JP2927292B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention converts an image information signal obtained by subjecting a document image to photoelectric scanning or the like by a novel gradation conversion method, and has excellent gradation reproducibility. The present invention relates to an image forming apparatus capable of forming a recorded image.

More specifically, the present invention is applicable to various types of original images (continuous tone images such as monochrome or color photographs, video images obtained from television or computer video signals, etc.) which are to be reproduced on a recording sheet. The same applies to the following.) An image obtained from the same is converted using a new gradation conversion formula under a gradation adjustment mechanism, and a gradation is output on a recording sheet based on the gradation-converted output signal. The present invention relates to a method and apparatus for forming a recorded image having a distribution of pixels corresponding to a document image having excellent tone and color tone reproducibility.

(Prior Art) When an image is formed on a recording sheet from a document image having a continuous tone such as a photograph by using an image forming apparatus such as a copying machine, a method using photosensitive paper as the recording sheet is analog processing of the document. By (exposure), an image having a continuous tone corresponding to the document is formed (silver halide photographic recording). On the other hand, with an electrophotographic copying machine that records an image on plain paper instead of photosensitive paper, it is not possible to form an image by analog processing, and it is difficult to reproduce density gradation (gradation). In the case of formation, it is not easy to adjust the color tone (color balance) together with the density gradation.

Therefore, recently, efforts have been actively made to improve the reproducibility of gradation and color tone in an image forming apparatus.

The formation of a recorded image in an image forming apparatus of this type is performed by photoelectrically scanning a document image having a continuous tone, such as a photograph, similarly to a method of converting the continuous tone of photoengraving in printing into a halftone dot tone. The obtained image information signal is processed, and the signal is used to form an image having a distribution of pixels having gradations and colors corresponding to the original image on a recording sheet.

However, in the current image forming apparatus, when the gradation characteristics of the original image are reflected on the recorded image based on the final distribution of pixels, what characteristics should the distribution of the pixels have? At present, satisfactory reproducibility of gradation and color tone has not been obtained because no scientific study has been made on how to obtain the distribution.

That is, for a density value of a predetermined sample point on a document image,
In the pixel block on the recorded image corresponding to the sample point, a scientific correlation formula has been developed for what ratio should be recorded with respect to the number of unit images constituting the pixel block. At present, these equipment manufacturers have to rely on experience, intuition, or what has been determined based on a limited number of fixed conditions.

For this reason, originals with image quality that the equipment manufacturer did not expect, such as non-standard
In the case of, for example, a color film original with an underexposed underexposure), it is difficult to obtain a desired recorded image excellent in gradation and color tone. Therefore, a recorded image of a desired image quality can be obtained not only from a document having a standard image quality but also from the above-mentioned non-standard document, and the image quality of the document can be arbitrarily changed or corrected (change or correction of gradation or color tone). ) Cannot develop a flexible image forming apparatus.

This means, as described above, what pixel density value the conventional image forming apparatus should correspond to the density value of a predetermined sample point on the original image on the corresponding recorded image. Has not been scientifically elucidated.

(Problems to be Solved by the Invention) The cause of the above-mentioned problem in the conventional image forming apparatus is that when a recording image based on a final pixel distribution is formed from an original image such as a continuous tone image, The idea is to consider the process of gradation conversion of an image which plays an important role at the first stage. That is, the density value of a predetermined sample point on a document image is calculated by calculating the ratio of the number of unit pixels to be recorded to the number of unit pixels forming the pixel block in the corresponding pixel block on the recorded image (hereinafter referred to as pixel density). Value), the conventional way of thinking about gradation conversion does not mean that it must be done based on scientifically reasonable means of converting gradations, but is based solely on experience and intuition. It was dependent on.

The present inventor pays attention to such a situation, and in order to ultimately rationalize the image forming process and form a recorded image with excellent quality, a rational image gradation conversion technology must be established. We conducted intensive research based on the basic understanding that

(Means for Solving the Problems) In general, according to the present invention, an image information signal obtained by photoelectric scanning or the like from a document image is processed by a gradation adjustment mechanism, and a recording sheet is formed based on the processed signal. In an image forming apparatus for forming a recorded image based on a distribution of pixels corresponding to an original on the image forming apparatus, the gradation adjustment mechanism may include a base for an arbitrary sample point on the original based on an image information signal obtained from the original. The density value (x) (the difference between the density value at the sample point and the density value at the brightest part on the same image) is determined by the number of unit pixels forming a pixel block corresponding to the sample point in a recorded image to be formed. And a ratio (y) of the number of unit pixels to be recorded with respect to the following formula (1).

<Relational expression> Hereinafter, the configuration of the present invention will be described in detail.

In photolithography, the density of an image having continuous tone is represented by the size of a halftone dot by dot processing, but in an electrophotographic image forming apparatus by digital image processing, the density of a latent image on a photoreceptor is reduced. In this case, it is very difficult to change the diameter of the beam to represent a multi-step gradation. For this reason, the beam diameter is kept constant, and in a pixel block which is a matrix-like aggregate of pixels as the minimum unit formed by the beam, the image is recorded as an image with the number of unit pixels constituting the pixel block (that is, an image) By changing the ratio with the number of unit pixels (recorded with toner, etc.), the size of the halftone dot is often corresponded. Of course, in the digital image processing technique, in addition to the gradation expression method (dot density modulation method) that changes the number and arrangement of dots (having a specified size) as described above, a method of changing the size of dots ( Dot size modulation method). Here, the former will be described as an example.

By the way, what can be considered as a basic component for expressing an image is a pixel density value represented by the above-mentioned pixel distribution (the number of pixels to be recorded among the unit pixels constituting a predetermined pixel block) And the distribution form thereof) and the surface reflection density of the image recording material (toner or the like), of which 1% of the halftone dot area in the printed image As can be seen from the fact that it has the ability to easily identify differences in density as density differences, the pixel density value represented by the distribution of pixels, which has the same relationship as the size of the dot area, plays an extremely important role as an image forming means. Fulfill. That is, when the influence of the change in the amount of toner applied to them and the change in the size of the pixel on the gradation of the pixels recorded in a certain predetermined pixel block is examined, the latter is much larger, How to set the pixel density value is a very important issue.

