JPH1075359A - Dot forming method and device and recording medium recording program to execute the processing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To comparatively increase gradation number of a dot image even when the number of picture elements in one dot area is comparatively small by generating a 3rd threshold compared with an image signal from a threshold pattern with a comparatively small size and a threshold pattern with a comparatively large size. SOLUTION: A 1st threshold generating section 30 generates a 1st threshold pattern with a comparatively multi-valued gradation and a comparatively small size. A 2nd threshold section 32 generates a 2nd threshold pattern with a comparatively small gradation and a comparatively large size. A multiplier section 34 multiplies a 1st threshold TP1 generated by the 1st threshold generating section 30 with a positive integer N. An adder section 36 adds the TP1*N being the result of multiplication and the 2nd threshold TP2 generated by the 2nd threshold level generating section 32 to generate a 3rd threshold TP3. A comparator 38 compares the 3rd threshold TP3 with an image signal Im to generate a binary dot signal Sd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for forming a halftone dot according to an image signal.

[0002]

2. Description of the Related Art Binary halftone signals representing halftone dots are obtained by comparing an image signal with a threshold pattern.
A threshold pattern (also called a “threshold matrix”) is usually set so as to cover an area where one halftone dot is formed. In this case, the number of gradations of the halftone image is at most equal to the number of pixels included in the threshold pattern + 1, which
It is also equal to the number of pixels +1 included in one halftone dot area (the area where one halftone dot is formed).

[0003]

However, in recent years,
There is an increasing demand for faster image output processing, and since it is necessary to output halftone dots having the same number of lines at a lower resolution, it is required to reduce the number of pixels included in one halftone dot region. ing. However, when the number of pixels in one halftone dot region is reduced, there is a problem that the number of gradations that can be expressed by one halftone dot also decreases.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem in the prior art. Even when the number of pixels in one dot area is relatively small, the number of gradations of a dot image can be compared. The aim is to provide a technology that can be highly targeted. Another object of the present invention is to provide, in addition to the above objects,
It is to generate a halftone image having no unevenness and smooth gradation reproducibility.

[0005]

In order to solve at least a part of the above-mentioned problems, a first invention is a method of forming a halftone dot in accordance with an image signal,
(A) generating a first threshold value forming a first threshold pattern having a relatively large number of gradations and a relatively small size;
(B) The second having a relatively small gradation and a relatively large size
(C) generating a third threshold value by performing an operation using the first threshold value and the second threshold value, and generating a third threshold value. (D) generating a dot signal representing a dot by comparing the third threshold value with the image signal.

[0006] The third threshold value to be compared with the image signal is generated from a first threshold pattern having a relatively small size and a second threshold pattern having a relatively large size. Therefore, if the first threshold pattern having a relatively small size is made to correspond to one halftone area, even if the number of pixels in one halftone area is relatively small, the number of gradations of the halftone image can be reduced. Can be relatively high.

In the first aspect, the step (c)
Preferably includes a step of performing an interpolation operation for interpolating the first threshold value with the second threshold value.

[0008] Further, the interpolation operation step includes a step of multiplying the first threshold value by an integer N, and a step of creating a third threshold value by adding the result of the multiplication and the second threshold value. And it is preferable to include

By performing such an interpolation operation, a third threshold having a more complicated threshold pattern can be generated from the first and second threshold patterns.

[0010] The second threshold pattern may be 0 to
It is preferable that the threshold value is composed of (N-1) N threshold elements.

In this case, the first threshold pattern can be smoothly interpolated by the second threshold pattern.

[0012] In the first aspect of the present invention, the second aspect is provided.
Is preferably configured by randomly arranging a plurality of types of two-dimensional matrices having different arrangements of a plurality of threshold elements.

In this case, since the spatial frequency of the second threshold pattern is increased, the spatial frequency of the third threshold pattern can be increased. If the halftone signal is generated by comparing the third threshold value with the image signal, the unevenness due to the periodic structure is eliminated, and a halftone image having a smooth gradation expression can be obtained.

In the first aspect, the integer N is 4
Preferably, each two-dimensional matrix is a 2 × 2 matrix containing four threshold elements 0-3.

Further, the plurality of types of two-dimensional matrices are such that the threshold elements 0 and 1 are arranged diagonally to each other, and the threshold elements 2 and 3 are also arranged diagonally to each other. It is preferably a matrix.

This makes it possible to further increase the spatial frequency of the second threshold pattern.

According to a second aspect of the present invention, there is provided an apparatus for forming a halftone dot according to an image signal, wherein the first threshold value forming a first threshold pattern having a relatively large number of gradations and a relatively small size is generated. And a second threshold pattern forming a second threshold pattern having a relatively small number of gradations and a relatively large size.
A second threshold value generating means for generating a third threshold value; a threshold value calculating means for generating a third threshold value by performing an operation using the first threshold value and the second threshold value; Comparing means for comparing the threshold value with the image signal to generate a halftone signal representing a halftone dot.

