JP3301164B2 - Full-color image forming method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a full-color image forming method and apparatus for reproducing a full-color halftone image using an electrophotographic system, and more particularly, to a single-color image forming unit for each color component of a full-color image. The present invention relates to a full-color image forming method for superposing color component images formed in each single-color image forming unit and an improvement of the apparatus.

[0002]

2. Description of the Related Art Conventionally, in a full-color image forming apparatus such as a full-color copying machine or a full-color printer of an electrophotographic system, a method of reproducing a halftone image using a line screen generally known in the printing technique has been adopted. Have been. A line screen of this type is usually called a "conformal line screen", and has the same screen angle for each color component of a full-color image, for example, yellow, magenta, cyan (and black). Was used. That is, many of the above full-color image forming apparatuses usually perform image writing by raster scanning using a laser beam, and continuously write image information written in a scanning operation for each line in the sub-scanning direction, thereby forming a line screen structure. is intended to realize, because the screen angle is 90 ° in a coordinate system taking the main scanning direction to 0 °, what is referred to as a "90 ° the angular line screen" is generally employed Was.

[0003]

However, a so-called tandem type full-color image has a single-color image forming unit for each color component of a full-color image, and superposes the respective color component images formed by the single-color image forming units. In the forming apparatus, since the image forming characteristics and the positional accuracy of each single-color image forming unit vary, the accuracy of the color registration (each color image writing position) in the main scanning direction often decreases. There has been found a technical problem that the overlapping state of the line screens of the respective color components is not constant, and the reproduced colors are visually different.

In order to solve such a technical problem,
Using screen rotation technology known in the printing technology, the line screen angles for each color component are made different from each other, and the degree of overlap between the line screens of each color component is randomized in advance, and the color condition to be reproduced is determined. A method for maintaining the uniformity is conceivable (see JP-A-53-19201).

In this case, in order to obtain a screen angle other than 90 °, when raster scanning is performed for each line, it is only necessary to shift a normal equiangular line screen by a fixed amount for each scan. By making the shift amount different for each component, it is possible to realize a screen rotation in which the screen angle differs for each color component.

However, when the screen angle is greatly changed by using such a method (for example, by setting the shift amount to の of the pitch of the line screen to obtain a screen angle of 45 °), the screen angle becomes larger than that of the original line screen. 1 / sinα
(Α is the screen angle, in this example, 45 °), the number of lines is increased (in this example, the number of lines is increased to 1.41 times), and a high resolution is required for image reproduction. . At this time, usually, the number of lines of the basic line screen is set as high as possible within the image reproducibility of the image forming apparatus, and an image is reproduced in a state where the resolution is as high as possible to achieve high image quality. If the image forming apparatus is required to have a high resolution due to the screen rotation that has been performed and exceeds the image reproducibility of the image forming apparatus, the resolution of the full-color image forming apparatus based on the electrophotographic method becomes insufficient, and the image such as a decrease in gradation is reduced. A technical problem of deterioration has been found.

If the line screen pitch is greatly different for each color component, the resolution will be greatly different for each color component image. Even if these color component images are superimposed, the color component image having a lower resolution is emphasized. For example, there is a concern that the quality of a full-color image is impaired.

The present invention has been made to solve the above technical problems, and can maintain good color reproducibility and effectively prevent image deterioration such as deterioration in gradation. It is an object of the present invention to provide a full-color image forming method and apparatus capable of preventing such a problem.

[0009]

That is, the present invention provides:
Having a plurality of single-color image forming units corresponding to each color component of the full-color image, forming each color component halftone image subjected to line screen processing in each single-color image forming unit, and then sequentially superimposing each color component halftone image In the full-color image forming method, the line screen angles for color component halftone images other than at least the black component halftone image are different from each other, and the line screen pitch for each color component halftone image is set to be substantially equal. It is characterized by.

