JPH11215372A - Shading correction device and shading correction method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To properly discriminate abnormalities in shading reference values thereby realizing satisfactory shading correction. SOLUTION: Normal white reference values of a noted pixel p10 are determined to be within the range of AVE ±α, based on a mean value AVE of white reference values of several pixels P2-P9 immediately before the noted pixel. When a white reference value of the noted pixel P10 is one outside of the normal range, the value is corrected to a corrected value CV, which may be set equal to the mean value AVE. Since the range of normal white reference values for each pixel and the corrected value are determined, fault discrimination and correction of white reference values are conducted properly by taking the local situation of each pixel into account.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a shading correction device and a shading correction method for reducing density unevenness in a read image in an image reading device such as a facsimile device and an image scanner.

[0002]

2. Description of the Related Art In an image reading apparatus such as a facsimile apparatus or an image scanner, in order to compensate for uneven pixel density caused by uneven illumination of a reading light source, etc.
So-called shading correction is performed. In the shading correction, correction based on the following equation (1) is performed on digital data obtained by converting an analog signal output from an image sensor by an analog / digital converter.

[0003]

(Equation 1)

In the equation (1), the "black reference value" is a value obtained by converting a pixel signal output from the image sensor into digital data when a black black reference image is read, and a "white reference value". "Is a value obtained by converting a pixel signal output from the image sensor into a digital data when a pure white reference image is read. For example, the black reference value can be obtained by turning off the reading light source of the image sensor and performing a reading operation, and the white reference value can be obtained by turning on the reading light source and reading the previously provided white reference plate. Can be obtained.

In this manner, the white reference value and the black reference value are obtained for each pixel read by the image sensor. Therefore, when reading an original, the pixel data output from each pixel read out of the image sensor corresponds to the pixel. By performing the correction of the above equation (1) using the white reference value and the black reference value, it is possible to compensate for uneven density between read pixels. However, if local stains adhere to the white reference plate with use of the image reading apparatus, an accurate white reference value cannot be obtained for a pixel corresponding to the spot of the stain. As a result, shading correction becomes poor.

Accordingly, for example, Japanese Patent Application Laid-Open No.
As disclosed in Japanese Patent Application Laid-Open No. 8-208, processing for correcting a white reference value having an abnormal value is performed. In particular,
When the white reference value of a certain pixel exceeds a certain threshold value, the white reference value of that pixel is replaced with a predetermined correction value. This publication further discloses that the threshold value and the correction value are respectively corrected in order to solve the problem when dirt adheres to the entire white reference plate. The value and the correction value are uniformly applied to all pixels of the image sensor.

[0007]

The image sensor is composed of, for example, a line CCD, and as shown in FIG.
By forming an image on the light receiving surface 52a of D52, reading of the document is achieved. In this case, the optical path length from the document surface 50 to the light receiving surface 52a of the line CCD 52 is different between the vicinity of the center and the vicinity of both ends of the reading target area 55. Therefore, the white reference
The white reference value obtained by reading with the CD 52 generally takes a value as shown in FIG. That is, a large value is obtained for pixels near both ends of the line CCD 52,
The value is small for pixels near the center of 52.

The output of the line CCD includes electrical noise,
Noise N1, caused by adhesion of dust, contamination of the reference plate, etc.
N2,... Are included. Therefore, in order to appropriately perform shading correction on pixels corresponding to these noises N1, N2,..., It is necessary to correct a white reference value. Therefore, a threshold value TH smaller than a general white reference value is set. The white reference value having a value smaller than the threshold value TH is a constant correction value C
To be corrected.

However, as described above, the white reference value is small in the pixels near the center and large in the pixels near both ends, so that the noise N that appears in the pixels near the center is small.
Although the detection and correction of 1 can be performed well, the detection and correction of the noises N2 and N3 that appear in the pixels near both ends cannot always be performed properly. If the threshold value TH is set to a large value, noise detection and correction in pixels near both ends may be appropriately performed. However, in a pixel near the center, a normal white reference value is determined to be an abnormal value. There is a risk.