Also, in connection with the above, when a recording image is to be formed by the image forming apparatus, the quality contents of the original image may vary widely, and the image forming process may have various characteristics. Furthermore, there is a background that the evaluation criteria of the image quality are not uniform, and it is necessary to overcome these complicated and unstable factors.

For this reason, when converting a document image such as a continuous tone image into a recording image based on pixel distribution, the density ratio (y _H ) of the pixel block of the brightest part (H) in the recording image based on the pixel distribution to be created. ) and concentration ratio of the pixel block of darkest (S) (y _S) and optionally can be selected, moreover reasonable and the gradation of the image leading to darkest part (S) from the brightest part (H) It is absolutely necessary to provide a means that allows easy adjustment and management.

The tone adjustment method of the present invention devised based on such a concept, specifically, the tone adjustment method defined by the above <Relational expression (1)>.

First, the process of deriving <Relational expression (1)> will be described.

The present inventors reasonably perform gradation conversion (conversion of continuous gradation to halftone gradation) when creating a halftone printed image from a continuous gradation color film original. To this end, a tone conversion equation, which is the predecessor of the above <Relationship equation (1)>, has been previously proposed. (Japanese Patent Application No. 62-148912, Japanese Patent Application No. 63-2590)
No.).

The tone conversion formula (hereinafter referred to as <Relational Expression (2)>) proposed by the inventors of the present invention is not limited to the creation of a print image, but also includes various types of printing images such as creation of a recorded image based on pixel distribution according to the present invention. Although it can also be used when creating a duplicate image, the description below is limited to creating a print image.

<Relational expression (2)> By comparing <Relational expression (1)> and <Relational expression (2)>,
The meanings of the β value and the k value are different, and there is no regulation of the γ value in <Relational expression (2)>. These differences will be described later, and the derivation process of <Relational expression (2)> will be described to help the understanding of the present invention.

The <relational expression (2)> for calculating the numerical value (y) of the dot area percentage used when creating the print image is a generally accepted density formula (photo density, optical density), that is, Is derived from

Applying this general formula for density D to plate making and printing gives the following:

<Relational expression (2)> makes it possible to arbitrarily set halftone dots of a desired size in the H portion and the S portion of the print image in the density expression (D ') relating to plate making and printing, and The base density value (x) at an arbitrary sample point on the halftone image and the numerical value (y) of the halftone dot area percentage of the halftone dot at the corresponding sample point on the halftone image are rationally associated with each other. The request was incorporated so that the theoretical value and the measured value approximately matched.

When the above <Relational expression (2)> is applied to a gradation conversion method of an image when a print image is created, the reflectance (α) of the printing paper, the surface reflectance (β) of the printing ink, and the print image density Based on the numerical value of the ratio (k) of the area / original image density area, while arbitrarily selecting the halftone dot size (y _H , y _S ) desired to be placed in the H and S parts of the print image, It is operated so as to obtain the numerical value (y) of the dot area percentage of the halftone dot at the corresponding sample point (Y) on the print image from the basic density value (x) of an arbitrary sample point (X) on the image. As a result, the density gradation of the original image (continuous gradation image) can be faithfully reproduced 1: 1 on the printed image (halftone gradation image).

In the case of multicolor plate making (generally, four sets of cyan (C), magenta (M), yellow (Y), and black (BL) are considered to be one set), a reference plate (multicolor plate making) If
As is well known, the cyan plate (C) is the reference plate. ), I.e., a halftone gradation characteristic curve which is a reference for converting the density information value of the original image into the halftone dot area value of the print image (obtained by graphing the above-mentioned x value and y value). Once the curve has been determined, this is the basis for the work of converting continuous tone to halftone.) Once the working reference characteristic curves for the other color plates have been determined, By multiplying by an appropriate adjustment value based on the gray balance ratio of, it can always be determined rationally. The working reference characteristic curve of each color plate determined in this way is, of course, each a reasonable characteristic curve, and furthermore, the interrelationship between the characteristic curves regarding gradation and color tone is also reasonable. And appropriate.

That is, when a halftone print image is created from a continuous tone original image, if the tone conversion is performed based on the above <Relationship (2)>, an image based on conventional experience and intuition will be used. Departing from the gradation conversion method, the gradation of the image can be arbitrarily and rationally converted, and accordingly, the color tone closely related to the gradation can be rationally adjusted. As a result, it is possible to obtain a printed image having a density gradient and a color tone that are natural for human visual perception. The above is the content proposed by the present inventors.

However, subsequent research has revealed that there is a certain limit in the operation of <Relational expression (2)>.

The limitations are as follows: very effective if the original image is of standard quality, but non-standard quality, especially those of extremely poor quality (for example, the Or something that is under).

This will be described in terms of the operation of <Relational expression (2)>. In the case of a standard quality (standard original), a yellow ink having a large stimulus value with printing ink is used as a numerator for determining the k value. (Typical density values are 0.9 to 1.0) when performing gradation conversion (in the case of multicolor plate making, the C plate is manufactured using this value). Although it is extremely effective, it is not satisfactory enough for the non-standard originals having the above-mentioned poor quality. ・ When the β value corresponds to the non-standard originals, the printing ink (as described above) , Yellow ink is the standard), and it is not satisfactory even if the surface reflectivity and other numerical values are arbitrarily selected and adopted.

In order to overcome the above-mentioned limitations, it is necessary to make the halftone gradation characteristic curve serving as a work reference for the gradation conversion correspond to not only the standard original but also the non-standard original, and the shape of the curve can be arbitrarily determined with a reasonable ratio. Must be able to be changed to As a result of the study, the present inventors have found that satisfactory results can be obtained when gradation conversion is performed under the following conditions.