According to a third aspect of the present invention, there is provided a computer-readable recording medium on which a computer program for forming a halftone dot in accordance with an image signal is recorded. Generating a first threshold value forming a small first threshold pattern; and (b) generating a second threshold value forming a second threshold pattern having a relatively small number of gradations and a relatively large size. (C) performing a calculation using the first threshold value and the second threshold value to generate a third threshold value; and (d) comparing the third threshold value with the image signal. And generating a halftone signal representing the halftone dot by the computer. A computer-readable recording medium that records a computer program for causing a computer to execute the process.

In the second and third inventions, similarly to the first invention, the number of gradations of the halftone dot image can be increased.

[0020]

Other Embodiments of the Invention The present invention includes the following other embodiments. In a first aspect based on the second aspect, the threshold value calculation means includes an interpolation means for performing an interpolation calculation for interpolating the first threshold value with the second threshold value.

According to a second aspect, in the first aspect,
The interpolation means includes means for multiplying the first threshold value by an integer N, and means for creating a third threshold value by adding the multiplication result and the second threshold value.

A third aspect is a program supply device which is executed by a microprocessor of a computer system and supplies a software program (computer program) for realizing each step or each means of the above-mentioned invention via a communication line. It is.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION

A. Outline of Embodiment: Next, an embodiment of the present invention will be described based on an embodiment. FIG. 1 is an explanatory diagram showing an outline of a halftone dot forming apparatus according to an embodiment of the present invention. This halftone dot forming apparatus includes a first threshold value generation unit 30 and a second threshold value generation unit 3
2, a multiplication unit 34, an addition unit 36, and a comparison unit 38. The first threshold generator 30 generates a first threshold pattern having a relatively large number of gradations and a relatively small size. The second threshold value generating section 32 generates a second threshold pattern having a relatively small gradation and a relatively large size. The multiplication unit 34 multiplies the first threshold value TP1 generated by the first threshold value generation unit 30 by a positive integer N (referred to as “interpolation gradation number”). Adder 3
6 is the multiplication result TP1 * N and the second threshold value generation unit 3
A third threshold value TP3 is generated by adding the second threshold value TP2 generated in step 2 to the third threshold value TP2. The comparing unit 38 generates a binary halftone signal Sd by comparing the third threshold value TP3 with the image signal Im.

The level of the dot signal Sd is determined, for example, as follows. When TP3 <Im: H level (exposure, lighting); When Im ≦ TP3: L level (non-exposure, non-lighting).

The multiplying section 34 and the adding section 36 realize a function corresponding to the threshold value calculating means in the present invention. As the threshold value calculating means, it is possible to adopt another configuration, and in particular, it is also possible to adopt a configuration for performing other operations other than the four arithmetic operations.

The function of each unit in FIG. 1 can be realized by hardware.
By executing a computer program (application program). Such a software program is transferred from a portable recording medium (portable recording medium) such as a floppy disk or a CD-ROM to a main memory or an external storage device of a computer system. Alternatively, the program may be supplied from the program supply device to the computer system via a communication line.

As a recording medium, a flexible disk, CD-ROM, magneto-optical disk, IC card,
Various computer-readable media such as a ROM cartridge, a punched card, a printed matter on which a code such as a barcode is printed, an internal storage device (memory such as RAM and ROM) and an external storage device of the computer can be used.

FIG. 2 is an explanatory diagram showing an example of the first threshold pattern TP1 generated by the first threshold generator 30. The first threshold pattern TP1 has a size of 8 × 8 pixels and includes a threshold TP1 in a range of 0 to 62. The area occupied by the first threshold pattern TP1 is equal to one dot area. Since the level of the halftone dot signal Sd is determined by the inequality described above, in the first threshold pattern TP1, when the image signal Im is 0, all become L level, and when the image signal Im is 63, all become H level. Become. Accordingly, the first threshold pattern T shown in FIG.
The reproduction gradation number of P1 is 64. Further, since the first threshold pattern TP1 has a square shape surrounded by sides parallel to the main scanning direction Y and the sub-scanning direction X, the screen angle (also referred to as “screen angle”) is 0. °.

FIG. 3 is an explanatory diagram showing an example of the second threshold pattern TP2 generated by the second threshold generator 32. The second threshold pattern TP2 has a size of 32 × 32 pixels, which is equal to the size of 4 × 4 halftone dot areas. The coordinates in the sub-scanning direction X are represented by A to Z hexadecimal numbers, and “AF” corresponds to decimal number 32. In addition, it is preferable that the size of the second threshold pattern TP2 generally includes tens to hundreds of halftone dot regions. FIG. 4 shows the second threshold pattern TP2 (N = 4)
FIG.

As can be understood from FIG. 3, the second threshold pattern TP2 is constituted by randomly arranging blocks of a 2 × 2 matrix including four threshold elements 0 to 3. I have. There are 24 combinations of 0 to 3 threshold elements arranged in a 2 × 2 matrix. By randomly selecting and arranging these 24 types of 2 × 2 matrices, a second threshold pattern TP2 as shown in FIGS. 3 and 4 can be configured.

