JPH07212587A - Device and method for correcting shading - Google Patents

Abstract

PURPOSE:To express density in multi-level by selecting one of a specified expressions in accordance with a set mode and executing a shading correction operation for image data in the arithmetic part of a shading correcting circuit. CONSTITUTION:Carrying an original is started and image data of the original is successively read, processed in CCD and given to the shading correcting circuit. In the shading correcting circuit, the set mode is discriminated at the time of inputting image data, one of the expressions I-IV is selected in accordance with the set mode and the shading correction operation based on the selected expression is executed. That is, the shading correction operation is executed based on the expression I when the set mode is the mode '0', based on the expression II when the set mode is the mode '1' and based on the expression III or IV when the set mode is the mode '2' or '3'.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shading correction method and a shading correction apparatus for reducing density unevenness in a read image in an image reading apparatus such as a facsimile machine or an image scanner.

[0002]

2. Description of the Related Art In an image reading apparatus such as a facsimile apparatus or an image scanner, so-called shading correction is performed in order to compensate for uneven density between pixels due to uneven lighting of a reading light source. In shading correction, the analog signal output from the image sensor is converted to analog /
The digital data converted by the digital converter (hereinafter referred to as "A / D converter") is corrected based on the following equation (5).

[0003]

[Equation 3]

In the equation (5), the "black reference value" is a value obtained when the signal output from the image sensor when the completely black reference image is read is converted into digital data, and the "white reference value". Is a value when a signal output from the image sensor when a pure white reference image is read is converted into digital data. For example, the black reference value can be obtained by turning off the reading light source of the image sensor to perform the reading operation, and the white reference value is turned on the reading light source to read the white reference plate provided in advance. Can be obtained.

A basic configuration for performing shading correction is shown in FIG. That is, the white reference value WS
The subtracter 151 calculates the difference (WST-BST) between T and the black reference value BST. Further, the subtracter 152 calculates the difference (ID-BST) between the input data ID and the black reference value BST. The outputs of the subtracters 151 and 152 are the division circuit 15
3 and the output data (ID-BS
The correction data is obtained by dividing T) by the output data (WST-BST) of the subtractor 151.

In the above shading correction, the minimum value of the input data is the white reference value. Therefore, when the white reference value is larger than the minimum output of the A / D converter, the dynamic range of the correction data is narrow. Become. For example, if the white reference value is "0000111" even if the A / D converter has a data depth of 7 bits,
The correction data can only vary substantially over a 4-bit range. For this reason, the density resolution is deteriorated, and the density expression can be performed only with a small number of gradations.

In order to solve the above-mentioned drawbacks of the prior art, the applicant of the present application has previously made a shading correction method and apparatus capable of increasing the dynamic range of correction data and expressing density in multiple gradations. And filed a patent application (see Japanese Patent Application No. 4-209651). In the shading correction method according to the prior application, the predetermined offset OFFS
Using ET (preferably OFFSET = white reference value), shading correction is performed according to the following expression (6).

[0008]

[Equation 4]

According to the shading correction method according to this prior application, the offset OFFSET is subtracted from the dividend and the divisor in the division operation performed at the time of correction.
Therefore, for example, when the A / D converter has a data depth of 7 bits, the white reference value is "000" which is the minimum value.
If the value is larger than the originally possible minimum value, such as “0000111” instead of “0000”, the offset OFFSET is subtracted to correct the density to be represented by 7 bits. it can.

[0010]

However, in the shading correction method according to the prior application, the dynamic range of the corrected data can be widened in a certain direction (white side in the above case), and the density expression can be expressed in multiple gradations. However, the dynamic range cannot be expanded in any direction.

That is, it is not possible to arbitrarily expand the dynamic range to the black side or the white side. Therefore, there is a problem in that it is not possible to freely change the obtained image to an image shaken to the white side or an image shaken to the black side. Therefore, according to the present invention, in performing shading correction on input data, density expression can be performed in multiple gradations, and the density expression can be freely shifted to the black side or the white side, or the black level in the density expression can be changed. To provide a shading correction method and apparatus capable of increasing the number of gradations on the side or increasing the number of gradations on the white side and obtaining effective and abundant image data of density expression. To aim.