K value = γ / (density range value of original image) γ value = any positive or negative numerical value β value: a numerical value obtained by β = 10− ^γ from the γ value defining the above k value.

By operating the above <Relational expression (2)> under the above conditions, a standard and non-standard original is formed into a print image having excellent reproducibility of density gradation and color tone inseparable from the density gradation. be able to.

As described above, the description has been made mainly with respect to the creation of the halftone print image, but it goes without saying that the theory supporting the tone conversion work described above can also be applied to the creation of a recorded image based on the distribution of pixels. That is.

It goes without saying that the gradation conversion formula suitable for creating a recorded image based on the pixel distribution becomes <Relational expression (1)> when the above-described study results are incorporated and arranged.

Next, the meaning of each item of the <Relational expression (1)> of the present invention,
The characteristics of operation will be explained.

In the operation of <Relational expression (1)> of the present invention, the basic density value (x) must be obtained from the image information signal of the original image. The density information value may be any value as long as it reflects a physical quantity related to the density of each pixel of the document image, and should be interpreted in the broadest sense. Synonyms include reflection density, transmission density, brightness, light intensity,
Voltage value, and the like. These density information values may be extracted as density information signals by photoelectrically scanning an original image. In the <relational expression (1)> of the present invention, the value of the basic density value (x) measured by a densitometer (for example, a positive color film having a density value of 0.2 to 2.70 as a portrait) is used. ), And y _H [pixel density value set in the pixel block of the brightest part (H)] and y _S [pixel density value set in the pixel block of the darkest part (S)]. (For example, a numerical value of 5% or 95%), y [pixel density value recorded in the pixel block (Y) corresponding to an arbitrary sample point (X) on the original image] is calculated as a percentage numerical value. You.

In the operation of the above <Relational expression (1)> of the present invention, not only the following deformation and use, but also any processing,
It can be used by deforming or guiding.

In the above modification, α = 1. this is,
The surface reflectance of the recording paper (base material) is 100%. The value of α may be practically 1.0.

Further, according to the modification (alpha = 1.0), a y _H in the brightest part H of the recording image by the distribution of the pixel can be set exactly as scheduled to y _S to darkest portion S. This means that x = 0 in the brightest portion H on the recorded image, and x = [document image density area] in the darkest portion S, that is, Therefore, it is clear from -kx = -γ.

In the operation of <Relational expression (1)> of the present invention, α,
The values of β and γ (which define the β value by β = 10− ^γ as described above) take various values. In the present invention, by appropriately selecting these numerical values, it is possible to rationally convert the gradation of an image regardless of the quality characteristics of the original image.

That is, the tone conversion processing method of the image based on the <Relationship (1)> of the present invention reproduces the tone and color tone of the original pixel, that is, the tone of the original image is 1: 1 to the recorded image. Although very useful in reproducing, its usefulness is not limited to this. The <relational expression (1)> of the present invention is represented by
In addition to faithfully reproducing the characteristics of the original image, α, β, γ
Value, furthermore is extremely useful to modify or change the characteristics of the original image reasonably by selecting y _H, a y _S values as appropriate.

To be more specific, it should be noted that the user (operator) has the following degrees of freedom in the operation of the <Relational expression (1)>.

<Part 1>: <Relational expression (1)> is used for the purpose of forming an image faithful to a document image. That is, <Relational expression (1)> should be used with the primary consideration of obtaining exactly the same visual sensory image when observed with the human eye. In the present invention, such an attitude of the tone adjustment is described by the term “(tone) conversion of (image)”.

<Part 2>: Using <Relational expression (1)> so as to change or modify the original image according to the technical necessity of image formation, artistic request, or needs of the ordering side. That is, the <relational expression (1)> is used with the primary consideration of obtaining a corrected or changed visual sensory image itself as observed by the human eye. In the present invention, such an attitude of the gradation adjustment is described by the term “(image) gradation (of) correction (or change)”.

When a multicolor image is formed using the above <Relational expression (1)>, for example, when a color original is reproduced by an image forming apparatus, color separation known in the field of printing or the like, that is, reflected light from a color original And the like are separated into blue (B), green (G), and red (R) to obtain a density information signal for each color, and this is processed by a gradation adjustment mechanism using the <relational expression (1)>. An image may be formed based on this processing information. At that time, a reference color plate (for example, C
The y value for the color plate, that is, the tone characteristic curve of the reference color plate (the y value is calculated, and the y value with respect to the x value is plotted. Characteristic curve is obtained) and other color plates (M plate, Y
Can be determined rationally by multiplying the y value of the reference color plate by an appropriate adjustment value based on the gray balance ratio of each ink. An image may be formed using the curve.

Y value for each color plate determined as described above,
That is, the gradation characteristic curve for each color plate is expressed by the following relational expression (1).
As a result, not only are the characteristic curves reasonable, but also the mutual relation between the characteristic curves relating to the gradation and the color tone is rational and appropriate.

In the image forming apparatus of the present invention, the beam of the exposure scan is modulated by the gradation adjusting mechanism according to the algorithm of <Relational expression (1)>, and is arithmetically processed according to a predetermined pixel distribution pattern. Pixels may be formed.

For example, as shown in FIG. 5, in the case of the column (a) in FIG. 5, the distribution of the recorded pixels has a positional relationship of being mutually dispersed within the pixel block as the number of recorded pixels increases. In addition, for example, it is conceivable that the pixel block is sequentially spirally spread outward from the center of the pixel block. In this case, the pixel block approximates a halftone dot in photolithography. In the column of FIG. 5B, halftone dots having an area corresponding to the number of pixels in the column of FIG. 5A are shown.

Although the pixel block has been described as a 4 × 4 matrix type here, 17 levels of gradation are expressed.
In general, an n × n matrix type pixel block is n ² +
One level of gradation (0 to 100%) is expressed.

Such a method of expressing the density of a document image such as a continuous tone image by the distribution of pixels formed in a matrix type pixel block is generally called a dither matrix method and is well known. (For example, see JP-A-58-85
No. 434, No. 58-114569, No. 59-52969, No. 60-141585
Nos. 62-186663. ).

