JPH0439059B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a copy density control device that detects the density of a document and controls copy density.

(Prior Art) Generally, it is desirable that the background color of a copy made by a copying machine or the like is the same as the background color of the copy paper (generally white), regardless of the background color of the original. but,
For example, if the copy density is adjusted so that the background color of the copy becomes white for an original with a relatively dark background color, the quality of the copied image deteriorates when copying an original with a pale image such as pencil drawings.

Many of today's copying machines have built-in manual density control devices, and in cases like the one mentioned above, the operator has to make trial copies and adjust the density to the optimum density, or rely on experience and intuition to make adjustments. However, I was unable to prevent unnecessary copying.

In addition, in recent years, copying machines equipped with automatic document transfer devices (ADF devices) have rapidly become popular in order to speed up and improve the efficiency of copying operations. It is necessary to operate the density control device quickly and frequently each time the density condition of the original changes. However, such an operation is nearly impossible in reality.

Therefore, various copy density control devices have been proposed that detect the density of the original and automatically adjust the copy density (Japanese Patent Laid-Open No. 53-93834, etc.). However, in these methods, the density conditions are controlled from the maximum and minimum density values, so for originals that have an image darker than the most necessary image density, such as a pencil original with printed ruled lines, the printed ruled lines are There was an inconvenience that the concentration conditions were adjusted.

Furthermore, as an improvement, () the density distribution of the original is sampled at a constant cycle, and the maximum and minimum density values are also sampled, and the frequency distribution for the density values is obtained by mixing the two. Those that utilize the statistical properties of distribution to determine concentration conditions (Japanese Patent Application Laid-open No. 57-45564),
() Sampling the density distribution of the original at regular intervals, finding the frequency distribution function for density values, and smoothing the maximum value in this distribution contributes to the determination of density conditions (Japanese Patent Application Laid-Open No. 1983-1999)
No. 89751) is proposed.

In the case of any of these () and (),
Because we use a constant periodic sampling method,
In order to obtain accurate character and background density, it is necessary to increase the sampling frequency. However, as the sampling frequency increases, the overall frequency increases, which requires an increase in the capacity of the data memory that stores the frequency. In addition, by sampling the middle part of the output waveform's rise and fall,
Apparently, there is a drawback that the intermediate density portion increases. In addition, in a method that uses a combination of constant period sampling and peak value sampling as shown in (), sampling is performed at a frequency that is 1/N (N is an integer greater than or equal to 2, for example) the sampling frequency at which the peak value is sampled. As a result, the output is as flat as the background (detected concentration signal).
Although this prevents the increase in frequency obtained from the waveform, it still has the drawback that it may sample the middle part of the rise or fall.

(Object of the Invention) The present invention eliminates such drawbacks, obtains accurate background density and character density, and obtains accurate background density and character density while saving data memory for storing frequencies for density values without increasing the sampling frequency. An object of the present invention is to provide a copy density control device that controls copy density.

(Structure of the Invention) The present invention achieves this object in a copy density control device that detects the density of a document and controls the copy density, and includes a document density detector that scans the document surface and detects the document density; A sampling circuit that samples the output of the original density detector and three consecutive sampling values of the sampling circuit are
−2, Xk−1, and Xk, the following conditions are satisfied: <Xk, and the relationship
(integer greater than or equal to) continues, and a concentration discrimination circuit that outputs a signal to count up the frequency of the concentration division of Xk-1 every time any of the following conditions is satisfied; A counting circuit that counts frequencies in a density category and a frequency distribution based on the frequency values counted by the counting circuit, depending on whether the frequency distribution on the low density side is shifted to the high density side or to the low density side. The shading of the background color is detected, and the shading of the image is detected based on whether the frequency distribution on the high density side shifts to the high density side or to the low density side. It is characterized by comprising an image quality control circuit that controls the developing section to lower the density to bring the background color closer to white, and for originals with pale images on white backgrounds, to increase the copy density to further emphasize the image. That is.

(Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram showing an embodiment of the present invention.
This shows the application to a copying machine equipped with an ADF device (the developing unit is a magnetic brush type). Therefore, this figure also shows components other than the copy density control device. In the figure, EX is the part other than the copy density control device, that is, the copying machine part,
In this EX, 1 is a photosensitive drum, and an electrostatic latent image formed on the photosensitive drum 1 by an exposure optical system 2 is developed in a developing section 3. Note that the copy density is adjusted by switching the bias voltage applied to the developing section 3. Also, the original 4 in the original stocking section 5 is
The ADF device 6 feeds the documents onto the document placement surface 7 one by one.

Reference numeral 8 denotes a document density detector for detecting the density of the document 4. The document density detector 8 is moved by the document density detector driving device 9 in a direction (main scanning direction) perpendicular to the transport direction (sub-scanning direction) of the document 4, and detects the density of the document 4. Detects background color and image reflectance. The output of the original density detector 8 is the average value of the amount of reflected light in the irradiation spot area, and if the spot diameter is relatively large compared to the density distribution of image information (for example, character line width), the output of the original density detector 8 is The average density value of the spot area shows a higher reflectance than the image area, making it impossible to faithfully capture the density distribution of the original. Therefore, it is necessary to reduce the diameter of the detection spot. Further, it is desirable that the output of the document density detector 8 changes linearly within the range of the density distribution of the document. The document density detector 8 used for this purpose is preferably a photodiode or the like from the viewpoint of ease of handling, but other types of document density detectors may be used.
When the ADF is not used, the original density detector 8 is connected to the optical system, for example, the first mirror portion to be scanned or the second mirror portion to be scanned.
It may be provided in a mirror section or the like to read the density of a document that is stationary or moving at a predetermined position.

10 is a sampling circuit, which is a document density detector 8;
The output is sampled at time intervals long enough to approximately capture the maximum and minimum levels. Reference numeral 11 denotes a concentration discriminator circuit which receives the output of the sampling circuit 10, and selects local maximum values, local minimum values, and concentration values that have been continuously output a predetermined number of times (for example, N times) from among the signals output from the sampling circuit 10. It discriminates which of the predetermined concentration categories these belong to and sends an output indicating which concentration category they belong to to the counting circuit 12. The counting circuit 12 receives the output of the density discrimination circuit 11, calculates the frequency of the density value in each density division, and constructs a density histogram based on the density divisions. This histogram has characteristics depending on the characteristics of the document, such as a document document with dark writing on a white background, a document with pencil writing on a relatively dark background color, etc. Of this density histogram, that is, the frequency distribution based on the frequency values counted by the counting circuit 12,
The frequency distribution (peak position, etc.) on the low-density side indicates the shading of the background color, so whether the frequency distribution on the low-density side shifts to the high-density side or to the low-density side determines the background color of the original. Can detect shades of color.
Also, since the frequency distribution (peak position, etc.) on the high density side indicates the shading of the image, the shading of the image can be determined depending on whether the frequency distribution on the high density side has shifted to the high density side or to the low density side. Can be detected. Reference numeral 13 denotes an image quality control circuit that detects whether the document has a dark image on a dark background or a light image on a white background, and controls the developing section 3 according to the document. The magnetic brush bias adjuster 14 lowers the copy density for an original with a dark image on a dark background color to bring the background color closer to white, and increases the copy density for an original with a pale image on a white background to further emphasize the image. This is to control the bias voltage of the

The operation of the apparatus of the present invention having the above-mentioned structure will be clear from the explanation of the above-mentioned structure, and in particular, the operations of the concentration discrimination circuit 11 and the counting circuit 12 are shown in the flowchart of FIG. The meaning of each character in FIG. 2 is as follows.