[0012]

The invention according to claim 1 is
A shading correction device for performing a predetermined correction on input image data, comprising: a reading unit that reads the image data of an image data reading unit, an illumination unit that illuminates the image data reading unit, and a density reference provided in the image data reading unit. Plate, density reference offset reading means that reads the reflected light of the density reference plate when the plate is illuminated by the illumination means and holds the value as an offset, density reference when the density reference plate is not illuminated by the illumination means A black reference value holding means for reading the reflected light of the plate by the reading means and holding the value as a black reference value, a reading means for reading the reflected light of the density reference plate when the density reference plate is illuminated by the illuminating means, and A white reference value holding unit that holds the value as a white reference value, and a document located in the image data reading unit is illuminated by the illumination unit and read by the reading unit. Using the offset held by the offset holding means, the black reference value held by the black reference value holding means, and the white reference value held by the white reference value holding means for the image data of the obtained original Four arithmetic expressions (1) to (4)

[0013]

[Equation 5]

It is characterized in that it includes any one of the above, and a correction means for correcting the image data of the document read by the reading means. The invention according to claim 2 is
2. The shading correction device according to claim 1, wherein the offset holding unit reads the reflected light of the density reference plate when the density reference plate is illuminated by the illumination unit by the reading unit, and the predetermined line for the reading by the reading unit. The feature is that the minimum value of the average value of data is held as an offset.

The invention according to claim 3 is the invention according to claim 1 or 2.
In the shading correction apparatus described, the reading means includes a line sensor, the black reference value is an average value of data for a predetermined line read by the line sensor,
The white reference value is an average value of data of a predetermined line read by the line sensor.

The invention according to claim 4 is the following four arithmetic expressions (1) to (4) using a predetermined white reference value, a predetermined black reference value, and a predetermined offset.

[0017]

[Equation 6]

A shading correction method is characterized in that any one of the above is applied to perform shading correction of input image data.

[0019]

Operation: Four arithmetic expressions (1) to (4) used in the present invention
Are arithmetic expressions using the black reference value, the white reference value, and the offset. The offset can be set to a value equal to the white reference value or a value slightly smaller than the white reference value, for example. Then, even if the white reference value read by the reading unit is not the minimum value that can be read by the reading unit, the difference can be absorbed by the offset. Also, the offset can be used to positively increase the difference. Therefore, by properly using the arithmetic expression, the image can be shifted to the white side or the black side as necessary while maintaining the number of gradations of the density of the shading-corrected data, or the white side or the black side can be changed. The number of gradations can be increased.

[0020]

[Embodiment] The following is an example of a facsimile machine.
An embodiment of the present invention will be described in detail. Figure 1
FIG. 4 is a block diagram showing an electrical configuration of an image reading mechanism of a facsimile apparatus in which an embodiment of the present invention is incorporated. First, with reference to FIG. 1, an image reading process in this facsimile apparatus will be briefly described.

The original set on the facsimile is
It is read by the scanner 1. The scanner 1 includes an image sensor such as a CCD image sensor or a CIS image sensor for reading an image. The image sensor may be an area image sensor that reads two-dimensional data or a linear image sensor that reads line data. Usually, a line image sensor is used in order to make the device inexpensive.

The image data of the original read by the scanner 1 is given to the input interface section 2, and the interface section 2 performs sample hold processing of signals and the like. The input interface unit 2 is composed of an analog circuit in this embodiment, and the above processing is performed in an analog manner. The image data processed by the input interface unit 2 is then given to the digital AGC circuit 3 to perform gain control for keeping the level of the signal (image data) within a desired range, and the image data is an analog signal. Is converted into a digital signal.

Next, the gain-controlled image data is supplied to the shading correction circuit 4, where shading distortion is reduced or removed. The shading distortion is uneven density between pixels due to uneven lighting of a reading light source when reading a document with the scanner 1. The image data from which the shading distortion has been reduced or removed is then supplied to the area separation circuit 5. In the area separating circuit 5, whether the input image data is image data obtained by reading characters (character data), image data obtained by reading a photograph (photograph data), or printed photographs such as newspapers and magazines. It is discriminated whether or not it is the image data (halftone dot data) of the halftone dot obtained by reading the photographic image.

