JPS6161566A - Device for forming image - Google Patents

Abstract

PURPOSE:To obtain a stable device for forming images with uniform gradient by combining with a beam multi-values output with the modulation of the pulse width and a dither means and thickening a dot to the main scanning direction of a beam without thickening the dot to the opposite direction. CONSTITUTION:Image input data is inputted to a dither converting part 1 for the dither conversion so that plural gradient signals can be outputted by each one element of an image outputs. The output of the converting part 1 is inputted to a pulse width modulating circuit 2 and synchronized with a clock signal (a). When a dot is thickened with 1/2 and 1 dot widths corresponding to the dither matrixes of 1/2 and 1dot gradient conversions 1a, 1b respectively and to the main scanning direction, an image signal pulse-moduled and formed with (1+1/2) dot width. The dot is formed so as to be thickened hardly to the main scanning and the opposite direction. The output of the circuit 2 drives a laser 3, and a binary modulating beam is image-formed on a photosensitive drum 9 through a lens 4, revolutional polyhedron 5, lens 6. An reflected light from a mirror 7 is detected by an optical detector 8 to form a synchronous signal of the main scanning.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus using a gradation recording method that combines a dither conversion method and multi-value output using pulse width modulation.

[Prior Art] Digital printers often record gradations using a dithering method that outputs binary values of 1 and 0. However, this method has the effect that when trying to improve gradation, the resolution deteriorates.
A 4×4 dither matrix is required to provide 17 gradations including white, and an 8×8 dither matrix is required to provide 65 gradations. In this way, if the number of gradations is increased, the dither conversion area becomes wider and the resolution deteriorates. For this reason, a method is being considered that combines the dither method and single-pixel beam multivalue output to improve gradation without reducing resolution. According to this method, for example, in the case of a 4x4 dither matrix, only up to 17 gradations can be obtained with binary beam output, but if the beam is output with 3 levels at the same resolution, it will be up to 33 gradations, and with 4 levels of beam output, it will be up to 49 gradations. up to the key,
If you set the beam to 5-level output, you will be able to get up to 65 gradations.

On the other hand, dither patterns can be broadly classified into minute ITk type and dot concentration type. It is known that a dot concentrated type (for example, spiral type) has better gradation than a dispersed type (for example, Bayer type) for beam binary output. However, if a spiral dither pattern is used when a pulse width modulated beam multilevel output is adopted, the following problems will occur.As an example, a 4x4 dither matrix and a beam with a pulse width of 0.1/2.1 dots are used. Let's consider ternary output. Figure 2 is a diagram showing an example of a spiral dither pattern in relation to beam main scanning and sub-scanning. Sub-scanned. Further, in FIG. 3, a pulse width modulated beam ternary output signal is shown, in which a is a clock signal synchronized with pixel formation, b is an independent image signal modulated to 1/2 dot width, C is an image signal modulated into an independent one-dot width, d
For example, when the dots become thicker in the main scanning direction from 1 to 2 in the dither pattern in Figure 2, (1+1/2)
The image signal e modulated to the dot width also shows that the dot becomes thicker from 3 to 4 in the opposite direction to the main scanning direction.
This is an image signal modulated to a dot width of (1/2+1). This explanation follows the convention that the width of one pixel always increases by 172 dots from the left side along the main scan.

Figure 4 (a) to (C) are main scanning (X) and sub-scanning (Y)
In Figure 0, which is a diagram showing the energy distribution when the photoreceptor is main scanned with a laser beam spot approximated by a Gaussian distribution in the direction, signals b' to e' are the image signals b to e in Figure 3, respectively. This corresponds to the case where the photoreceptor is exposed. Now, when a photosensitive drum exposed with such an energy distribution is visualized with toner, (1+1/2)
The resulting print 7a degree is different between exposure using the dot width signal d and exposure using the (1/2+1) dot concentration type e. Moreover, if the beam power fluctuates or the various conditions related to the development process fluctuate, continuous (
It can be seen that exposure using the 1+1/2) dot width signal d is less affected by the resulting density than exposure using the discontinuous (1/2+1) dot width signal e. More specifically, the signals d' and e' in FIGS. 4(b) and (C)
In each case, the portion of the light energy above the visualization level d1 is visualized to form a dot. In this case, it is easy to understand that, for example, when the beam power is increased, the optical energy increases in proportion to it, the visualization level d1 relatively decreases to d2, and the dot becomes larger. In electrophotographic printers, such a state occurs not only due to variations in beam power but also due to variations in various conditions such as photoreceptor potential and developing bias. However, in any case, the variation in dot size, that is, the variation in density, is continuous (1+1/2). Exposure using the dot width signal d is better than exposure using the discontinuous (1/2+1) dot width signal e. I understand that it's small. Moreover, depending on the laser driver, there is a delay in the rise of light emission in response to the pulse drive signal, and in the case of exposure using the (1/2 + 1) dot width signal e, there are two rises, which increases the loss of the rise and increases instability. As the beam output is further increased to 3-value, 4-value, 5-value, etc., the number of gradations can be increased, but the instability of image quality increases. In this way, when a conventional pulse width modulated beam multilevel output and a spiral dither pattern are used together, the gradation is different when the dots get thicker in the main scanning direction and when the dots get thicker in the opposite direction. The latter has the disadvantage that image quality is easily affected by variations in beam energy and electrophotographic process conditions.

