JPH04330861A - Image forming device - Google Patents

Abstract

PURPOSE:To make a density difference between an original and a second generation copied object small as much as possible even when the gradation reproduction suitable for an original image is realized and the copied object is prepared again based on the copied object of an original. CONSTITUTION:The digital image data equivalent to the original read by a CCD 1 are stored into a page memory 11 at an image unit, the stored image data are read by a CPU 12 and a histogram is prepared. By using a LOG conversion table to prepare the density data which can reproduce the gradation of a high density part from the prepared histrogram to the original with many high density parts, an image is formed and to the comparatively light original, by using the LOG conversion table to prepare the density data to guarantee the gradation reproduction up to the maximum density of the original even when the high density part cannot be reproduced, the image is formed, and thus, the gradation reproduction suitable for the original image can be always realized.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and particularly to an image forming apparatus that reproduces halftones.

0002

2. Description of the Related Art Conventionally, a copying machine equipped with a laser beam printer is well known as this main apparatus. Some laser beam printers perform pulse width modulation on an image signal using a reference signal (for example, a triangular wave signal), and reproduce tone by making the obtained pulse width correspond to the laser emission time.

[0003]Furthermore, in recent years, full-color images have been increasing not only in the printing and design fields but also in offices, etc., and color copying machines that faithfully read and output color originals have become widespread, but here too color tones have become more common. The above-mentioned laser beam printer using pulse width modulation is well known for faithfully reproducing images. FIG. 2 is a schematic configuration diagram of the image reading section. In the figure, 41 is a document, 42 is a document table glass on which the document 41 is placed, 43 is a lamp for illuminating the document, 44 is an imaging element array, 45 is an infrared cut filter,
1 is a contact type CCD sensor (CCD), 47 is a lamp 4
3. This is an optical system unit that integrally holds the CCD 1.

As described above, when the copy key (not shown) is pressed, the lamp 43 illuminates the original 41, and the reflected light from the original 41 is transmitted to the imaging element array 44 and the infrared cut filter 45.
The image is formed on the CCD 1 through the CCD 1. In this way, the optical system unit 47 is sub-scanned in the direction of the arrow, and the main-scanning and sub-scanning electrical signals from the CCD 1 are sent to the signal processing circuits shown in FIGS. 3-1 and 3-2.

FIGS. 3-1 and 3-2 are block diagrams showing the configuration of an image forming apparatus. As shown in the figure, a signal from a CCD 1 is amplified by an amplifier 2 and converted into digital image data by an A/D conversion circuit 3. The image data is then converted into image (density) data by the LOG conversion circuit 29, subjected to various image processing by the image processing circuit 30, and then input to the D/A converter 14. This digital signal is again converted into an analog signal, and is compared with a signal of a predetermined period generated from the triangular wave generation circuit 15 by a comparator 16, and is pulse width modulated. This pulse width modulated binary image signal is input to the laser drive circuit 17,
It 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 delivered to a photosensitive drum 22, which is an image carrier, and rotates in the direction of the arrow. will be irradiated onto the surface, forming an electrostatic latent image.

On the other hand, after the photosensitive drum 22 is uniformly neutralized by an exposure device 28, it is uniformly negatively charged by a charger 23. Thereafter, it receives the aforementioned laser beam to form an electrostatic latent image on the surface according to the image signal. In addition, as a developing method often used in laser beam printers,
Since the so-called image scanning method is performed in which the portion to be developed (black pixels) is exposed, the developing device 24 uses a well-known reversal development method to transfer toner having negative charge characteristics to the portion of the photosensitive drum 22 that has been neutralized by the laser. is attached and visualized.

The developed image (toner image having a negative charge) formed on the photosensitive drum 22 by the above method is transferred onto a transfer material (generally paper) 26 by a transfer charger 25. The visible toner image transferred to the transfer material 26 is fixed by a fixing device (not shown). Further, the residual toner remaining on the photosensitive drum 22 is then scraped off by the cleaner 27, and the above-described series of processes are repeated again.