As described above, when a recorded image is formed by the image forming apparatus of the present invention, it is incorporated in hardware or software that performs gradation conversion based on the <Relationship (1)> in the gradation adjustment mechanism. As a result, it is possible to obtain a recorded image having excellent tone reproduction as well as gradation, or a recorded image in which the image quality of a document image is arbitrarily corrected or changed.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples as long as it does not exceed the paper of the present invention.

As described above, according to the present invention, when a recorded image is formed by the image forming apparatus according to the present invention, the gradation adjustment mechanism section performs the gradation conversion according to <Relational expression (1)>. Has features. Therefore, <Relational expression (1)>
Will be described from the aspect of perfecting the operation of, particularly the handling of the γ value.

(Example) (1) A method of determining a γ value adopted in <Relational expression (1)>.

According to the present invention, when a recorded image is formed by the image forming apparatus of the present invention, core tone conversion work in the process of creating a recorded image is performed under the above <Relational expression (1)>. Has the greatest features.

In this case, it is desirable that the same quality content as that of a recorded image formed from a standard standard document be formed even from a document image whose quality content varies widely, such as light and dark. It is not even.

For this purpose, it is necessary to obtain a gradation characteristic curve that defines the relationship between the x value and the y value that gives the same quality as the recorded image obtained from the standard document, without being affected by the quality of the document image. In the <relational expression (1)> of the present invention, the shape of the gradation characteristic curve can be largely changed because γ
Value.

Hereinafter, a method of determining a γ value that is extremely important in the operation of <Relational expression (1)> will be described. The image forming apparatus of the present invention can form a recorded image having excellent reproducibility of gradation and color tone only by rationally determining the γ value.

Y values (ie, pixel density values) for various γ values
Table 1 shows how the values change. First
In the table, while changing the γ value (adopting γ value = 2.00 to −0.20 as shown in Table 1), the above <Relational expression (1)>
_{The, y H = 3%, y} S = 95%, α = 1.00, β = 10 -γ, k = y /
Each density step (calculated under the condition of (density area value of original image) = γ / (2.8−0.2)) (Table 1 shows the density area of the original image divided into nine steps. ) Is shown.

According to Table 1, when the γ value is changed, individual gradation characteristic curves corresponding to the respective γ values are obtained. Therefore, gradation conversion may be performed by setting an optimum one based on the quality content of the given document image. The results in Table 1
Illustrated in the figure.

Therefore, when a document image having a predetermined quality content is given, the optimal γ to be adopted in <Relationship (1)>
The question is how to determine the value rationally.

We decided to use a monochrome or color film, whose tone and color reproduction is considered faithful, as the original image, and establish a method for determining the γ value to be adopted according to the image quality of the original. I do. This is because if a method for determining an effective γ value under a monochrome or color film original with high tone reproducibility is established, it will be useful for other original images. .

A detailed analysis of the image quality of the color film used as the original shows that the image quality, such as high-key (photographed with over-exposure) and low-key (photographed with under-exposure), is standard color photographed with standard exposure. Compared to film manuscripts, there are many differences. However, differences in the image quality of the color film original, the exposure amount of the differences seen from the fact that a direct impact on the brightest portion density value H _n of the original,
By paying attention to this point, it can be objectively defined. As the present invention previously proposed, when the standard document (which exposure is performed properly.), The γ value is matched consider taking a value of 0.9 to 1.0, the H _n and γ What is necessary is to find the correlation. Note that push-back chose H _n is because density region the brightest portion near is important in reproduction of gradation. Theoretically, it is needless to say that may choose darkest density value S _n of the original.

Therefore using various color film original, to form an excellent recorded image quality, experiments were conducted to determine the relationship between H _n and γ values. The experimental data are shown in Table 2. In Table 2, Experiment No. 2 was for a standard manuscript, and 0.9 was adopted as the γ value.

From these experiments, the γ value can be rationally determined by the following equation.

(I) When the relationship between γ _n and H _{n in} Table 2 is graphed as shown in FIG. 2 (all logarithmic graph), γ _n is obtained by the following equation.

(Ii) In addition, the standard document (concentration range 0.20~2.80) γ _o
An experiment was carried out in which a recorded image was formed under the condition of 1.00, and a recorded image of the same quality was obtained from various color film originals. As a result, the relationship between γ _n and H _n could be defined as follows.

_{(B) γ n = 1.70-2.2961 (logH n} +1) (γ n, H n together relationship obtained when displayed on a logarithmic scale) _{(ii) γ n = 1.70-2.3 (logH n} +1) (γ _n normal scale, the H _n from the equation) than that obtained when displayed on a logarithmic scale, the gradation reproducibility by an image forming apparatus of the present invention from a document image having a quality contents of infinite variety excellent to duplicate the recorded image, first from H _n values of the original image gamma _n
, And then adopt this as the γ value of <Relational expression (1)> to perform the gradation conversion process.

In the gradation adjustment mechanism of the image forming apparatus of the present invention,
In order to operate the relational expression (1)> under the γ value determined as described above, the image forming apparatus needs to
Mechanism for measuring the H _n, may be added a mechanism for calculating the γ value from the H _n. Alternatively, these measurements and calculations may be left to an operator.

(2) Method for fixing (constantizing) the γ value employed in <Relational expression (1)> In applying the <Relational expression (1)> of the present invention,
The method of determining the γ value described above is complicated, and a recorded image created by this method is strictly different from a recorded image obtained from a standard original. Is that of course the fact that ,, the brightest portion density values of the color film original (H _n) is different from that the brightest portion density values of the standard document (H _o) because.

As described above, in the relational expression (2) useful for gradation conversion of a standard document, a value of γ = 0.9 (or a value between 0.9 and 1.0) is excellent in tone reproducibility as well as gradation. A duplicate image can be created. Therefore, the relational expression (1)
In the operation of <>, in order to make the value of γ a constant such as γ = 0.9, the density gradation of the original image must be adjusted (corrected) to the density gradation of the standard image. Hereinafter, γ
A method for converting a value into a constant will be described.

