JPS6041343B2 - Multicolor mesh printing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a multicolor halftone plate printing method for printing color images, and in particular to a multicolor halftone plate printing method applied to a color scanner operation in which a color original image is photoelectrically scanned and a color separation halftone plate is made. This invention relates to an improved printing method for halftone plates.

Generally, color printed matter is produced by overprinting four color separation mesh plates of yellow (Y), magenta (M), cyan (C), and black (K) with each color ink, as is well known. The silk dot pitch of each mesh plate is the same for all four versions.
Print with different screen angles (directions in which silk dots are arranged).

For example, the screen angle for Y version is oo, the M version is 1y,
M, C, -150 for C version, 450 for K version, etc.
Moiré is prevented by printing with the screen angles of the three K plates sliding one by one, and by setting the Y plate at an intermediate angle.

When such color separation mesh plates are photographically plate-made using a plate-making camera, one contact screen is rotated to a required angle according to each color separation plate, or,
A halftone plate at the desired angle can be easily obtained by using a set of four contact screens, each made at the desired angle, to obtain a halftone image, but a color scanner can simultaneously separate the colors and adjust the gradation of the original image. The formation of corresponding halftone dots to record color separation halftone plates involves extremely difficult problems.

That is, when recording a halftone plate with a color scanner, the recording image area is scanned line-by-line by the recording light beam, and the brightness of the recording light beam is modulated according to the desired dot pattern and the density distribution of the image. There is a need.

In this case, the brightness modulation according to the dot pattern is
If the screen angle is oo or 45o with respect to the scan direction, the process is relatively simple and can be repeated, but if the screen angle is 11y or -150, the positional relationship between each scan line and each fine point can be repeated. Since there is no periodicity in the dots, it is necessary to calculate the recording position of each halftone dot by computer, and in order to prevent moiré, it is necessary to determine the position of each dot with extremely high precision. As a result, the equipment becomes very complicated and expensive, and the work speed is slow and inefficient.

The present invention aims to overcome these drawbacks by providing a multicolor halftone printing method suitable for recording color separation halftones by a scanning device such as a color scanner.

The main point of the method of the present invention is that it is extremely difficult to implement with scanning equipment such as a color scanner.
In place of the screen angle of 150 degrees, a screen angle that is relatively easy to implement is used, and the mesh pitch in each color separation mesh plate is adjusted respectively to prevent the occurrence of moiré.

A detailed explanation will be given below based on the drawings. Figure 1 shows the screen angles for the four mesh plates at 00 and 450, respectively.
, Ten Shin 0, Isshin. This shows the direction of each screen line when the screen line is 1, and the screen lines in two orthogonal directions are shown for one halftone plate, and the lowercase letters a and b are attached. In the case of the mesh plate shown in the figure, the screen angle of 00 is usually the yellow plate, and the screen angle of 450 is the black plate.

Isshin. are applied to the magenta and cyan versions, respectively, so in the following explanation, each screen line is
Ya, Yb, Ka, Kb, Ma, Mb, Ca, respectively
It is written as Cb. When considering moiré caused by four sets of orthogonal halftone dots arranged at these angles, the color of the yellow plate is weak, so even if moiré actually occurs, it will hardly be a problem, and it will not affect the actual multicolor printing result. In order to prevent moire that affects the image, it is sufficiently practical to consider the moire that occurs between the black, magenta, and cyan versions. First, let's consider moiré caused by two sets of halftone dots arranged at screen angles of 1000 and 1000, namely magenta and shear.

Moire caused by two sets of intersecting halftone dots can be determined geometrically, and when the pitches of the two sets of halftone dots are equal, it is expressed by the following equation. Ps,. ．． 、． ．． ．． ．．
．． (1) pm=turbid ginseng Qm=90.

10A ......(0) However, Pm: Pitch of moire, ps: Pitch of halftone dots a: Crossing angle of halftone dot group Qm: Direction of moire Therefore, moire due to halftone dot groups Ma and Cb in Fig. 1 is the halftone dot pitch p for the two sets of silk dots, and P-? are equal, PJPm0 = book i by equations (1) and (0)
n(45o-nuutashi, Pmma: Moire pitch Qm○=9oo10(45o isshin.

) = 13 zars. However, the moire angle for Qmgu:N wind, that is, the moire angle in this case is (10J.

For the halftone dot group at the angle a), it will occur in the direction of (1350 single center), and this direction coincides with the direction of the silk dot group (45 ob), as is clear from FIG. That is, the moire caused by the halftone dot groups Ma and Cb coincides with the direction of the halftone dot group Mb of the black version.