If the input image data includes character data, photographic data, and halftone dot data, the areas of the respective data are separated. The reason why the region separation is performed according to the type of data is to perform an appropriate process suitable for the type of data in the subsequent process. On the output side of the area separation circuit 5, in order to perform different processing depending on the type of the area-separated data, the differential filter 6, the integration filter 7 and the pass-through circuit (pass through the signal without any processing). (Only circuit) 8 is connected. The character data separated into regions is given to the differential filter 6, and the contour is sharpened by the differential filter 6. The halftone dot data is given to the integration filter 7, and the data is smoothed. The data other than the character data and the halftone dot data, that is, the photographic data is given to the pass-through circuit 8 and sent as it is to the next circuit.

In this way, the image data is subjected to a predetermined process according to the type or a process of not performing the process. Then, these data are given to the zoom smoothing circuit 9. When enlarging or reducing an image, the zoom / smoothing circuit 9 performs the enlarging or reducing process and the process of correcting the image distortion associated therewith. If the image is not enlarged or reduced, the zoom / smoothing circuit 9 does not process the image data.

The image data that has been processed up to the zoom / smoothing circuit 9 is then processed by one of the following circuits according to the type. That is, when the image data is photographic data or halftone dot data and halftone output processing is performed, the γ correction circuit 1
0, and the sensitivity characteristics of the data are complemented so as to match the human eye. Further, it is given to the error diffusion circuit 11 and subjected to processing for good halftone expression.

On the other hand, when the image data is character data and is data to be binarized, the binarization circuit 1
Given to 2. The binarization circuit 12 adjusts the slice level for binarization to separate the background from the characters and line drawings. At this time, automatic density adjustment processing is also performed so that the density becomes appropriate. Then, the output of the binarization circuit 12 is given to the isolated point removal circuit 13 to remove the isolated black points, white points, etc. that appear due to noise or the like.

The data that has undergone the above processing is DMA (Dire
ct Memory Access) circuit 14 to output to a transmission circuit (not shown) or to a printing circuit.
This embodiment relates to the shading correction circuit 4 of the image reading processing circuits described above. Figure 2
It is a figure for demonstrating the structure of the shading correction circuit 4 concerning this Example.

Referring to FIG. 2, in the facsimile apparatus in which the shading correction circuit 4 according to this embodiment is adopted, a predetermined position on the conveyance path 22 of the document conveyed by the roller 21 is the reading position 23. It is said that.
At the reading position 23, for example, a white reference plate 24 as a density reference plate extending in a direction orthogonal to the conveyance direction of the document conveyance path 22.
Are arranged. Further, a lamp 25 for illuminating the document conveyed through the white reference plate 24 and the document conveying path 22 at the reading position 23, and the contents of the document at the white reference plate 24 or the reading position 23 illuminated by the lamp 25 are read. A CCD 26 is provided for this purpose.

The image data read by the CCD 26 is supplied to the shading correction circuit 4 according to this embodiment via the input interface section 2 and the digital AGC circuit 3 as described with reference to FIG. The shading correction circuit 4 includes a white reference value storage register 3
1, a black reference value storage register 32, an offset storage register 33, and a calculation unit 34 are provided.

The calculation unit 34 can be constituted by, for example, a microcomputer. In that case, a calculation process for shading correction is performed on the input image data based on one of the calculation formulas (1) to (4) for shading correction described later based on a predetermined program. The corrected data is output.

The arithmetic unit 34 can also be composed of a hardware circuit using a subtractor, an adder, a divider and the like. A signal from the mode setting switch 36 is applied to the shading correction circuit 4. The mode setting switch 36 may be, for example, an adjustment switch operated by a service person or a user operation switch. The switch 36 outputs a signal for selecting an arbitrary mode such as a mode in which the density expression of the read image data is shifted to the black side, a mode in which the white side is shifted, or a mode in which the number of gradations on the black side is increased. It is for giving.

FIG. 3 is a flow chart for explaining the processing operation of the shading correction circuit 4 described in FIG. Next, the processing operation in the shading correction circuit 4 shown in FIG. 2 will be described with reference to FIG. When transmitting a document by a facsimile apparatus, after the document is set at the setting position, the transmission start button is pressed and a start signal is input.