[Objective] The present invention has been made in view of the above-mentioned drawbacks of the prior art, and its object is to provide a stable image forming apparatus with uniform gradation.

[Example] Hereinafter, an example according to the present invention will be described in detail based on one drawing.

FIG. 1 is a block diagram showing the main configuration of a laser beam printer according to an embodiment of the present invention. In the figure, image input data is input to a dither converter 1 (this is input to an image output 1).
Dither conversion is performed to output a multi-tone signal for each pixel. That is, in the figure, 1a is a dither matrix for 1/2 dot gradation conversion, and 1b is a dither matrix for 1 dot gradation conversion.The gradation output of the 0th order dither conversion section 1 is input to the Nockles width modulation circuit 2. be done. Here, the gradation output of the dither converter l is synchronized with the clock signal a.Based on this, the image signal is pulse width modulated to a width of 172 dots,
An image signal pulse-width-modulated to a one-dot width, and an image signal pulse-width-modulated to a (1+1/2) dot width when the dot becomes thicker in the main scanning direction are formed.

However, the arrangement of the dither matrix prevents the formation of an image signal that is pulse width modulated to the (1/2+1) dot width when the dots become thicker in the direction opposite to the main scanning direction. Regarding this point, the image output signal of the zero-order pulse width modulation circuit 2, which will be described later, is
(fi drive. The binary modulated light beam from the semiconductor laser 3 is incident on the rotating polygon mirror 5 via the collimating lens 4, where it is deflected in the main scanning direction (arrow H), and the deflected light beam is focused. An image is formed on the photosensitive drum 9 by the image lens 6. At this time, the reflected light from the mirror 7 arranged at the leading end of the main scanning is detected by the photodetector 8, and a synchronizing signal for the main scanning is formed. At the same time, since the photosensitive drum 9 is rotated in the direction of arrow V, a latent image is formed on the surface of the drum 9 by main scanning and sub-scanning.

FIGS. 5(a) to 5(f) show that dots are dithered to become thicker in the beam scanning direction from the density center that should appear in the output image by repeated dithering along the main scanning and sub-scanning.
FIG. 6 is a diagram showing a dither pattern of an embodiment in which dither conversion is performed so as not to make dots thicker in the direction opposite to the beam scanning direction. The same figure (a) shows a pattern of dither conversion to 1/2 dot gradation in 3×3, and the same figure (b) shows the same pattern in 3×3 as 1/2 dot gradation.
This is a pattern for dither conversion to dot gradation. These two patterns form a Norculus width modulation signal with three beam values (0, 1/2, 1) output. Now, if we look at both <turns (a) and (b), we can see that the dot thickening direction l-1 is always to the right (scanning direction) and downward (sub-scanning direction). However, since dithering turns are repeated two-dimensionally along the main scanning and sub-scanning directions, if we look at the dither pattern centered around a certain output pixel, the patterns shown in (a) in the same figure will become different from those shown in (C) to (C) in the same figure, respectively. It looks like the pattern f). Therefore, the relationship that prevents the dots from becoming thicker in the opposite direction of beam scanning cannot be easily determined by simply observing how the dots in the matrix become thicker, and the same is true for the dither pattern shown in FIG. 3(b). That is, in this embodiment, the repetition of the dither pattern is also taken into consideration, so that the dots are not made substantially thicker in the opposite direction of beam scanning.

Figures 6(a) to (C) show the order of dots for other types of dither conversion, and show examples where it is prohibited to make dots thicker in the opposite direction of beam scanning. It is a diagram. The patterns in the same figure (・) are 2 and 3, 1 and 4
This pattern cannot be used because it thickens the dot in the opposite direction of the beam scan at points 1 and 3, which are the repetition points of the main scan. Although the pattern shown in FIG. 2C is thicker at the top at points 1 and 2, it can be used in this embodiment because it corresponds to the sub-scanning relationship.