As shown in FIG. 1, the apparatus invention embodying such a method invention includes a plurality of, for example, four, single-color image forming units 1 for forming each color component image corresponding to each color component of a full-color image (specifically, FIG. 1). 1a to 1d) and halftone information for each color component image is provided through a line screen periodically arranged in the main scanning direction provided for each of the single-color image forming units 1 (1a to 1d). A full-color image including line screen processing means 2 (specifically, 2a to 2d), wherein each color component halftone image formed by each single-color image forming unit 1 (1a to 1d) is sequentially superimposed. Assuming the forming apparatus, at least a write timing of a line screen for a color component halftone image other than at least a black component halftone image is a fixed timing defined corresponding to each color component. Only one scanning screen writing control means 3 for shifting for each line (specifically 3a~3
d) and the screen frequency adjusting means 4 (specifically, 4a to 4d) for adjusting the line screen spatial frequency in the main scanning direction for each color component so that the line screen pitch for each color component halftone image is substantially equal. And characterized in that:

In such technical means, the screen writing control means 3 sets the angle of the line screen. The screen angle is determined only by the shift amount of the writing position of the line screen and the raster pitch in the sub-scanning direction. And does not depend on the pitch of the line screen. Therefore, the screen angle can be set independently of the number of lines of the line screen.

The line screen angles for the respective color components need only be basically different from each other at least for the line components for the color components other than the black component. In this case, it is preferable that the line screen angles for all the color components are different from each other in order to keep the hue better, and when the color components other than the black component are different, the black component Even if the line screen angle is set to be the same for any of the color components, the effect is relatively small. Further, when the screen angles of the respective color components are different, if the difference between the screen angles of the respective color components is small, there is a possibility that moiré may occur. From the viewpoint of suppressing the occurrence of moiré, the screen angle of each color component is reduced. Is preferably set as large as possible.

Further, screen writing control means 3
For the specific configuration of, if emphasis is placed on accuracy, it may be designed to use an element capable of selecting an arbitrary shift amount, and if emphasis is placed on simplification, the element in the main scanning direction may be used. What is necessary is just to design so as to use a delay means for delaying by a shift amount of 1 / n (n is a positive integer of 2 or more) of the standard line screen pitch A.

The screen frequency adjusting means 4 may be designed so as to use a means for individually generating a spatial frequency for each color component, if importance is placed on accuracy.
If emphasis is placed on simplification, a frequency at a screen angle of 90 ° is set as a standard frequency, and a common clock generating means generates a clock N times the standard frequency as a common clock. The frequency of the N-times clock is appropriately divided by the frequency dividing means, so that the number of lines in the main scanning direction of the line screen of each color component may be adjusted.

The screen frequency adjusting means 4
The degree of adjustment is such that the line screen pitch of each color component halftone image is substantially the same, but the line screen pitch differs for each color component within a range where the difference in resolution cannot be discerned by human eyes. It does not matter what, specifically, it depends on the size of the standard line screen pitch,
What is necessary is just to set it within the range of about ± 15% with respect to the standard line screen pitch.

[0016]

The operation of the above-described technical means will be described with reference to the apparatus configuration shown in FIG. In FIG. 1, the screen writing control means 3 (3a to 3d) adjusts the writing timing of the line screen for each color component halftone image by a fixed amount d defined for each color component, as shown in FIG. The shift is performed for each scanning line, and the line screen angle θ for each color component is made different from each other. On the other hand, the screen frequency adjusting means 4 (4a to 4d) adjusts the line screen pitch A in the main scanning direction by adjusting the line screen spatial frequency in the main scanning direction for each color component as shown in FIG. The line screen pitch B at a predetermined screen angle θ with respect to each color component halftone image is set to be substantially equal in size as appropriate for each color component. Therefore, for example, as shown in FIG. 3, when the line screen S of a certain color component halftone image G0 has a screen angle θ = 90 ° and a line screen pitch of A, the other color component halftone image G1 The line screen S has a screen angle θ = θ1 ≠ 9
If 0 ° and the line screen pitch B ≒ A, even if the image writing position of the corresponding color component by the monochrome image forming unit 1 is shifted, these color component halftone images are uniformly shifted. Since they do not overlap in the state, and originally overlap in a randomized state, the color tone due to the overlap of the color component halftone images is kept good, and the resolution of each color component halftone image is almost the same, so There is no situation where the halftone images of the color components are emphasized, and the halftone images of the color components overlap with the same degree of influence.

