JP3059857B2 - Correction table generation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to an image processing apparatus or the like for correcting halftone dot image data based on image data read from a document image with reference to a correction table and converting the same into a film original composed of halftone images. To a correction table generation method.

[0002]

2. Description of the Related Art In the field of printing and plate making, for example, in order to streamline work processes and improve image quality, image information recorded on a read original is electrically processed to form a film comprising a halftone image. 2. Description of the Related Art An image processing apparatus such as an image reading and reproducing system for creating an original is used.

In such an image processing apparatus, when producing a film master according to the purpose, image information is photoelectrically read from an original image to obtain image data, and sharpness, gradation conversion, Image processing such as color correction is performed, and the image data subjected to the image processing is corrected by a correction table in order to tune to the characteristics of an output device that converts the image data into a halftone image. The corrected image data (hereinafter also referred to as QL data) is obtained. The corrected image data is sent to an output device, a laser beam or the like is controlled based on the corrected image data, and a film original is formed by scanning the film.

In generating such a correction table,
A value between the maximum value max (for example, 255) and the minimum value min (for example, 0) of a value that can be sent to the output device is divided into 20 equal parts, and the number of screen lines, the screen angle, the screen shape, and For example, 21-stage reference image data sequentially increasing, for example, {(max-min) i / 20} schematically shown in FIG.
The image data of (i = 0 to 20) is sent to the output device as reference image data, and the output device performs a conversion process to a halftone image based on the input reference image data, for example, a predetermined image shown in FIG. A halftone dot test image is created, and a halftone dot area value (hereinafter also referred to as halftone% data) of the created image is measured.

[0005] Reference image data is plotted with respect to the dot% data of the measurement result, and interpolation is performed between plot points over a predetermined section by Lagrange interpolation, Hermite interpolation, spline interpolation, etc., and is defined as a piecewise function between the measurement points. This is performed over the entire section to generate a correction table for the output device based on the defined function formula.

[0006]

When the above-mentioned conventional correction table is prepared, interpolation is performed so as to always pass the measured value of the dot area ratio, and the dot area ratio of the dot test image with respect to the reference image data is plotted. FIG. 7 interpolated so as to pass through the measurement point is the output characteristic of the output device. When a correction table is generated based on the characteristic shown in FIG. 8 which is the inverse characteristic of FIG. 7, local distortion occurs. There was a problem.

Further, even if the correction table is created as described above, a process subsequent to the output device, ie, plate collection,
For example, as shown in FIG. 9, there is a problem that the input / output characteristics in the process such as the contact exposure include gradation distortion, and a tone jump occurs due to the gradation distortion.

[0008] The tone jump generated in the post-process is reduced by adjusting the gradation characteristics of the output device. However, when the correction table is generated as described above, the tone characteristics optimized by the output device are canceled, and a tone jump occurs. In order to solve this problem, if an attempt is made to generate a correction table smoothly so that a tone jump does not occur, an output predicted value (halftone dot area value) displayed for a gradation setting operation or the like in an image processing apparatus and an output apparatus However, there is a problem that the error between the measured halftone dot area ratio value of the image at the corresponding position on the film output from the image processing device becomes large and the operability of gradation setting in the image processing apparatus is deteriorated.

Now, let the function of the correction table be F ₁ (x),
The characteristic of the output device is F ₂ (x), and the characteristic of the post-process is F
₃ (x). x is the respective input data, and the characteristic G (x) of the printed matter output through the post-process is F ₃ (F ₂
(F ₁ (x))). When the inverse characteristics of the output device characteristics are accurately reproduced as a correction table,

[0010]

(Equation 1)

## EQU1 ## So the overall characteristic G (x) is

[0012]

(Equation 2)

As a result, the characteristics of the post-process remain.

If the characteristics of the output device are configured to have the inverse characteristics of the characteristics of the post-process, the characteristics after the output device become linear. However, at this time, if the correction table is installed in the reverse characteristic of the output device,

[0015]

(Equation 3)

[0016]

(Equation 4)

[0017]

(Equation 5)

As a result, the characteristics of the subsequent process remain as a whole.

On the other hand, if the characteristics of the output device are set to be the inverse characteristics of the characteristics of the post-process, and the characteristics of the correction table, that is, F ₁ (x) are linear, the overall characteristics are linear. The error between the output predicted value (dot area ratio value) to be displayed on the image processing device and the actually measured halftone dot area value of the image at the corresponding position on the film output from the output device becomes large, and image processing is performed. Since the correction table is completely useless for the tone setting operation in the apparatus, there is a problem that the halftone dot area value expected by the operator of the image processing apparatus cannot be obtained from the output apparatus.

