JP2788920B2 - Gradation conversion method for producing duplicate image from original image - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Purpose of the Invention] (Industrial application field) The present invention is intended to rationally reproduce a document image having no density region which plays a guiding role in work when producing a duplicate image. It is intended to provide a method for producing an image.

More specifically, the present invention relates to a continuous tone original image such as a color film original (transparent original) having only one of black and white regions or both of the regions in the original image. The present invention relates to a method for producing a reproduced image such as a halftone dot having a desired density gradation (gradation) and color tone (tone), and particularly to a gradation conversion method as a basic technology thereof.

(Prior Art) Duplicate images such as print images, copy images, and printer images are produced from continuous-tone original images by various duplication techniques. In the production of these duplicate images, it is extremely important to reproduce the density gradation and the tone of the original image on the duplicate image in a rational, efficient and faithful manner with work regularity. . However, despite the recent progress in reproduction technology, the above-mentioned reproducibility cannot be achieved rationally and efficiently with work regularity. That's enough.

In this regard, continuous tone color film original images (hereinafter, referred to as
It is called a color original image. ) Will be discussed in detail with reference to a technique for producing a printed image having a halftone dot gradation.

In general, when producing a color print image, a color original image is subjected to color separation by a color scanner, and multi-color plate making (generally C, M, Y, and K plates) is performed to produce a halftone print image. Be replicated.

At present, almost all color separation work in plate making in Japan is processed using a color scanner, and the number of operating scanners reaches about 2,400. Approximately 460 total scanners that can also perform processing such as plate collection after color separation are used.

Although these color scanners and total scanners are extremely expensive devices that are mechatronized, one of the major problems in the industry is that the operating rate is about 30 on average.
%. The reason for keeping such a low operation rate is that the setup time (scanner setup time) for operating the color scanner is long, and the quality of the product obtained by the color separation operation is insufficient, so that rescanning is performed. There are many things.

Considering this from a slightly technical point of view, as described above, although a high-resolution color scanner or the like made into mechatronics is used as a tool for color separation work, multiple element technologies of color separation work, for example, Color correction (Color Correc
) and density gradation (Gradation) are not systematically coordinated, which causes the low operation rate of the color scanner. As is well known, of the two elemental technologies described above, color correction has been very rigorously scientifically pursued, such as a masking equation and a Neugebauer equation. However, the latter gradation (which is based on a predetermined pixel in a color original image) , Which is attributable to the question of what size of halftone dot should be corresponded.) Has been left without supporting a reasonable theory, and this part is inferred from human experience. Is heavily dependent on In other words, eliminating operator's guess work, operator's experience and intuition while using an advanced electronic color separation device (without operatorevalua
and without operator's guess werk), they cannot always produce stable quality products.

Next, the problem of a color separation device such as a color scanner is removed, and attention is paid to a color original image.

In general, in a color document image, a white area (which literally means the brightest portion in the document image, which serves as a guide for work in producing a print image. Even when there is no white portion, it is used when expressing the brightest portion relatively, so the following description uses the term white region to distinguish between the two, and the black region (literally in the original image). ) Is usually included.

However, many color original images include only one of the white and black regions, or do not include both. In such a case, as described above, scientific theories are given as to what size of a halftone dot (which is the above-mentioned gradation problem) should correspond to a predetermined pixel in a color original image. Since it has not been done, it relies entirely on the experience and intuition of the scanner operator. This greatly increases the set-up time of the color scanner, increases the number of rescans due to the dark scanner operation, and lowers the effective utilization of the color scanner.

(Problems to be solved by the invention) In order to solve the above problems, the present inventors
We believe that there is a leading important technology that must be solved before solving these problems.

These prior important technologies include image tone conversion technology (a technology that faithfully converts the tone of continuous tone pixels into a halftone image at a 1: 1 ratio) and color fog (color
cast, fog) removal technology, and more than simply converting the tone of the color original image to the tone of the printed image, producing a printed image with the desired tone of the color original image with regular work. Technology to do.

In the development of these advanced important technologies, the present inventors have proposed a gradation conversion formula for an image which is a predecessor of the <gradation conversion formula> used in the present invention (the theoretical basis is the same for both). However, it is necessary to keep in mind that their application aspects are different.) (Japanese Patent Application Nos. 63-2590, 63-114599, 63-114599)
207326, 63-259360, etc.).

The present invention utilizes a tone conversion formula of an image, which is the core of these prior arts, and has either a white area or a black area in a color original image or a density area of both of them. It is an object of the present invention to provide a technique for producing a printed image rationally and efficiently with a regular work even from a color original image having no image, that is, excluding the experience and intuition of a scanner operator. In addition, the present invention
The present invention is not limited to the production of a print image, but is applicable to the production of all kinds of duplicate images, as described later.

