JP3077204B2 - Image reading device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading apparatus used for a copying machine, a facsimile, a printer, and the like.

[0002]

2. Description of the Related Art In recent years, a full-color sensor for generating red (R), green (G) and blue (B) color signals has been used in an image recording apparatus such as a copying machine, a facsimile, a printer, or the like. 2. Description of the Related Art An image recording apparatus for performing image recording or an image recording apparatus of a so-called plus one color system for performing image recording using two colors of black and one color other than black has been actively developed. In an image recording apparatus using such a full-color sensor, so-called free registration is achieved by performing digital processing on image data output from the full-color sensor. That is, as shown in FIG. 8, a mark indicating a registration position (hereinafter, referred to as a registration position) is attached to a predetermined position of the platen 1 as shown in FIG. 8, but digital processing is performed. In the image recording apparatus, the document 3 can be placed at any position on the platen 1. In this case, the minimum value X _min and the maximum value X _{max in the} sub-scanning direction (SS) are determined based on the read image data. By detecting the minimum value Y _min and the maximum value Y _max in the main scanning direction (FS), the position where the document 3 is placed and the document size can be recognized. This is called a free cash register. In this case, X _min , X
Various proposals have been made to detect four values of _max , _Ymin , and _Ymax .

One of them relates to a document holding surface of a platen cover (hereinafter, simply referred to as a document holding surface). That is, as a method of document detection, a method of performing document detection using a density difference between a document and a document pressing surface, and a method of performing document detection based on a difference in color between the document and the document pressing surface are known. In the former, the original pressing surface is formed in a black or mirror surface, and in the latter, the original pressing surface is given a predetermined color such as yellow.

[0004]

However, in the former method, it is necessary not only to take care not to stain the original pressing surface when using the original, but also to transmit the original such as tracing paper, OHP paper or thin paper. When using paper with a high ratio, the density difference between the original and the original pressing surface becomes smaller, making it difficult to detect the original.Furthermore, even if the original can be detected, the black of the original pressing surface will show through to the original There is a problem that other secondary failures occur.

In the latter method, there is no problem when the platen cover is closed. However, when the platen cover cannot be closed, such as a thick document, the depth of focus of the full-color sensor is small. There is a problem that the color of the holding surface cannot be detected well.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, and an object of the present invention is to provide an image reading apparatus capable of detecting a document satisfactorily without causing a secondary obstacle.

[0007]

In order to achieve the above-mentioned object, an image reading apparatus according to the present invention comprises: For a pixel whose density is equal to or less than the first threshold, the density value is output as zero, and for a pixel whose density exceeds a second threshold larger than the first threshold, the input density value is output. The image processing apparatus further comprises: means for increasing the density value of a pixel exceeding the first threshold value and being equal to or less than the second threshold value and outputting the result; and document detecting means for performing document detection processing based on image data passed through the means. And

According to the present invention, since the original detection is performed after the background density level of the original is reduced, the density difference between the background density of the original and the background density can be increased, and as a result, the original detection can be performed. Accuracy can be improved.

[0009]

Embodiments will be described below with reference to the drawings.

FIG. 1 is a schematic block diagram showing the configuration of an embodiment in which the image reading apparatus according to the present invention is applied to a plus one color copying machine. In the drawing, reference numeral 4 denotes an image input terminal (hereinafter referred to as an image input terminal). , IIT), 5 is a digitizing processing unit, 6 is a color separation unit (hereinafter, referred to as CS), 7 is a background removal unit, 8 is a digital filter (hereinafter, referred to as DF), and 9 is a document detection unit. Show.

Hereinafter, each block will be described.

IIT4 is a platen glass. It includes a platen cover, a scanning optical system for irradiating a document, a full-color sensor, and a driving circuit for these components, and outputs three-color analog density signals of green (G), blue (B), and red (R). The original pressing surface of the platen cover is given a predetermined density of gray. The density of this gray can be set as required.However, if the density is too high, it will be difficult to detect the original if it is a document with high transmittance as described above. The density is such that the density difference can be clearly recognized from the background of the original document and that the show-through can be avoided. Therefore, in this embodiment, when the density is set to 256 gradations from absolute white to absolute black, a gray density of about 170 is applied to the original pressing surface. It is also possible to use a contact-type full-color sensor as the full-color sensor. It is also possible to use other full-color sensors. Further, in FIG. 1, the G, B, and R analog density signals are shown to be output simultaneously, but they may be output in a dot-sequential manner.