Therefore, if the end point pitch of the black version is the same as the pitch P of the magenta version and the cyan version, since the pitch of the moiré is different from this, it means that silk points with different pitches are overlapped at the same angle, which is noticeable. Secondary moiré occurs. In order to prevent this secondary moire, the dot pitch of the black plate should not be the same as that of the P medium, but should be made equal to the pitch of the moire caused by Ma and Cb. If the error between the angle and the dot pitch is 0.1% or less, moiré will not be noticeable at all in actual work.

In other words, the screen angles of the magenta and cyan versions are (
Ten J. ), (1J.), and when the silk point pitch is P less, the screen angle is 45o, P? The silk point pitch of the rack version is P45 (45.

-v, the occurrence of secondary moiré can be prevented. The above explanation also applies to the relationship between the moiré caused by the halftone dot groups Mb and Ca and the halftone dot group Ka.

Next, when the dot pitch of the black plate is set as described above, moiré caused by this and the end point group of the magenta plate or cyan plate will be considered.

The pitch and angle of moiré caused by a group of halftone dots with different pitches are expressed by the following equation.

*P;ノP. 20P Keizo P ticket. p2C melon.

Q = 900 Coyl matrix) (The compound sign is determined by which of the two sets of silk dot pitches is larger) where, P: Pitch of moire P, P2: Pitch of halftone dot group 8: Crossing angle, Q: P , the moire angle for the halftone dot group Ka of the black version and the halftone dot group C of the cyan version.
The moiré caused by a is calculated based on the above formula as follows.

However, cos (450 tenshin.

)=sin(450-small O), so the above equation also becomes qm2 9000 COS-. { B■ Sin(450
Ten little.

}P■2Sinku450 all.

) = 9000 COS-. {F-2 Sin(4
50 ten.

) Sin(450-)}P■=90010CO
S-1 COS 2 small 1 COS 900) = 900 twelve.

That is, the moire pitch is equal to the silk dot pitch of the magenta and cyan versions, and the angle is (900
12 small 0), which corresponds to the direction of the silk point group M5 of the magenta version.

In other words, the moiré caused by Ka and Ca, and the second one caused by Mb.
It can be seen that moiré does not occur. The same applies to moire caused by the black version and the magenta version.

As described above, in the mesh plate for multicolor printing, the screen angles of 150 and -150, which are generally used for magenta and cyan plates, are replaced by ten-center screen angles.

By using Isshin 〇 and selecting the end point pitch of the black plate as described above, it is possible to prevent the occurrence of secondary moiré, but when applying this method to an actual color scanner, It is desirable that the screen angles of the magenta plate and the cyan plate are such that the dot pattern along the scanning line is repeated in the simplest form possible. For that purpose, the value of the nen core is 1/31/41/5...
It is appropriate to select a screen angle that takes a simple integer ratio such as . This will lead to a decrease in the resolution of the effect, and conversely, if it is too fine, problems such as poor printability will occur.
The value is 115 degrees, which is the conventional screen angle.
, -150 must be selected.

The ones that meet this condition are tan. =1/4
(Min.=14o02) is the most appropriate. Figure 2 is a schematic diagram of a color separation mesh plate based on the above-mentioned theory, in which A shows a magenta plate, the opening shows a black plate, and C shows a yellow plate.

The cyan version is a horizontally rotated version of the magenta evening version. In each figure, the 50% silk dot area is indicated with diagonal lines.・ First, the magenta version is centered in the scanning direction.

(Matsu N Elementary School: Chapter) It is a complicated mesh plate, and in this case, it is clear that the relationship between the silk dot pitch P◇ and the silk dot interval 1 along the scanning line is 1 = P◇ COS center. It is.

Also, the silk dot pattern at this time is L=41 in the scanning direction.
It can also be understood from the diagram that the pitch is repeated at a pitch of .

Next, regarding the black version, the halftone dot pitch P45 can be determined as P or lcOS center by the above-mentioned formula (1).

P45 = public in(45o -shinzu-2(sin45oc)
Cos45QsinJ rudder to osJ It will be the direction of the 0 shout fox boat side of lcos.

That is, as shown at the beginning of Figure 2, in this case as well, the halftone dot pattern is repeated with a period of L in the scanning direction (of course, the minimum repetition period is (111/31), but if 1 is The repetition period when used as a unit is 41:L).
As for the yellow plate, as mentioned above, the effect of moiré can be practically ignored, so it is only necessary to select a dot pitch that is convenient for the actual work.For example, as shown in Figure 2C, , 1 in the first half may be used as the silk point pitch.

