JPS63214076A - Image forming device - Google Patents

Abstract

PURPOSE:To form an image excellent in gradation by correcting inputted multigradation image data by a correcting means and selecting one image data from the corrected data to form an output image. CONSTITUTION:An original 9 is read by a CCD 1, obtained analog image signal is amplified up to a prescribed level by an amplifier 2 and then converted into a digital signal by an A/D converter 3. The digital image signal is passed through gamma correction tables (gamma1-gamma3) 11-13 and one of the obtained the signals is selected by a selector 10 and applied to a D/A converter 14. The signal is converted again into an analog signal and the pulse width of the signal is modulated by a comparator 16 and a triangle wave generating circuit 15. The signal modulated at its pulse width is inputted to a laser driving circuit 17 to control ON/OFF of light emission of a laser diode 18. Consequqently, an electrostatic latent image is formed on a photosensitive drum 22.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image forming apparatus that forms an output image from an input image, and more particularly to an image forming apparatus that forms a halftone image.

[Prior Art] Well-known techniques for expressing halftone images include the dither method and the density pattern method. However, in either case, sufficient gradation cannot be obtained with a small-sized threshold matrix. Therefore,
It is conceivable to use a threshold matrix of a larger size, but in this case, there is a problem that the resolution will be extremely reduced.

On the other hand, the applicant of the present application has already proposed a method of improving gradation while maintaining high resolution using a relatively simple device configuration. This method involves first converting the digital image signal into an analog signal and then converting it to an analog signal in order to obtain halftone gradation when the digital image signal is binarized and image formed using a laser beam printer or the like. By comparing the signal with a periodic pattern signal such as a triangular wave, a pulse width modulated binary signal is generated, and this binary signal is used as a drive signal for a laser light source. be. At this time, the level of the analog signal and triangular wave, and the result of pulse width modulation result in 2
An example of a value conversion code is shown in FIG.

By pulse width modulating the digital image signal in this manner, it becomes possible to achieve both high resolution and high gradation. Of course, when performing such conversion, if an attempt is made to reproduce gradations faithful to the original, corrections must be made taking into account the input characteristics of the document reading system or the output density characteristics of the printer.

FIG. 7 is a main block diagram of an image forming apparatus including γ correction (gradation correction) based on the pulse width modulation method already proposed by the applicant of the present invention.

Optical image information reflected from a document (not shown) is transmitted to the CCD 7.
1 into an analog electrical signal. The analog electrical signal output from the CCD 71 is amplified to an appropriate level by an amplifier 72, and converted from an analog signal to a digital signal by an A/D converter. This digital signal is used by a gradation (γ) corrector 74 to correct gradation fluctuations that occur between image input and image output. In general, by using a ROM etc. that stores tone correction constants for the entire system, a specific input tone signal can be obtained by referring to a lookup table in which the tone correction constants at that time are written. It is converted into a gradation signal that has been subjected to a predetermined correction. The corrected digital image signal is again converted into an analog signal by the D/A converter 75 and compared with the triangular wave signal obtained by the triangular wave generating circuit 7. Reference numeral 76 designates a comparator for this purpose, and the output of the comparator 76 results in a binary image signal pulse width modulated according to the density. This binary image signal is input to the printer 78 and is used, for example, to control ON10FF of laser emission, and outputs an image with halftone expression. That is, a halftone image is formed by controlling the laser emission time.

By the way, the main factors that determine the γ characteristics are the input characteristics (i.e., the characteristics when converting the C0D 71 to an analog electrical signal) and the output characteristics (i.e., the characteristics when forming the final recorded image from the amount of pulse width modulation of the printer 78). concentration characteristics). The CCD 71 generally has a characteristic that shows a monotonous change with respect to the amount of light, and the printer 78 has various characteristics depending on the type of each device.

FIG. 8 shows a typical example of input characteristics and human output characteristics when using an electrophotographic laser beam printer, and an example of a γ correction table for this case.

For the concentration shown in the 1st I quadrant in the figure, CCD71
The concentration of has a characteristic shown in the first quadrant.

Therefore, in order to obtain output characteristics that are faithful to the input concentration, the first I
The γ correction shown in the I quadrant must be corrected. Therefore,
It can be seen that it is sufficient to provide the gradation (γ) corrector 4 in FIG. 7 with the correction table shown in the second quadrant.