The present invention relates to an output predicted halftone dot area ratio value and an image processing apparatus which display a halftone image based on image data read from an original image while reducing gradation distortion as an overall characteristic including a post process. It is an object of the present invention to provide a correction table generation method capable of reducing an error between the halftone dot area value of the output of the correction table.

[0021]

According to a first aspect of the present invention, there is provided a correction table generating method for correcting halftone dot image data based on image data read from a document image by referring to a correction table, and performing a subsequent process. A correction table generating method in an image processing apparatus for converting a halftone image into a halftone image under the output condition of the inverse characteristic of the first step, wherein a first step of forming a halftone test image under the output condition based on reference image data; A second step of measuring the dot area ratio value of the dot test image, and a second step under the condition that the dot area ratio value measured in the second step passes through a predetermined start point and end point. When obtaining an approximate curve by the least square method for the dot area ratio value obtained in the above,
Partial so as to reduce distortion of the image output from the process
A third step of multiplying the difference between the measured halftone dot area value by the weighting coefficient to be corrected to obtain the least squares equation, and a correction table based on the weighted least squares curve obtained in the third step. And a fourth step of generating.

According to a third aspect of the present invention, there is provided a correction table generating method for correcting halftone dot image data based on image data read from a document image with reference to a correction table to obtain a reverse characteristic of a characteristic in a subsequent process. A method of generating a correction table in an image processing apparatus for converting into a halftone image under an output condition, comprising: a first step of forming a halftone test image under the output condition based on reference image data; A second step of measuring the halftone dot value, and a third step of approximating a set of points of the halftone dot value measured in the second step by a quadratic curve.
And the difference between the halftone dot area value measured in the second step and the value obtained by the quadratic curve approximated in the third step is detected.
The difference is reduced by the halftone dot area value measured in the second step.
A fourth step of correcting the set of halftone dot area values corrected in the fourth step with a cubic curve, and a third step obtained in the fifth step. And a sixth step of generating a correction table based on the curve.

[0023]

According to the first aspect of the present invention, a halftone dot test image is formed on the basis of the reference image data under the output condition of the inverse characteristic of the characteristic of the subsequent process, and the halftone dot area ratio value of the halftone dot test image is formed. Is measured, under a condition passing the predetermined start point and end point of the measured halftone dot area value, a least square approximation curve for the halftone dot area value obtained by the measurement is obtained. Since the predetermined range based on the characteristics of the above post-process is reduced in weight, a smooth correction table is generated.

As a result, since the least squares curve corresponds to the output condition, the correction data forming the correction table based on the least squares curve has the inverse characteristic of the output condition. Is corrected, and there is almost no difference between the expected halftone dot area value and the halftone dot area value output from the image processing apparatus. Further, since the portion where the distortion is caused by the characteristics of the subsequent process is smoothly replaced, the distortion due to the characteristics of the subsequent process is suppressed, and the expected halftone dot area ratio value in this portion and the halftone image output from the image processing device are reduced. The difference from the halftone dot area value is small.

According to the third aspect of the present invention, a halftone dot test image is formed on the basis of the reference image data under the output condition of the inverse characteristic of the characteristic of the subsequent process, and the halftone dot area value of the halftone dot test image is calculated. The set of each point of the measured dot area ratio value is 2
A difference between the measured halftone dot value approximated by the quadratic curve and the value approximated by the quadratic curve is detected, and the measured halftone dot value is corrected to reduce the difference, and the correction is performed. The set of the obtained dot area ratio values is approximated by a cubic curve, and a correction table is generated based on the cubic curve.

As a result, since the curve represented by the set of measured dot area ratio values approximated by the quadratic curve corresponds to the output condition, the correction data constituting the correction table based on the quadratic curve is The characteristics are the reverse of the conditions, and 2
The halftone dot area value approximated by the quadratic curve is corrected corresponding to the difference between the halftone dot area value approximated by the quadratic curve and the measured halftone dot area value, and the corrected halftone dot area is corrected. Since the set of rate values is approximated by a cubic curve, the occurrence of distortion is suppressed by the characteristics of the subsequent process, and the difference between the expected dot area value and the dot area value output from the image processing apparatus is small. I can do it.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An image processing apparatus to which a correction table generated by a correction table generation method according to the present invention is applied will be described in detail with reference to the accompanying drawings.

