JPH0332264B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a color image plotter device that plots a color image using dots of a plurality of colors, and more particularly to an improvement for preventing moiré from occurring on the plotted color image.

To create a color image, dot images of cyan, magenta, and yellow colors are usually plotted one on top of the other on the same sheet of paper. It is known that in color images plotted in this manner, moiré occurs in which the dots of the dot images for each color are of equal pitch, resulting in a significant drop in image quality.

In order to prevent the occurrence of moiré, a color image plotter using a contact mesh screen employs a method in which the mesh screen is rotated little by little when plotting a dot image for each color. However, such rotation of the mesh screen generally requires laborious manual labor, and there is also the problem that moiré cannot be prevented if the rotation angle is inappropriate.
Furthermore, this method is not applicable to devices that do not use contact mesh screens, such as electronic color image plotter devices.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a color image plotter device which prevents the occurrence of moiré without relying on the rotation of the contact mesh screen as described above.

According to the present invention, pixels of a color image to be plotted are plotted as dots of corresponding colors for each color,
In a color image plotter device which creates the above-mentioned color image as a set of dots of a plurality of colors, means is provided for irregularly varying the plotting timing of each pixel over time. By the action of this means, the plot positions of the pixels of each color are changed relatively at random, thereby preventing the occurrence of moiré.

In the case of this occurrence, in addition to the above-mentioned means, a means for correcting the density of the pixel according to the amount of variation in the plot timing of the pixel by the means is provided, and the pixel position is adjusted to prevent moiré. Further improvement in image quality is achieved by compensating for variations.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram of an electronic color image plotter device embodying the present invention.

The apparatus of this example includes a drum 10 covered with a photoconductor.
The drum 100 is electrically charged in advance and rotated by a motor 101, and the surface of the drum 100 is scanned with a recording light beam to plot dot images of different colors in the form of electrostatic latent images. This latent image is then printed on paper using a known PPC process. Normally, dot images of cyan, magenta, and yellow are printed on drum 1.
00 and then print them overlappingly on the same sheet of paper to obtain the desired color image.
The main scanning of the recording light beam is performed on the drum 100 by a rotating prism 103 and an f-θ lens system 102.
The sub-scanning is performed in the axial direction of the drum 100, and the sub-scanning is performed in the drum circumferential direction by the rotation of the drum 100 itself.

Note that the latent image of the dot image on the drum 100 is
The mechanism for making a hard copy using the PPC process may be a known mechanism, so it is omitted from the diagram.

The recording light beam is emitted from a light source 104, which is driven by a driver 106 in the following manner.

The color image to be plotted is stored in the image memory 114 as an image sampled pixel by pixel for cyan, magenta, and yellow. Under the control of the control unit 109, pixel information indicating the density level of each cyan pixel is sequentially read out from the image memory 108 in synchronization with the scanning on the drum 100.
The signal is input to the calculation unit 107. The driver 106 drives the light source 104 in accordance with the output of the calculation unit 106, whereby a latent image of a cyan dot image is recorded on the drum 100, and a mechanism (not shown) prints it as a cyan dot image on the paper. Photographed.

Next, the pixel information of the magenta image is read out from the image memory 108, and the drum 108 is similarly read out.
0 as a latent image. This latent image of the magenta dot image is printed on the same sheet of paper as a magenta dot image, superimposed on the cyan dot image. Finally, a yellow dot image is printed on the same sheet of paper through a similar process. In this way, the desired color image is obtained.

Although the calculation section 107 has not been described so far, this calculation section 107 is the characteristic part of the apparatus according to the present invention. That is, it is a part that controls the plot position of pixels and corrects pixel density. The operation of this calculation section 107 will be explained with reference to FIG.

FIG. 3 shows an example of a dot plot, with circles indicating cyan dots and triangles indicating magenta dots. Further, the sizes of the circles and triangles indicate the size of the dots, and visually correspond to the density of the pixels.

When plotting a cyan dot image, the calculation unit 1
07 provides the driver 106 with a density signal of a level corresponding to the density level indicated by the pixel information inputted from the image memory 108, and also provides the driver 106 with a constant periodic plot timing signal. Therefore, for a cyan dot image, dots (pixels) are plotted at equal intervals with a basic pitch Pc that matches the period of the plot timing signal. The plot position of this dot is called the normal position of the pixel. Furthermore, since the driver 106 drives the light source 104 so as to emit a recording light beam with an intensity corresponding to the level of the input density signal, the size of the dot corresponds to the density level itself of the corresponding pixel on the image memory 108. are doing.

On the other hand, when plotting a magenta dot image,
The calculation unit 107 randomly varies the period of the plot timing signal over time, and moves the plot position of the dot from the normal position as shown in FIG. The dot pitch P _n at this time is expressed by the following equation.

P _n =P _c ±δP _n ...Formula (1) Here, δP _n is a quantity that changes randomly over time, and satisfies the condition δP _n <P _c /2.