FIG. 5 is an explanatory diagram showing eight preferable 2 × 2 matrices. These 2x2 matrices are
The threshold elements 0 and 1 are arranged diagonally to each other, and the threshold elements 2 and 3 are arranged diagonally to each other. When such a 2 × 2 matrix is used, the spatial frequency of the halftone dot illuminated pixel can be increased, so that there is an advantage that a fine portion of an image can be reproduced more faithfully.

Note that there is a more preferable method for creating the second threshold pattern TP2, which will be described later.

As shown in FIG. 1, the first threshold pattern T
Each of the threshold values in P1 is multiplied by N by the multiplication unit 34. In this example, it is assumed that N = 4. It is preferable that the value of the number N of interpolation gradations is set to be equal to the number (= 4) of threshold elements constituting the second threshold pattern TP2. In the first threshold pattern TP1 shown in FIG.
, The multiplication result (TP1 × N) generated by the multiplication unit 34 has discrete values such as 0, 4, 8,..., 248. The result of the multiplication is added to the threshold value in the second threshold value pattern TP2 to become a third threshold value TP3. Therefore, the second threshold pattern TP2 is set to 4
If one threshold element is used, the first threshold pattern T
P1 can be smoothly interpolated with the second threshold pattern TP2, and the third threshold TP3 can be generated so that the value does not jump.

FIG. 6 is an explanatory diagram showing the distribution of the third threshold value TP3 (third threshold value pattern). However, in FIG.
Only the area of 6 × 16 pixels is illustrated. Since the threshold TP2 of the second threshold pattern TP2 shown in FIG. 3 has a value in the range of 0 to 3, the third threshold TP2 obtained by the addition is
Has a value of 0, 1, 2,..., 251. Therefore, if the image signal Im has a level of 0 to 255, a halftone dot of 253 gradations is formed by the halftone signal Sd obtained by the comparing unit 38. Also, the third
Since the threshold value TP3 has a continuous integer value in the range of 0 to 251 without a jump in value, it is possible to smoothly reproduce the gradation of halftone dots to be reproduced.

In order to form a halftone dot of 256 gradations, the third threshold value TP3 needs to include a value in the range of 0 to 254. However, the number of gradations that can be reproduced as described above is 25.
Even if it is 3, there is no practical problem if the tone correction curve (tone curve) is appropriately set in advance for the image signal.

By setting a value in the range of 0 to 63 as the threshold value of the first threshold pattern TP1 shown in FIG.
The third threshold value TP3 obtained by the adding unit 36 is in the range of 0 to 255. On the other hand, a range of 0 to 254 is appropriate as a threshold value for reproducing 256 gradations. Therefore, in this case, “255” is added between the addition unit 36 and the comparison unit 38.
Is added, a threshold value T in the range of 0 to 254 is added.
P3 can be obtained, and 256 gradations can be reproduced.

FIG. 7 shows the threshold pattern TP of FIGS. 2 and 4.
FIG. 4 is an explanatory diagram showing halftone dots formed when the value of an image signal Im is 18 using 1, TP2. FIG. 8 is an explanatory diagram showing halftone dots formed when the value of the image signal Im is 125. As can be seen from these figures, the shape of each halftone dot is not the same, and the number of pixels constituting each halftone dot is slightly different. As a result, 254 gradations can be reproduced by a plurality of halftone dots (4 × 4 = 16 halftone dots in this case).

FIG. 9 is an explanatory diagram showing a second threshold pattern TP2 'when the number N of interpolation gradations is two. This second threshold pattern TP2 'is composed of two types of threshold elements 0 and 1. In particular, the second threshold pattern TP2 ′ shown in FIG. 9 is equal to the pattern of the upper bits when each threshold of the threshold pattern shown in FIG. 4 is represented by 2 bits. If the second threshold pattern TP2 'and the first threshold pattern TP1 shown in FIG. 2 are used, and if the number N of interpolation gradations in FIG. 1 is set to 2, the third threshold TP3 becomes 0 to 125. Range. Therefore, a halftone image of 127 gradations can be formed.

FIG. 10 shows the second case where the number N of interpolation gradations is five.
Is an explanatory diagram showing a threshold pattern TP2 ″ of the second embodiment.
The threshold pattern TP2 ″ is composed of five types of threshold elements 0 to 4. The second threshold pattern TP2 ″ and the first threshold pattern TP1 shown in FIG. 2 are used. If the number N of interpolation gradations is set to 5, the third threshold value TP3 is in the range of 0 to 314. Therefore, a halftone image of 316 gradations can be formed.

As can be seen from the above example, it is preferable that the value of the number N of interpolation gradations is set to be equal to the number of types of threshold elements of the second threshold pattern TP2. If you do this,
By using the first threshold pattern TP1 having L threshold elements in the range of (L-1), {L × N + 1}
A halftone dot image can be formed.