[Configuration of the invention]

(Means for Solving the Problems) To summarize the present invention, the present invention provides a continuous tone original image having only one of a white area and a black area, or a continuous image having no density area. A tone conversion method for producing a duplicate image such as a halftone tone using a tone original image: (i). A hypothetical individual density characteristic curve tentatively indicating a density characteristic from a white area (density value Hn) to a black area (density value Sn) of the original image. (A) In the case of an original image having only a white area, By assuming an area, (b) in the case of a document image having only a black area, by assuming a white area, (c) in the case of a document image having neither a white area nor a black area, By assuming a white area and a black area, they are respectively defined, and (ii). On the other hand, in the production of a duplicate image, a reference density characteristic curve of a standard original used as a standard original having both a white area (density value Ho) and a black area (density value So) is defined; (iii). The (i) hypothetical individual density characteristic curve (assumed density range Hn-Sn = DRn) of the original image is adjusted to the (ii) standard density characteristic curve (density range Ho-So = DRo) of the standard original. Obtaining a density information value (Do) on the adjusted reference density characteristic curve corresponding to the density information value (assumed Dn) at an arbitrary point on the assumed individual density characteristic curve; (iv). From the Do value, the density information value of the document image is converted into a tone intensity value (y) such as a dot area% by the following <tone conversion formula>.
(V). Based on the gradation conversion curve obtained in (iv),
Converting a density information value (actually measured Dn) obtained by actually measuring an arbitrary pixel on the original image into a gradation intensity value (y) such as a halftone dot area%; A gradation conversion method for producing an image.

<Tonal conversion _{formula> y = y H + [α} / (α-β)] · (1-10 -k · (Do-Ho)) · (y S -y H) where the <tonal conversion formula >, Each symbol means the following: (Do-Ho): From the adjusted density information value (Do) corresponding to the density information value (actually measured Dn) of an arbitrary pixel on the original image, Basic density value obtained by subtracting the white area density value (Ho) of the image.

y: A gradation intensity value such as a dot area% of a pixel on the duplicate image corresponding to an arbitrary pixel on the original image.

y _H: white area actually exists in the original image or tone intensity value, such as dot area percent are previously set to the brightest portion of the reproduced picture corresponding to the hypothesized white regions.

y _S: tone intensity value, such as halftone dot area% is preset in the darkest part of the reproduced picture corresponding to the black area or hypothesized black area, actually exists in the original image.

α: surface reflectance of a substrate used to represent a duplicate image.

β: Numerical value obtained by β = 10 ^-r .

 k: Numerical value obtained by γ / (So−Ho).

 γ: arbitrary coefficient.

 Hereinafter, the configuration of the present invention will be described in detail.

The tone conversion method for producing a duplicate image from a document image according to the present invention will be described with reference to a case of producing a color print image as a duplicate image. Therefore, this is for convenience of explanation and does not mean that the present invention is applied only to reproduction of a color print image.

At present, as described above, a color scanner color separation apparatus is very commonly used in the duplication work of a color print image, and a color original image subjected to color separation processing by the apparatus is a standard one. In some cases, a print image having a relatively stable image quality is produced even when the scanner operator changes. That is, as a color original image, both white and black areas exist in the color original image and the density area (De
When a document having an appropriate nsity range (D, R) (hereinafter, referred to as a standard document) is used, a color print image with a relatively good tone can be produced.

Here, characteristics of a color original image used for producing a color print image including a standard original will be described based on a density characteristic curve of a photographic material of a color film. First
The figure shows a density characteristic curve (color film manufactured by F Company) of a color film photographic light-sensitive material, in which the vertical axis represents the object density (D) on a color original image, and the horizontal axis represents the exposure (X).
Is shown.

As shown in FIG. 1, the density characteristic curve of the color film photographic light-sensitive material is schematically represented by a downwardly convex foot portion (between AB),
It has a substantially linear portion (between BC) and an upwardly convex shoulder portion (between CD). Then, on the density characteristic curve of the color film photographic material, the density characteristic curve of each individual color original image including the standard original is defined. This is because if the development conditions of a color film that has been photographed are kept constant (this condition can be regarded as being kept constant), the density characteristic of each original image is determined by the exposure amount. This is because of the difference.

That is, the density characteristic curve of the color film photographic light-sensitive material shown in FIG. 1 is a basic one that defines the density characteristic of each individual document, and is hereinafter referred to as a basic density characteristic curve.
The density characteristics of each individual document can be roughly defined on the basic density characteristic curve as follows.

(I) Standard original The exposure is appropriate and positioned between BCs in the basic density characteristic curve (having the lightest density Ho and the darkest density So). As a standard manuscript, for example, the measured value of the densitometer is Ho = 0.3, So = 2.80
Are given.

(Ii) Overexposed document Due to overexposure, the brightest portion is positioned at point A and the darkest portion is positioned between BC (having the brightest portion density Hn).

(Iii) Underexposed document Due to underexposure, the darkest part is positioned at point D and the brightest part is positioned between BC (having darkest part density Sn).

Among these color original images, aside from the standard original of (i), the white (or brightest) area in the color original image of (ii) goes beyond Ho-Hn beyond B in FIG. , Where the black area is not near the So value, or (ii)
In the case of processing a so-called non-standard original with a color scanner, i.e., the black area enters the area of So to Sn beyond C in FIG. 1 and the white area is not near the Ho value, the color separation of the scanner is reasonable. Cannot be performed, and the production of a print image having good image quality depends heavily on the experience and intuition of the operator. In addition, when a color document image does not have both a white area and a black area (however, it is needless to say that it has a relative brightest portion and a darkest portion), it is extremely difficult to perform a plate making operation. It is. Here, the white area is the brightest portion Ho in the standard document (specifically, about 0.
3) or a region having a density value close to it, and the black region is the darkest portion density So in the standard original.
(Specifically, it is a region having a density value close to about 2.8 as measured by a densitometer) or a density value close thereto.