In such technical means, it is preferable that the angles of the line screens with respect to the halftone images of the respective color components are different from each other in order to maintain a good color tone.
Even though the angle of the line screen for the black component halftone image is set to be the same as the line screen for the other color component halftone images, the line screen angle for the other color component halftone images is set to be the same as each other. Compared to when
The degree of color change is small.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the embodiments shown in the accompanying drawings. Embodiment 1 FIG. 4 shows an embodiment of a full-color printer to which the present invention is applied. In FIG. 1, a full-color printer includes four single-color image forming units 10 (specifically, yellow image Y, magenta image M, cyan image C, and black image K) for forming each color component image of a full-color image (specifically, yellow image Y, magenta image M, cyan image C, and black image K). 10a to 10d), and a sheet transport belt 20 for sequentially transporting the recording sheet 25 to the image transfer portion of each monochrome image forming unit 10 at a predetermined timing.

In this embodiment, each of the single-color image forming units 10 includes a photosensitive drum 11, a charger 12 for pre-charging the photosensitive drum 11, and a halftone image for each color component on the pre-charged photosensitive drum 11. A laser 13 for writing a latent image of each color, a developing device 14 of each color system for developing a latent image of each color component formed on the photosensitive drum 11 with toner corresponding to each color component, and a toner formed on the photosensitive drum 11 The image forming apparatus includes a transfer device 15 for sequentially transferring an image to a recording sheet 25 that has been conveyed, and a cleaner 16 for removing residual toner on the photosensitive drum 11.

The sheet conveying belt 20 is stretched between a driving roll 21 and a driven roll 22 and sequentially moves in accordance with the driving rotation of the driving roll 21. The single color image forming unit 1 of the color components is sequentially transported while being electrostatically attracted.
0, the unfixed toner image on the recording sheet 25 is fixed by the fixing device 26 and the discharge tray 2
7 is discharged to the side.

Further, in this embodiment, reference numeral 30
(Specifically, 30a to 30d) is a laser controller that modulates the pulse width of the laser beam from the laser 13 according to the image information of each color component, and reference numeral 40 (specifically, 40a to 40d) denotes image data generation. This is a screen generator that outputs image data for each color component sequentially sent from the device 50, by performing line screen processing.

FIG. 5 shows a specific configuration of the screen generator 40 used in this embodiment. In the figure,
Reference numeral 41 denotes a digital-to-analog (D / A) converter for converting halftone digital image data sent from the image data generation device 50 into an analog image signal, and reference numeral 42 removes a high frequency component of the signal from the D / A converter 41. The filter 43 synchronizes with a main scanning synchronization (SOS) signal indicating a writing start timing in the main scanning direction and a page synchronization (PS) signal indicating a writing start timing of one page, and a reference clock of a line screen for each color component. A line clock generating circuit for generating a triangular wave in synchronism with the line clock from the line clock generating circuit 43;
A comparator 45 compares the halftone image signal passed through the filter 42 with the triangular wave from the triangular wave generation circuit 44 and extracts a line screen image signal having a pulse width corresponding to the magnitude of the halftone image signal. In this embodiment, a triangular wave is used to form a line screen. However, the present invention is not limited to this, and a sine wave or a signal having a waveform similar thereto may be generated.

In particular, in this embodiment, each screen generator 40 has a raster pitch in the sub-scanning direction of 400 lines / line.
An image signal is output based on a line screen equivalent to inches. The timing of generating a line clock from the line clock generation circuit 43 differs for each screen component 40 of each color component. Are set so that the screen angles are different from each other while the screen pitches are substantially equal.

That is, in this embodiment, the screen generator 40a for the monochrome image forming unit 10a for generating the yellow image (Y) has a screen angle of 4 ° when the main scanning direction is 0 ° as shown in FIG.
At 5 °, a line screen having a screen pitch of 188 lines / inch is set, and the screen generator 40b for the single-color image forming unit 10b that generates the magenta image (M), as shown in FIG. The screen angle is 6 when the main scanning direction is 0 °.
A line screen with a screen pitch of 224 lines / inch at 3.5 ° is set, and a cyan image (C) is set.
As shown in FIG. 8, in the screen generator 40c for the single-color image forming unit 10c that generates, the screen angle is −63.5 when the main scanning direction is 0 °.
In ゜, a line screen having a screen pitch of 224 lines / inch is set. In the screen generator 40d for the single-color image forming unit 10d that generates the black image (K), as shown in FIG. A line screen is set at 90 ° when the main scanning direction is 0 ° and the screen pitch is 200 lines / inch.