Next, FIG. 3 is a perspective view schematically showing a recording section of a color scanner to which the present invention is applied.

A film 2 is wound around a recording cylinder 1, and while rotating it in the direction of the arrow, the cylinder surface is irradiated with a laser beam 3 through an acousto-optic element 4 and a lens 5, and the film is moved at a constant pitch in the axial direction of the cylinder. .

When ultrasonic waves are supplied to the acousto-optic element 4, a diffraction grating is formed in the element 4 according to the frequency of the ultrasonic waves, and the laser beam 3 is deflected. It is possible to vibrate the deflection with the amplitude, and for example, it is possible to record the exposure of silk dots as shown in the third figure. A method for recording each color separation halftone plate shown in FIG. 2 using such a color scanner will be explained with reference to FIGS. 4 and below.

FIG. 4 shows the case where nJ=1/4, that is, the magenta version.

In this case, in the interval of repeating unit length "L" of the halftone dot pattern,
The four dots are arranged at an angle of 10 degrees with respect to the scanning line, and the pitch of the scanning line is set as Ps〒P as shown in the figure.
If the halftone dot pitch is set to P, two dots will be recorded in the interval "L" by one scan. That is, in the scanning of the scanning line S, the squares A,
, A2, G, A3 (first halftone dot) and square E. , E2, day, silk points included in the range of E3 (
A second silk point group) is recorded.

These squares are the silk spot area when the silk spot area is 100%. Similarly, in the scanning of the scanning line S2, the third halftone dot and the fourth halftone dot are recorded.

Scan line S. In order to record the first and second halftone dots by scanning, the following procedure may be used.

In the figure, the square indicated by the dotted line, that is, the area is 100%
Although the halftone dot area is constant, the actual halftone dot area (hatched area) changes depending on the density of the original image.

Therefore, for the first silk point, the distance {a},'c}
, 'f}' are constant, and {pair, ■ is an amount that changes depending on the original image density. Therefore, the scanning line S. During the scanning, the exposure light beam is shielded by appropriate means during the distance {a'' from the female point U in the section "L", and no deflection is applied in the left-right direction.

Next, (A.) The light beam is still blocked for a distance {b} from the point, but in the scanning direction? Deflected at an angle of 100 degrees. When the scanning point reaches point [B],
The light beam is unblocked and exposure starts, and at the same time the light beam moves leftward from the line A, , M connecting the centers of each end point (
i), it is deflected to the right to vibrate with an amplitude of (i).

The quantities (i) and (j) are controlled so that the loci at both ends of the amplitude of the light beam as the scan progresses gradually increase to 450 for lines A, , M, respectively. As the scanning further progresses and travels a distance (c+d), the first turning point B4
reach.

(j), which has been gradually increasing up to this point, will then conversely change to the lines A,,M
It is controlled so that it gradually decreases at an angle of 45 degrees with respect to the angle of 45 degrees. Subsequently, as the scanning progresses over the distance (fd), it reaches a second turning point B2, and (i), which had been gradually increasing up to that point, is controlled to gradually decrease thereafter. Thus, (i) and (j)
Both become 0 at point (B3), and the recording of the first silk point ends. In addition, the first turning point B4 and the second turning point B3
When the scanning advances the distance 'e) between E. and E. of the second silk point area. , the control for recording the second halftone dot is started from this time.

This is done by converting the distance {a- to the distance {e} at the first silk point, and also for the distances 'c' and 'f}, respectively (
By changing the calendar to c+e-a) and (f1e-a), the control can be performed in the same way as the first silk point. For this purpose, two sets of control devices may be used in parallel, and one set may be applied to light beam control when recording the first halftone dot and the other to record the second halftone dot, and the details will be described later. Thus, in the section of length "L", the first and second silk dots are recorded by scanning the scanning line S.

Scan line S. In the scanning, two halftone dots are recorded in the same manner as above in the next section, and this is repeated to perform the first scanning over the entire area of the image. Next, when the recording cylinder 1 of the color scanner rotates once and the fourth illustrated area is scanned again, it advances by one pitch in the sub-scanning direction and the scanning line S
In the second scan, the third silk point and the fourth silk point are recorded. These are the above-mentioned distances {a} and {e', respectively (g)
and (h), and is controlled in the same manner as in the case of the scanning line S. The same applies to the scanning line S&bottom. Nene n
The medium: -1/4 cyan version is similar to the magenta version described above. Next, FIG. 5 shows a case where tan is 0, that is, a yellow plate. In this case, four halftone dots are arranged in a direction parallel to the scanning line in a section of the repeating unit length "L" of the silk dot pattern.