This is because, in a non-standard original, what size halftone dot should be associated with a missing white (or lightest part) area or a black (or darkest part) area, or both. This is because there is no rational means for determining how to set the concentration gradient between.
Note that the density characteristic curve of a standard document defined in a specific range on the basic density characteristic curve is referred to as a reference density characteristic curve, and is referred to as a first density characteristic curve.
In the figure, it is set between BC. The density characteristic curve of the non-standard document is also defined in another range on the basic density characteristic curve, and is called an individual density characteristic curve.

The present inventors have conducted various studies with the aim of automating the color separation operation using a color scanner as described above. As a result, (1) the tone (density level) of a continuous tone original image Tone and color tone) can be faithfully converted into a halftone duplicate image with a 1: 1 correspondence with work regularity (this is the <tone conversion formula> of the present invention). (2) The individual density characteristic curve of a color original image is subjected to mathematical processing to the reference density characteristic curve of a standard original. Adjustment, that is, the density information value of any pixel in the color original image is adjusted to the corresponding density value on the reference density characteristic curve of the standard original, and then processed by the tone conversion technique of (1) above.
We have developed a technology (Japanese Patent Application No. 63-207326) that always produces print images of the same quality as those obtained from standard manuscripts.

The present invention uses an extremely inappropriate color original image having only one of a white area and a black area in the image or having no density area for both as a color original image. An object of the present invention is to provide a method for rationally producing a printed image having excellent tone reproducibility.
For this purpose, the present invention makes full use of the above-mentioned techniques (1) and (2) proposed by the present inventors, but here, the characteristics of the color original image to be processed by the present invention will be described. Further explanation will be given.

The present inventors have developed a color original image having only one of a white area and a black area in a color original image, or using a color original image lacking both of them, to perform color separation work rationally. I believe it is important to build on the following assumptions.

That is, it is assumed that there is a white area and a black area in a specific color original image to be processed, and it is sufficient that the density area on the color original image can be specified.

In this case, the assumed individual density is plotted on the density characteristic curve (basic density characteristic curve) of the color film photographic light-sensitive material of the color original image shown in FIG. A characteristic curve (referred to as an assumed individual density characteristic curve in order to distinguish it from the individual density characteristic curve described above) can be defined, from which density information of an image necessary and sufficient to perform color separation can be obtained. It is. After such an assumption, as described above, the technique of adjusting the individual density characteristic curve previously proposed by the present inventors to the reference density characteristic curve, and furthermore, always use the density information of the adjusted image to change the standard image from the standard image. What is necessary is just to combine the gradation conversion technology which guarantees to produce a print image of the same image quality as that obtained.

For example, when a color original image having a white area and no black area (the area having a density value close to So of the standard original as described above) is subjected to color separation by a color scanner, the darkest part ( It should be noted that this is a relative darkest part and not a black area.) It is important to always consider how large a halftone dot should be set.

In the present invention, the density range of the standard image (Ho to
The case of So = DRo is shown. ), It is reasonable to determine what size of halftone dot should be set in the (relative) darkest part of the color original image in relation to the size of the halftone dot set in So of the standard original. To decide,
It is intended to completely eliminate the guesswork, experience and intuition of the scanner operator.

In order to determine the assumed density area of the specific color original image, the objective fact that the density difference between many color original images having both a white area and a black area takes a substantially constant value may be used.

However, the density difference between the white and black areas in the color document image varies from 0.10 to 0.20 depending on the brand of the color film, the type of the subject, the criterion for determining the black and white area, exposure conditions such as photographing, and the like. Therefore, in order to accurately grasp the density difference between the white and black areas, the density values of the white area and the black area in the color original image are collected, arranged, and organized for each film brand.
It is necessary to process and obtain data for specifying the concentration range and concentration gradient.

A setting example of the assumed individual density characteristic curve will be described with reference to the drawings.
FIG. 2 shows a method for setting an assumed individual density characteristic curve by assuming the density area of the color original image from the density value of the white area (lightest part) or the density area of the black area (darkest part) of the color original image. FIG.

According to the examination of the color original image having the white area and the black area, in the case of the color film manufactured by Company F, the color original image having a large exposure amount (overexposure) at the time of photographing and a light tone color original image or a standard exposure (suitable exposure) The density range of the original image is about 2.
It is possible to grasp a general tendency that the density is 60 and that the density range of the color original image having a low tone (under exposure) and a dark tone is about 2.50. These empirical facts are used in the construction of FIG. That is, in a color document image having a white area H ₁ defines the assumed individual density characteristic curve assuming such that the black density in _{S 1 = H 1 +2.60 (H} 1 ~S 1), the black area S _In a color original image having a value of ₂ , an assumed individual density characteristic curve (H _{2 to} S ₂ ) is defined on the assumption that white density H ₂ = S ₂ −2.50.

In the case where both a white area and a black area are missing in a color original image (of course, there is a relative brightest part and a darkest part). Can be defined on the basic density characteristic curve by measuring the relative density of the brightest portion and the darkest portion in the color original image. The individual density characteristic curve of such a color original image is defined by a specific range on the reference density characteristic curve of the standard original. This is because if one of them exceeds the density value at both ends of the density area of the standard document, it belongs to the category of the color document image that lacks either the white area or the black area. In the present invention, it goes without saying that the density range of the assumed density characteristic curve described in the above three examples can be set arbitrarily.