[Problems to be solved by the invention] However, as can be seen from FIG. 8, in order to obtain the linear gradation shown in the 1st quadrant, the digital input shown in the - It was found that there is a factor that causes a large quantization error in the output characteristics, and that noticeable false contours occur, especially in light density regions. In other words, the slope of the digital input-output characteristic in the faint region in the second quadrant is extremely low, with a slope of 115 to 1/10.
It is in the position. For this reason, the gradation of the input image is, for example, 6
Even if there are 4 gradations, the number will drop to 12 gradations at the time of output, or even 6 gradations in the worst case. In other words, if the slope is 115, the input will change by 5 steps and the output will finally change by 1 step, so the gradation reproducibility will drop to 115 etc. due to this quantization error. There was a drawback.

The present invention has been made in view of these problems, and it is an object of the present invention to provide an image forming apparatus with good image reproducibility.

[Means for solving the problem] In order to solve this problem, the present invention has the configuration shown below.

That is, an input means for inputting multi-tone image data for each scanning line, a plurality of correction means for correcting the gradation level of the input multi-tone image data based on the characteristics of the input means, and corrected respective image forming means for forming an output image with the gradation of the selected image data.

[Function] In the configuration of the present invention, the correction means corrects each of the multi-tone image data inputted by the input means, and one image data is selected from among them by the selection means, and the selected image data is selected by the selection means. An output image is formed by an image forming means based on the data.

[Embodiments] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[Description of Main Structure (FIG. 1)] FIG. 1 is a block diagram of an image forming apparatus according to this embodiment. In this embodiment, a case where the image forming apparatus is applied to a laser beam printer will be described. That is, a case will be described in which a wire image is formed by scanning a laser beam on a photosensitive drum in synchronization with reading a document.

First, the original 9 is read by the CCD 1, and the obtained analog image signal is amplified to a predetermined level by the amplifier 2, and converted to a digital image signal by the A/D converter 3. Next, this digital image signal passes through the γ correction tables (γ violet to γ,) 11 to 13, and then the selector 1
0 selects one of them. This selection is
This is performed for each main scan, and the details will be described later.

Now, the digital image signal is γ1. γ2 and γ.

Although it passes through the γ correction tables 11 to 13 and is γ corrected,
The selector 10 selects one of these three (corrected) image data and outputs it to the D/A converter 14. D/
The A converter 14 converts the signal into an analog signal again, and the comparator 16 compares the signal with the signal generated from the triangular wave generating circuit 15 to perform pulse width modulation. This pulse width modulated 2
The digitized image signal is input as is to the laser drive circuit 17, and is used as a signal for controlling on/off of light emission of the laser diode 18. The laser light emitted from the laser diode 18 is scanned in the main scanning direction by a well-known polygon mirror 19, passes through an f/θ lens 20 and a reflection mirror 21, and is irradiated onto a photosensitive drum 22 rotating in the direction of the arrow. An electrostatic latent image will be formed. In this embodiment, the photosensitive drum 22 is an a-sipf4 optical drum whose potential is stable against changes over time, and the exposure device 28
After being uniformly neutralized by the charger 23, it is uniformly charged positively. Thereafter, it receives the aforementioned laser light to form an electrostatic latent image on the surface in accordance with the image signal. Further, in this embodiment, the portion to be developed (black pixels) is exposed.

Since the so-called image scanning method is used, the developing device 24
Now, by a well-known reversal development method, toner having a positive charge characteristic is adhered to the portion of the photosensitive drum 22 that has been neutralized by the laser, and this is visualized. Then, the developed image (toner image having a positive charge) formed on the photosensitive drum 22 is transferred onto a transfer material (generally paper is used) 26 by a transfer charger 25 with a negative corona charge. Also, due to transfer efficiency, some parts may not be transferred to the photosensitive drum 2.
The residual toner remaining on the toner 2 is then scraped off by the cleaner 27, and the above-described series of processes are repeated again.

[Description of γ correction table (FIGS. 2(a), (b), FIG. 3)] Next, the functions of the γ correction tables 11 to 13 in FIG. 1 will be explained.

FIGS. 2(a) and 2(b) show comparison diagrams between the case where there is one γ correction table in the conventional example and the case where there are three γ correction tables according to this embodiment.