In the case of a color film original, the above-described adjustment of the density gradation can be performed extremely rationally. This will be described with reference to FIG.

As is well known, exposure amount of color film photosensitive material (X)
(Note that the sample point X is different from X described above.)
The relationship of the color film density (D) at that time is as shown by the basic density characteristic curve in FIG.

The standard document and the non-standard document depend on whether the exposure amount is appropriate or not. The respective density characteristic curves are shown as having a specific range on the basic density characteristic curve. In FIG. 3, the former is shown as a reference density characteristic curve, and the latter is shown as an individual density characteristic curve (note that FIG. 3 shows an underexposed document as a non-standard original). Adjusting the density characteristics of a document to the density characteristics of a standard document can be performed very easily by making the basic density characteristic curve a function.

The above-mentioned basic density characteristic curve is defined by a function of D = f _D (X), as shown in Table 3 below (Table 3 also shows an inverse function). It should be noted that the method of functionalizing the basic density characteristic curve in Table 3 should be taken as an example.
A more simplified formula may be used.

In order to match the individual density characteristic curve of the color original with the reference density characteristic curve, the following procedure may be used (see FIG. 3).

(I) H and S density values of a color original image and a basic density characteristic curve (D = f _{D) of} a color film photosensitive material of the color original
(X)), the individual density characteristic curve of the color original image is defined, and (ii) the density values D _{Hn to} D _Sn of the color original are expressed by X =
Substituting it into F _X (D) to determine the value range of the color original image on the X axis, X _{Hn to} X _Sn , and (iii) the value range on the X axis of the density characteristic curve based on this, X _{Ho to} X _So To match. (Iv)
Next, the D axis value range of the reference density characteristic curve, D _{Ho to} D _So, is determined.

As a matter of course, when the individual density characteristic curve of the color document coincides with the reference density characteristic curve, it is needless to say that the matching between the two is unnecessary. Alternatively, an arbitrary allowable range may be defined for the reference density characteristic curve, and when it is within the allowable range, image processing may be performed by regarding the same as the reference density characteristic curve.

In the matching procedure of the individual reference density characteristic curves described above, it is usual that XR _o (exposure range of the standard original) and XR _n (exposure range of the non-standard color original) do not match. , XR _n must be matched to XR _o (see steps (ii) and (iii) above). XR _n to XR _o
Simple matching (an attitude that matches the brightest part density value to the same value and makes the darkest part unmatched) and proportional matching (an attitude that matches both the brightest part density value and the darkest part density value) .)
There is. FIG. 3 shows a case where mathematically proportional matching is performed.

As shown in FIG. 3, the density information value (density information value between D _{Hn and} D _Sn , D _n ) of the individual density characteristic curve is substituted for the basic density characteristic curve D = f _D (X), and XR seek _n, which density information value of a color original with adjustments by X value was adjusted to XR _o (D _Ho ~D density information values between _So, D _o) but not to obtain, XR _n equation to determine the X values after adjusting the XR _o is a
A simple calculation gives:

(1) In the case of simple matching X = f _D (D _n ) ± | m | (2) In the case of proportional matching 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 individual density characteristic of non-standard individual original on X axis Color film original As standard quality (D _Ho =
0.20, D _So = 2.80), high key (over exposure) (D _Hn = 0.10, D _Sn = 2.70) and low key (under exposure) (D _Hn = 0.60, D _Sn = 3.20) Table 4 below shows matching data when the individual density characteristic curves are matched with the reference density characteristic curves under the basic density characteristic curves shown in Table 3.

In the above-mentioned matching experiment, the density regions (DR) of the three color film originals used were all 2.60, so that one was based on simple matching and the other was based on proportional matching.

In the density value of D _n and D _o of the Table 4 were determined based on the D _o <relation (1)> by y values (pixel density value,%). The results are shown in Table 5. Also shows the correlation between the y values and D _n value of Table 5 in Figure 4. The curve shown in FIG.
7 is a gradation characteristic curve that defines a correlation between an x value and a y value, which can create a recorded image having excellent gradation reproducibility from a non-standard image quality original image.

In the gradation adjustment mechanism of the image forming apparatus of the present invention,
In order to operate with the γ value of the relational expression (1)> fixed (constantized), a mechanism for measuring the density of the original image in the image forming apparatus (measurement of density over H and S, and H to S) It is necessary to incorporate software or hardware for matching the individual density characteristic curve of the original image with the reference density characteristic curve, but this makes it possible to create a recorded image with excellent gradation and color tone from an original of any quality. .

(3) Image Forming Apparatus The image forming apparatus of the present invention will be described with reference to FIGS.

FIG. 6 is a block diagram of the image forming apparatus according to the first embodiment of the present invention.

As shown in FIG. 6, the image forming apparatus of the present invention
Detecting unit 1 that reads transmitted light or reflected light of a document image by separating it into R (red), G (green), and B (blue).
And a color separation unit 2 that converts the output signal of the detection unit 1 into color separation signals of Y (yellow), M (magenta), C (cyan), and K (black), and using <Relational expression (1)> Adjusting mechanism (hereinafter referred to as a tone adjusting unit) 3 for performing a tone conversion for forming a duplicated image with an appropriate pixel distribution, and a laser beam based on an output signal of the tone adjusting unit 3. And an output unit 4 for exposing the electrophotographic photoreceptor to the latent image formed on the photoreceptor. The latent image formed on the photoreceptor is developed in a developing unit to become a toner image, transferred to a recording sheet, and fixed in a fixing unit.

In order to form a color image, an independent photoconductor and a developing unit are provided for each component color, and the formed toner images are sequentially transferred to a recording sheet, or a latent image of the component color is formed on one photoconductor. After forming and developing to form a toner image, the toner image is transferred to a recording sheet, and this process is repeated for each color component.