Further, although there is a difference between the normal white reference values of the pixels near both ends and the normal white reference values of the pixels near the center, the abnormal white reference values of these pixels are uniformly determined. If the correction value C is corrected to a constant value, it is not always possible to perform appropriate shading correction. Such a problem is caused not only by an image reading device configured to form a document image on a line CCD by an optical system, but also by a contact type that has no imaging optical system and reads a document image in close contact with a document surface. It can also occur in an image reading device using an image sensor.

That is, the contact type image sensor is constituted by a sensor unit having a length covering the entire width of the document, but cannot be constituted by a one-chip semiconductor element because of its size. , A plurality of chips are connected in series. For this reason, when the white reference plate is read due to the variation in the reading characteristics between the chips, a step occurs in the white reference value at the boundary between the chips. Therefore, there is a possibility that the shading correction cannot be appropriately performed by the above-described conventional technology in which the white reference value is corrected using the fixed reference value and the correction value for all the pixels.

Accordingly, an object of the present invention is to solve the above-mentioned technical problems, and to appropriately determine whether a shading reference value is abnormal, thereby achieving a good shading correction. It is to provide a correction method. It is a further object of the present invention to provide a shading correction device and a shading correction method that can appropriately correct a shading reference value and thereby can perform shading correction satisfactorily.

[0013]

According to the first aspect of the present invention, there is provided a reading means (scanner 1) for reading an image for each pixel and outputting density data of each pixel. A shading correction device that corrects the output data of (1) based on a shading reference value (white reference value or black reference value), and reads the density reference plate (white reference plate 24) by the reading means. A shading reference value storage means (line memory 41) for storing the density data of each pixel output from the means as a shading reference value for shading correction; The shading reference values of a plurality of pixels having a positional relationship of A normal value range setting unit (normal value range setting unit 44) for determining a normal value range of the shading reference value for each pixel of interest based on the output shading reference values of the plurality of pixels; If the value is within the range of the normal value set by the normal value range setting means, the shading reference value is determined to be a normal value, and the shading reference value of the pixel of interest is set by the normal value range setting means. If the value is outside the normal value range, the shading reference value is determined to be an abnormal value.
And 3) a shading correction device. Note that the components in parentheses indicate corresponding components in the embodiment described later. The same applies to the following.

The plurality of pixels having a predetermined positional relationship in the vicinity of the target pixel are a series of pixels including pixels located on one side of the target pixel when the reading means is a one-dimensional image sensor (line CCD 26). Is preferred. Also,
The normal value range setting means calculates an average value of the shading reference values in a plurality of pixels having a predetermined positional relationship in the vicinity of the target pixel, and sets a certain range around the average value as a range of the normal value. There may be.

According to the above configuration, the range of the normal value of the shading reference value is determined based on the shading reference values of a plurality of pixels in the vicinity of the pixel of interest. Criterion values are determined. Therefore, even if the shading reference value greatly varies within the reading range of the reading unit, it is possible to appropriately determine whether the shading reference value of the target pixel is normal or abnormal.

[0016] Thereby, an abnormal shading reference value can be surely eliminated, so that good shading correction can be performed. In the invention according to claim 2, the normal value range setting means determines that the abnormal reference value determining means is a normal value among the shading reference values of a plurality of pixels having a predetermined positional relationship in the vicinity of the target pixel. 2. The shading correction device according to claim 1, wherein a range of the normal value of the shading reference value is determined based only on the shading correction value.

According to this configuration, since the range of the normal value is determined using only the shading reference value determined as the normal value, the range of the normal value can be determined more appropriately. According to a third aspect of the present invention, there is provided a correction value calculating means for calculating a correction value corresponding to the normal value range set by the normal value range setting means.
E) and a shading reference value correcting means (white) for correcting the shading reference value determined to be an abnormal value by the abnormal reference value determining means to a correction value determined by the correction value calculating means. The shading correction device according to claim 1, further comprising a reference value correction unit (45).

