.00 to 2.500 (DR−X = 2.50) y _H = 5 % Y _S = 95% γ = 0.90% and the basic gradation characteristic curve was set by using the <gradation conversion formula> (1) (this characteristic curve is shown in FIGS. 1 to 4 described later). Illustrated). The setting data (y value) of the basic halftone dot characteristic curve is shown in Table 4 described later.

(Ii) Setting of halftone dot characteristic curve for gradation adjustment With reference to the basic halftone dot characteristic curve obtained as described above, what range (range of X _n values relating to the light amount value on the X axis) is determined. Whether to adjust the gradation is determined in advance. In this embodiment, the light amount value range on the X-axis is 0.00 to 2.
50 was divided into three ranges, and individual dot characteristic curves were obtained for each range.

Table 3 shows the conditions set for each gradation adjustment range.

As shown in Table 3, for the basic halftone dot characteristic curve,
Conditions are set in consideration of various cases of gradation adjustment. That is, (1) a case where the tone of the highlight portion is adjusted particularly with respect to the basic dot characteristic curve, and (2) a case where the tone of the shadow portion is adjusted particularly with respect to the basic dot characteristic curve. (3) a case where the gradation of a highlight portion and a shadow portion is to be adjusted with respect to a basic halftone dot characteristic curve;
(4) Although related to the above (3), conditions are set for a case where the tone of both the highlight portion and the shadow portion is to be adjusted more strongly than in the case of (3).

In Table 3, each symbol means the following: light quantity value DR-X of darkest (S) in the light value X _S ...... each range of the brightest portion (H) in the X _H ...... each range ... … Light value range value (range value) = X _S −X _H y _H … Dot area% value set in H section of each range y _S … Dot area% value set in S section of each range DR-y ...... dot area% range values in each range (range _{value) = y S -y H γ ......} γ values for setting the individual dots characteristic curves each range.

Note that γ = 0.50 is a positive value, Means a negative value.

Tables 4 to 7 show the setting data of the gradation adjustment halftone dot characteristic curves calculated under the <gradation conversion formula> (1) under the setting conditions in Table 3. FIGS. 1 to 4 show gradation characteristic halftone dot characteristic curves based on these setting data. From these results, the gradation characteristics of the basic halftone dot characteristic curve can be arbitrarily and rationally adjusted, that is,
It can be seen that a halftone dot characteristic curve for tone adjustment that is indispensable for tone adjustment is obtained rationally.

Embodiment 2 In this embodiment, a basic halftone dot characteristic curve (X-axis color separation curve) typical in plate making practice, and a typical tone adjustment halftone characteristic curve obtained by adjusting the characteristic curve (Gradation adjustment X-axis color separation curve).

(I) Setting of Basic Halftone Characteristic Curve The color film original used in this example is manufactured by EK (Ektachrome 64) and has an X obtained from a D value of 4 ″ × 5 ″.
It was an indoor still life with a value range (light amount range) of 0.00 to 2.50. Table 8 shows the setting data of the basic halftone dot characteristic curve.

(Ii) Setting of halftone characteristic curve for gradation adjustment In this embodiment, gradation is set to a light amount value range of 0.00 to 0.30, 0.3.
The adjustment was made in three regions, 0 to 1.80 and 1.80 to 2.50. The gradation adjustment in the present embodiment is to emphasize the contrast between the highlight part and the shadow part. Table 8 also shows data for setting tone characteristic curves for gradation adjustment.

FIG. 5 shows the X-axis color separation curves for basic and gradation adjustment based on Table 8.

(Iii) Image quality evaluation Magnascan M- as a color scanner for color separation
Color separation was performed using 645 (manufactured by Crossfield). Color proofing is performed by Chromaline proofing machine (Dupont)
A color proof print image was produced by the method described above, and the image quality was evaluated. The result was in accordance with the content of the gradation adjustment determined in advance, and was sufficiently satisfactory.

(Embodiment 3) In this embodiment, a method of adjusting gradation using a color original which is a high key will be described. The high key manuscript used in the present embodiment is typically a wedding commemorative photograph. That is, when a manuscript is a color photograph in which white and black live together, as seen in a bride's wedding dress and a groom's morning, the subtle gradation of white (highlight area) (woven pattern, lace pattern, Is very difficult to reproduce.

(I) Setting of Basic Halftone Characteristic Curve As a wedding commemorative photograph, which is a high-key color original described above, manufactured by EK (Ektachrome 64), an X value range (light amount range) obtained from a D value of 4 ″ × 5 ″ Used was 0.25 to 2.80. Table 9 shows the setting conditions of the basic halftone dot characteristic curve.

(Ii) Setting of Halftone Characteristic Curve for Tone Adjustment In this embodiment, consideration was given to faithfully reproducing the subtle tone of the bride's wedding dress. In consideration of the preferable result of the second embodiment, the gradation is changed to the light amount range of 0.2500 to 0.2.
Adjustment was made in three areas of 5560, 0.556 to 2.0860, and 2.0860 to 2.800. Table 9 also shows setting data of gradation characteristic curve for gradation adjustment. Table 9 shows the light intensity value range of 0.25 to 2.
The halftone dot area% value (y value) with respect to the light amount value after adjusting 80 to 0.00 to 2.55 is shown.

FIG. 6 shows the X-axis color separation curves for basic and gradation adjustment based on Table 9.

(Iii) Image quality evaluation Color separation and color proofing were performed in the same manner as in Example 2.
It was satisfactory enough, and it was possible to produce a high-key tone sufficiently reproduced without lowering the tone of the wedding dress part as well as the tone of the morning part.