When the color original image is a reflection original (it should be noted that the above description is based on a transparent original).
When a watercolor or the like is subjected to color separation by a color scanner, the basic density characteristic curve becomes substantially linear, and in this case, the individual density characteristic curve of the color original image is also determined by an arbitrary predetermined value on the linear basic density characteristic curve. It will be specified by the range. For example, it is set between a density range of white density of color paper and drawing paper and a density range of black density of paint, black ink and cloth.

Therefore, the assumed individual density characteristics of the color original image lacking both the white area (the area having the density value close to Ho of the standard original) and the black area (the area having the density value close to So of the standard original) The curve can be assumed to be a reference density characteristic curve of a standard image in the case of a transparent original, and can be assumed to be a linear basic density characteristic in the case of a reflective original. That is, in the case of a color document image having neither a white area nor a black area in the color separation operation, the procedure for adjusting the assumed individual density characteristic curve to the reference density characteristic curve is omitted. In the case of such a color original image, steps (iii) to (v) of the present invention, which will be described later, are largely omitted, and color separation may be performed using <gradation conversion formula> as it is.
This is because, when a standard original is subjected to color separation under the <gradation conversion formula>, a printed image with a tone that is natural for human vision is always obtained with work regularity. This is because the density characteristic curve is defined on the reference density characteristic curve. It goes without saying that the individual density characteristic curve of the color original image in which both the white area and the black area are missing can be arbitrarily moved within the range of the reference density characteristic curve. This is because the color original image is not produced under proper exposure conditions (in such a case, there is a case where an individual density characteristic curve can be defined within a range on the reference density characteristic curve). This is a necessary means when trying to meet the conditions.

As described above, in the individual color original image to be subjected to the color separation operation by the color scanner, the assumed density area is specified, and the assumed individual density characteristic curve is specified. That is, the steps (i) to (ii) of the present invention are completed.

Next, a method of adjusting the assumed individual density characteristic curve to the reference density characteristic curve (the above-mentioned step (iii) of the present invention) will be described. This adjustment method has been previously proposed by the present inventors (Japanese Patent Application No. 63-207326), and the gist of the adjustment method is as follows.

FIG. 3 shows the principle of adjusting the assumed individual density characteristic curve to the reference density characteristic curve.

First, in order to perform the adjustment mathematically, it is necessary to define the basic density characteristic curve as a function. This can be defined, for example, by a function of D = f _D (X) shown in Table 1 below. The function definition of the basic density characteristic curve in Table 1 should be understood as an example, and it goes without saying that it can be arbitrarily defined. In addition, in Table 1, in addition to D = f _D (X) for obtaining the density value (D) from the exposure amount (X), X for obtaining the exposure amount (X) from the density value (D) which is an inverse function thereof is shown. = F _X
(D) is shown.

As shown in FIG. 3, the reference density characteristic curve of the standard original and the assumed individual density characteristic curve of the color original image are defined on the basic density characteristic curve of the color film photographic material, and the density value D of the color original image is defined. _{Hn to} D _Sn are projected on the X axis, and the value range (X _{Hn to} X _Sn ) of the color original image on the X axis is matched with the value range (X _{HO to} X _SO ) on the X axis of the standard density characteristic curve.

Next, the density values D _HO to _SO on the D axis of the reference density characteristic curve are obtained. In this way, the density information value (Dn) of the assumed density area of the color original image is converted to the density information value (Dn) on the reference density characteristic curve.
o) can be adjusted. As a matter of course, when the assumed individual density characteristic curve of the color document coincides with the reference density characteristic curve, it is needless to say that the two need not be matched.

Generally, XR _O (exposure range of the reference density characteristic curve of the standard document) is used as a means for matching the assumed individual and the reference density characteristic curve.
And XR _n (exposure amount range of the assumed individual density characteristic curve of a non-standard individual document), it is necessary to mathematically and proportionally match both characteristic curves. Of course, a simple matching method may be adopted in which the start point (or the end point) of the range is simply matched. This lacks both white and black areas as described above,
This is necessary means for moving the individual density characteristic curve within the reference density characteristic curve in a color original image whose individual density characteristic curve is defined in the reference density characteristic curve.

In addition, in order to simplify the adjustment procedure, the assumed individual density characteristic curve may be directly projected on the X-axis, and the value on the X-axis may be directly read as the density value, or the read density value may be set in the same manner. The tone conversion of the image may be performed by using the <tone conversion formula> after performing a desired process such as matching the density value of the reference density characteristic curve read again.

As shown in FIG. 3, from the relational expression used for the mathematical proportional matching operation, that is, from the density information value (D _n ) of the assumed individual density characteristic curve, the basic density characteristic curve D = f _D (X) is obtained. A relational expression for obtaining the X value for obtaining the density information value (D _O ) of the color original adjusted and substituted is as follows.