The document detection is performed at the time of prescan.
Therefore, the speed of prescan is an important issue. The prescan is generally performed at high speed in order to shorten the prescan time and increase the number of copies per unit time. Therefore, the prescan is performed at several times the speed of the copy scan. At this time, by setting the speed of the prescan corresponding to the sampling period of the image data in the document detecting means 9, a simple operation is performed. Document detection can be performed with a circuit configuration in a short time. Therefore, in the present embodiment, when the sampling of the image data by the document detecting means 9 is performed at every N (where N is a power of 2) pixels, the prescan speed is set to be N times as fast as the copy scan. To do.

The details are as follows. As will be described later, the document detection unit 9 samples image data every four pixels in the FS direction in order to detect white at the edge of the document, and after the density changes from the density of the document pressing surface to white. When three white pixels continue, the third point is determined to be the document edge. Therefore, since the pixel density is set in advance, the document detecting means 9 increases the FS of the document by multiplying the number of points in the range determined as the document edge by four.
The dimension in the direction can be determined. The process of quadrupling the number of points only requires shifting by 2 bits. It can be performed at high speed with a simple circuit.

On the other hand, the dimension in the SS direction is determined by counting a clock supplied to a stepping motor of a scanner or a signal corresponding thereto, which is located within the range of the document edge in the SS direction, as described later. . In the case of using a stepping motor, the number of clocks represents the distance. If the speed at the time of copy scanning is assumed to be 100% and the number of clocks directly represents the size at this time, the scanning speed is quadrupled. In this case, the dimension can be obtained by multiplying the counted clock number by four. Therefore, when the speed of the pre-scan does not correspond to the sampling interval of the pixel data in the document detecting unit 9, for example, when the pre-scan is performed at five times the copy scan, the size in the SS direction is determined. , The number of clocks within the range of the document edge is multiplied by 5, but such an operation is complicated.
It takes time. However, since the copy scan is started immediately after the pre-scan is completed, it is not preferable that time is required for document detection. Therefore, the prescan speed is set so that the calculation for obtaining the dimensions can be performed by the same calculation as in the FS direction. Therefore, in this embodiment, the pre-scanning speed is four times the copy scanning speed. As a result, the size of the document can be easily obtained in a short time simply by performing a 2-bit shift in both the FS direction and the SS direction.

The analog density signals of G, B, and R output from the IIT 4 are input to the digitizing means 5, and after a gain adjustment and an offset adjustment are performed, a predetermined number of bits,
For example, an 8-bit digital density signal is formed, and after further shading correction, the three-color digital signal is input to CS6 as a simultaneous signal.

Based on the input G, B, and R densities, the CS 6 determines whether the pixel is a designated color (hereinafter referred to as a chromatic color) or another color (hereinafter referred to as an achromatic color). )
The density data, various flags, a main color flag (MCF) and a sub color flag (S
CF) are output. MCF and SCF are flags indicating whether a pixel is chromatic or achromatic. MCF is an achromatic pixel and its density is a predetermined value, for example, when the density range is 256 gradations from absolute white to absolute black. 10
A value of about 20 is set to 1 only when the value is equal to or more than 0, and the value is set to 0 otherwise. Shall be.

In a plus one color copying machine,
Since a pixel is either achromatic or chromatic, only one flag is required to indicate whether the pixel is achromatic or chromatic. However, as described above, the two types of flags, MCF and SCF, are used as follows. For such reasons. That is, a portion which looks white to a human has a density value which is extremely small when read by a full color sensor but is not zero. Therefore, when an achromatic color and a chromatic color are distinguished by one flag, the image is output at that density, and is output with a certain non-zero density, not absolute white. However, this is not desirable, and it is desirable that pixels having an achromatic color and a small density value are actually output in absolute white. Therefore, a flag for distinguishing between achromatic colors and chromatic colors, in this case, SCF, and a flag for distinguishing whether to output the achromatic pixels in absolute white or the density obtained by reading, in this case, MCF, Two types of flags are used. Therefore, in the final stage of the image output unit, the pixel for which MCF = 0 is actually provided with a certain density, but is treated as absolute white, that is, zero density.