Conventionally, in the above-mentioned image forming apparatus, LOG
The conversion circuit 29 performs digital conversion using a look up table method. For example, two types of tables are prepared and switching is performed between text mode, photo mode, etc.

[0009]

However, in the conventional example described above, the user has to manually switch the LOG conversion table that determines the output density while looking at the document, which is inconvenient. Furthermore, in the case of photographic originals, the maximum density differed greatly depending on whether the original was a silver halide photograph or a print.

[0010] Even in silver halide photography, there are various types, such as completely dark photographs and bright photographs with almost no high density, and it is difficult to select the mode each time. Also, appropriate LOG
If no conversion table is selected, the following problems will occur. In other words, a dark photo original with many areas with a reflection density of 1.7 to 2.0 or higher is set to LOG 1.7 (density 0.05
If processing is performed using a table (reproducible up to 1.7), it will be impossible to reproduce the gradation of the portion with a density of 1.7 or higher, and the gradation will be distorted. Conversely, if you select LOG1.7 for a light original with a maximum density of about 1.0, the maximum copy density will be 1.0.
This results in an overall image with low density.

[0011] In the case of generational copying in which a copied image is used as a document to be copied again, the above problems are exacerbated with each generation, causing a significant deterioration in image quality. The present invention has been made to solve the above problems, and an object of the present invention is to provide an image forming apparatus that can realize gradation reproduction suitable for an original image.

0012

Means for Solving the Problems and Operations In order to achieve the above object, an image forming apparatus of the present invention has the following configuration. That is, in an image forming apparatus that converts luminance information based on an original image into density information and performs image formation, a generation means for generating shading information of the original image and a frequency distribution are created based on the shading information in the generation means. The method is characterized in that the processing coefficient of the converting means is controlled according to the distribution state of the frequency distribution prior to image formation.

[0013] Furthermore, the distribution state of the frequency distribution is characterized in that frequencies in a high density output level range are extracted.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will be described in detail below with reference to the accompanying drawings. FIGS. 1-1 and 1-2 are block diagrams showing the configuration of the image forming apparatus in the embodiment, and explanations of the same numbers as in FIGS. 3-1 and 3-2 will be omitted. In the figure, numeral 11 is a page memory, which stores digital data after reading the original. Also, the page memory 11 transfers digital data to the CPU 12 under the control of the CPU 12, or
It is possible to switch whether to transfer the data to the OG conversion circuit 29 or not. 13
is a RAM that stores the calculation results of the CPU 12 and allows the CPU 12 to retrieve data as necessary.

In the configuration shown in FIG. 1-1, first, the CC
Digital image data corresponding to the document read by D1 is stored in the page memory 11 in units of images. Here, when the digital image data for each pixel is as shown in the schematic diagram shown in FIG. 4, for example, the image data is sequentially written in 8-bit units in the direction of the arrow at the address corresponding to each pixel on the page memory 11. It will be done.

Next, the CPU 12 reads out the above data and creates a histogram of the image data as shown in FIG. At this time, the CPU 12 performs arithmetic processing according to the flowchart shown in FIG. 6, and calculates the frequency S(n) of each nth level of digital data "0 to 255". First, in step S1 shown in FIG. 6, one pixel data (
n level) is read from the page memory 11, and in the following step S2, the frequency of that level is incremented.

Then, in step S3, it is determined whether or not the image data has been completed, and if it has not been completed, the above-mentioned processing is repeated. Note that the memory of the RAM 13 is used as necessary, and the final frequency distribution S(n) is also stored in the RAM. On the other hand, the LOG conversion circuit 29 converts the CCD sensor output, which is the luminance information of the original image, into density data, as shown in FIG. First, the CCD sensor output is amplified through the amplifier 2 and converted into 8-bit (0 to 255) digital data. At this time, data "0" is the output when the original illumination lamp is not lit (black), and data "255" is the output for paper (white) when the lamp is lit.