[Effect of the Invention] The gradation conversion method for an image of the present invention rationally adjusts (corrects and changes) the gradation of a duplicate image such as a print image produced based on the basic dot characteristic curve set initially. Including.)

That is, when it is determined in which range of the basic halftone dot characteristic curve the tone adjustment should be performed, the halftone dot halftone characteristic curve corresponding to the required degree can be rationally set.

This means that, at present, there has not been developed a rational tone conversion technology that is the core of producing duplicate images such as halftone print images from continuous tone original images.
Therefore, the present invention makes an extremely large contribution in view of the current situation in which duplicate images are produced or tone adjustments are made with a great request based on the experience and intuition of the operator.

[Brief description of the drawings]

FIG. 1 illustrates a halftone dot characteristic curve for adjusting the gradation of a highlight portion set by the gradation conversion method of an image according to the first embodiment of the present invention. FIG. 2 shows a halftone dot characteristic curve for adjusting the gradation of a shadow portion set by the gradation conversion method of an image according to the first embodiment of the present invention. FIG. 3 illustrates a halftone dot characteristic curve for adjusting the gradation of a highlight portion and a shadow portion set by the gradation conversion method of an image according to the first embodiment of the present invention. FIG. 4 is a dot characteristic curve for adjusting the gradation of a highlight portion and a shadow portion in a different mode from that of FIG. 3 set by the gradation conversion method of an image according to the first embodiment of the present invention. Is illustrated. FIG. 5 shows a halftone characteristic curve for basic and gradation adjustment set according to the second embodiment of the present invention. FIG. 6 shows a halftone dot characteristic curve for basic and gradation adjustment set according to the third embodiment of the present invention.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-3477 (JP, A) JP-A-2-167580 (JP, A) JP-A-2-166968 (JP, A) JP-A-2-167 57066 (JP, A) JP 6-14683 (JP, B2) JP 6-14682 (JP, B2) JP 6-91612 (JP, B2) JP 5-64909 (JP, B2) JP-B-6-34499 (JP, B2) JP-B 5-47156 (JP, B2) Patent 2743070 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) H04N 1/40- 1/409 H04N 1/46 H04N 1/60

Claims (11)

(57) [Claims] 1. An image gradation conversion method for converting a continuous gradation image to a halftone gradation image for producing an arbitrary desired halftone gradation image, wherein: (i) performing gradation conversion Information value and halftone dot area% from H part (lightest part) to S part (darkest part) of original image
Setting a basic halftone characteristic curve that governs the correlation by operating the following <gradation conversion formula> based on the initial setting conditions; (ii) replacing the grayscale conversion by the basic halftone characteristic curve with
At least one control point for setting an area for performing tone adjustment in order to perform tone conversion so as to have a desired halftone dot tone is set in the image information values ranging from H to S. (Iii) For each area divided by the control point, the following <gradation conversion formula> is operated as follows to determine an individual halftone dot characteristic curve for each area; (b) Each control point desired y _S values for determining the one of the adjacent individual dots characteristic curve the value of dot area percent of, be y _H values for determining the other individual dots characteristic curve, (b) A desired value is employed as a γ value for determining an adjacent individual halftone dot characteristic curve. (C) In a predetermined gradation adjustment region, a desired halftone dot area% set for each control point is set. At least one of the numerical value and the γ value for determining the individual halftone dot characteristic curve is determined based on the basic halftone dot characteristic curve. It is different from the numerical value of the halftone dot area% at the control point and the γ value used to determine the basic halftone dot characteristic curve, and (iv) integrating the individual halftone dot characteristic curves determined as described above. A halftone dot characteristic curve for gradation adjustment from H to S, and converting the continuous tone image to a halftone dot image based on the halftone adjustment dot characteristic curve. A gradation conversion method for an image, characterized in that: <Tone conversion formula> 2. The gradation conversion method for an image according to claim 1, wherein the image information value related to the density information is a density information value of a document. 3. The method according to claim 1, wherein the image information value related to the density information is a photographic density of a color or monochrome photographic original. 4. The gradation conversion of an image according to claim 1, wherein the image information value related to the density information is related to an exposure value obtained from a photographic density characteristic curve of a color or monochrome photographic original. Law. 5. An image information value relating to density information relates to a light amount value of a subject obtained from a density characteristic curve which governs a correlation between an image density of a medium on which a document image is recorded and a light amount value of the subject. The method for converting gradation of an image according to claim 1. 6. The image floor according to claim 1, wherein the control point for managing the gradation adjustment area is set so as to emphasize or suppress the highlight portion of the original image. Tone conversion method. 7. The image floor according to claim 1, wherein the control point for managing the gradation adjustment region is set so as to enhance a shadow portion of the original image density region or to suppress the shadow portion. Tone conversion method. 8. The control point according to claim 1, wherein the control point for managing the gradation adjustment area is set so as to emphasize or suppress only the darkest part of the original image density area to the middle part. Image gradation conversion method. 9. The method according to claim 1, wherein the control point for managing the gradation adjustment area is set so as to emphasize or suppress only the darkest part density area from the middle part of the document image density area. Image gradation conversion method. 10. A control point for managing the gradation adjustment area is set so as to emphasize the darkest area from the middle to the darkest area while emphasizing the middle area from the brightest part of the original image density area. The method of claim 1, wherein the gradation conversion is performed. 11. A control point for managing a gradation adjustment area is set so as to suppress the middle area density area from the brightest part of the original image density area and also suppress the darkest area density area from the middle part. The method of claim 1, wherein the gradation conversion is performed.