According to this configuration, a correction value corresponding to a range of normal values determined based on the shading reference values of a plurality of pixels around the pixel of interest is obtained. Is corrected to Therefore, since the correction value can take a value reflecting the local situation around the pixel of interest, it is possible to appropriately correct the shading reference value. Thereby, good shading correction can be performed.

The correction value calculating means may use the center value of the normal value range as the correction value. Further, the correction value calculating means may determine an average value of the shading reference values of a plurality of pixels having a predetermined positional relationship in the vicinity of the target pixel as the correction value, regardless of the normal value range.

According to a fourth aspect of the present invention, there is provided a shading correction method for reading an image for each pixel and correcting output data of reading means for outputting density data of each pixel based on a shading reference value. Acquiring density data of each pixel output when the density reference plate is read by the means as a shading reference value for shading correction; and sequentially setting each pixel as a target pixel and determining a predetermined value in the vicinity of the target pixel. Determining a range of the normal value of the shading reference value for each pixel of interest based on the shading reference values of the plurality of pixels having the positional relationship described above, and determining whether the shading reference value of the pixel of interest is a value within the range of the normal value. If the shading reference value of the pixel of interest is determined to be a normal value, If outside the range of values of the serial normal value, the shading reference value is a shading correction method characterized by comprising the determining that an abnormal value.

According to this method, the same effect as that of the first aspect can be achieved. In the invention according to claim 5, the range of the normal value is determined based only on the shading reference values of a plurality of pixels having a predetermined positional relationship in the vicinity of the target pixel, which are determined to be normal values. The shading correction method according to claim 4, wherein:

According to this method, the same effect as that of the second aspect can be achieved. The invention according to claim 6 further includes a step of obtaining a correction value corresponding to the normal value range, and a step of correcting a shading reference value determined to be an abnormal value to the correction value. A shading correction method according to claim 4 or claim 5.

According to this method, the same effect as that of the third aspect can be achieved.

[0024]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a block diagram showing an electrical configuration of a portion related to image processing of a facsimile apparatus to which an embodiment of the present invention is applied. The document set on the facsimile machine is read by the scanner 1. The scanner 1 includes a line CCD image sensor.

The image data of the document read by the scanner 1 is supplied to an input interface unit 2, which performs a signal sample-and-hold process and the like. In this embodiment, the input interface unit 2 is configured by an analog circuit, and the above processing is performed in an analog manner. The image data processed by the input interface unit 2 is applied to a digital AGC circuit 3, where a gain control for keeping the level of a signal (image data) within a desired range is performed, and an analog signal is converted to a digital signal. Is performed.

Next, the gain-controlled image data is supplied to a shading correction circuit 4 to reduce or eliminate shading distortion. The shading distortion refers to unevenness in illumination of a reading light source when reading an original by the scanner 1 or unevenness in density between pixels due to other causes. This embodiment is characterized by the configuration and processing of the shading correction circuit 4.

The image data from which the shading distortion has been reduced or removed is next supplied to a region separating circuit 5. In the region separating circuit 5, the input image data is image data (character data) obtained by reading characters, image data (photo data) obtained by reading a photograph, or a printed photograph, such as newspaper or magazine. It is determined whether the image data is image data (halftone data) of halftone dots obtained by reading the photographic image.

If character data, photograph data, and halftone data are mixed in the input image data, area separation of each data is performed. On the output side of the region separation circuit 5, a filter unit 6 that performs different processing for each type of region-separated data is connected. The filter unit 6 includes a differentiation filter 6A, an integration filter 6B, an integration differentiation filter 6C, and a data through filter 6D. Then, the filter unit 6 receives, from the region separation circuit 5, a region separation signal indicating which region each pixel belongs to, and performs an appropriate filter process on each pixel. That is, the character data is stored in the differential filter 6.
A, and a process for sharpening the outline is performed. The halftone data is supplied to the integration filter 6B, and the data is smoothed. Also, the boundary pixels of each area are provided to the integral / differential filter 6C, and are subjected to processing for alleviating a sudden change in the density of different pixel information. Further, data other than the character data and the halftone dot data, that is, the photographic data, is given to the data through filter 6D and sent to the next circuit as it is.