The image data output by the CS 6, ie, the density data, is as follows. Regarding the density of black and color, the density of the color is smaller than that of the color, and this causes a difference between the density of the chromatic color and the density of the achromatic color. Therefore, for the achromatic pixel, for example, the following (1) ) Is output as density data, and the density of a chromatic pixel is output by multiplying by a coefficient K as in equation (2).

(G / 2) + (3R / 8) + (B / 8) (1) ((G / 2) + (3R / 8) + (B / 8)) × K (2) K Varies depending on the color and is determined experimentally for each color.

Next, the background removing means 7 will be described.
The background removing unit 7 includes a background removing unit 11 and a background level detecting unit 12, as shown in FIG. Background level detector 12
Performs the following operation at the time of pre-scan to obtain the background removal threshold level THRD. First, the sampling unit 13
Samples the input density data at predetermined intervals in the main scanning direction (FS) and the sub-scanning direction (SS). The sampling in the FS direction does not take in density data at predetermined intervals as in normal sampling, but monitors density data of a predetermined number of pixels.
Sampling is performed by taking the minimum density value among them as the density of the sampling range. For example, if sampling is performed at an interval of 96 pixels, the sampling unit 13 monitors the first 96 pixels of the main scanning line and captures the minimum density therein, and then monitors the next 96 pixels, that is, the first 96 pixels. The operation of monitoring the 97th pixel to the 192nd pixel and taking in the minimum density value among them is repeated for one line.

In the SS direction, gates are performed at a predetermined number of lines, for example, at intervals of 96 lines, and sampling is performed by enabling the above-described sampling in the FS direction.

The density data sampled by the sampling section 13 is supplied to a histogram creating section 14, where a histogram is created for each predetermined density area. FIG. 3 shows an example of the histogram. In FIG. 3, the density gradation is 0
In the case of 256 gradations of 255 to 255, the density is 0 to
For the range 143, nine density areas 0 to 8 are set. That is, in this case, the background level has a density value of 143 or less. The density range of each density area is as shown in FIG. 3B.

When the prescan is completed, the creation of the histogram is completed, and the background removal level determination section 15 determines the background removal level based on the created histogram. The operation is as follows.

First, the background removal level determination unit 15 searches the frequency of the histogram from the high density side, detects a density area that first exceeds a preset density check value, and detects a background area belonging to the detected density area. A threshold level (DEC) (see FIG. 3B) is obtained, and a preset offset value (OFST) is added to the background threshold level (DEC) to obtain a background removal threshold level (THRD). Output to Also,
If there is no frequency exceeding the density check value, a predetermined threshold level (OHPT), for example, a level at which the background can be effectively removed even if the original is an OHP sheet, is output as a background removal threshold (THRD).
If the density check value is set to a frequency of about 100 cm ^{2 in} terms of area, an optimum background removal level can be obtained for a document of a normally used size.

The background removal threshold (THRD) determined at the time of pre-scan as described above is given to the background removal unit 11, and the background removal is performed at the time of copy scan. The operation is as follows.

The background removal unit 11 sets the input / output characteristics shown in FIG. 4 using the background removal threshold level THRD. That is, when the input image data (density) is equal to or less than THRD, the output image data is set to zero, and the input image data is set to THRD.
If the input image data exceeds 1.5 times RD, and the input image data exceeds THRD and is 1.5 times or less than THRD, the difference between the input image data and THRD is calculated by three.
Multiply and output.

The background removal unit 11 performs the above-described background removal operation and corrects the values of MCF and SCF. That is,
As described above, the MCF is set to “1” when the density is higher than a predetermined level for achromatic pixels, and the SCF is set to “1” when the density is higher than a predetermined level for chromatic pixels. However, the density may become zero or less than the predetermined level due to the background removal described above. Therefore, when the density becomes lower than the predetermined level due to the background removal, the background removal unit 11 corrects the MCF to zero for achromatic colors and corrects the SCF to zero for chromatic colors.

The operation of the background removing unit 11 during copy scanning has been described above.
Is set in advance, for example, 80, and the input / output characteristics shown in FIG. 5 are set. Therefore, at the time of pre-scanning, the background removal is performed according to the input / output characteristics shown in FIG. 5, which can reduce the background density level even in the case of a document having a high background density. The density difference can be increased, and as a result, the accuracy of document detection by the document detection means 9 at the subsequent stage can be improved.