Where m: Required translation amount XR _o : Exposure range of reference density characteristic curve of standard original on X axis XR _n : Exposure range of assumed individual density characteristic curve of non-standard individual original on X axis When the work of matching the assumed individual density characteristic curve with the reference density characteristic curve is completed, the density information value (D _o ) of the color original adjusted from all the density information values (D _n ) on the assumed individual density characteristic curve ). When the _Do value obtained in this way is substituted into the <gradation conversion formula>, the color separation curve, that is, the density information value of the color original image (the density value of continuous tone) and the image information value of the color print image (Numerical value of halftone dot area% of halftone tone) A tone conversion curve that governs the correlation (this is also called a halftone tone characteristic curve)
Is obtained. This step corresponds to the above (iv) of the present invention.

This completes all the preparation work for performing color separation using a color scanner on a continuous tone color original image that does not have one or both of the white area and the black area in the color original image. Become. Only the steps corresponding to the above (v) of the present invention are left. This (v)
In the step (3), the density information value (actually measured D _n ) of an arbitrary pixel on the original image obtained by scanning the color original image is adjusted as described above (actually measured D _n → D _o ), and the adjusted D _{o is} adjusted. This is performed by substituting the value into the <gradation conversion formula> and converting it into a dot area% (y value).

As described above, assuming that the above-mentioned <gradation conversion formula> of the present invention is used, the assumed individual density characteristic curve of the color original image and the requirement for creating a print image having desired image quality are satisfied. If the reference density characteristic curve of the standard document is set and the two are linked by a rational method such as mathematical processing, the color document image can be used to reproduce both the white area and the black area that serve as a guide for the work. Even if they are not included at the same time, a print image having a desired image quality (having a tone that is natural for human vision) can always be created with work regularity.

In addition, since there are seemingly many parameters in the <gradation conversion formula> of the present invention, there arises a question that the operation of the <gradation conversion formula> may be arbitrarily entered. However, in the replication technique, such as printing the image, y _H of the <tonal conversion formula>, parameters y _S is almost constant of, for example, in the case of the print image, 5% C version of y _H, y _A dot area% of 95% is used for _S. Incidentally, in the operation of the <tonal conversion formula>, using the densitometer measurements to the density value, using the percentage number in y _H and y _S, y values is also calculated as a percentage value. α is the surface reflectance of the printing paper, and α may be set to 1.0.

In the present invention, a characteristic parameter is a γ value. This is to adopt a solid density value of yellow ink at the time of color separation of a standard document and to fix it to a value of γ = 0.9 to 1.0. And in many cases γ = 0.
Excellent results are obtained with values between 9 and 1.0. In the present invention, since the means for adjusting the assumed individual density characteristic curve of the color film original image to the reference density characteristic curve of the standard image is provided, the γ value may be fixed to the above value. It is important to be able to convert the γ value into a constant, and it is possible to almost convert the term of the <gradation conversion formula> into a constant, thus simplifying the calculation of the <gradation conversion formula> and a mechanism for the purpose. In addition,
The numerical value setting of these parameters is different depending on the standpoint of reproducing the tone of a given color original image faithfully into a printed image and the standpoint of producing a printed image with a different tone consciously. For example, by changing the γ value, the shape of the color separation curve can be changed to a desired one, so that printed images of various tones can be obtained.

Note that the <gradation conversion formula> of the present invention is applied to multi-color plate making (generally, four plates of a C plate, an M plate, a Y plate, and a K (black) plate are considered as one set). To set each color separation curve, the following may be performed. The <gradation conversion formula> is derived from the viewpoint of rationally determining the most important plate C among multicolor plate making. Therefore, what is derived by the operation of the <gradation conversion formula> is the color separation curve for the C plate, and the color separation curves for the M and Y plates are to maintain the gray balance and the color balance. It may be determined by multiplying by an appropriate adjustment value. The decomposition curve for the K (black) plate may be determined according to a standard method in the art from the viewpoint of reducing the consumption of C, M, and Y inks.

Next, features of the <gradation conversion formula> of the present invention, such as operation aspects and application aspects of the tone conversion method of the present invention, will be described.

In the operation of the <gradation conversion formula> of the present invention, the density information value refers to each pixel of an original image (the present invention is not limited to a color original image when a color print image is created as described above). Any value that reflects the physical quantity related to the concentration it has may be used, and should be interpreted in the broadest sense. Synonyms include reflection density, transmission density, luminance, intensity, light quantity, amplitude, current / voltage value, and the like.

In the operation of the <gradation conversion formula> of the present invention, the present invention can be used by being modified as described below, as well as being freely modified, deformed, or guided.

In the above modification, α = 1. this is,
For example, the surface reflectance of printing paper (base material) used to represent a printed image is set to 100%. The value of α can take any value, but may be 1.0 in practice. This is the same for a luminance image such as a video image.

Further, according to the modification (alpha = 1.0), a y _H to the brightest portion H (i.e. H _o of the standard image) on the printed image, darkest part S
(Ie, the _S0 part of the standard image) _ys can be set as planned, which is a major feature of the present invention. This means that in the brightest part H on the print image,
(D _o −H _o ) = 0 by definition, and (D _o −H _o ) = [reference image density area] in the darkest part S;
That is, Therefore, it is apparent from -k · [(D _o −H _o )] = − γ. Thus, the <gradation conversion formula> (α
= 1) is always as expected
It is possible to set the y _H and y _S on the printed image is very important in terms of user to consider the work result. For example, set the value to desired to y _H and y _S in a printed image, changing the γ value (where, alpha = 1.0), various color separation curve obtained. Then, the printed image obtained under these color separation curves can be easily evaluated in relation to the γ value.

