JP2715935B2 - Image sensor shading correction method and correction 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 sensor for optically reading an original on which image information is recorded to obtain electric signal data, and more particularly to shading for correcting shading characteristics such as variations in the obtained data. The present invention relates to a correction method and a correction device.

[0002]

2. Description of the Related Art Conventionally, in a facsimile transmitter or the like,
When an image sensor is caused to read a document via a lens system, a convergence difference due to the lens system, a variation in sensitivity of a plurality of pixels (photoelectric conversion elements) constituting the image sensor,
Alternatively, due to variations in the illuminance of the light source that illuminates the original, the output level of the original may vary even when the original has the same density and is read by the image sensor.
Further, even when a document is read by a perfect contact image sensor that does not pass through a lens system, the output level of the image sensor varies due to the various variations described above. These variations include variations in the output level (bright output level) when reading the white reference surface and variations in the output level (dark output level) when the light source that illuminates the document is turned off.

In order to electrically correct such a variation in the output level of each bit of the image sensor, various methods have been studied, one of which is as follows. That is, first, the document is read by the image sensor with the light source for illuminating the document turned off, and then the white reference surface is read by the image sensor with the light source turned on. The output level of each pixel of the image sensor obtained at this time should originally be the same value. Therefore, by obtaining a difference between these output levels, this can be obtained as a correction coefficient. In this method, the output level of each pixel of the image sensor when an original is actually read is corrected using the above-described correction coefficient.

However, in this method, it is necessary to obtain a correction coefficient corresponding to each pixel of the image sensor over the whole area of the original reading area and store the correction coefficient individually. Therefore, the correction coefficient is stored. There is a problem that the memory capacity is increased and the image sensor becomes expensive due to the need for a large-capacity memory.

To solve such a problem, Japanese Patent Laid-Open Publication No. 3-136
In the first shading correction method described in Japanese Patent Application Publication No. 466, the original data is stored for the correction data corresponding to the first pixel, and the correction data one pixel before is stored for the correction data corresponding to the pixel following the first pixel. In performing the shading correction, the shading correction calculation processing is performed by restoring the shading correction data from the correction data of the first pixel and the difference data of the pixels subsequent thereto.

A second shading correction circuit described in Japanese Patent Application Laid-Open No. 60-218982 relates to correction of a waveform error of a two-dimensional screen composed of m × n sections.
Each horizontal reference value is stored in a reference value memory, and each horizontal shading correction value is set to a difference value from the reference value in the section at the start point, and the shading correction value of the preceding section is used in the remaining sections. Is stored in a difference value memory as a difference value between the reference value and the reference value, and the shading correction is performed using the reference value and the difference value.

On the other hand, as a method of compressing data using a difference, a difference PCM in a digitally encoded audio signal is used.
And the quasi-instantaneous companding method. JP-A-62-190
The digital audio transmission device described in Japanese Patent Publication No. 930 will be described as an example. FIG. 9 is a block diagram of the digital audio transmitting apparatus. The audio signal is first linearly quantized by the A / D converter 101, and then the difference data between adjacent data is obtained by the differentiator 102. The scale detector 103 divides the input difference data into blocks each of a predetermined number of data, and extracts a scale value corresponding to the maximum absolute value in the difference data in this block. On the other hand, the sample data of the audio signal is passed through the delay unit 104 for one block, and the difference between adjacent data is obtained in the variable-precision difference unit 105.
At 6, the bit at the position corresponding to the scale value is extracted.

In this system, the A / D converter 101 has 14
If it is a bit type, the difference data of the output of the differentiator 102 needs 15 bits. If this is compressed to 8 bits, eight types of scale values are required, and the position of the 8 bits extracted from the difference data corresponding to each scale value is shown in FIG. That is, in the sample data output from the delay unit 104, a predetermined number of lower bits are truncated in advance according to the scale value when taking the difference between adjacent data in the variable precision difference unit 105. For example, in the scale value (1), the least significant one bit, the scale (2)
Is truncated to 2 bits from the least significant bit. As described above, the difference between adjacent data is obtained with an accuracy corresponding to the scale value, and variable-precision difference data is obtained. Finally, the bit at the position corresponding to the scale value is taken out by the compressor 105 and sent out together with the scale value as compressed difference data.