The detection unit 1 detects transmitted light or reflected light of each part of the original 5 by a photoelectric conversion element such as a photomultiplier or a solid-state imaging device (CCD), and outputs R, G, B, and USM signals as currents. The signal is converted into a voltage signal in the A / V converter 6.

The color separation unit 2 uses the log amplifier 7 to detect R,
Each of the voltage signals of G, B, and USM is logarithmically calculated and converted into a density. In a basic masking (BM) 8, a black (K) component is separated from the density, and each of Y, M, and C components is further separated. Next, a color correction (CC) unit 9 controls the Y, M, and C components for each of the R, G, B, and Y, M, and C document colors, and furthermore, converts the black component of the document to UCR /
UCR (under control removal) of UCA section 10, or UCA
In (under control addition), the ratio expressed by three components of Y, M, and C is determined. After these Y, M, C, and K components are obtained, conventionally, the area ratios ye ', me', ce ', and ke' of the pixels occupied by the gradation control unit in the gradation adjustment unit (IMC) are determined. This was the inverse log conversion.

In the present invention, as shown in FIG. 6, the gradation adjusting unit 3 uses an adjusting unit 11 instead of the above-mentioned conventional gradation control unit and inverse log conversion unit, where Y, M,
Conversion from C and K to ye ', me', ce ', ke'.
The adjustment unit 11 has the algorithm of <Relationship (1)> inside, applies <Relationship (1)> for each of Y, M, C, and K, and converts ye ′, me ′, ce ′, ke ′ Ask.

The adjustment unit 11 is a general-purpose computer having the algorithm of <Relationship expression (1)> as software and having an A / D and D / A I / F (interface), and the algorithm is embodied by a general-purpose IC as internal logic. Circuit, ROM-containing electric circuit holding algorithm operation results, PA implementing algorithm as internal logic
It can take various forms such as L, gate array, custom IC, etc. In order to form an image corresponding to the density of the original image with the laser beam, the diameter and intensity of the laser beam are fixed as described above, and are formed in the image block as corresponding to the area of the halftone dot in the case of photoengraving. Calculate the number of unit pixels and their distribution and output the data.

The area ratio of the pixels obtained by the adjustment unit 11 is input to the color channel selector 12, and the color channel selector 12 selectively outputs ye ', me', ce ', and ke' in the order of order. This output is A / D converted by the A / D converter 13 and input to the output unit 4. The output unit 4 controls the laser beam in the dot control unit 14 based on the output of the gradation adjustment unit 3.

FIG. 7 shows a second embodiment, in which the conventional inverse log converter 16 is used as it is, so that the adjuster 11 outputs ye ', me', ce ', ke' in logarithmic form. You. As a result, <Relationship (1)> can be applied only by converting one component of the conventional device, and the existing system can be remodeled to the system according to the present method with less changes than in the first embodiment.

FIG. 8 shows a third embodiment, in which the conventional gradation control (IMC) unit 17 is left as it is, and
This is a format in which the og conversion unit 16 and the gradation control unit 17 can be used together. An adjusting unit M similar to that of the second embodiment, that is, an adjusting unit 11 for outputting ye ', me', ce ', ke' in logarithmic form is employed. The adjustment unit 11 takes the Y, M, C, and K signals from the preceding stage of the gradation control unit 17, and outputs the value after gradation conversion to the inverse log conversion unit 16.

FIG. 9 shows a fourth embodiment, in which a conventional reverse log is used.
The conversion unit 16 and the gradation control (IMC) 17 are left, and these can be used together. And the adjustment unit 11 is a gradation control unit.
Y, M, C, and K signals are taken from the previous stage of 17 and directly connected to the color channel selector 12, so that ye ', me', ce ', and ke' can be implemented without being restricted by the conventional system. Ye ', me', c with the same optimal processing mode as in the adjustment unit in the example
e ′ and ke ′ can be obtained. Then, similarly to the third embodiment, the system can be realized with a slight modification of the conventional system.

FIG. 10 shows a fifth embodiment, in which the entire conventional tone conversion unit is constituted as a new adjustment unit 11, and this adjustment unit 11
<Relational expression (1)> can be used.

In the example of FIGS. 6 to 10, the image forming unit is an electrophotographic type in which an electrostatic latent image is formed by scanning a photoconductive photoreceptor with a laser beam, but a recorded image is formed by the distribution of pixels. As a method, various methods such as an electrostatic recording method and a magnetic recording method can be adopted.

In the case of electrostatic recording, as shown in FIG. 11, each electrode of a recording head in which a large number of electrodes are arranged in a direction perpendicular to the moving direction by approaching or in contact with a recording body made of a sheet-like dielectric that moves. To form an electrostatic latent image. The steps after the step of adding toner to the latent image and developing the same are the same as those of the electrophotographic type. The dot control unit 14 of the output unit 4 corresponds to the recording head as an aggregate of electrodes.
Is applied as an output signal corresponding to the image to be recorded. In the case of the magnetic recording type, for example, a recording body in which a magnetic body is uniformly coated on the surface of a drum body is used, and a magnetic recording head is applied while applying a voltage as an image information signal to the magnetic recording head in contact with the surface. And the recording medium surface are relatively moved to form a magnetic latent image on the recording medium surface. In order to develop the latent image, a toner made of a magnetic material is used.

By modifying the gradation adjustment unit of the conventional device as described above, it is possible to integrate <Relational expression (1)> with other processing, and achieve high speed and compactness by optimizing the system. In addition, cost performance per system can be improved.

In the above embodiment, the color separation unit has the same configuration as the conventional one. However, by using <Relational expression (1)>, the color correction (CC) unit 9 and the USR / USA unit 10 are unnecessary. A color separation unit in which these are omitted may be employed.

In the above embodiments, explanations of commonly used effects such as blur masks and sharpness effects not directly related to the present invention are omitted.

(4) Usefulness of <Relational expression (1)> Next, the usefulness of <Relational expression (1)> applied to the gradation adjustment mechanism of the image forming apparatus of the present invention will be supplementarily described.