Further, the conversion method of image characteristics based on the <gradation conversion formula> of the present invention includes reproduction of gradation and color tone of a document image,
That is, the tone of the manuscript image is applied to the printed image with work regularity 1:
Although it is extremely useful for reproducing in 1, the usefulness is not limited to this. As described above, the <gradation conversion formula> of the present invention can be obtained by appropriately selecting α, β, γ values, and further, y _H , y _S values, in addition to faithful reproducibility of the characteristics of the original image. This is extremely useful in reasonably changing or correcting the image characteristics of a color original image.

Although the operation of the <gradation conversion formula> of the present invention has been described particularly in relation to the creation of a print image, its application is not limited to the creation of a print image.

That is, the application aspects of the <gradation conversion formula> of the present invention include: (i) 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) (This is also called the area gradation method), as well as (ii) a sublimation transfer type thermal transfer image, (using silver salt) a heat development transfer image, and a constant area which is seen in a conventional gravure image, etc. When it is intended to express a gradation or a color tone 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). iii) Recording density per unit area found in digital copying machines (such as color copying machines), printers (such as ink jet machines and bubble jet machines), and all types of facsimile machines When the gradation is to be expressed by changing the degree, for example, the number of dots, the number or size of the ink particles (this is similar to the area gradation in (i) above), (iv) ) From electrical signals related to image information such as video signals, television signals, and high-definition signals, try to obtain a CRT image that expresses an image by adjusting the intensity of brightness per unit area, and to obtain a printed matter or hard copy with gradation from this. (V) Not only in the case of the image conversion processing between the original image and the duplicate image in the above-mentioned substantially equal density (luminance, illuminance) area, but also in the spatial, luminance, wavelength, and temporal invisible areas. For example, in the case of input conversion of image information in a low illuminance region (such as imaging with a high-sensitivity camera), for example, when the contrast of the original image is extremely low and the density range difference between the original image and the duplicate image is small, (In such a case, the emphasis conversion of the contrast of the image is more important than the conversion of the gradation of the image.) (Vi) In addition, the dot area% and the like are also displayed together with the density display. -It can be applied to densitometers with a gradation conversion mechanism, for preparatory inspection of color separation (for example, color proof for calibration), and for printing-related crises such as simulators for color separation education.

When the image characteristic conversion method using the <gradation conversion formula> of the present invention is applied to the above-described various application fields, it is obtained from continuous gradation (including both hard originals and soft originals). Image information and / or image information electrical signal relating to image density (either analog or digital)
Is performed by the image conversion processing unit (gradation conversion unit) of the device in the various application fields described above, and the current of the recording unit (recording head) of the device is correlated with the processed value y value (gradation intensity value). The density gradation of the original image is controlled by controlling the value, voltage value, or application time thereof, and changing the halftone dot area, the number of dots per fixed area (1 pixel), the density per fixed area (for example, 1 dot), and the like. It is only necessary to output a duplicate image such as a halftone gradation, which corresponds to 1: 1.

For example, in order to produce an original of a print image that is a halftone image, that is, an original for printing, by using a conversion processing method of image characteristics based on the <gradation conversion formula> of the present invention, it is required in the art. A well-known existing system may be used, and this is achieved by incorporating the image conversion processing 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. Is done. More specifically, a small spot light is applied to a document image which is a continuous tone image such as a color photograph, and the reflected light or transmitted light (image information signal) is received by a photoelectric conversion unit (photomultiplier). , Converts the intensity of light into the intensity of voltage, converts the obtained image signal electric signal (electric value) into a desired arrangement and processing by a computer, and outputs the processed image information electric signal (voltage value) output from the computer In a well-known existing system for controlling an exposure light source light based on a laser beam and then applying a laser spot light to a raw film to produce a printing master, for example, a computer for organizing and processing image information electric signals of a document image In the calculation processing mechanism, the assumed individual density characteristic curve is adjusted to the reference density characteristic curve, and the continuous tone image is obtained by using the <tone conversion formula> of the present invention. Only it may be incorporated software that can be made to information electrical signal to the image information electrical signal halftone gradation. As such software, the hypothetical individual density characteristic curve of the present invention is adjusted to a reference density characteristic curve, and the algorithm of the <gradation conversion formula> is held as software, and A / D (analog-digital conversion), D / A I / F
General-purpose computer with an (interface), an electric circuit embodied by a general-purpose IC as an algorithm as logic, an electric circuit including a ROM holding algorithm operation results, and a PA embodied as an algorithm as internal logic
It can take various forms such as L, gate array, custom IC, etc. Particularly in recent years, modularization has been developed, and it is necessary to adjust the assumed individual density characteristic curve of the present invention to a reference density characteristic curve and to perform a conversion process of image characteristics in a density region based on the <gradation conversion formula>. Can be easily manufactured as a module such as a dedicated IC, LSI, microprocessor or microcomputer. Then, the spotlight for photoelectric scanning is sequentially advanced while being divided into points, and if the laser exposure unit is also synchronized with this,
It is possible to easily create a halftone printing original plate having a halftone dot area percentage value (y) derived by the <tone conversion formula>.