More specifically, the line screen for the black image (K) is a commonly used 200-line screen.
It has a basic structure of 0 °.

The line screen for the yellow image (Y) has a line width of 133 lines 90 ° (a pitch width 1.5 times as large as a line line screen of 200 lines). ), And the line screen for each line is shifted by 万 of the line screen pitch of 133 lines 90 ° (in other words, it is equivalent to の of the 200 line line screen pitch of the basic structure), and rasterized. The scanning is performed, and as a result, a line screen of 188 lines 45 ° is obtained.

Further, the line screen for the magenta image (M) has a line screen which is a source in the main scanning direction.
The basic structure of the line 90 ° is used, and the line screen for each line is shifted by １ ／ of the line screen pitch of the basic structure, and raster scanning is performed.
As a result, a line screen having the number of lines of 224 lines and 63.5 ° and the screen angle is obtained.

Further, the line screen of the cyan image (C) is a line screen which is the source in the main scanning direction.
The basic structure of the line 90 ° is used, and the line screen of each line is shifted by ／ of the line screen pitch of the basic structure, and raster scanning is performed.
As a result, a line screen having 224 lines, a line number of -63.5 ° and a screen angle is obtained.

When the halftone images of the respective color components were superimposed on each other using such a line screen, a color image having extremely good hue and gradation was obtained. In this case, the number of lines of the line screen for the yellow image (Y) is 188, and the number of lines of the line screen for the magenta image (M) and the cyan image (C) is 224. Although it is reduced or increased by about 10% from several 200, both are obtained as new line screens having a line number close to the optimal line number of 200 lines, and no large image deterioration phenomenon is observed at all. Was.

Here, as a comparative example, for example, as the line screen for the yellow image (Y), the line screen for each line is set to 1/1 of the line screen pitch of the basic structure.
Raster scanning was performed by shifting by two. In this comparative example, the line screen for the yellow image (Y) is obtained as a line screen having 282 lines and 45 ° as shown in FIG. The number of lines increases by more than 40%, and the number of lines increases significantly. As a result, image deterioration (gradation deterioration and deterioration of graininess) due to the increase in number of lines occurs. Also, in this comparative example, since the shift amount of 上 記 is just half of the line screen cycle of the basic structure, the resulting screen structure is not a line but a dot. Would. Usually, there is a great difference in the image quality (texture, graininess, etc.) of the reproduced image between a line screen having a line structure and a halftone dot having a dot structure. It has been confirmed that the use of such a method has many inconveniences, and furthermore, the deterioration of gradation such as tone jump occurs.

Next, a specific configuration of the line clock generation circuit 43 of the screen generator 40 will be described. FIG.
Shows a line clock generation circuit 43 of the screen generator 40a for yellow images. In the figure, the line counter 431 counts the main scanning synchronization (SOS) signal after the page synchronization (PS) signal becomes valid,
The 2-bit signal of _{L a0, L a1 [0,0]} , [0,
1], [1,0], [0,0], [0,1], [1,
0]... Are repeated every three lines.

The 133-line rotation clock generator 432 outputs L _a0 , L a when the SOS signal is input.
_If the two bits of _a1 are [0,0], 0, if [0,1], 2
If [1,0], an offset amount of 4 is set in the counter circuit, and a clock (2-BC) having a frequency twice as high as the synchronization signal BCLK for each basic pixel corresponding to 400 lines (400 SPI) is used.
LK) is divided by 6. By such processing, a rotation line clock which is shifted by one BCLK clock for each line and divided by three is obtained. FIG. 12 shows a timing chart of the above operation.