Xk: Current sampling value Xk-1: One previous sampling value Xk-2: Two previous sampling value n: Number of repetitions of the same level N: Upper limit of number of repetitions of the same level Cmax: Saturation value of frequency Shown in this flowchart In operation, after reading the sampling value Xk, if Xk-1 is an extreme value or the same density value that has been output N times in a row, these are adopted as the accumulation target and a histogram is created. . That is, when the output of the original density detector 8 changes (the sampling value Xk changes), only its extreme values (maximum value and minimum value) are sampled. In a state where there is no change (Xk=Xk-1), the frequency obtained by sampling at a fixed period (the number of consecutive times of the same value) is compressed to 1/N and added to the frequency at that level.

Each step in FIG. 2 will now be explained in detail. Note that the entire device is initialized when the power is turned on. At the end of this initialization, the memories in the concentration discrimination circuit 11 and the counting circuit 12 are cleared, n is reset to 0, and the frequency in each concentration category is also reset to 0. In this state, the process according to the flowchart of FIG. 2 is started. In the process shown in FIG. 2, the k-th sampling value Xk sampled by the sampling circuit 10 is
1 shows the operation when reading is performed. First, read the sampling value Xk (step 1),
It is determined whether the k-1st sampling value Xk-1 is less than this, that is, whether Xk-1≦Xk (step 2). If Xk-1≦Xk, after resetting n (step 3), the process moves to step 4.
Conversely, if Xk-1≦Xk, proceed to step 5.

In step 4, it is determined whether Xk-2<Xk-1, and if this holds true, the process moves to step 11, where the counting circuit 12 counts up the frequency in the density category of Xk-1. Moving to step 11 via steps 2 and 4 means that Xk-1
Greater than Xk or Xk−2, that is, Xk
−1 means the maximum value. On the other hand, if Xk-2<Xk-1 does not hold in step 4, the process moves to step 14 without counting up the frequency.

In step 5, it is determined whether Xk-1 =
-1, and if Xk-2>Xk-1, the process moves to step 11, where the frequency in the density category of Xk-1 is counted up. Steps 5 and 7
Moving to step 11 via
That Xk−1 is smaller than Xk and Xk−2,
That is, it means that Xk-1 is the minimum value.
On the other hand, if Xk-2>Xk-1 does not hold in step 7, the process moves to step 14 without counting up the frequency.

Also, if Xk-1=Xk holds in step 5, after adding 1 to the value of n (step 8), n
Determine whether or not becomes N (step 9),
If n has not yet become N, proceed to step 14 without counting up the frequency. On the other hand, if n becomes N, after resetting n (step 10),
Proceeding to step 11, the frequency in the concentration category of Xk-1 is counted up. Moving to step 11 via steps 5, 8, and 9 means that for consecutive sampling values, even if you update the sampling values N times and compare Xk-1 and Xk,
That Xk−1 was the same as Xk, that is,
This means that the sampling value does not change. As is clear from the above explanation, the frequency count up in the concentration category of Xk-1 in this case is as follows:
It will be compressed to 1/N.

Steps 14 and 15 perform Xk-2 prior to returning to step 1 and reading new sampling values.
This is also a step for updating the data of Xk-1, and the previous Xk is set as the new Xk-1, and the previous Xk-1 is set as the new Xk-2.

Steps 3 and 6 are for counting up the frequency in the concentration category of Xk-1 only when Xk-1 = Xk continues N times, and before reaching N times, Xk-1≠Xk. , to reset n when proceeding to step 4 or step 7. Also, step 10 is
Prior to counting the next consecutive number of Xk-1=Xk, n
This is for resetting the .

Steps 12 and 13 are steps for preventing the frequency in the concentration category of Xk-1 from becoming excessively large; when the frequency exceeds a predetermined value, the frequency in that concentration category is fixed at Cmax.