(Examples) Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

At the time of the experiment, ○ operation of adjusting the actually measured density area of the color original image to a predetermined assumed density area; ○ adjustment of the assumed individual density characteristic curve having the assumed density area to the reference density characteristic curve; Converting the actually measured density value (D _n ) of a predetermined pixel into a corresponding density value (D _o ) on the reference density characteristic curve; ○ using the adjusted D _o value of the D _n value, the <tone conversion> A series of tasks of creating a color separation curve using the <Eq.
The required calculations were performed on an 800-type personal computer.

Example 1 As a document, a color document image of a 4 × 5 ″ copy target image manufactured by Company F, which is a still life in a room, was used.

The color original image has a white area (density value 0.17) in the image.
And the density value of the darkest darkest part (not the black area) in the image was 2.40.

Assuming that the density range of the color original image is 2.70, the assumed density range was 0.17 to 2.87.

The hypothetical individual density characteristic line having this hypothetical density area is shown in FIG. 3, and a basic density characteristic curve (a color film density characteristic curve manufactured by Company F is used as a basic density characteristic curve. This is specified in the above.) On the other hand, a basic density characteristic curve in a density range of 0.30 to 2.90 was defined on the basic density characteristic curve as shown in FIG.

Next, the assumed individual density characteristic curve was adjusted to a reference density characteristic curve, and a color separation curve (for C plate) was created using the <gradation conversion formula> of the present invention. The conditions of the parameters at the time of creating the color separation curve are as follows.

_{y H = 5%, y S} = 95%, α = 1.00, γ = 1.00, β = 10 -γ = 0.10, k = 1 / 2.60, then Magna scan M-645 as a color scanner for color separation ( (Crossfield), and color proof printed images were produced by DuPont's Chromalin proofing method, and the image quality was evaluated. Incidentally, M plate, Y _{plate, y H = 3%, y} S = 90%, the halftone area so as to set the halftone dot to be 10% less than the C plate, K version according to a conventional method Was.

The proof print image produced as described above had the tone as expected as a whole, had a good gray balance, and had an appropriate dark portion density. In other words, the tone of the proof print image has a higher density as a whole than the tone of the color original image, and in particular, the contrast in the highlight portion is emphasized, and the tone of the image to be copied is reproduced more faithfully. there were. Incidentally, in the darkest part where the density was 2.40 on the color original image, about 92% of halftone dots were present.

In this experimental procedure, the qualities possessed by <tonal conversion formula> parameter _{y H, y S, α,} k, by appropriately setting the value of gamma, as was expected a tone proofing image It was confirmed that it can be adjusted with regularity.

Example 2 As a document, a color document image of a still life in a forest where the image to be copied does not include a white area was used. The density value of the black area of the color original image is 3.06, and the density value of the brightest portion is 0.87.
Met.

The density area of the color original image is 2.50, and the assumed density area is
From 0.56 to 3.06.

Next, in the same manner as in Example 1, the density information of the color original image was converted and adjusted to density information on a reference density characteristic curve (density range 0.30 to 2.90), and a color proof print image was produced.

In the color proof print image produced in this way, an image of the tone predicted in advance is obtained, and the density gradient from the shadow part to the highlight part is natural for human visual perception and is satisfactory. there were. That is, the tone is brighter overall than the color original image, and the tone of the shadow portion is well expressed. By the way, the portion where the density was 0.87 on the color original image had about 23% of halftone dots.

(Example 3) A watercolor in which both a white area and a black area are missing was used as a document. The brightest part density value of this watercolor was 0.50, and the darkest part density value was 1.19. As is well known, a watercolor is a reflection original, and its density characteristic curve is defined by a straight line. Also, the reference density characteristic curves (density range 0.30 to 2.90) defined in the first and second embodiments can be regarded as substantially linear, so that the watercolor density characteristic curve is regarded as being defined on the reference density characteristic curve. Thus, although not may be anyway color separation, where the nature of the <tonal conversion formula>, _{i.e., y H, y S, α} , γ, any tone of a printed image by appropriately setting the k The experiment was carried out utilizing the characteristic that the work can be adjusted with regularity.

Therefore, the tone of the proof print image is made darker as a whole (in this case, the fact that the γ value should be set to a large value is derived from the characteristics of the <gradation conversion formula>) and high. In order to fulfill the requirement of increasing the contrast of the light portion (in this case, y _{S is set} to a small value), the values of the parameters were set as follows.

y _H = 5%, y _S 90%, α = 1.00, γ = 1.20, β = 10- ^γ = 0.063, k = 1.20 / 2.60 Scanner color separation and color proofing are performed under the above conditions, and the color proof print image is obtained. Made. The tone of the color proof print image produced in this way was exactly as expected, the density gradient was natural for human visual perception, and the color balance was well-balanced. Note that γ = 1.
00, y _H = 5%, y _S = 95%, and the size of the halftone dot after color separation from the highlight to the middle tone is larger by about 2 to 3% in this experiment. And consistent with the observations.

〔The invention's effect〕

The application of the gradation conversion method of the present invention is versatile, but when applied to the production of a printed image, the following effects are obtained.

At present, an expensive mechatronic scanner device is used for producing a printed image, but the operation rate is extremely low. This is largely due to the long setup time of the scanner for determining the conditions under which color separation is performed on a document image such as a color document, and a high rate of rescanning.