FIG. 13 shows a timing chart of the screen generator 40 in which such a line clock generation circuit 43 is employed. In the figure, as the lines proceed from the start of the page to the first line, the second line, the third line, the fourth line,... Is formed on the basis of, and when an analog signal corresponding to the triangular wave and the image data is input to the comparator 45, the above-mentioned line screen image of 188 lines 45 ° is obtained. In FIG. 11, the 133 / -133 switching signal input to the line counter 431 is a signal for selecting whether to set the screen angle of the 133 line to + 45 ° or −45 °.

Further, parallel line clock generating circuit in the screen Jefferies Ne <br/> regulator 40b for magenta image 43 1
1, but different from those for yellow images, as shown in FIG.
7, two-bit signals of _{La 0} and La ₁ are [0, 0],
[0,1], [1,0], [1,1], [0,0],
[1, 0]... Are output in a 4-line cycle, while the 200-line rotation clock generator 438 generates L _a0 ,
0 if 2 bits are [0,0] of L _a1, [0, 1] If 1, 2 if [1,0], and set in the counter circuit offset amount of 3 if [1,1], 400 lines ( 400SP
I) A clock (2-BCLK) that is twice as large as the synchronization signal BCLK for each basic pixel is divided by four. By such processing, a rotation line clock which is shifted by 1/2 clock of BCLK for each line and divided by 4 is obtained. In FIG. 14, the 200 / −200 switching signal input to the line counter 437 is a signal for selecting whether to set the screen angle of 200 lines to + 63.5 ° or −63.5 °. It is. Further, the line clock generation circuit 43 in the screen generator 40c for cyan image is composed of the same elements as in FIG. 14, and the 200 / −200 switching signal changes the screen angle of 200 lines to −63.5 °. What is necessary is just to set it.

Embodiment 2 This embodiment has basically the same construction as that of Embodiment 1, but differs from Embodiment 1 in that the number of lines of the yellow image line screen is changed to other The number of lines on the line screen for color component images is made closer to the line number, so that the difference in the number of lines on the line screen of each color component image is reduced to further improve the image quality. FIG. 15 shows the line clock generation circuit 43 of the yellow image screen generator 40a (see FIG. 4). In the figure, the line counter 439 counts the main scanning synchronization (SOS) signal after the page synchronization (PS) signal becomes valid, and _{La 0} , L _a0 , L
_a1 and _La2 are represented by [0, 0, 0], [0,
[0,1], [0,1,0], [0,1,1], [1,
[0,0], [0,0,0],.

The 160-line rotation clock generator 440 outputs L _a0 , L a when the SOS signal is input.
_a1, L 3 bit of _a2 is 0 if [0, 0, 0], [0,
[0,1] is 4, [0,1,0] is 8, [0,1,
1] is set to 12 and [1,0,0] is set to 16 in the counter circuit, and 400 lines (400 SPI) are set.
A clock (4-BCLK) having a frequency four times the frequency of the synchronization signal BCLK for each corresponding basic pixel is divided by ten. In other words, a clock (2-BC) having twice the frequency of the synchronization signal BCLK for each basic pixel corresponding to 400 lines (400 SPI)
LK) is divided by 5. By such processing, a rotation line clock which is shifted by one BCLK clock for each line and divided by 5 is obtained.

FIG. 16 shows a timing chart of the screen generator 43 in which such a line clock generation circuit 43 is employed. In the figure, as the lines proceed from the start of the page to the first line, the second line, the third line, the fourth line,... Is formed, and when an analog signal corresponding to the triangular wave and the image data is input to the comparator 45, a line screen image of 226 lines 45 ° as shown in FIG. 17 is obtained. Note that, in FIG.
The 160 / -160 switching signal input to 9 is 160
This is a signal for selecting whether to set the screen angle of the line to + 45 ° or −45 °.

Embodiment 3 In this embodiment, unlike Embodiments 1 and 2, the line screen pitch of each color component is set to, for example, 200 lines, which is the optimum number of lines, and the screen angle of each color component is made finer. It can be set. More specifically, as shown in FIG. 18, if the pitch of the line screen in the main scanning direction is A, and the pitch of the line screen rotated by the screen angle θ is B, there is a gap between the two. , A = B / cos θ. Therefore, in order to obtain a desired B, A should be selected so as to satisfy the above expression. A is determined by the cycle of the reference triangular wave of the screen generator, and the cycle of the triangular wave is determined by the frequency of the line clock from which the triangular wave is based. Therefore, a desired B can be obtained by appropriately selecting the frequency of the line clock. As a specific example, for example, with a line clock of 36 MHz,
Taking the case where a line screen equivalent to 200 lines 90 ° is realized as an example, for example, when the screen angle θ is to be set to 45 °, the frequency of the line clock is set to 2
The frequency may be set to 5.5 MHz.