Now, the pixel information stored in the pixel memory 108 indicates the density level when the pixel is plotted at a normal position. If a pixel is plotted at a position deviated from the normal position as shown above while keeping this density level, the visual pixel density will change.
This may cause a change or deterioration of the image quality. Therefore, when plotting magenta dot pixels, the arithmetic unit 107 corrects the pixel density according to the deviation of the plot position from the normal position.

This correction is performed, for example, using the following equation. However, the basic pitch P _c =1.

S _n = {f(x)・S ₀ +f(1-x)S ₁ } /{f(x)+f(1-x)} ...Equation (2) Here, x is the plot position of the dot of interest. The amount of deviation from the normal position (see Figure 3), S ₀ , S ₁
is the density level of two pixels corresponding to the normal positions before and after the plot position of the dot of interest (image memory 108).
(the agricultural level expressed by the pixel information read out). S _n is the corrected pixel density level, that is, the level of the density signal applied to the driver 106 corresponding to the dot of interest.

Various functions can be considered as f(x), but in this embodiment, the following function is used.

f(x)=sin(πx)/πx (3) The above plot timing control and pixel density correction are performed in the same way when plotting a yellow dot image.

FIG. 2 shows an example of the arithmetic unit 107 as described above and will be described.

The timing generation circuit 200 includes a drum 100.
A clock pulse synchronized with the scanning (FIG. 1) is supplied from the control section 109 (FIG. 1). The timing generation circuit 200 has a built-in counter with a modulus of 10, and counts up based on the input clock pulse. 201 is a random number generator, which operates only while the inhibition signal from the control unit 109 is "0". A random number generated by the random number generator 201 is converted by a counter 202 into a random number with a probability of +1 or -1, and is provided to the timing generation circuit 200. The built-in counter of the timing generation circuit 200 counts up by 1 when +1 is input from the counter 202, and counts down by 1 when -1 is input. The carry output of the built-in counter is sent to driver 1 as a plot timing signal.
Sent to 06.

When plotting a cyan dot image, control unit 1
The inhibition signal from 09 is set to "1", and the random number generator 201 stops. Therefore,
The plot timing signal (b) with a period (referred to as the fundamental period) that is 10 times that of the clock pulse (a) shown in Fig. 4 is
It is generated by the timing generation circuit 200. Each pulse of this plot timing signal (b) corresponds to the normal position of the dot, and its fundamental period corresponds to the fundamental pitch _Pc of the dot.

When plotting magenta and yellow dot images, the inhibition signal is reset to "0" and the random number generator 201 operates. Therefore, the timing generation circuit 200 generates the above plot timing signal.
A signal obtained by randomizing (b) is output. Fourth
(c) in the figure is an example of a plot timing signal generated when plotting a magenta dot image.
(d) in the figure is an example of a plot timing signal generated when plotting a yellow dot image.

The timing generation circuit 200 also outputs the amount of deviation from the basic period of the generated plot timing signal (an amount corresponding to the above-mentioned x). This shift amount (x) is input to the arithmetic unit 203 together with the pixel information read out from the image memory 108. Using these inputs, the arithmetic unit 203 executes the above-mentioned equations (2) and (3), and sends a concentration signal to the driver 106. This arithmetic unit 203 is configured using, for example, a ROM (read only memory).

In FIG. 1, 105 is a sensor for detecting the recording start position of the drum 100.

In the embodiment described above, dot images of three colors are sequentially plotted using a common mechanism, but the dot images may be plotted using different mechanisms for each color. For example, the entire mechanism in FIG. 1 or just the drum 100 and printing mechanism can be
Systems may be provided, and dot images of corresponding colors may be plotted in each system in synchronization with the movement of the paper.

As described above, the present invention changes the plot position of each color pixel relatively irregularly.
It is possible to prevent the occurrence of moiré, and
Since the density is corrected according to changes in the plot position, a color image with extremely good image quality can be created.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention;
FIG. 2 is a block diagram showing an example of the calculation section in FIG. 1, FIG. 3 is a diagram showing an example of a dot plot, and FIG.
The figure is a diagram illustrating clock pulses and plot timing signals generated during plotting of each color. 100...Drum, 101...Motor, 102
... f-theta lens system, 103 ... rotating prism,
104...Light source, 106...Driver, 107...
...Arithmetic section, 108... Image memory, 109... Control section, 200... Timing generation circuit, 201...
...Random number generator, 202...Counter, 203...
Arithmetic unit.

Claims (1)

[Scope of Claims] 1. In a color image plotter device that plots pixels of a color image to be plotted as dots of corresponding colors for each color and creates the color image as a set of dots of a plurality of colors, the plotting position of each pixel is A means for generating a timing signal as a reference for a pixel, and a means for determining a plot position for pixels of a specific color based on the timing signal, and for pixels of other colors, plotting by irregularly varying the timing signal over time. The present invention includes means for determining the position and relatively irregularly varying the plot position of the pixel of each color, and means for correcting the density level of the pixel according to the amount of variation in the plot position of the pixel of each color. Characteristic color image plotter device.