Next, the LOG conversion circuit 29 performs density conversion on this output to convert it into data corresponding to the density. Here, a LOG curve is used as the density conversion table. The concentration Dn can be expressed by the following formula.

[0019]

[Math 1]

[0020] Here, Db and Da are the maximum density and minimum density that can be reproduced, respectively. And Db,D
By selecting a, the density reproduction range can be changed. In the above-mentioned histogram, areas with low image data levels (for example, data 0 to 7) correspond to dark areas of the document, and if the data frequency in this area is high, there are many high-density areas in the document. I can say that it is.

A method for selecting an optimal LOG conversion table depending on the original will be described below with reference to the flowchart shown in FIG. The concentration Dx is expressed by the following formula. Here, x is image data.

[0022]

[Math 2]

When x=1, Dx =2.4 When x=4, Dx =1.8 When x=7, Dx =1.6 At this time, the histogram is divided into several groups depending on the image data. to find the cumulative frequency for each group (
Step S11). For example, n1 is the cumulative frequency of image data 0 to 4, and n is the cumulative frequency of image data 5 to 7.
2, n1 is the frequency of pixels with a density of 1.8 or more, n
2 represents the sum of frequencies of pixels with a density of 1.6 to 1.8.

Next, in step S12, the obtained n1
is compared with a predetermined threshold m1, and if n1 ≧ m1, L
OG table (1) is selected (step S13). However, if n1<m1, the process proceeds to the next step S13, where the obtained n2 is compared with the threshold value m2, and n2 ≧m2
If so, LOG table (2) is selected (step S15). However, if n2 < m2, LOG table (3) is selected (step S16) and image processing is performed.

[0025] Note that the LOG tables (1) to (
3) is set according to the data shown in FIG. 9 and stored in the ROM 31 in advance. That is, LOG
Table (1) is an image in which there are many parts with a density of 1.8 or more, and Da = 0.1, Db = 2.0 of the above-mentioned formula 1.
Set it to . LOG table (2) has few high-density parts of 1.8 or higher, but it is an image with many densities of about 1.6 to 1.8, and similarly, Da = 0.1, Db = 1.8
I'll leave it as that. Also, the LOG table (3) shows the maximum density 1.
It is a relatively pale image of 6 or less, and here Da = 0.
05, Db = 1.5.

Then, the CPU 12 retrieves an optimal LOG conversion table from the ROM 31 in accordance with the results of the flowchart shown in FIG. 8, and uses it for image formation. As explained above, according to the present embodiment, by using the gradation information of the original image as the output level and analyzing its frequency distribution, it is possible to form an image using a LOG conversion table suitable for the original image. .

That is, for an original with many high-density areas, an image is formed using a LOG conversion table that generates density data that can reproduce the gradation of the high-density areas.
For relatively light originals, by forming images using a LOG conversion table that generates density data that guarantees gradation reproduction up to the maximum density of the original even if high-density areas cannot be reproduced, the original can always be maintained. It is possible to achieve gradation reproduction suitable for images.

Furthermore, even when a copy is made again based on a copy of the original, there is an effect that the difference in density between the original and the second generation copy can be made as small as possible. This is clear from the gist of the present invention.

[0029]

[Other Embodiments] Next, another embodiment according to the present invention is shown in FIG.
This will be explained below with reference to the block diagrams shown in FIG. 0-1 and FIG. 10-2. In the figure, numeral 32 is a counter circuit, and the rest has the same configuration as in FIGS. 1-1 and 1-2 described above.

In the embodiment described above, the frequencies of all levels of digital data are stored, but in this embodiment, the frequencies of two groups, for example, levels 0 to 3 and levels 4 to 7, are accumulated. Therefore, the counter circuit 32 becomes as shown in FIG. In the figure, image data sent in 8 bits is first judged by a comparator 321, data corresponding to 0 to 3 sends an ON signal to counter 1, and 4
Data corresponding to 7 sends an ON signal to counter 2. Then, in response to this ON signal, two counters 3
22 performs addition.