The data from the filter section 6 is applied to a zoom / smoothing circuit 9. In the case of enlarging or reducing an image, the zoom / smoothing circuit 9 performs an enlarging or reducing process and a process of correcting image distortion accompanying the enlarging or reducing process. If the image is not enlarged or reduced, the zoom / smoothing circuit 9 does not perform any processing on the image data.

The image data that has undergone the processing up to the zoom / smoothing circuit 9 is then subjected to processing in one of the following circuits depending on the type, and multi-valued data (for example,
Conversion from 8 bits (0 to 255) to binary data ("0" or "1") is performed. That is, when the image data is photograph data or halftone data and is to be subjected to halftone processing, the image data is supplied to the γ correction circuit 10 and the sensitivity characteristics of the data are adjusted to match the characteristics of human eyes. Will be corrected. Further, after being given to the error diffusion circuit 11 and subjected to a process for good halftone expression,
It is binarized.

On the other hand, if the image data is character data and data to be subjected to simple binarization processing, it is supplied to the binarization circuit 12. The binarization circuit 12 adjusts a slice level for binarization to distinguish a background from characters, line drawings, and the like. At this time, an automatic density adjustment process is also performed so that the density becomes appropriate. The output of the binarizing circuit 12 is supplied to an isolated point removing circuit 13 to remove isolated black points and white points that appear due to noise or the like.

The binarized data generated by the above processing is supplied to the output circuit 14 and output to a transmission circuit (not shown) or to a printing circuit. FIG. 2 is a diagram for explaining the configuration of the shading correction circuit 4.
In the facsimile apparatus according to this embodiment, the roller 21
A predetermined position on the conveyance path 22 of the document conveyed by is set as a reading position 23. At the reading position 23, for example, a white reference plate 24 as a density reference plate extending in a direction perpendicular to the transport direction of the document transport path 22 is arranged. Further, a lamp 25 for illuminating the original conveyed through the white reference plate 24 or the original conveyance path 22 at the reading position 23, and reading the original content at the white reference plate 24 or the reading position 23 illuminated by the lamp 25. A line CCD 26 is provided for this purpose.
An image forming optical system 27 for forming a document image presented at the reading position 23 on the light receiving surface of the line CCD 26 is disposed between the line CCD 26 and the reading position 23.

The image data read by the line CCD 26 is supplied to the shading correction circuit 4 via the input interface unit 2 and the digital AGC circuit 3 as described with reference to FIG. The shading correction circuit 4 includes a line memory 41 for storing a white reference value WST and a black reference value BST respectively corresponding to a plurality of read pixels of the line CCD 26, and a white reference value WST and a black reference value stored in the line memory 41. Reference value BST
And a correction operation unit 42 that performs a shading correction operation on the input image data D based on the following formula (2) to calculate data SOUT after the shading correction.

[0034]

(Equation 2)

In the above equation (2), OFFSET represents an offset value. For example, when the analog / digital converter provided in the digital AGC circuit 3 has a data depth of 7 bits, the minimum value of the white reference value is “00”.
If it is "00111", it means that the gradation expression is substantially performed by 4 bits. Therefore, by subtracting a constant offset value OFFSET from the divisor and the dividend in the shading correction operation expression, the data representing the density in 7-bit gradation is represented by the data S after the shading correction.
OUT can be obtained. That is, the dynamic range of the corrected data SOUT can be increased by effectively utilizing the data depth of the analog / digital converter.

The shading correction circuit 4 further includes:
Abnormality determination unit 43 for performing abnormality determination of white reference value WST
A normal value range setting unit 44 for determining a normal value range of the white reference value WST; and a white reference value correction unit 45 for correcting the abnormal white reference value WST. FIG.
Is the white reference value WST in the shading correction circuit 4.
FIG. 4 is an illustrative view for explaining a principle of correction of FIG.