B. Preferred Method for Creating Second Threshold Pattern TP2: FIG. 11 is an explanatory diagram showing a multi-tone large-size threshold pattern Sr used in a more preferred method for creating the second threshold pattern TP2. This multi-tone large-size threshold pattern Sr is the second threshold pattern TP
It has the same size as 2. In the example of FIG.
The threshold pattern Sr includes a threshold in the range of 0 to 254, but the range of this threshold is arbitrary.

The threshold pattern Sr shown in FIG. 11 is created by the method described in Japanese Patent Application Laid-Open No. Hei 7-264403 disclosed by the present applicant. That is, this threshold pattern Sr (threshold matrix) is obtained by dividing a threshold matrix area of an M × M threshold matrix (M is an integer of 2 ^N and N is an integer of 2 or more) into a plurality of sub-matrices having the same size. It is created by setting a difference between a plurality of thresholds included in a sub-matrix to a predetermined value and arranging the plurality of thresholds at random in each sub-matrix. At this time, each 2 × 2 submatrix Tij (i, j) included in the M × M matrix area
Indicates the coordinates of the 2 × 2 sub-matrix Tij in the M × M matrix area) in accordance with the following equation (1), and sets integers aij, bij, cij at each coordinate (i, j). , Dij are randomly determined.

[0043]

(Equation 1)

[0044] Alternatively, using equation 2 shown below in place of the equation (1), as M × M threshold matrix the matrix TM _MxM represented by Equation 2, 2 × 2 basic sub-matrix S ¹ within the component a, b, The combination of the values of c and d may be randomly determined for each 2 × 2 sub-matrix included in the M × M threshold matrix.

[0045]

(Equation 2)

The threshold pattern S thus created
As for r, the randomness of the threshold distribution is large and the spatial frequency is high, as described in JP-A-7-264403 described above. Here, “large in randomness” means that there is little periodicity and there is no deviation. Further, “high spatial frequency” means that a spatial frequency spectrum obtained by performing a Fourier transform on a threshold pattern shows a high value. The spatial frequency of an image is defined by the number of cycles per unit length (cycles / mm) when a pair of black and white in a black and white striped pattern is applied to one cycle of a sine wave. The spatial frequency of the threshold pattern is a similar concept.

Since the detailed procedure for creating the threshold pattern Sr is described in detail in this publication, the description is omitted here.

The second threshold pattern TP2 is calculated from the multi-tone large-size threshold pattern Sr according to the following equation 3:

[0049]

(Equation 3)

Here, N is the number of interpolation gradations (N in the example of FIG. 4).
= 4), Nr is the number of threshold types of the threshold pattern Sr (Nr = 256 in the example of FIG. 11). Operator INT ()
Indicates an operation for converting a value in parentheses into an integer (rounding down). Note that the same result can be obtained even if the denominator in the operator INT () is replaced with Nr. Equation 3 above defines the threshold value of the multi-tone large-size threshold pattern Sr as N in order of its size.
It means that it is equally divided and converted into N integers from 0 to (N-1). For example, among the thresholds in the range of 0 to 254 in the threshold pattern Sr in FIG. 11, Sr = 0 to 63 are converted to TP2 = 0, and Sr = 64 to 127 are converted to T
P2 = 1, Sr = 128-191 for TP2 = 2, Sr
r = 192 to 254 are converted to TP2 = 3, respectively.

FIG. 4 described above shows the threshold pattern S of FIG.
This is the result obtained by converting r in this way. Since the threshold pattern Sr in FIG. 11 is a pattern having a large randomness and a high spatial frequency, the second threshold pattern TP2 created by Expression 3 also has a large randomness and a high spatial frequency. These features are shown in the lighting patterns shown in FIGS. FIG. 12 to FIG. 14 show lighting patterns when the presence or absence of lighting of each pixel is determined by directly comparing the threshold pattern Sr with the image signal Im. FIG. 12 shows that Im = 64
Therefore, the area ratio of the lighting pixel is 25%. FIG. 13 shows the case where Im = 128 and the area ratio of the lit pixel is 50%, and FIG. 14 shows the case where Im = 192 and the area ratio of the lit pixel is 75%. In terms of correspondence with the second threshold pattern TP2 in FIG. 4, the lighting pattern (area ratio 25%) in FIG. 12 is equal to the distribution of TP2 = 0, and FIG.
The lighting pattern (area ratio 75%) of the lighting pattern (area ratio 75%) of FIG.
%) Is equal to the sum of the distributions of TP2 = 0, 1, 2. FIG.
It can be seen that the black-and-white pattern of FIG. 14 has a large number of black-and-white periods per unit length, and thus has a high spatial frequency distribution.