Figures 7(a) and (b) show 1 in the case of three-level beam output, respectively.
2 is a diagram showing the order of dots for dither conversion between 2 dots and 1 dot. Note that the dots are placed in different orders in Figures (a) and (b); these 172 dots and 1 dots do not necessarily have to be placed in the same order, and 1/2 dots do not necessarily have to be placed in the same order. You can omit the dot level and hit one dot all at once, or you can hit multiple dots at the same time.

FIGS. 8(a) to 8(d) are diagrams showing the order of dots for 4×4 dither conversion. Of these, the same figure (a)
~ (C) shows the same pattern from a different perspective, but in the same way, just by following the thickening of the dots in the matrix, the dots do not get thicker in the opposite direction of beam scanning. It turns out that it is not easy to judge.

Furthermore, as a method to improve gradation without reducing resolution, there is a method of expressing gradation using multiple basic matrices.For example, use four 3x3 basic matrices as the unit of gradation expression, and use two on the diagonal. If the dots are placed one by one, the number of gradations will be twice as many as in the case of one 3×3 matrix.

FIG. 9 is a diagram showing an example in which the present invention is applied to this method in the order in which dots are placed. The unit in the dot nest related to resolution is a 3x3 basic matrix, and the unit for gradation expression is four 3x3 basic matrices. i.e. 6×6
This is the matrix of By placing two dots on this 6x6 matrix, the number of gradations becomes twice that of one 3x3 matrix. In this example, two dots were placed in a 6x6 matrix, but if dots were placed one at a time, the number of tones would be four times that of one 3x3 matrix, but the number of dots would be smaller. Texture patterns appear more easily in highlighted areas. -The present invention can also be realized as a thermal printer, an inkjet printer, etc.

[Effects] As described above, according to the present invention, when employing a gradation recording method that combines a dither method and beam multilevel output using pulse width modulation, the repetition of the dither pattern is taken into consideration, and the By making the dots thicker from the center of the density that should appear in the output image, and making the dots thicker only in the main scanning direction of the beam, as in the example, the gradation of the output image can be made uniform and significantly improved. It has the effect of Further, when an electrophotographic method is employed, it is effective to maintain stable gradation even in the face of fluctuations in process conditions due to changes in various operating environments. Furthermore, when the present invention is applied to a color printer, since the gradation of each color is uniform and stable, high-quality full-color prints with little variation in hue can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the main configuration of a laser beam printer according to an embodiment of the present invention, FIG. 2 is a diagram showing an example of a spiral dither pattern in relation to main scanning and sub-scanning of the beam, and FIG. 3 Figure 4 shows the pulse width modulated beam ternary output signal, and Figures 4 (a) to (C) are diagrams showing the energy distribution when the photoreceptor is main scanned with the main scanning (X) and sub scanning beam spots. , FIGS. 5(a) to (f) show that dots are dithered to become thicker in the beam scanning direction from the density center that should appear in the output image by repeated dithering along the main scanning and sub-scanning.
Figures 6(a) to 6(c) show a dither pattern of an embodiment in which dithering is performed so as not to thicken dots in the direction opposite to the beam scanning direction. Figures 7(a) and 7(b) show the order in which it is prohibited to thicken the dots in the opposite direction of beam scanning.
/2 Figure showing the order of dots for dot and 1-dot dither conversion, fJ18 Figures ('a) to (d) are diagrams showing the order of dots for 4X4 dither conversion, Figure 9 is the basic FIG. 7 is a diagram showing the order of dots in another embodiment of dither conversion in which gradation is expressed using a plurality of matrices. Here, 1... dither conversion unit, 2... pulse width modulation circuit, 3... semiconductor laser, 4... collimating lens, 5... rotating polygon mirror, 6... imaging lens, 7...
-Mirror, 8... photodetector, 9... photosensitive drum. Patent Applicant: Canon Co., Ltd. Figure 2 Support - k Figure 3 - Figure 4 □× □ Q) (b) Part 8 (C) Fig. 9 (d)

Claims (1)

[Claims] a dither converting means for dither converting an input image signal; a pulse width modulating means for outputting a multivalued image signal pulse width modulated for each output pixel according to the output of the dither converting means;
In an image forming apparatus including a driving means for driving a scanning output in two values according to a multivalued image signal output from the pulse width modulation means, the dither conversion means performs repetitive dither conversion to change the density in the scanning direction from the center of density to appear in the output image. An image forming apparatus characterized in that dither conversion is performed to make dots thicker in the first direction, and dither conversion is performed in a direction opposite to the scanning direction so as not to make dots thicker.