FIG. 2 shows an image reading / recording / reproducing system 10 as an image processing apparatus to which the correction table generating method according to the present invention is applied. This image reading / recording / reproducing system 1
0 is a transmission type scanner 12 for reading image information recorded on a transmission type original such as a film, a reflection type scanner 14 for reading image information recorded on a reflection type original such as a photograph,
An output device 16 for reproducing and outputting image information read by the transmission scanner 12 or the reflection scanner 14 on a film; a console unit 18 for operating the transmission scanner 12, the reflection scanner 14 and the output device 16; It is basically composed of

The transmission type scanner 12 scans an image on a film.
Transmission type original S on which information is recorded_TRead image information of
Operation control of the reading unit 20 and the mechanism of the transmission type scanner 12
Mechanism control unit 22 to perform, overall control, generation of correction table
Calculation of the function expression for the
The CPU 24 is a main component. The reading unit 20
Transmission type original S_TLight source 26 for irradiating illumination light to
And the transparent original S _TTo collect the illumination light transmitted through
An optical lens 28, and a CC for converting the illumination light into an electric signal.
And a D unit 30. The CCD unit 30
Generates an image signal that separates the illumination light into three primary colors of RGB
Then, the data is output to the A / D converter 31.

On the other hand, the mechanism control unit 22 and the CPU 24 are connected to a system bus 32. The system bus 32 includes a frame memory 34, a CCD compensation circuit 36, an image preprocessing circuit 38, a color processing circuit 40, a magnification / sharpness adjustment circuit 42, a correction table storage circuit 44, and a reference for generating reference image data as pattern data. The image data generating circuits 46 are respectively connected. The frame memory 34 temporarily stores image information read by the reading unit 20, and this image information is supplied to a CRT display 48. Also, console unit 1
Reference numeral 8 also has an input port for a mouse 50 and a keyboard 52.

CCD compensation circuit 36, image preprocessing circuit 3
8. The color processing circuit 40, the magnification / sharpness adjustment circuit 42, the correction table storage circuit 44, and the reference image data generation circuit 46 have the following functions under the control of the CPU 24.

The CCD compensating circuit 36 has a function of performing CCD dark correction, shading correction, and the like on the image data supplied from the A / D converter 31. The image preprocessing circuit 38 has a function of converting output data of the CCD compensating circuit 36 into a density value inside the scanner, and switching between a transmission input unit and a reflection input unit. Color processing circuit 40
Has a function of performing desired color processing on image data based on image processing conditions. Magnification / sharpness adjustment circuit 4
Reference numeral 2 has a function of adjusting magnification and sharpness of image data.

The correction table storage circuit 44 stores the correction data of the image data with respect to the dot% data as a table. The correction table storage circuit 44 receives the dot% data output from the magnification / sharpness adjustment circuit 42, It has a function of reading out image data corrected for the halftone% data with reference to the% data and outputting it to the output device 16.

The reference image data generation circuit 46 has a function of sequentially outputting reference image data, for example, reference image data having a value obtained by dividing 0 to 255 into 20 based on an instruction.

In addition to the above, the CPU 24 functionally functions as a halftone dot image measured from the halftone dot test image obtained by the processing by the output device 16 receiving the reference image data at a predetermined interval output from the reference image data generation circuit 46. % data received from the keyboard 52, under conditions through a predetermined start point p _a and the end point p _b of each measurement network% data, the measurement dot% data at the measurement point and the least square approximation, than said minimum square approximation A calculating means 24A for calculating a curve obtained by multiplying the obtained curve by a weighting coefficient; and image data corrected for the dot% data based on the curve calculated by the calculating means 24A, and a correction table storage circuit 44 as a correction table. And a write control means 24B for storing the information in the memory. here,
The correction data is obtained from the inverse characteristic of the curve calculated by the calculation means 24A.

Further, the CPU 24 displays the original image on the CRT display 48 based on an instruction from the keyboard 52, and places the halftone data at the position designated by the mouse 50 based on the halftone output data of the magnification / sharpness adjustment circuit 42 of the original image portion. Is provided.