As described above, the multi-tone large-size threshold pattern Sr having a high randomness and a high spatial frequency is prepared, and is converted into an integer of N thresholds of 0 to (N-1) to obtain the second threshold pattern. By generating the threshold pattern TP2, the spatial frequency of the second threshold pattern TP2 can be increased. Since the third threshold pattern TP3 (FIG. 6) includes the component of the second threshold pattern TP2,
The third threshold pattern TP3 has appropriate randomness and spatial frequency. If the halftone signal Sd is formed by comparing the third threshold pattern TP3 with the image signal Im, a halftone image having no unevenness and smooth gradation reproducibility can be generated.

The threshold range (0 to 254) of the multi-tone large-size threshold pattern Sr shown in FIG. 11 has no direct relationship with the threshold range of the third threshold pattern TP3.
Optional. However, it is preferable to set the range of the threshold value of the multi-tone large-size threshold pattern Sr larger than the range of the threshold value of the third threshold pattern TP3.

Note that the second threshold pattern TP2 is "N
Threshold elements are changed for each pixel (that is, the same threshold does not continue in the X and Y directions), and all N threshold elements appear collectively for each predetermined small area. Is a threshold pattern of a neighborhood convergence type. " The multi-tone large-size threshold pattern Sr is obtained by converting the threshold pattern Sr into N thresholds of 0 to (N−1), thereby obtaining the second threshold pattern TP2.
May be used as long as the threshold pattern is generated.

C. First Embodiment FIG. 15 is a block diagram showing a configuration of a halftone dot forming apparatus to which the first embodiment is applied.
This halftone forming apparatus includes a CPU 100, a main memory (ROM and RAM) 102, a floppy disk device 104, an SPM memory 106, and an interpolation value memory 10
8 and two sub-scanning address counters 110a, 110
b and two main scanning address counters 112a, 112
b, an interpolation gradation number register 114, a multiplier 116,
An adder 118 and a comparator 120 are provided. Also,
The dot forming apparatus also includes an exposure device (not shown) for recording a dot image on a recording medium with a light beam.

The SPM memory 106 is a memory for storing the first threshold value pattern TP1, and the interpolation value memory 108
Is a memory for storing the second threshold pattern TP2. The area of each threshold pattern is hereinafter referred to as a “repetition unit block”. This name is derived from the fact that the threshold pattern is repeatedly assigned on the image plane when the halftone image is generated.

The sub-scanning address counter 110a receives a sub-scanning start signal Rx and a sub-scanning clock signal Cx. The sub-scanning start signal Rx is a signal that generates one pulse when the sub-scanning coordinates of the light beam are reset to the initial position. The sub-scanning clock signal Cx is a signal that generates one pulse each time the sub-scanning coordinates of the light beam are updated. The sub-scanning address counter 110a responds to these signals Rx and Cx by a first threshold pattern TP
A sub-scanning coordinate of the light beam in one repetition unit block is generated and supplied to the SPM memory 106 as a sub-scanning address. Main scanning address counter 112a
Similarly, in accordance with the main scanning start signal Ry and the main scanning clock signal Cy, the main scanning coordinates of the light beam in the repeating unit block of the first threshold pattern TP1 are generated, and this is stored in the SPM memory 106 in the main scanning. Supply as address. These two address counters 110a, 1
One threshold is read from the threshold pattern in the SPM memory 106 according to the address given from 12a,
The signal is supplied to the multiplier 116. Similarly, one threshold value is read from the threshold value pattern in the interpolation value memory 108 according to the address given from the two address counters 110b and 112b, and supplied to the adder 118.

The multiplier 116 is provided for the interpolation gradation number register 11
4 and the SPM memory 106
, And supplies the multiplication result (N × TP1) to the adder 118. Adder 11
8 indicates the multiplication result (N × TP1) and the interpolation value memory 1
08 and a threshold value TP2 given from the step 08. This addition result corresponds to the third threshold value TP3 in FIG.

The comparator 120 compares the threshold value TP3 with the input image signal Im and generates a binary output (halftone signal) according to the comparison result.

An exposure device (not shown) exposes a photosensitive recording medium (for example, a photosensitive film) with a light beam in accordance with the binarized output, thereby forming a halftone image on the recording medium. In this way, a halftone image of each color plane of YMCK is created, and these halftone images are overprinted with inks of the respective colors, whereby a multicolor print can be obtained.

The SPM memory 106, the X address counter 110a, and the Y address counter 112a shown in FIG.
Implements the function of the first threshold value generation unit 30 in FIG. Further, the interpolation value memory 108, the X address counter 110b, and the Y address counter 112b in FIG.
The function of the second threshold generator 32 in FIG. 1 is realized.

FIG. 16 is a flowchart showing the operation of the apparatus shown in FIG. In step S 1, the CPU 100 sends the first threshold pattern T from the floppy disk drive 104.
P1 (FIG. 2) is read and written to the SPM memory 106. Further, the X-direction size and the Y-direction size of the first threshold pattern TP1 are set in the address counters 110a and 112a, respectively.