The white reference value WST is output data of the line CCD 26 when the white reference plate 24 is read. As described in the section of [Prior Art], the white reference value WST takes a small value near the center of the reading range and takes a large value near both ends of the reading range. Therefore, as shown in FIG. 3, the white reference values WST of the pixels P1, P2,... Along the way, it will gradually take smaller values.

In the shading correction circuit 4, the line C
After the white reference value WST from the CD 26 is stored in the line memory 41, a plurality of pixels are sequentially set as a target pixel one by one from one end pixel, and it is determined whether or not the white reference value WST of the target pixel is a normal value. However, if the value is an abnormal value, a process for replacing the value with an appropriate correction value is performed. For example, the pixel P in FIG.
If 10 is the pixel of interest, the normal value range setting unit 44 sets the immediately preceding pixels (eg, 8 pixels) P9, P8,.
The average value AVE of the two white reference values WST is obtained, and a certain range around the average value AVE, that is, AVE-α
Is set as the range of the normal value of the pixel of interest P10. In other words, AVE + α is the upper limit threshold of the normal value, and AVE−α is the lower threshold of the normal value.

The abnormality determining section 43 compares the white reference value WST of the pixel of interest P10 read from the line memory 41 with the upper threshold value AVE + α and the lower threshold value AVE-α, and the white reference value WST is normal. Determines if the value is within the value range. If the white reference value WST is a value within the range of the normal value, the abnormality determination unit 43 determines that the white reference value WST is a normal value, finishes the process for the target pixel P10, and returns to the next pixel. Similar processing is performed using P11 as the pixel of interest.

On the other hand, in FIG.
As shown by 10b, contamination of the white reference plate (P10a),
Electrical noise, adhesion of dust, or contamination of the reference plate (P10
b) and the like, the white reference value WST of the pixel of interest P10
Is outside the normal value range, the abnormality determination unit 43 determines that the white reference value WST is an abnormal value, and outputs an abnormality determination signal indicating this to the white reference value correction unit 45. Give to. In response, the white reference value correction unit 45
Replaces the white reference value WST with the correction value CV, and writes the correction value CV into the line memory 41 as the white reference value of the pixel of interest P10. After that, similar processing is performed with the next pixel P11 as a target pixel.

The correction value CV may be equal to the above-mentioned average value AVE. In this case, the average value AVE is given from the normal value range setting section 44 to the white reference value correction section 45. Note that the normal value range setting unit 44 does not use the white reference value data of the pixel for which the initial white reference value WST is determined to be an abnormal value when calculating the average value of the immediately preceding pixels. An average value calculation is performed using the white reference value WST for several pixels. Specifically, when the normal value range setting unit 44 receives from the abnormality determination unit 43 a result of the determination that the white reference value WST of the target pixel is a normal value, the normal value range setting unit 44 responds accordingly. In order to set the value range, an average value of the pixel of interest at that time and several pixels immediately before the pixel of interest is obtained. On the other hand, when the determination result that the white reference value WST of the target pixel is abnormal is received from the abnormality determination unit 43, the average value calculation is not performed and the normal value range is not updated.

The average value calculation in the normal value range setting section 44 may be a calculation for obtaining a simple average value of the pixel of interest and several pixels immediately before the pixel of interest or a moving average value obtained by the following equation (3). The required calculation may be used. (Moving average value) = Total− (previous moving average value) + (white reference value of target pixel) (3) Here, Total is the white reference value of the target pixel and several pixels immediately before it. Is the sum of

Since the number of pixels for calculating the average value is insufficient for some pixels at the beginning of the process, the normal / abnormal judgment and correction of the white reference value are not performed for these pixels, or The normal / abnormal judgment and correction may be performed based on a predetermined normal value range and a predetermined correction value. As described above, according to this embodiment, the normal value range and the correction value are set for each pixel of interest based on the average value of the white reference values of several pixels immediately before the pixel of interest. With this, the line CCD
It is possible to appropriately determine whether or not the white reference value is normal / abnormal for any read pixel from one end to the other end of the read range 26 and correct it. As a result, good shading correction becomes possible.