This is a supplementary explanation for facilitating the understanding of the present invention, and mainly describes the operation of <Relational expression (1)> applied to the gradation adjustment mechanism of the image forming apparatus of the present invention and the significance of the result. State.

(A) Operation Experiment of <Relational Expression (1)> The following two experiments were performed as basic experiments for incorporating <Relational Expression (1)> into the gradation adjustment mechanism of the image forming apparatus.

a) First of all, using a simple computer, namely Sharp Pythagorean EL509A (manufactured by Sharp Corporation), and applying the desired numerical value to <Relational expression (1)>, the simple computer described below is operated. 6 tables (1) (2)
(3) The gradation adjustment tables of the images shown in Tables 7 and 8 were prepared.

As a result, the time required for these operations was 3 hours, 2 hours, and 2 hours, respectively, including the inspection time of the calculation results.

b) The following experiment was also performed.

Simple personal computer (PC-9801-M manufactured by NEC)
In 2), the desired software separately obtained is input as function data, and the basic density value (x) of the original image (continuous tone image) is recorded in the pixel block on the recorded image by the distribution of the corresponding pixels. An operation was performed to adjust the number ratio (y) (hereinafter, referred to as the area ratio of recorded pixels).

As a matter of course, the same numerical value as the result of manual calculation using the simple calculator was obtained.

Moreover, in this experiment, there was no need to use special software to create the above software for adjusting the gradation of the image to be input to the personal computer, and it was created using the N88-BASIC attached to the personal computer. The work took only one hour to complete.

Further, conversion or correction of the gradation of the target image can also be performed by software capable of directly inputting a measurement value from a highlight (H) to a shadow (S) and a densitometer of the original image instead of the basic density value of the original image. Can be performed.

Using these software, a desired density interval (for example, 0.00 to 1.00 in 0.05 steps, 1.00
3.03.00 in 0.10 increments), and the value was input to the personal computer to obtain the target pixel area ratio (y).

Further, by inputting density values at a plurality of locations from highlights to shadows on the original image, the area ratio (y) of desired recorded pixels corresponding to the density values is input.
Could be obtained.

The area ratio (y) of the pixels recorded by the software described above is such that either a positive image or a negative image can be output alone or simultaneously.

(B) Calculation results obtained from <Relational expression (1)> and their usefulness Next, Table 6 (1) (2) (3), Table 7,
The usefulness of Table 8 will be described. (Note that in each table, the y value is used as the pixel density value.) [Table 6 (1), (2), and (3)] Table 6 shows a case where a monochrome image is formed from an original image by an image forming apparatus. Density of image recording material such as toner (in the table,
This is displayed as a recording image density area, which corresponds to the γ value in <Relational expression (1)>. ) And the use range of the area ratio of the pixels to be recorded (in the table, the maximum pixel density value is displayed,
Three cases are shown, 0-100%, 0-98%, 0-95%. ) Changes how to set the pixel area ratio (y) (pixel density value) at each sample point to obtain an ideal gradation characteristic curve. It is.

Also, from this table, even if the density of the toner or the like is the same, when the use range of the area ratio of the recorded pixels is changed (that is, when the γ value is changed), the ideal gradation characteristic curve And how it changes and must be changed.

In Table 6, the β value that determines the ε value is determined by β = 10− ^γ . Incidentally, when the recording image density range = γ value = 1.0, ε = 1 / (1−β) = 1.11111. The values in the same row as the pixel density value (%) are theoretical values when β = 0 (ε = 1.0).

It is not possible to form a black-and-white image based on a distribution of pixels corresponding to 1: 1 from a document image such as a continuous-tone image, and to learn techniques and methods that can arbitrarily adjust the gradation characteristics of a black-and-white image. It is also the basis of multicolor image formation.

[Table 7] Table 7 shows the case where the density of the image forming material (toner or the like) changes when a black-and-white image is formed from the original image by the image forming apparatus (that is, the recorded image density area), as in Table 6. = When the γ value changes), the use range of the maximum pixel density value is 0% to
Aside from 100% contrast, apart from the contrast of the entire image, the tone ratio of the same image to the human visual perception, and the area ratio of pixels at each sample point required to form an image having similar image quality (y) (Pixel density value) in a table.

In other words, when the precondition is ideal, the area ratio (y) of the pixel recorded at each sample point on the ideal gradation characteristic curve corresponding to the density value of the image forming material (toner or the like) to be used. Is a list.

[Table 8] Table 8 has the same basic conditions as Table 7, but when the use range of the maximum pixel density value (5% to 95%) is used, the ideal gradation characteristic curve 9 is a table showing what percentage of the area of a pixel to be recorded (y) (pixel density value) should be set at each sample point.

To date, color separation work, such as the creation of print images, has been performed using masking techniques for color correction.
The tone adjustment work of an image basically basically relies solely on human experience and intuition, or a limited number of fixed preliminary documents. For this reason, it is necessary to make the gradation conversion technology for creating a duplicate image scientific based on the side of the image to be duplicated such as a print image.

<Relational expression (1)> of the present invention is to perform gradation conversion at the time of creating a duplicate image by a rational method. Also <
The data in Tables 6 to 8 showing the correlation between the basic density value (x) of the original image obtained by the relational expression (1)> and the area ratio (y) of the image pixels to be recorded are shown in FIG. It is a useful basic material for scientific consideration of various basic matters in color separation work at the time.

From each of these tables, pay attention to what essence and principles exist between the original image and the color separation work, and what should be considered in order to rationally match the essence and principles with practice. You can extract what you have to go.

(C) Application of <Relational expression (1)> to gradation correction (or change) Next, <Relational expression (1)> is for conversion of image gradation (that is, faithful conversion from a continuous gradation original image). This is effective not only for conversion to a gradation image by the distribution of pixels having a high degree, but also for so-called gradation correction (or change) for correcting the original image itself.