FIG. 3 shows the constituent blocks of the reflection type scanner 14. The main components of the reflection scanner 14 are a reading unit 56 for reading image information recorded on the reflection original S _R and a CPU 58 for controlling the entire reflection scanner 14. An illumination light source 60 is reading section 56 for irradiating illumination light to the reflection originals S _R, a plurality of reflecting mirrors 62 for guiding the illumination light from the illumination light source 60 that is reflected in a predetermined direction by the reflection originals S _R And a condenser lens 64 for condensing the illumination light, and a CCD unit 66 for converting the illumination light into an electric signal. The CCD unit 66 generates an electric signal in which the illumination light is separated into three primary colors of RGB,
/ D converter 68.

On the other hand, the CPU 58 has a CCD compensation circuit 70
Is connected. The CCD compensating circuit 70 performs CCD darkness correction, shading correction, and the like on the image data supplied from the A / D converter 68,
2 to the image preprocessing circuit 38. That is, the image data read by the reflection scanner 14 is processed by the transmission scanner 12 shown in FIG.

FIG. 4 shows the structural blocks of the output device 16. The output device 16 includes a CPU that controls the entire operation.
72, a mechanism control unit 74 that controls the operation of the mechanism of the output device 16, and a recording unit 76 that records image information on the film F are basic components. A halftone generation circuit 78 is connected to the CPU 72. The dot generation circuit 78 generates a dot signal from image data supplied from the correction table storage circuit 44 or the reference image data generation circuit 46 of the transmission type scanner 12, and the dot signal is an LD.
It is supplied to the driver 80. The LD driver 80 drives a laser diode 82 constituting the recording unit 76 based on the dot signal. The recording unit 76 includes the laser diode 82, an optical deflector 84 that deflects and scans the laser beam L in the main scanning direction, and a film F that deflects the deflected laser beam L.
And a condensing lens 86 for condensing light.

The image reading / recording / reproducing system 10 of the present embodiment
Is basically configured as described above. Next, a method of generating a correction table in the image reading / recording / reproducing system 10 will be described according to its operation.

The characteristics of the output device 16 are configured to have the opposite characteristics to those of the post-process.

When a new correction table needs to be generated, such as when the connected output device 16 is updated, a correction table generation routine is executed by pressing a key on the keyboard 52.

When entering the correction table generation routine, the reference image data from the reference image data generation circuit 46 is, for example, "0", "13",
The reference image data of 21 stages of “26”, “48”... Is sequentially generated and output to the output device 16.

In the output device 16 having received the reference image data, under the control of the CPU 72, the halftone data is output by the halftone generation circuit 78, and the laser diode 82 is driven via the LD driver 80. 8
2 outputs a laser beam L based on the halftone dot data, the laser beam L deflected by the optical deflector 84 is condensed by a condensing lens 86, and a test image is formed on the film F. An example of a test image to be formed is schematically shown in FIG.

The test image generated by the correction table generation routine was obtained by reading dot% data of each stage of the test image using a separately prepared measuring instrument, and was read from the order of the size of the reference image data, for example, from the left of the test image. The network% data is input by the keyboard 52. By this input, the dot% data corresponds to the reference image data, which corresponds to the characteristics of the output device 16. When the dot% data with respect to the reference image data is plotted, it is schematically indicated by x in FIG.

[0046] by dot percentage data to reference image data CPU24 shown in FIG. 1, the start point p _a (white data), the end point p
_{Under the} condition passing _b (black data), the least squares approximation including the weight coefficient is performed to obtain an m-th order function expression as described below.

The operation of this function expression will be described.

The general form of the following equation is

[0049]

(Equation 6)

Assume that (6) Starting point p determined by equation
_a (x _a, y _a) and the end point _{p b (x b, y b} ) and passing through two points (but x _a ≠ x _b). Then, y _a , y
_b is

[0051]

(Equation 7)

Is as follows.

When m = 1, equation (6) is uniquely determined.

By solving equation (7),

[0055]

(Equation 8)

Is as follows.

[0057] When m ≧ 2, is required two c _0, c ₁ of coefficient c _k from equation (7). That is,

[0058]

(Equation 9)

By solving for the coefficients c ₀ and c ₁ ,

[0060]

(Equation 10)

Thus, the coefficients c ₀ and c ₁ can be obtained.

Here, the order m is set to a value sufficiently smaller than the number of measurement points. If the degree m is too large, the number of inflection points increases, which causes distortion. When the order is increased, distortion near the midpoint is accurately reproduced. Therefore, m = “3” or “4” is preferable. Further, the weighting coefficient is set as t _i , and the value in the vicinity where distortion occurs in a process after the output device 16 is set to be small. For example, when there is distortion near i = 12, t _i = “10” to “14” and t _i =
It is assumed that t _i = 1.0 for other i and 0.1.