According to the present invention, a color original image in which the arbitrariness of the scanner operator, experience, intuition, etc. enters and is forced to rescan, particularly a color original image without a density portion corresponding to either white or black, or both. Even if there is, it can be processed automatically. Therefore, since the setup time for such a color document image can be greatly reduced, the operating rate of the scanner can be significantly improved. Needless to say, waste of materials and labor can be reduced by reducing rescanning.

Further, as described above, the present invention enables the color separation technology using a scanner to automatically respond to a specific problem in daily life, which is important in establishing full automation of the scanner separation technology. It is something.

In addition, in the production of a printed image based on the gradation conversion method of the present invention, the core gradation conversion work is performed using <gradation conversion formula> that matches theory and practice. Education and practical training can be conducted rationally. That is, based on the <gradation conversion formula> of the present invention, in addition to faithfully reproducing the tone (density tone and color tone) of the original image into a printed image in a 1: 1 ratio, production of a printed image with a changed tone Because you can do creative work such as
A wide range of platemaking techniques can be educated and practiced.

The gradation conversion method of the present invention is applied not only to the production of the above-described print image but also to the production of a variety of duplicate images, and has the same effects as described above.

[Brief description of the drawings]

FIG. 1 shows a density characteristic curve (basic density characteristic curve) of a color film photographic light-sensitive material (manufactured by Company F). FIG. 2 shows a method of setting a reference density characteristic curve and an assumed individual density characteristic curve on the basic density characteristic curve. FIG. 3 shows the principle of matching between the assumed individual density characteristic curve of the color original image and the reference density characteristic curve.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G03F 5/00 G03F 3/08

Claims (7)

(57) [Claims] 1. A method of using a continuous tone original image having only one of a white area and a black area, or a continuous tone original image having no density area for both of them, to obtain halftone dot gradation and the like. In a gradation conversion method for producing a duplicate image, (i). A hypothetical individual density characteristic curve tentatively indicating a density characteristic from a white area (density value Hn) to a black area (density value Sn) of the original image. (A) In the case of an original image having only a white area, By assuming an area, (b) in the case of a document image having only a black area, by assuming a white area, (c) in the case of a document image having neither a white area nor a black area, By assuming a white area and a black area, they are respectively defined, and (ii). On the other hand, in the production of a duplicate image, a reference density characteristic curve of a standard original used as a standard original having both a white area (density value Ho) and a black area (density value So) is defined; (iii). The (i) hypothetical individual density characteristic curve (assumed density range Hn-Sn = DRn) of the original image is adjusted to the (ii) standard density characteristic curve (density range Ho-So = DRo) of the standard original. Obtaining a density information value (Do) on the adjusted reference density characteristic curve corresponding to the density information value (assumed Dn) at an arbitrary point on the assumed individual density characteristic curve; (iv). From the Do value, the density information value of the document image is converted into a tone intensity value (y) such as a dot area% by the following <tone conversion formula>.
(V). Based on the gradation conversion curve obtained in (iv),
Converting a density information value (actually measured Dn) obtained by actually measuring an arbitrary pixel on the original image into a gradation intensity value (y) such as a halftone dot area%; A gradation conversion method for producing an image. <Tonal conversion _{formula> y = y H + [α} / (α-β)] · (1-10 -k · (Do-Ho)) · (y S -y H) where the <tonal conversion formula >, Each symbol means the following: (Do-Ho): From the adjusted density information value (Do) corresponding to the density information value (actually measured Dn) of an arbitrary pixel on the original image, Basic density value obtained by subtracting the white area density value (Ho) of the image. y: A gradation intensity value such as a dot area% of a pixel on the duplicate image corresponding to an arbitrary pixel on the original image. y _H : A tone intensity value such as a halftone dot area% preset at the brightest part on the duplicated image corresponding to a white area existing in the original image or the assumed white area. y _S: tone intensity value, such as halftone dot area% is preset in the darkest part of the reproduced picture corresponding to the black area or hypothesized black area, actually exists in the original image. α: surface reflectance of a substrate used to represent a duplicate image. β: Numerical value obtained by β = 10 ^-r . k: Numerical value obtained by γ / (So−Ho). γ: any coefficient. 2. The gradation conversion method according to claim 1, wherein in the case of a document image having only a white area (density value Hn), the density value Sn of the black area is assumed to be Sn = Sn + (2.20 to 3.00). . 3. The gradation conversion according to claim 1, wherein in the case of a document image having only a black area (density value Sn), the density value Hn of the white area is assumed to be Hn = Sn- (2.20 to 2.80). Method. 4. The gradation conversion method according to claim 1, wherein the original image is a color original or a monochrome original. 5. The gradation conversion method according to claim 1, wherein the original image is selected from a reflective original, a transparent original, and an electrically or magnetically recorded image. 6. When the original image is a color film original (transparent original), the assumed individual density characteristic curve and the reference density are plotted on the density characteristic curve of the photographic light-sensitive material (curve indicating the correlation between the exposure amount and the film density). 2. The gradation conversion method according to claim 1, wherein a characteristic curve is defined. 7. The gradation according to claim 1, wherein when the original image is a reflection original, the assumed individual density characteristic curve and the standard density characteristic curve are defined on a substantially linear density characteristic curve. Conversion method.