In step S2, the CPU 100 reads out the multi-tone large-size threshold pattern Sr (FIG. 11) from the floppy disk device 104, executes the operation of the above equation 3 according to the number N of interpolation gradations, and executes the second operation. A threshold pattern TP2 (FIG. 4) is created. This second threshold pattern TP2 (also referred to as “interpolated value”) is written to the interpolated value memory 108. The number N of interpolation gradations used for the calculation is stored in the interpolation gradation number register 114. Second threshold pattern T
The X-direction size and the Y-direction size of P2 are set in address counters 110b and 112b, respectively.

In step S3, the sub-scanning start signal R
X address counters 110a and 110b are initialized according to x. In step S4, the main scanning start signal R
Y address counters 112a and 112b are initialized according to y.

In step S5, the main scanning clock signal C
According to y, the input image signal Im is received, and the comparator 120 performs a binarization process. At this time, the Y address counters 112a and 112b are also incremented, and return to the initial values when the count value reaches the respective size in the Y direction. In step S6, it is determined whether the binarization processing on one main scanning line has been completed. This can be determined by whether or not a pulse of the sub-scanning clock signal Cx has occurred. If the processing on one main scanning line has not been completed, the process proceeds from step S6 to step S6.
Returning to step S5, on the other hand, if the processing has been completed, the process proceeds to step S7. In step S7, the X address counters 110a and 110b are incremented according to the sub-scanning clock signal Cx. X address counters 110a, 110b
Is returned to the initial value when the count value reaches each X-direction size. In step S8, it is determined whether or not the binarization processing for all the sub-scan lines has been completed. If not completed, the process proceeds from step S8 to step S8.
Return to 4. By repeating steps S4 to S8 in this manner, a binarized output (dot signal) representing one dot image is generated.

D. Second Embodiment FIG. 17 is a block diagram showing a configuration of a halftone dot forming apparatus to which the second embodiment is applied.
In this halftone dot forming apparatus, the multiplier 116 of the first embodiment shown in FIG. 15 is replaced by a bit shifter 130, and the adder 118 is replaced by a bit combining OR circuit 132. The interpolation gradation number register 114 shown in FIG.
Although it is omitted in FIG. 7, it need not be omitted. If the interpolation gradation number register 114 is provided, this register 11
4 to the bit shifter 130 and the bit synthesizing OR circuit 132
Is supplied with the number N of interpolation gradations.

The apparatus shown in FIG. 17 is a simplified apparatus which can be used when the number N of interpolation gradations is a factorial of 2. That is,
When N is 2 ^n, the same result as the multiplier 116 in FIG. 15 can be obtained by shifting the first threshold pattern TP1 to the left by log ₂ N (= n) bits by the bit shifter 130. In the bit combination OR circuit 132, the second
By taking OR (logical sum) of the lower log ₂ N (= n) bits of the threshold pattern TP2 and the output of the bit shifter 130 for each bit, the same result as 118 in FIG. 15 can be obtained.

E. Third Embodiment FIG. 18 is a block diagram showing a configuration of a halftone dot forming apparatus to which the third embodiment is applied.
This halftone dot forming apparatus is obtained by adding a bit mask 140, a second comparator 142, and a clock gate 144 to the apparatus of the second embodiment shown in FIG. In this device, updating of the Y-direction address of the interpolation value memory 108 is controlled by a clock gate 144.

The bit mask 140 is a circuit for masking the lower log ₂ N bits of the image signal Im to 0. For example, when N = 4, the lower two bits are masked. The masked image signal Im ′ is compared with the output (N × TP1) of the bit shifter 130 in the second comparator 142. Then, the output of the bit shifter 130 (N × TP
When 1) becomes equal to the masked image signal Im ′, the clock gate 144 is opened for one clock. For example, when the masked image signal Im ′ is 96, the output (N × TP1) of the bit shifter 130 becomes 9
What is equal to 6 is the position of TP1 = 24 (the threshold value of the position (X1, Y2) in FIG. 2). Therefore, when the binarization processing is performed on the main scanning line whose sub-scanning coordinate is X1, every time this position (X1, Y2) is referred to, that is, every eight pixels, the clock gate 144 is output.
Opens to update the Y address of the second threshold pattern TP2. On the other hand, on the main scanning line whose sub-scanning coordinates are other than X1, Im ′ = (N × TP1) does not hold.
The first threshold (threshold whose Y-direction coordinate is 1) on each scan line of the second threshold pattern TP2 shown in FIG. 4 is used over the entire length of each scan line.

FIG. 19 is a flowchart showing the processing procedure of the third embodiment. This procedure is obtained by replacing step S5 of the procedure of the first embodiment shown in FIG. 16 with steps S11 and S12.

In step S11, the input image signal Im is received according to the main scanning clock signal Cy. Further, the first comparator 120 performs binarization processing of the image signal Im. At this time, the Y address counter 112a for the SPM memory 106 is incremented. When the count value of the Y address counter 112a reaches the size in the Y direction, the value returns to the initial value.