In the figure, the horizontal direction is the main scanning direction (laser scanning), and the vertical direction is the sub-scanning direction (rotation of the photosensitive drum). Furthermore, the second
The figure is for explaining the effect of this embodiment in principle, and is described based on electrical signals, so it is different from the electrostatic latent image and developed image formed on the photosensitive drum in the embodiment. . This is because the laser drive circuit, laser response, laser spot diameter, MTF of the photosensitive drum, etc. are involved when forming a latent image, and the toner particle size, development characteristics (halftone reproducibility), edge This is because effects, etc. are involved. The reason why the final density reproducibility is significantly different from the shaded area in Figure 2 is that the output characteristics of the printer shown in quadrant 1II in Figure 8 have a complex nonlinear shape. It can be seen from

Now, FIG. 2(a) shows a case where relatively light density is reproduced using the single γ correction table 74 explained in FIG. 7 of the conventional example. The size of one dot is indicated by a dotted line. Further, as shown in FIG. 6, the triangular wave for comparison has a cycle of 3 dots, and therefore forms 3 horizontally long pixels in the main scanning direction. On the other hand, FIG. 2(b) shows a halftone with the same density as FIG. 2(a) formed by the configuration of this embodiment, in which γ1 to γ are repeated in the main scanning direction, γ2 is large area, γ2 is , and γ are the same and have a smaller area than γ2.

Macroscopically, the density values are the same as in Figure 2 (a), but microscopically, the areas of the shaded areas (i.e., black pixels) in Figures 2 (a) and (b) are not necessarily the same. No. This is due to the non-linear output characteristics of the printer as described above.

Referring to the digital input-output characteristics in quadrant (2) of FIG. 8, it can be seen that in the halftone region of light density, the output information is extremely small relative to the input signal. In other words, this shows that in the method shown in FIG. 2(a), false contours are likely to occur at light densities. On the other hand, Figure 2 (
In b), the correction value of the correction table for γ2 is applied to other γ1. γ
As the concentration is higher than the part 3, it appears to be 3 vertically and horizontally.
This is to give the pixel center when a x3 pixel unit is formed. Therefore, as is clear from Figure 2(b),
The black pixel formed by the γ2 correction table can be made larger than the black pixel in the main scanning direction in FIG. 2(a). Therefore, in the correction table for γ2, the linearity of the pale portion can be improved compared to the quadrant (2) of FIG.

FIG. 3 shows γ correction tables 11 to 13 in this embodiment.
It is a figure which shows the correction curve of. The solid line in the figure shows the correction curve (in the case of a single correction table) using the correction table 74 shown in FIG. I can see that it is. Also, at this time, γ1. γ3 has a lower slope in pale areas than conventional γ, but from a visual standpoint, reproducibility in the center of the pixel is particularly important for tone reproducibility in pale areas, and improving the linearity of γ2 improves image quality. This is most important for improvement. Ideally, it would be desirable to have a perfect straight line as shown in γ4, but in order to achieve this, the apparent matrix size should be 3 x 3 or more, for example, the number of types of γ should be further increased, or the triangular wave should be regressed. If you make the cycle longer,
It becomes possible. However, in this case, the resolution tends to decrease, so it is desirable to set the apparent matrix size to a maximum of about 4×4 matrix.

However, if there is a printer with higher resolution than the current image reproduction, the matrix size is not limited to this.

[Explanation of other γ corrections (Figures 4 and 5)] In the above embodiment, γ2 is used as a correction table for forming the pixel center, and γ
1 and γ are used as correction tables with the same content to reduce the total quantization error of the input and output of γ2, but it goes without saying that γ1 and γ do not necessarily have the same content. .

In this example, in order to mainly improve the reproducibility on the low density side, γ1. Although correction was performed such that γ is always at a low density, depending on the characteristics of the printer, the density may rise rapidly in the high density region and eventually reach saturation.

In such a case, on the high concentration side, γ1.
Conversely, setting γ3 to a high concentration reduces the quantization error. That is, γ2 and γ1. If γ3 is reversed in a relatively high density region as shown in FIG. 5, it becomes possible to obtain good reproducibility in all density regions. On the image, light areas become halftone dots, and dark areas become white halftone dots.