Further, in order to obtain a desired screen angle θ, as shown in FIG. 18, it may be each time of the start position of Rasutasuki catcher two emissions <br/> grayed to constant weight shift . However, this shift amount is 1 / n of A (n is 2
It is necessary that the number of the positive
Shows an example in which n = 4.

FIG. 19 shows a specific configuration of the screen generator 40 according to the third embodiment. In FIG. 19,
The same components as those in the first embodiment are denoted by the same reference numerals as in the first embodiment, and the detailed description thereof will be omitted. In the figure, the line clock generation circuit 43 of the screen generator 40 differs from the first embodiment in that the main scanning synchronization (SO
S) A clock oscillator 433 that oscillates an individual clock for each color component in synchronization with the signal, a 2-bit counter 434 that is cleared by input of a page synchronization (PS) signal and counts in synchronization with the SOS signal, 4
The clock from the clock 33 is supplied to a predetermined delay time (t ₁ , 2t ₁ , 3
a delay circuit 435 for delaying the output by t ₁ ), and a clock directly sent from the clock oscillator 433 and one of the three clocks from the delay circuit 435 to output [L ₀ , L ₁ from the 2-bit counter 434. ] And a selector 436 selected in accordance with Then, if Re be inputted to the comparator 45 and the analog signal based on a triangular wave and the image data generated based on the parallel line clock generated by the line screen clock generating circuit 43, as shown in FIG. 18, the desired Of the screen pitch B (= corresponding to 200 lines) and a desired screen angle θ.

Embodiment 4 This embodiment is different from the above embodiments in that when the screen angle is set to 0 ° in the main scanning direction by the screen generator 40b for generating a magenta image (see FIG. 4).
A line screen with a screen pitch of 224 lines / inch is set at 3.5 °, and the screen angle is set to 0 ° in the main scanning direction by the screen generator 40c (see FIG. 4) for generating a cyan image. -63.5
In 、, a line screen having a screen pitch of 224 lines / inch is set, but when the screen angle is set to 0 ° in the main generator in the screen generators 40a and 40d for generating yellow and black images. 90
゜ is a line screen having a basic structure in which the screen pitch is 200 lines / inch. In this embodiment, the line screen angles for the yellow image and the black image match each other, but when any of the line screen angles for the yellow image, magenta image, and Ian image match, In comparison, it was confirmed that the hue of the color image was not extremely impaired.

In this embodiment, the line screen angles for the yellow image and the black image are made to match, but the line screen angles for the magenta image (or cyan image) and the black image are made to match. Also, it was confirmed that the color tone of the color image was not extremely impaired. However, it was confirmed that the one in which the line screen angles with respect to the yellow image and the black image were the same changed the color tone least. This is considered to be due to the fact that the yellow image has a smaller degree of visual influence than the magenta image or the cyan image.

[0043]

As described above, according to the first or second aspect of the present invention, at least the line screen angles in the halftone image of each color component other than the black component are made different from each other, and Since the line screen pitch for the halftone image was set to be substantially equal, and the color component halftone images were sequentially superimposed, the writing position of each color component halftone image between the single color image forming units was temporarily changed. Even if it is shifted, it is possible to homogenize the degree of overlap of the halftone images of each color component as a whole while maintaining the resolution of the halftone images of each color component mutually, and accordingly, the color reproducibility Can be maintained satisfactorily, and image deterioration such as a decrease in gradation can be effectively prevented.

In particular, according to the third aspect of the present invention, since the configuration is adopted in which the elements for generating the line screen of each color component are shared as much as possible, the configuration of the apparatus can be simplified.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a configuration of a full-color image forming apparatus according to the present invention.

FIG. 2 is an explanatory diagram showing the principle of the present invention.