In step S12, the input image signal Im is received in accordance with the main scanning clock signal Cy, and the second comparator 142 outputs the masked image signal Im '.
And the output of the bit shifter (N × TP1).
If Im ′ = N × TP1, the clock gate 144
Opens, and the Y address counter 1 for the interpolation value memory 108
12b is incremented. Y address counter 1
When the count value of 12b reaches its Y-direction size, it returns to the initial value.

The timing for opening the clock gate 144 may be various other than the above. For example, regardless of the value of the input image signal Im, the clock gate 144 may be opened for each Y-direction size of the first threshold pattern TP1 (every eight pixels in FIG. 2). In this way, the reference position of the second threshold pattern TP2 is updated for each size of the first threshold pattern TP1 in the Y direction.

As described above, the second threshold pattern TP2
Need not be updated in accordance with the update of the reference position of the first threshold pattern TP1, but can be updated in accordance with a desired timing. In this case, a favorable halftone image may be formed over a wider image area than the size of the second threshold pattern TP2.

F. Fourth Embodiment: FIG. 20 is a block diagram showing the configuration of a halftone dot forming apparatus to which the fourth embodiment is applied.
This halftone dot forming apparatus is obtained by adding a clock gate 146 in the X direction and a set / reset gate 148 to the apparatus of the third embodiment shown in FIG. This device is
The update of the Y-direction address and the X-direction address of the interpolation value memory 108 is controlled by two clock gates 144 and 146.

The added X-direction clock gate 146
Opens in response to a signal provided from the set / reset gate 148 when the set / reset gate 148 is in the set state. The set / reset gate 148 is set when the output (N × TP1) of the bit shifter 130 becomes equal to the masked image signal Im ′, and is reset when the main scanning start signal Ry is supplied. Therefore, the clock gate 146 in the X direction is connected to the bit shifter 1
30 (N × TP1) is the masked image signal Im ′
Open only after equals. As described in the third embodiment, when Im ′ = N × TP1, the Y-direction clock gate 144 opens and the Y address counter 112b is incremented. At this time, the clock gate 146 in the X direction opens at the same time. Thereafter, the sub-scanning clock signal C
When one pulse of x occurs, the X address counter 110b
Is incremented. Thus, in the device of the fourth embodiment, the X address counter 110b for the second threshold pattern TP2 is updated only when the Y address counter 112b has been updated even once. Conversely, X
The address counter 110b is a Y address counter 11
Until 2b is updated, it is kept the same value.

FIG. 21 is a flowchart showing the processing procedure of the fourth embodiment. In this procedure, steps S4, S7, and S12 of the procedure of the third embodiment shown in FIG. 19 are changed to steps S4a, S7a, and S12a, respectively.

In step S4a, the Y address counters 112a and 112b are set in accordance with the main scanning start signal Ry.
And the set / reset gate 148
Reset. In step S12a, the Y address counter 112b is incremented, and the set / reset gate 148 is set. In step S7a, only when the set / reset gate 148 is set, the clock gate 146 in the X direction is opened and the X address counter 110b is incremented.

FIG. 22 is an explanatory diagram showing an example of a halftone dot formed by the fourth embodiment, and corresponds to FIG. 8 of the first embodiment. As can be seen by comparing FIG. 22 and FIG. 8, in the halftone image of FIG. 22, the lighting pixels vary more than in the halftone image of FIG. Therefore, in the fourth embodiment,
It can be understood that there is a possibility that the gradation can be reproduced more smoothly as a whole.

The present invention is not limited to the above examples and embodiments, but can be implemented in various modes without departing from the gist of the invention.
For example, the following modifications are possible.

(1) In the above embodiment, the second threshold pattern TP2 is obtained by calculation from the multi-tone large-size threshold pattern Sr in accordance with the number N of interpolation gradations. A plurality of sets of second threshold patterns TP2 respectively corresponding to a plurality of values of the number N may be prepared in advance, and one of them may be selected according to the number N of interpolation gradations.

(2) The first threshold pattern TP1 and the second threshold pattern TP2 are stored in the respective threshold memories 106, 1
08, an arithmetic unit such as the CPU 100 performs an arithmetic operation to obtain the first threshold pattern TP1
Alternatively, a second threshold pattern TP2 may be generated.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an outline of a halftone dot forming apparatus according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an example of a first threshold pattern generated by a first threshold generator 30;

FIG. 3 is an explanatory diagram showing an example of a second threshold pattern generated by a second threshold generator 32;

FIG. 4 is a diagram showing an entire second threshold pattern TP2 (N = 4).

FIG. 5 is an explanatory diagram showing eight preferable 2 × 2 matrices.

FIG. 6 is an explanatory diagram showing a distribution of a third threshold value TP3 (third threshold pattern).

FIG. 7 is an explanatory diagram showing halftone dots formed when the value of an image signal Im is 18.

FIG. 8 is an explanatory diagram showing halftone dots formed when the value of an image signal Im is 125.