When the m-th order equation of the equation (6) is a least squares equation of N points (m <N + 2), this problem is caused by the measured dot%.
Data p _i (x _i , y _i ) (i = 0 to N−1, p _i ≠ p
_j , i ≠ j), the measured dot% data (y _i )
Multiplied by the weighting factor t _i

[0064]

[Equation 11]

The coefficients c ₂ to _cm that minimize the above are obtained. That is, for n = 2 to m,

[0066]

(Equation 12)

Is satisfied. here,

[0068]

(Equation 13)

[0069]

[Equation 14]

Using equations (13) and (14), (1)
2) When rearranging the equation

[0071]

(Equation 15)

Is obtained.

[0073]

(Equation 16)

[0074]

[Equation 17]

In other words, the normal equation is

[0076]

(Equation 18)

Then, simultaneous equations are established for n = 2 to m and solved for c _k to obtain a functional equation.

This function formula is calculated by the calculation means 24A, and based on this function formula, a correction table is generated under the control of the writing control means 24B, and is written in the correction table storage circuit 44.

The curve a based on the function equation thus obtained
Is as shown in FIG. 1 and i = “10” to “14”
In other cases, the coefficient t _i is 1.0, and during this time, the correction curve is a correction by least squares approximation. Further, the coefficient t _i is 0.1 in the range of i = “10” to “14”, and during this period, the correction curve is substantially straight because the coefficient t _i is set small.

However, as described above, the characteristics of the output device 16 are set so as to be the inverse characteristics of the characteristics of the post-process, and the characteristics of the correction table are not output except for i = “10” to “14”. Since the characteristics of the output device 16 are set to substantially the opposite characteristics, the characteristics of the output device 16 are corrected by the correction table. Will not be done.

When the characteristic of the output device 16 is the reverse characteristic of the post-process, if i = “10” to “14”, the portion where distortion occurs in the post-process, and correction is made in this portion. Since the characteristics of the table are set almost linearly, distortions do not occur due to linear characteristics as a whole including the subsequent steps. Further, in this portion, since the characteristics of the correction table are linear, the output predicted value to be displayed on the CRT display 48, that is, the dot%
Although the error between the data (halftone dot area value) and the actually measured halftone dot area value of the image at the corresponding position on the film output from the output device 16 becomes large, the area is small.

Next, an embodiment of another method for generating a correction table will be described.

Another method for generating the correction table is as follows: dot% data versus each point (x _i , y _i ) of the reference image data shown in FIG.
Is approximated by a quadratic curve, and the function value f
₂ The difference between (x _i ) and y _i ,

[0084]

[Equation 19]

[0085] In contrast, defines the h (Δ _i) as a correction function, that was modified by the correction function _{h (Δ i) (y i} ',
determine the function value f ₃ to 3-order curve approximation of the x _i).

That is, y _i ′ is

[0087]

(Equation 20)

Then, a set of points (x _i , y _i ′) is approximated by a cubic curve to obtain a function value f ₃ .

Here, the correction function h (Δ _i ) is appropriately determined by, for example,

[0090]

(Equation 21)

By making the above determination, a desired curve can be obtained. A correction table is generated based on this curve.

Here, the reason why the set of each point (x _i , y _i ) is approximated by a quadratic curve is that the vicinity of each point (x _i , y _i ) is smoothly connected by approximation using a quadratic curve, and the output is obtained. It corresponds well to the characteristics of the device 16. As a result, the expected halftone dot area value and the output halftone dot area value are well approximated.

Further, a correction function h (Δ _i ) for the difference Δ _i
And (x) obtained by the correction function h (Δ _i )
_i , y _i ′) is approximated by a cubic curve,
The difference between the approximation by the quadratic curve and the approximation by the cubic curve acts as a weighting factor for the approximation by the quadratic curve, thereby suppressing the distortion due to the characteristics of the subsequent process.

Here, the correction function h as shown in equation (21)
By setting (Δ _i ), points (x _i , y _i ) and 2
By displacing approximately one half of the difference from the approximation by the secondary curve from the approximation value by the secondary curve to obtain the approximation by the cubic curve, the distortion due to the subsequent process is suppressed.