When sampling in this way, the sample value counted by the counting circuit 12 is, for example, the third
As shown in the figure, the values are indicated by ○ marks (extreme values) and × marks (continuous values). Based on these values, a histogram like the solid line A in FIG. 4 is constructed. Note that if the output of the document density detector 8 is sampled at a constant period as in the conventional method, the rise of the output,
Sampling at the middle part of the fall results in an apparent increase in the middle density part of the histogram, and also an apparent increase in the frequency of the extreme values, as shown by the dotted line B in the histogram in Figure 4. distribution. On the other hand, even in a method in which fixed period sampling and peak sampling are performed together, intermediate density values appear to be mixed into the histogram, as shown by the dotted line C in FIG. 4.

The histogram obtained in this way shows a characteristic pattern depending on the type of copy document. As described above, the image quality control circuit 13 adjusts the copy density by applying a preset bias voltage to the magnetic brush according to such a characteristic pattern.

The characteristics of this histogram may be determined after removing the frequency Hn due to noise or the like from the histogram. In that case, as is clear from FIG. 4, the maximum density value decreases from _D1 to _D2 , but the difference is substantially negligible.

In the above embodiment, the bias voltage of the magnetic brush is adjusted as the density adjustment means, but it is also possible to control the density by adjusting the amount of lamp light, the amount of exposure aperture, the amount of charge on the photoreceptor drum, etc. . Further, although the above embodiment was applied to a copying machine having an ADF device, it can also be applied to a general copying machine, whether the type is a movable document table type or a fixed type. It is. Further, the judgment corresponding to FIG. 2 may be processed by software using a microcomputer.

(Effects of the Invention) As explained above, according to the present invention, the following effects can be obtained.

() Accurate character density, etc. can be obtained by sampling peak values, and copy density can be accurately controlled.

() For a background part where the level change is small, it is recognized that the detection level does not change over time and is treated as the background density, so accurate background density detection can be performed and accurate copy density control can be performed.

() In the above (), based on the fact that the frequency of the background level is originally high, if the same level continues N times, it is stored as one, so the memory can be saved without degrading the detection performance.

[Brief explanation of drawings]

Figure 1 is a conceptual diagram showing one embodiment of the present invention, Figure 2 is a conceptual diagram showing an embodiment of the present invention.
FIG. 3 is a flowchart showing the procedure of signal processing and determination, FIG. 3 is an explanatory diagram of sampling, and FIG. 4 is an explanatory diagram showing an example of a histogram. 1... Photosensitive drum, 2... Exposure optical system, 3...
...Development section, 4...Document, 5...Document storage section,
6...ADF device, 7...Document placement surface, 8...Document density detector, 9...Document density detector drive device,
10... Sampling circuit, 11... Density discrimination circuit, 12... Counting circuit, 13... Image quality control circuit, 14... Bias adjuster.

Claims (1)

[Scope of Claims] 1. A copy density control device that detects the density of an original and controls copy density, comprising: an original density detector that scans the original surface and detects the original density; and an output of the original density detector. When the sampling circuit to sample and the three consecutive sampling values of the sampling circuit are defined as Xk-2, Xk-1, and Xk,
The following conditions: Xk-2<Xk-1 and Xk-1>Xk, Xk-2>Xk-1 and Xk-1<Xk, The relationship is repeated N times (N is 2)
(an integer greater than or equal to) continues, and a concentration discrimination circuit that outputs a signal to count up the frequency of the concentration category of Xk-1 every time any of the following conditions is satisfied; A counting circuit that counts the frequency in the density category, and a frequency distribution based on the frequency values counted by the counting circuit, depending on whether the frequency distribution on the low density side has shifted to the high density side or the low density side. The shading of the background color is detected, and the shading of the image is detected based on whether the frequency distribution on the high density side shifts to the high density side or to the low density side. an image quality control circuit that controls a developing section to lower the density to bring the background color closer to white, and for an original with a pale image on a white background, to increase the copy density to further emphasize the image; Characteristic copy density control device.