Therefore, four silk dots are recorded in a section of length "L" in one scan.

When recording the end point row on the left side in FIG. 5 by scanning the scanning line S, the bran MN connecting the center of each halftone dot is the scanning line S,
Since the exposure light beam is parallel to the halftone dot center line MN
Basically, the light is polarized by the amount corresponding to the distance (k) between the scanning line S, and the left and right amplitudes (i) (i) are set equally for each silk point, centering on the line MN, It is sufficient to deflect the amplitude.

That is, the light beam is blocked while the scan travels a distance (a + b) from the starting point U of the section of length "L" (however, the halftone dots in the section preceding the illustrated section are not recorded). However, for the sake of explanation, we will ignore this). Here, the distance (a} is constant, and the distance [b) changes depending on the density of the original image, as in the case of the magenta plate. Exposure is started from a point where the scanning of the scanning line S, has progressed by a distance (a+b) from the starting point U, and the light beam oscillates with a width (i) (i) around the line M plate.

The amounts of (i) and (j) are 4 on each side of the line MN'.
It is incremented as the scan progresses so that a total of 50 points are recorded. The scanning is done at a distance (
c}, the amounts of (i) and (j) gradually decrease to complete the recording of the first silk point.

Also, from the point where the scanning has progressed by a distance {c− from the starting point U,
The control operation for recording the second silk dot is started, and the third halftone dot is similarly recorded from the point that has progressed by the distance (g), and the third halftone dot is recorded at a distance (k).
Control operations are respectively initiated to record the fourth silk point from the point advanced by .

When recording these halftone dots, the area of each silk dot overlaps the area of the adjacent total dots, so the magenta version (
As in the case of (and
and a fourth halftone dot.

Next, FIG. 6 shows tan less=1, that is, a black version.

In the case of the black version, 45
A silk dot array exists at an angle of 0, that is, in a direction parallel to the scanning line, and three end points are recorded with one scanning line in an interval of repeating unit length "L" of the silk dot pattern. become.

In FIG. 6, when scanning the scanning line S, the light beam is basically deflected by a distance (k) from the scanning line S, of the vertical point center line MN, and the scanning is performed at a distance (k) from the starting point U. a+
The light beam is blocked while it travels by b), and then exposed while vibrating with an amplitude equal to the dot width (i+j), and is blocked again at the point where the scanning has progressed a distance c from the starting point U.

The second and third total points are also recorded in the same process. In the case of a black version, only one set of control devices is required because the halftone dot areas do not overlap for one scanning line. Note that it is advantageous for the structure of the apparatus to set the scanning line pitch P for the yellow plate and the black plate to be equal to the scanning line pitch for the magenta plate and the cyan plate.
In that case, in the yellow plate and black plate, the positional relationship between each scanning line and the total dot sequence should be constant, and there may be two scan lines that can be recorded for one silk dot area. (eg, scan lines S2 and S3 for the right halftone dot row in FIG. 6).

However, this problem can be solved by setting (K) in Figs. 5 and 6 to the required value, and recording each silk dot row by the scanning line closest to its center line. There is no practical problem if exposure is paused for lines.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the direction of screen lines of each color separation plate in four-color halftone printing, FIG. 2 is a schematic diagram showing each color separation halftone plate by the method of the present invention by a total of points with an area of 50%,
FIG. 3 is a diagram showing means for recording a halftone screen using a color scanner, and FIGS. 4 to 6 are diagrams showing the relationship between dot patterns and scanning lines in each mesh screen. 1...recording cylinder, 2...film, 3
. . . Laser beam, 4 . . . Acousto-optic element, 5. ．． ．． ．． ．． ．． Lens, S...Scanning line. Figure 1 Figure 3 Figure 5 Figure 6 Figure 2 Figure 4

Claims (1)

[Claims] 1 In a multicolor halftone printing method in which multiple types of color separation halftone plates are made using a scanning method with different screen angles for each separation color, and a color image is printed based on this, The screen angle of the first color mesh plate is 45°, and the screen angles of the second and third color mesh plates are +ψ° and −ψ° (however, tanψ°=1
/n, where n is an integer), and the halftone dot pitch in the first color halftone plate is P_4_5, and the halftone dot pitch in the second color and third color halftone plates is Pφ, then P_4_5
The halftone dot pitch of the first color halftone plate and the halftone dot pitch of the second and third color halftone plates are selected so that the following relationship is established: =(Pφ)/(2sin(45°−ψ°)) A multicolor halftone printing method characterized by printing.