Although one embodiment of the present invention has been described, the present invention can be embodied in other forms.
For example, in the above-described embodiment, a facsimile using the line CCD 26 composed of one chip is taken as an example. However, according to the present invention, a long scanner unit covering the entire width of a document to be read is composed of a plurality of chips. The present invention can also be applied to a device that reads a document using a contact image sensor. In this case, the influence of the step of the white reference value between the chips can be eliminated as much as possible, and the normal / abnormal judgment of the white reference value and the correction thereof can be appropriately performed.

In the above-described embodiment, the configuration for correcting the white reference value has been described. However, similar normal / abnormal judgment and correction may be performed for the black reference value. However, since the black reference value is generally detected in a state where the lamp 25 is turned off, there is a low possibility of noise as in the case of the white reference value. Is not necessary.

The present invention can be applied not only to a facsimile apparatus but also to an image scanner and a digital copying machine. In addition, it is possible to make various design changes within the technical scope described in the claims.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration related to image processing of a facsimile apparatus to which an embodiment of the present invention is applied.

FIG. 2 is a block diagram illustrating a configuration related to shading correction.

FIG. 3 is an illustrative view for explaining the principle of normal / abnormal judgment and correction of a white reference value;

FIG. 4 is an illustrative view showing a configuration for reading a document by a line CCD;

FIG. 5 is a diagram illustrating an example of a detection result of a white reference value.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Scanner 4 Shading correction circuit 26 Line CCD 41 Line memory 42 Correction operation part 43 Abnormality judgment part 44 Normal value range setting part 45 White reference value correction part

Claims (6)

[Claims] 1. A shading correction device for reading an image for each pixel and correcting output data of a reading means for outputting density data of each pixel based on a shading reference value. A shading reference value storage unit that stores density data of each pixel output from the reading unit when the reading is performed as a shading reference value for shading correction; The shading reference values of a plurality of pixels having a predetermined positional relationship are read from the shading reference value storage means, and based on the read shading reference values of the plurality of pixels, a normal value of the shading reference value for each pixel of interest is calculated. Normal value range setting means for determining the range; If the reference value is a value within a range of normal values set by the normal value range setting means,
It is determined that the shading reference value is a normal value, and if the shading reference value of the pixel of interest is a value outside the normal value set by the normal value range setting means, the shading reference value is an abnormal value. A shading correction device, comprising: an abnormality reference value determination unit that determines that 2. The method according to claim 1, wherein said normal value range setting means includes only a shading reference value of a plurality of pixels having a predetermined positional relationship in the vicinity of the pixel of interest determined to be a normal value by said abnormal reference value determining means. 2. The shading correction device according to claim 1, wherein a range of a normal value of the shading reference value is determined based on the following. 3. A correction value calculating means for obtaining a correction value corresponding to a normal value range set by said normal value range setting means; and a shading reference value determined to be an abnormal value by said abnormal reference value determining means. 3. The shading correction device according to claim 1, further comprising: a shading reference value correction unit that corrects the correction value to a correction value obtained by the correction value calculation unit. 4. A shading correction method for reading an image for each pixel and correcting output data of a reading means for outputting density data of each pixel based on a shading reference value. Acquiring the density data of each pixel output when read as a shading reference value for shading correction, and sequentially setting each pixel as a target pixel and a plurality of pixels having a predetermined positional relationship in the vicinity of the target pixel. Determining a normal value range of the shading reference value for each pixel of interest based on the shading reference value of the pixel; if the shading reference value of the pixel of interest falls within the range of the normal value, the shading reference value Is determined to be a normal value, and the shading reference value of the pixel of interest is out of the range of the normal value. If shading correction method thereof shading reference value, characterized in that it comprises a determining that an abnormal value. 5. The normal value range is determined based on only the shading reference values of a plurality of pixels having a predetermined positional relationship in the vicinity of the pixel of interest determined to be normal values. The shading correction method according to claim 4, wherein 6. The method according to claim 1, further comprising the steps of: obtaining a correction value corresponding to the normal value range; and correcting the shading reference value determined to be an abnormal value to the correction value. 6. The shading correction method according to 4 or 5.