In this way, C
Data corresponding to high density areas regarding CD output (for example,
0 to 3, 4 to 7) (add the results to n1, n2
After reading the document, the counter circuit 32 transfers the cumulative frequencies n1 and n2 to the CPU 12 under the control of the CPU 12. The following processing is similar to the processing in the embodiment described above.

[0032] This embodiment does not require the use of expensive page memory as in the above-mentioned embodiments, so there is an effect of cost reduction, and in terms of images, it is possible to obtain the same effect as the above-mentioned embodiments. can. <Modification> Next, a case where the present invention is applied to a color image forming apparatus will be described.

Note that the configuration of the image forming apparatus is not directly related to the essence of the present invention and will therefore be omitted. Similar to the embodiment described above, a CCD reads the original, but a color separation filter is provided in front of the CCD to read the color components of the original. At this time, frequency distributions of CCD outputs corresponding to red, green, and blue are created in the same manner as in the embodiment described above. FIGS. 12-1 to 12-3 are diagrams showing luminance data levels and frequency distributions in this example.

Next, as in the two embodiments described above, the cumulative frequency of each group corresponding to the high density portion is determined for each color. Then, the calculation result of the cumulative frequency of red is n1R,
The calculation results of n2R, green, and blue are each n1G.
, n2G, n1B, and n2B, and the average cumulative degree in each group is determined and set as n1 and n2.

[0035]

[Math 3]

The following processing is similar to that of the above-mentioned embodiment, and the CPU 12 extracts the optimum LOG conversion table for the document and performs image formation.

[0037]

[Effects of the Invention] As explained above, according to the present invention,
In an image forming apparatus that converts luminance information based on an original image into density information and performs image formation, a generation means for generating shading information of the original image, and a creation for creating a frequency distribution based on the shading information in the generation means. By controlling the processing coefficients of the converting means according to the distribution state of the frequency distribution prior to image formation, it is possible to realize gradation reproduction suitable for the original image.

[Brief explanation of the drawing]

[Figure 1-1],

FIG. 1-2 is a diagram showing the configuration of an image forming apparatus in this embodiment.

FIG. 2 is a diagram showing the configuration of an image reading section.

[Figure 3-1],

FIG. 3-2 is a diagram showing the configuration of a conventional image forming apparatus.

FIG. 4 is a schematic diagram showing the density distribution of a document in pixel units.

FIG. 5 is a diagram showing the relationship between frequency and CCD output level.

FIG. 6 is a flowchart showing a processing procedure for creating a frequency distribution.

FIG. 7 is a diagram showing a brightness-density conversion table (LOG conversion table).

FIG. 8 is a flowchart showing a processing procedure for selecting a LOG conversion table.

FIG. 9 is a diagram showing a LOG conversion table.

[Figure 10-1],

FIG. 10-2 is a diagram showing the configuration of an image forming apparatus in another embodiment.

FIG. 11 is a diagram showing a counter circuit.

[Figure 12-1],

[Figure 12-2],

[Figure 12-3] R, G, when reading a color original
B is the frequency with respect to the CCD output level of each color.

[Explanation of symbols]

1 CCD 11 Page memory 12 CPU 13 RAM 29 LOG conversion circuit 30 Image processing circuit 31 ROM 32 Counter

Claims (2)

[Claims] Claims: 1. An image forming apparatus that converts brightness information based on an original image into density information and performs image formation, comprising: a generating means for generating grayscale information of the original image; What is claimed is: 1. An image forming apparatus comprising: a generating means for generating a distribution; and prior to image formation, processing coefficients of the converting means are controlled according to a distribution state of the frequency distribution. 2. The image forming apparatus according to claim 1, wherein the distribution state of the frequency distribution is to extract frequencies in a high density output level range.