[0095]

As described above, according to the correction table generating method of the first aspect of the present invention, since the least square curve corresponds to the output condition, a correction table based on the least square curve is formed. The correction data has the inverse characteristic of the output condition, and the correction of the output condition is performed by the correction data, and the difference between the expected halftone dot area value and the halftone dot value output from the image processing apparatus is The effect that there is hardly any is obtained. Further, since the portion where the distortion occurs due to the characteristics of the subsequent process changes linearly, the effect of suppressing the distortion due to the characteristics of the subsequent process is obtained. In addition, there is an effect that the difference between the expected halftone dot area value in this part and the halftone dot area value output from the image processing apparatus can be reduced.

According to the correction table generating method of the third aspect of the present invention, since the curve represented by the set of measured dot area ratio values approximated by the quadratic curve corresponds to the output condition, the curve is represented by the quadratic curve. When a correction table based on the correction data is formed, the curve based on the correction data forming the correction table has the inverse characteristic of the output condition, and a portion having a large difference from the measured screen area ratio value is approximated by a quadratic curve. Since the halftone dot area value approximated by the cubic curve is corrected, the generation of distortion is suppressed by the characteristics of the post-process, and a part of the approximation part based on the quadratic curve is changed to the expected halftone dot area value and the image processing. The effect of reducing the difference from the halftone dot area value output from the apparatus is also obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of least square approximation according to the method of the present invention.

FIG. 2 is a block diagram illustrating a configuration of a transmission type scanner in an image processing apparatus to which the method of the present invention is applied.

FIG. 3 is a block diagram showing a configuration of a reflection type scanner in an image processing apparatus to which the method of the present invention is applied.

FIG. 4 is a block diagram illustrating a configuration of an output device in an image processing apparatus to which the method of the present invention is applied.

FIG. 5 is a schematic diagram illustrating an example of reference image data.

FIG. 6 is a schematic diagram illustrating an example of a halftone image corresponding to reference image data.

FIG. 7 is a characteristic diagram illustrating interpolation between halftone% data and image data in the case of a conventional method.

FIG. 8 is a schematic diagram for explaining interpolation between halftone% data and image data according to a conventional method.

FIG. 9 is a characteristic diagram for explaining a distortion generating portion based on characteristics of a post-process according to a conventional method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 12 ... Transmission type scanner 14 ... Reflection type scanner 16 ... Output device 24 ... CPU 24A ... Calculation means 24B ... Write control means 24C ... Calculation control means 44 ... Correction table storage circuit 46 ... Reference image data generation circuit 78 ... Dot dot generation Circuit 80 LD driver 82 Laser diode

Claims (3)

(57) [Claims] 1. A method of correcting halftone image data based on image data read from a document image with reference to a correction table,
What is claimed is: 1. A method of generating a correction table in an image processing apparatus for converting into a halftone image under an output condition having characteristics opposite to characteristics of a subsequent process, wherein a first step of forming a halftone test image under said output conditions based on reference image data. And a second step of measuring a dot area ratio value of the dot test image, under a condition passing a predetermined start point and end point of the dot area ratio value measured in the second step, When obtaining an approximate curve by the least square method with respect to the halftone dot area ratio value obtained in the second process, the distortion of the image output from the post-process is reduced.
The weighting factor for partially correcting the
Characterized by multiplying the difference, and a third step of the least squares equations, a fourth step of generating a correction table based on least squares curve weighting is obtained in the third step, further comprising a Correction table generation method. 2. The correction table generating method according to claim 1, wherein the weighting factor in the third step is smaller than other ranges in a range where distortion occurs in a subsequent process. 3. A halftone image data based on image data read from a document image is corrected by referring to a correction table.
What is claimed is: 1. A method of generating a correction table in an image processing apparatus for converting into a halftone image under an output condition having characteristics opposite to characteristics of a subsequent process, wherein a first step of forming a halftone test image under said output conditions based on reference image data. A second step of measuring the dot area ratio value of the dot test image; and setting each set of points of the dot area ratio value measured in the second step to 2
A third step of approximating with a secondary curve, and detecting a difference between a halftone dot area ratio value measured in the second step and a value of a quadratic curve approximated in the third step, A fourth step of correcting the measured halftone dot area value so as to reduce the difference , and approximating a set of halftone dot area value corrected in the fourth step by a cubic curve. A method of generating a correction table, comprising: a fifth step; and a sixth step of generating a correction table based on the cubic curve obtained in the fifth step.