FIG. 3 is an explanatory view showing a process of forming a full-color image according to the present invention.

FIG. 4 is an explanatory diagram showing a first embodiment of a full-color printer to which the present invention is applied.

FIG. 5 is an explanatory diagram illustrating a specific example of a screen generator used in the first embodiment.

FIG. 6 is an explanatory diagram illustrating a line screen for yellow images according to the first embodiment.

FIG. 7 is an explanatory diagram illustrating a line screen for a magenta image according to the first embodiment.

FIG. 8 is an explanatory diagram illustrating a line screen for a cyan image according to the first embodiment.

FIG. 9 is an explanatory diagram illustrating a line screen for a black image according to the first embodiment.

FIG. 10 is an explanatory diagram showing a line screen for yellow images according to a comparative example.

FIG. 11 is an explanatory diagram illustrating a specific configuration of a line clock generating circuit for yellow images according to the first embodiment;

FIG. 12 is a timing chart showing the operation of the line clock generation circuit of FIG. 11;

FIG. 13 is a timing chart showing the operation of the yellow image screen generator according to the first embodiment.

FIG. 14 is an explanatory diagram illustrating a specific configuration of a magenta image line clock generation circuit according to the first embodiment;

FIG. 15 is an explanatory diagram illustrating a specific configuration of a line clock generating circuit for yellow images according to the second embodiment;

FIG. 16 is a timing chart showing the operation of the yellow image screen generator according to the second embodiment.

FIG. 17 is an explanatory diagram illustrating a line screen for yellow images according to the second embodiment.

FIG. 18 is an explanatory diagram illustrating a line screen of each color component according to the third embodiment.

FIG. 19 is an explanatory diagram illustrating a specific configuration of a screen generator according to a third embodiment.

[Explanation of symbols]

1 (1a to 1d) ... single-color image forming unit, 2 (2a to 2d)
2d) ... line screen processing means, 3 (3a to 3d) ...
Screen writing control means, 4 (4a to 4d) ... screen frequency adjusting means

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-184136 (JP, A) JP-A-4-70164 (JP, A) JP-A-3-70267 (JP, A) JP-A-2- 244047 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04N 1/46-1/64 G06T 11/60 120

Claims (3)

(57) [Claims] A plurality of single-color image forming units (1: 1a to 1d) corresponding to respective color components of a full-color image;
In a full-color image forming method in which each color component halftone image is subjected to line screen processing in each single color image forming unit (1: 1a to 1d), and then each color component halftone image is sequentially superimposed, A full-color image forming method, wherein the line screen angles for color component halftone images other than the black component halftone image are different from each other, and the line screen pitch for each color component halftone image is set substantially equal. 2. A plurality of single-color image forming units (1: 1a to 1d) for forming each color component image corresponding to each color component of a full-color image, and a plurality of single-color image forming units (1: 1a to 1d). Line screen processing means (2: 2a to 2d) for providing halftone information for each color component image through line screens periodically arranged in the main scanning direction, and each single-color image forming unit ( In the full-color image forming apparatus in which the color component halftone images formed in 1: 1a to 1d) are sequentially superimposed, at least the writing timing of the line screen for the color component halftone image other than the black component halftone image is determined. Screen writing control means (3: 3a to 3d) for shifting by a fixed amount defined for each color component for each scanning line; Screen frequency adjusting means (4: 4a to 4d) for adjusting a line screen spatial frequency in the main scanning direction for each color component so that the line screen pitch for the component halftone image is substantially equal. Full-color image forming apparatus. 3. The monochromatic image forming unit according to claim 2, wherein the screen frequency adjusting means (4: 4a to 4d) sets a frequency at a screen angle of 90 ° as a standard frequency, and sets each of the monochromatic image forming units (1: 1a to 1d). ), A frequency lower than the standard frequency is generated in accordance with the screen angle, and the screen writing control means (3: 3a to 3d) adjusts the screen frequency adjusting means (4: 4a) of each monochromatic image forming unit (1: 1a to 1d). a full-color image forming apparatus, characterized in that the clock frequency made by ～4D) by the shift amount obtained by dividing a predetermined ratio is intended to delay.