The correction (or change) of the gradation of the image is performed by changing the reduction / enlargement ratio of the recorded image to be formed, the intention of the orderer, the type of the target image in the color original, the purpose of the image to be formed, and the whiteness of the recording paper. In some cases, it may be necessary to perform the adjustment depending on the degree and the density of the image recording material (ink). In any case, the operation of <Relationship (1)> can be rationally dealt with, and various colors can be handled. Disassembly work can be standardized and standardized.

Further, according to the present invention, correction (or change) of the gradation of an image in a highlight portion or a shadow portion can be similarly performed. This is apparent from the fact that the shape of the gradation characteristic curve (curve that regulates the correlation between the x value and the y value) can be arbitrarily changed according to the γ value employed, as shown in FIG. Further, it was confirmed that the color conversion in the highlight portion of the color original was automatically removed without taking a special countermeasure by the gradation conversion according to the <relational expression (1)> of the present invention.

[Effects of the Invention] The present invention has the following excellent effects.

1) Density values of an original image such as a continuous tone image and a surface wall ratio of a pixel to be recorded in a recorded image (an image recorded by pixel distribution), which are the most basic items for image formation. In the past, the correlation was determined by an irrational method based solely on the experience and intuition of workers or a limited number of fixed preliminary documents. On the other hand, according to the present invention, even under any precondition, this can be rationally determined under <Relational expression (1)>. When converting a document image such as a continuous tone image into a recorded image based on pixel distribution, the most important requirement of tone management (conversion, correction or change of tone) is simply that of the image. In addition to the above, the present invention has a direct relationship with the color tone of an image, so that the present invention makes it possible to rationally manage the gradation color tone. That is, the relational expression (1) of the present invention is applied to the gradation adjustment mechanism.
The image forming apparatus adopting the above algorithm can theoretically and rationally systematize the gradation conversion work (color separation work), simplify the work, and the effect is extremely large.

2) By adopting <Relational expression (1)> in the gradation adjustment mechanism of the image forming apparatus, the equipment can be rationalized and simplified, and the manufacturing cost can be reduced. Also,
The operation of these devices is also simplified and clarified, rework is extremely reduced, the consumption of consumables is greatly reduced, and the performance of the image forming apparatus can be greatly improved. In particular, in the performance of the image forming apparatus, there is a great merit that, regardless of the quality of the original image, a recorded image excellent in gradation and color tone can be formed.

3) By the gradation adjusting mechanism using <Relational expression (1)>,
It is possible to rationally and easily define the evaluation criterion for the quality of the recorded image based on the pixel distribution separately from the image information of the original image. Therefore, it is possible to rationally respond to the diversified needs of customers.

4) By adopting <Relational expression (1)>, the education and training of technicians required as the image forming equipment becomes more sophisticated can be effectively performed through the operation of <Relational expression (1)>. And eliminate unnecessary labor in daily work, and secure time for new creative research and development.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a γ value and a change in the shape of a gradation characteristic curve. FIG. 2 is a correlation diagram between γ _n and H _n . FIG. 3 is a diagram for explaining the principle of matching between an individual density characteristic curve of a color film original image and a reference density characteristic curve. FIG. 4 is a diagram showing a gradation characteristic curve set for a non-standard original. FIG. 5A shows an example in which a document image having continuous gradation is expressed by a distribution of unit pixels in a pixel block, and FIG. 5B shows a case corresponding to the case of FIG. FIG. 4 is a diagram showing a case where the image is expressed by the size of a halftone dot in a photoengraving process. FIG. 6 is a block diagram of the image forming apparatus according to the first embodiment of the present invention. FIG. 7 is a block diagram of the image forming apparatus of the second embodiment. FIG. 8 is a block diagram of the image forming apparatus of the third embodiment. FIG. 9 is a block diagram of the image forming apparatus of the fourth embodiment. FIG. 10 is a block diagram of the image forming apparatus of the fifth embodiment. FIG. 11 is a diagram showing an image forming unit in an electrostatic recording system.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N 1/40 G03G 15/00 G03F 3/08

Claims (5)

(57) [Claims] An image forming apparatus for processing an image information signal obtained from a document image by a gradation adjusting mechanism and forming a recorded image based on a distribution of pixels corresponding to a recording sheet based on the processed signal. The tone adjustment mechanism determines a basic density value (x) of an arbitrary sample point on the original image based on the image information signal (the difference between the density value at the sample point and the density value at the brightest part on the image)
To the ratio (y) of the number of unit pixels recorded to the number of unit pixels forming the pixel block corresponding to the sample point in the recorded image to be formed, the following <Relational expression (1)>
An image forming apparatus for performing a conversion process on the basis of an image. <Relational expression> 2. A latent image representing a distribution of said pixels is formed by scanning with a laser beam on an image forming body having a photoconductive layer uniformly charged for recording pixels on a recording sheet; 2. The image forming apparatus according to claim 1, wherein the latent image is transferred to a recording sheet after being developed with toner, and further fixed. 3. A series of operations including forming a latent image on the image forming body, developing with toner, and transferring to a recording sheet, or a part thereof, is performed using a specific color toner, and the same operation is performed using different colors. 3. The image according to claim 2, wherein the same operation is repeated by a required number of colors, and the color image is formed by performing positioning and transfer on the same recording sheet and then fixing. Forming equipment. 4. A voltage is applied to a large number of recording electrodes arranged in a direction perpendicular to the direction of movement of an electrostatic recording medium that moves for recording pixels on a recording sheet. 2. The image forming apparatus according to claim 1, wherein an electrostatic latent image is formed, transferred to a recording sheet after developing the latent image with toner, and further fixed. 5. A series of operations including formation of a latent image on the electrostatic recording medium, development with toner, and transfer to a recording sheet, or a part thereof, are performed using toner of a specific color, and the same operation is performed differently. 5. The color image according to claim 4, wherein the color image is formed by performing the same operation with the necessary number of colors by repeating the same operation for the required number of colors, and then performing the registration and transfer after fixing. Image forming device.