FIG. 4 shows an example in which the writing start position in the main scanning direction is further arbitrarily shifted to create a diagonal screen effect, which has the effect of making it even more difficult to visually see the pixel repetition period. In order to do this, it is sufficient to simply delay by, for example, 1.5 dots every three main scans, so the explanation will be omitted.

As explained above, according to this embodiment, a plurality of different gradation correction tables are provided for each main scanning line, and the gradation correction table is set to maximize the density of the output image relative to the input image. By providing a tone correction table as a correction table for pixel center formation and setting other tables in a direction that reduces the tone error of the input/output data of the pixel center formation table, the pseudo ring that has been a problem in the past can be solved. This makes it possible to significantly improve the reproducibility of halftones.

In addition, in this embodiment, the output pixels are determined by switching the correction table for each scan number of the input image, but if the printing device itself is capable of outputting higher resolution, for example, the image for one line can be changed. There is no problem even if the image is formed by scanning the output three times based on the image.

Furthermore, in this embodiment, an example was shown in which a laser beam printer was used as the image forming device, but for example, an ED array in which a large number of microscopic light emitting diodes (LEDs) are arranged is used, and each LED in this array is connected to a modulated signal. The gist of this embodiment can be applied as it is to an image forming apparatus (LED printer) that forms an image by exposing an electrophotographic photoreceptor to light with blinking control corresponding to .

Although this embodiment has been described in the case of a reversal development method using an image scan, this embodiment can be applied in exactly the same way to a normal development method using a background scan in which a white part of the background is scanned with light. It is. Furthermore, it goes without saying that the same effects can be achieved in other types of printers, such as thermal transfer thermal printers, by controlling the heat generation time of the head by pulse width modulation. That is, the present invention can be applied to all printers capable of area modulation.

[Effects of the Invention] As explained above, according to the present invention, when correcting an input image, an image is formed with corrections having different characteristics at any time, thereby forming an image with excellent gradation. becomes possible.

[Brief explanation of the drawing]

Fig. 1 is a block diagram for explaining one embodiment of the present invention, and Fig. 2 (a) and (b) compare the image formation previously proposed by the applicant and the image formation according to this embodiment. FIG. 3 is a diagram showing the curve of the γ correction table according to this embodiment. FIG. 4 is a diagram showing an image formed by an application example of this embodiment. FIG. 5 is a diagram showing other images in this embodiment. FIG. 6 is a diagram showing the generation principle of binary signal generation by pulse width modulation. FIG. 7 is a diagram showing an outline of the configuration of a device previously proposed by the applicant of the present application. FIG. 8 is a diagram showing an example of input characteristics and human output characteristics when using an electrophotographic laser beam printer. In the figure, 1,71...CCD, 2.72...amplifier,
3.73... A/D converter, 9... Original, 10...
・Selector, 11 to 13.74...γ conversion table,
14.75... D/A converter, 15.77... Triangular wave generation circuit, 16.76... Comparator, 17... Laser drive circuit, 18... Laser diode, 19...
Polygon mirror, 20...f/θ lens, 21...
Reflection mirror, 22... Photosensitive drum, 23... Charger, 24... Developer, 25... Transfer charger, 26...
- Transfer material, 27...Cleaner, 28...Exposure lamp, 78...Printer. 2nd I! I (0) Figure 2 (b) =ll (Town Figure 5 i+-+:, X to ■8 200 sting pressure

Claims (5)

[Claims] (1) an input means for inputting multi-tone image data for each scanning line; and a plurality of correction means for correcting the gradation level of the input multi-tone image data based on the characteristics of the input means;
An image forming apparatus comprising: a selecting means for selecting one of each corrected image data; and an image forming means for forming an output image with the gradation of the selected image data. (2) The image forming apparatus according to claim 1, wherein the input means optically inputs an image from a document. (3) The correction means are γ correction means, one of the γ correction means is used to form a center image, and the other γ correction means are used to reduce quantization errors of the center image. An image forming apparatus according to claim 1, characterized in that: (4) The selection means switches the selection object for each scan line when forming an image by the image forming means or for each scan line input by the input means. Image forming device. (5) A patent claim characterized in that the image forming means pulse width modulates the gradation in the image data selected by the selection means, and forms output pixels with this modulated pulse width. The image forming apparatus according to item 1.