In the facsimile machine, when the input of the start signal is judged (step S1), first, the lamp 25
Is turned on, the white reference plate 24 is illuminated, and the reflected color is reflected by the CCD2.
6, the read output of the CCD 26 is given to the digital AGC circuit 3 via the input interface section 2, and gain control (AGC processing) of the input signal is performed. The offset storage register 33 of the shading correction circuit 4 uses the minimum value of the AGC-processed signal as an offset.
(Step S2).

Next, with the lamp 25 turned off, the reflected light of the white reference plate 24 is read. In this case, since the white reference plate 24 is not illuminated, there is no reflected light, and the CCD 26 can obtain black reference data. The obtained data is input to the input interface unit 2 and the digital AGC circuit 3.
And is stored in the black reference value register 32 as the black reference value (step S3).

Then, the lamp 25 is turned on to illuminate the white reference plate 24, and the reflected light at that time is read by the CCD 26. The read data is processed by the input interface unit 2 and the digital AGC circuit 3, and the data is stored in the white reference value register 31 as a white reference value (step S4). The black reference value and the white reference value in steps S3 and S4 correspond to one line read by the CCD 26 or a plurality of predetermined lines (e.g., 8 lines).
The average value of the (line) data is used. This is because the black reference value and the white reference value are stabilized by using the average value of the line data. However, the black reference value is set to CCD2
6 may be the maximum value of the data read, or the white reference value may be the minimum value of the data read by the CCD 26.

Then, the roller 21 is driven to start the conveyance of the document, and in synchronization with this, the reading of the document is started (step S5). As a result, the image data of the original is sequentially read by the CCD 26, and the read image data is input to the input interface unit 2 and the digital AG.
It is processed by the C circuit 3 and given to the shading correction circuit 4.

When the image data is input (step S6), the shading correction circuit 4 determines the setting mode (step S7) and selects one of the following equations (1) to (4) according to the setting mode. Then, the shading correction calculation based on the selected calculation formula is performed (step S8).

[0039]

[Equation 7]

That is, when the setting mode is mode 0, the shading correction calculation is performed based on the equation (1),
When the setting mode is mode 1, shading correction calculation is performed based on equation (2), when the setting mode is mode 2, shading correction calculation is performed based on equation (3), and the setting mode is mode 3 In this case, the shading correction calculation is performed based on the equation (4).

Here, the mode 0 is a mode for outputting normal shading correction data. The mode 1 is a mode in which the image data after shading correction is shifted to the black side. Mode 2 is a mode in which the number of gradations on the black side of image data after shading correction is increased. Mode 3 is a mode in which the image data after shading correction is shifted to the white side. In this embodiment, any one of the four modes can be selected as described above, but another mode other than the above-mentioned modes may be set in advance and selected.

For reference, Table 1 shows the input image data I.I. A list of D, data after shading correction SHOUT, and a difference (correction amount) between data after shading correction and input image data is shown.

[0043]

[Table 1]

The list in Table 1 is an example of shading correction calculation when black reference value = 240 white reference value = 16 OFFSET = 16.

The OFFSET condition is OFFSE.
Any value may be used as long as T ≦ white reference value. FIG. 4 is a block diagram showing another configuration example of the calculation unit 34 in the shading correction circuit 4. The arithmetic unit 34 shown in FIG. 4 is a so-called hardware circuit using a subtractor, an adder, a divider, a multiplier and a selector.

Referring to FIG. 4, during the shading correction calculation, the input image data ID and the white reference value stored in the white reference value register 31 (see FIG. 2) are given to the subtractor 41, and the image data ID is obtained. From which the white reference value is subtracted. In addition, the black reference value register 32 (see FIG. 2)
The black reference value stored in and the white reference value stored in the white reference value register 31 are given to the subtractor 42, and the white reference value is subtracted from the black reference value.

Then, the output of the subtractor 41 is given to the adder 43 and the subtractor 44. In the adder 43,
OFFSET and the output of the subtractor 41 are added. Further, in the subtractor 44, from the output of the subtractor 41, OF
FSET is subtracted. On the other hand, the output of the subtractor 42 is given to the adder 45 and the subtractor 46. And adder 4
In 5, the output of the subtractor 42 and OFFSET are added,
The subtractor 46 subtracts OFFSET from the output of the subtractor 42.