FIG. 9 is an explanatory diagram showing a second threshold pattern TP2 ′ when the number N of interpolation gradations is two.

FIG. 10 is an explanatory diagram showing a second threshold pattern TP2 ″ when the number N of interpolation gradations is 5.

FIG. 11 is an explanatory diagram showing a multi-tone large-size threshold pattern Sr used in a more preferable method for creating a second threshold pattern TP2.

FIG. 12 is an explanatory diagram showing a lighting pattern based on a multi-tone large-size threshold pattern Sr.

FIG. 13 is an explanatory diagram showing a lighting pattern based on a multi-tone large-size threshold pattern Sr.

FIG. 14 is an explanatory diagram showing a lighting pattern based on a multi-tone large-size threshold pattern Sr.

FIG. 15 is a block diagram illustrating a configuration of a halftone dot forming apparatus to which the first embodiment is applied.

FIG. 16 is a flowchart illustrating a processing procedure according to the first embodiment;

FIG. 17 is a block diagram showing a configuration of a halftone dot forming apparatus to which the second embodiment is applied.

FIG. 18 is a block diagram showing a configuration of a halftone dot forming apparatus to which the third embodiment is applied.

FIG. 19 is a flowchart illustrating a processing procedure according to the third embodiment;

FIG. 20 is a block diagram illustrating a configuration of a halftone dot forming apparatus to which a fourth embodiment is applied.

FIG. 21 is a flowchart illustrating a processing procedure according to a fourth embodiment;

FIG. 22 is an explanatory diagram showing an example of a halftone dot formed by the fourth embodiment.

[Explanation of symbols]

 Reference Signs List 30 first threshold value generating unit 32 second threshold value generating unit 34 multiplying unit 36 adding unit 38 comparing unit 42 comparator 100 CPU 102 main memory 104 floppy disk device 106 SPM memory 1 threshold memory) 108 interpolation value memory (second threshold memory) 110a, 110b sub-scanning address counter 112a, 112b main scanning address counter 114 interpolation gradation number register 116 multiplier 118 adder 120 Comparator 130 ... Bit shifter 132 ... Bit combining OR circuit 140 ... Bit mask 142 ... Comparator 144,146 ... Clock gate 148 ... Set / reset gate

Claims (9)

[Claims] 1. A method for forming a halftone dot in accordance with an image signal, comprising: (a) generating a first threshold value forming a first threshold pattern having a relatively large number of gradations and a relatively small size; And (b) generating a second threshold value forming a second threshold pattern having a relatively small number of gradations and a relatively large size; and (c) the first threshold value and the second threshold value. And (d) generating a dot signal representing a dot by comparing the third threshold value with the image signal. The method of forming a halftone dot, comprising: 2. A halftone dot forming method according to claim 1, wherein said step (c) includes a step of performing an interpolation operation for interpolating said first threshold value with said second threshold value. Method. 3. The halftone dot forming method according to claim 2, wherein the interpolation calculation step includes: multiplying the first threshold value by an integer N; and calculating the multiplication result and the second threshold value. Creating a third threshold value by adding them together. 4. The halftone dot forming method according to claim 3, wherein the second threshold pattern is composed of N threshold elements from 0 to (N−1). 5. The halftone dot forming method according to claim 1, wherein the second threshold pattern includes a plurality of two-dimensional matrices having different arrangements of a plurality of threshold elements arranged at random. And a dot forming method. 6. The halftone dot forming method according to claim 5, wherein the integer N is equal to 4, and each two-dimensional matrix includes four threshold elements of 0 to 3.
A halftone dot forming method that is a × 2 matrix. 7. The halftone dot forming apparatus according to claim 6, wherein the plurality of types of two-dimensional matrices include threshold elements 0 and 1.
Are arranged diagonally to each other, and the threshold elements 2 and 3 are 2 × 2 matrices arranged diagonally to each other. 8. An apparatus for forming a halftone dot in response to an image signal, comprising: a first threshold value generating a first threshold pattern forming a first threshold pattern having a relatively large number of gradations and a relatively small size; Threshold value generating means; second threshold value generating means for generating a second threshold value forming a second threshold pattern having a relatively small number of gradations and a relatively large size; and the first threshold value and the second threshold value By performing a calculation using a threshold value, a threshold value calculating means for creating a third threshold value, and comparing the third threshold value with an image signal,
And a comparing means for generating a halftone signal representing a halftone dot. 9. A computer-readable recording medium on which a computer program for forming a halftone dot in accordance with an image signal is recorded.
A step of generating a first threshold value forming a first threshold pattern having a relatively small size; and (b) a second threshold value forming a second threshold pattern having a relatively small gradation and a relatively large size. Generating a third threshold value by performing an operation using the first threshold value and the second threshold value; and (d) generating a third threshold value and an image. Generating a halftone signal representing a halftone dot by comparing the signal with a signal; and a computer-readable recording medium storing a computer program for causing a computer to execute the halftone signal.