DF8 emphasizes the edges of characters and lines,
The halftone image is subjected to halftone dot removal, and is composed of, for example, a 5 × 5 matrix centered on a pixel of interest. At the time of copy scanning, a predetermined value is written to each pixel of the matrix. At the time of pre-scanning, the matrix shown in FIG. 6A is used for achromatic pixels, and FIG. A 6B matrix is used. As a result, the density data of the achromatic pixel is output as it is, and the density of the chromatic pixel is output by multiplying 1 / K. This K is a coefficient of the above equation (2).

This is done for the following reason. The CS 6 outputs the density data represented by the expression (2) for the chromatic color, in order to match the color of the copy image with the color of the original image. However, according to this, when a chromatic color, that is, a color paper of a currently designated color is used for a document, the background density of the document increases, and the difference between the background density of the document and the background density decreases. This is not preferable for performing document detection.
Therefore, in this embodiment, the density of the chromatic color is reduced by multiplying the inverse of the coefficient multiplied by CS6 in DF8. As a result, the accuracy of document detection can be improved even when color paper is used.

The original detecting means 9 detects density data, MCF, S
The CF is output to the subsequent stage, and X _min , X _max , Y _min , and Y _max shown in FIG. 8 are obtained by the following operation.

First, as shown in FIG. 7A, the input image data GD is binarized as shown in FIG. 7B using a predetermined density threshold TH. The document edge is obtained by sampling the binarized image data every four pixels, and comparing the sampled data with image data regarded as a document portion by 3 pixels.
When the points are continuous, the third point is determined as the document edge. Thus, for example, a white portion having a narrow range as indicated by reference numeral 20 in FIG. 7B is determined to be suspect data, and only a wide white portion as indicated by reference numeral 20 is determined to be document data.

The minimum value Y _min in the FS direction is obtained by latching the address of the image data in which the edge of the original document is first detected on the M-th line and comparing it with the address value of the edge on the (M−2k) -th line. The operation of rewriting and latching the smaller value is repeatedly obtained, for example, every 2k lines.

Similarly, _Ymax latches the address at which the edge of the document is first detected by looking at the image data from the end direction of the main scanning, and compares it with the address value of the edge on the (M-2k) th line. The operation of rewriting and latching a larger value is repeatedly obtained for every 2k lines, for example. Incidentally, k is the value when k times the speed of the copy scan speed prescan, also address latch when determining the Y _max is the address counted from the FS direction.

The minimum value X _min and the maximum value X _{max in the} SS direction
Is obtained by counting a stepping motor driving clock having document data of a predetermined number of points or more, and latching the first and last lines.

Note that these X _min , X _max , Y _min ,
In obtaining the Y _max, to represent the size of the registration position on the predetermined unit of mm or the like, it is natural to perform a quadrupling processing number count address or clock as described above.

X _min , X obtained as described above
_{The maximum} , _Ymin , and _Ymax are sent to control means (not shown), and are used for determining a magnification when the automatic magnification selection function is set, and for determining a copy paper size when the automatic paper selection function is set. You.

Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and it is apparent to those skilled in the art that various modifications are possible.

[0040]

As is apparent from the above description, according to the present invention, the density difference between the background density and the background density of the original is increased before the original is detected. Is what you can do.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram showing a configuration of an embodiment when an image reading apparatus according to the present invention is applied to a plus one color type copying machine.

FIG. 2 is a diagram illustrating a configuration example of a background removing unit.

FIG. 3 is a diagram for explaining an operation of a histogram creating unit of the background removing unit.

FIG. 4 is a diagram illustrating input / output characteristics set in a background removal unit during copy scanning.

FIG. 5 is a diagram illustrating input / output characteristics set in a background removal unit during pre-scanning.

FIG. 6 is a diagram for explaining the operation of the digital filter.

FIG. 7 is a diagram for explaining a document detection operation.

FIG. 8 is a diagram for explaining a free cash register.

[Explanation of symbols]

4 image input terminal, 5 digitization processing means, 6 color separation means, 7 background removal means, 8 digital filter, 9 document detection means.

Claims (1)

(57) [Claims] An image reading device that optically scans a document to output a density value of a pixel having a density equal to or lower than a first threshold value to zero with respect to the read image data; The input density value is output for a pixel exceeding a second threshold greater than the threshold of 1, and the density value is increased and output for a pixel whose density exceeds the first threshold and is equal to or less than the second threshold. An image reading device comprising: a document detecting unit that performs a document detecting process based on image data that has passed through the unit.