The output of the adder 43 and the output of the subtractor 44 calculate the numerator (dividend) of the above-mentioned equations (1) to (4), while the outputs of the adder 45 and the subtractor 46 are denominators. (Divisor) is calculated. The output of the adder 43 or the output of the subtractor 44 is selected by the selector 47, and the divider 4
Given to 9. Similarly, the output of the adder 45 or the output of the subtractor 46 is selected by the selector 48 and given to the divider 49. In the divider 49, the output selected by the selector 47 is divided by the output selected by the selector 48, and the result is given to the multiplier 50 and multiplied by the gradation number K.
Then, the output of the multiplier 50 becomes shading-corrected data.

With the above configuration, the selector 47 selects the output of the adder 43 or the output of the subtractor 44 based on the setting mode. Similarly, the selector 48 selects the output of the adder 45 or the output of the subtractor 46 based on the setting mode. Therefore, the selector 47 and the selector 48 are supplied with the setting mode signal. Further, the setting mode signal is also used to select whether or not to apply OFFSET to the adder 43 and the subtractor 44. That is, when the setting mode is the mode 2 or the mode 3, since the shading correction calculation based on the above formula (3) or (4) is performed, the numerator of the calculation formula does not include OFFSET.

As a result, the circuit shown in FIG.
The shading correction calculation based on any of the above equations (1) to (4) can be performed.

[0051]

According to the present invention, at the time of shading correction, the density expression can be performed in multiple gradations, and the density expression can be shifted to the desired side of the black side or the white side, or the desired side in the density expression can be obtained. The number of gradations can be increased.

[Brief description of drawings]

FIG. 1 is a block diagram showing an electrical configuration of an image reading mechanism of a facsimile apparatus incorporating an embodiment of the present invention.

FIG. 2 is a block diagram for explaining a shading correction circuit according to an embodiment of the present invention.

FIG. 3 is a flowchart for explaining a processing operation of a shading correction circuit in FIG.

FIG. 4 is a block diagram showing another configuration example of the shading correction circuit, which is an example of a so-called hardware configuration.

FIG. 5 is a block diagram showing a configuration of a conventional shading correction circuit.

[Explanation of symbols]

 1 Scanner 4 Shading Correction Circuit 26 CCD 31 White Reference Value Register 32 Black Reference Value Register 33 Offset Register 34 Arithmetic Section 36 Mode Setting Switch

Claims (4)

[Claims] 1. A shading correction device for performing a predetermined correction on input image data, comprising: a reading means for reading the image data of an image data reading portion, a lighting means for illuminating the image data reading portion, and an image data reading portion. The density reference plate provided, an offset holding means for reading the reflected light of the density reference plate when the density reference plate is illuminated by the illumination means, and holding the value as an offset, the density reference plate illuminated by the illumination means The black reference value holding means that reads the reflected light of the density reference plate when it is not turned on and holds that value as the black reference value, and the reflected light of the density reference plate when the density reference plate is illuminated by the illumination means Means, and the white reference value holding means for holding the value as a white reference value, and the document located in the image data reading section are illuminated by the illumination means. For the image data of the original obtained by reading with the reading means, the offset held by the offset holding means, the black reference value held by the black reference value holding means, and the white reference held by the white reference value holding means The following four arithmetic expressions (1) using values
(4) [Equation 1] A shading correction device comprising: a correction unit that corrects image data of a document read by the reading unit by applying any one of the above. 2. The shading correction apparatus according to claim 1, wherein the offset holding unit reads the reflected light of the density reference plate when the density reference plate is illuminated by the illumination unit, and the read light is read by the reading unit. It is characterized in that the minimum value of the average values of the data for a predetermined line is held as an offset. 3. The shading correction apparatus according to claim 1, wherein the reading unit includes a line sensor, and the black reference value is an average value of data of a predetermined line read by the line sensor, The white reference value is an average value of data of a predetermined line read by the line sensor. 4. The following four arithmetic expressions (1) to (4) using a predetermined white reference value, a predetermined black reference value, and a predetermined offset: A shading correction method characterized by applying any one of the above to perform shading correction of input image data.