[0009]

In the above-described first and second shading correction circuits, the frequency of occurrence of the range where the absolute value of the difference data is small is high, and the frequency of occurrence of the range where the absolute value of the difference data is large is high. Although the difference data is small, the data having a large absolute value of the difference data increases the correction data capacity, and is limited in reducing the memory capacity for storing the correction data, so that a sufficient memory capacity reduction effect is obtained. There is no problem.

Also, in the last-mentioned differential PCM and quasi-instantaneous companding method in the digitally encoded audio signal, this is applied to a shading correction circuit to reduce differential data having a small occurrence frequency and having a large absolute value. If coding is performed with high accuracy, the data capacity of the code is reduced, but this deteriorates the image quality of pixels corresponding to data having a large absolute value of the difference. It is difficult to actually apply it to shading correction of an image sensor.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to provide a shading correction method and a correction method for an image sensor which can reduce the amount of correction data required for shading correction and thereby reduce the memory capacity for storing correction data. It is to provide a device. It is another object of the present invention to provide a shading correction method and a correction device for an image sensor, which prevent deterioration in image quality of pixels corresponding to data having a large absolute value of difference data.

[0012]

According to a first shading correction method of the present invention, the output of the image sensor when the image sensor scans the density reference or when the incident light quantity is scanned under the specific conditions is scanned. The difference between the sensor output following the first pixel and the pixel data of the immediately preceding pixel is calculated, and the difference data value is set to a predetermined reference value.
With high precision when the values below and the difference data value the group
If the value is larger than the reference value , each is coded and stored with low precision, and the image is stored based on the coded data.
Image sensor output when the sensor scans the density reference
Wherein the shading correction of the output of the image sensor is performed so that the value after the shading correction becomes substantially constant .

A second shading correction method according to the present invention is characterized in that a sensor output correction corresponding to an image sensor output when the image sensor scans the density reference or when the incident light quantity is scanned under the specific condition is performed. Data is calculated, and a difference between the correction data following the first pixel of the correction data and the correction data one pixel before is calculated,
When this difference data value is less than or equal to a predetermined reference value
Is coded with high accuracy and, when the difference data value is larger than the reference value, is coded and stored with low accuracy, respectively. Based on the coded data, the image sensor
Shade of image sensor output when scanning reference
The shading correction of the output of the image sensor is performed so that the value after the shading correction becomes substantially constant .

A shading correction apparatus according to the present invention comprises: an image sensor; and a sensor output correction unit for correcting an image sensor output corresponding to an image sensor output when the image sensor scans the density reference or when the incident light quantity is scanned under a specific condition. A correction calculator that calculates data, a first memory that stores a head pixel of the correction data, and a subtractor that calculates a difference between correction data one pixel before the correction data following the head pixel; the difference data value is in advance
A coder for coding with high accuracy when the difference data value is equal to or less than a predetermined reference value, and with low accuracy when the difference data value is larger than the reference value; and a second memory for storing the coded data. .

[0015]

In the present invention, as shown in FIG. 2, the difference encoding is performed with high accuracy when the absolute value of the difference data is small, and with low accuracy when the absolute value of the difference data is large. I have. Thus, the difference data is reduced to 5 bits by coding the 6-bit portion, and the memory capacity can be reduced. Further, even when the absolute value of the difference data is large, encoding is performed with the same accuracy, so that the image quality of the pixel is not reduced.

[0016]

Next, an embodiment of the present invention will be described with reference to the drawings. Figure 1 is a block circuit diagram of a shading correction apparatus according to a first embodiment of the present invention, the correction device accessory
A light correction value detection circuit 10 as a positive operation unit, and a first memory
, And a second memory, bright correction.
Bright correction memory 20 containing the value memory 22, one pixel before the complement
The light correction circuit 30 includes a subtractor 11 for calculating a difference from the positive data . The configuration of each circuit and memory will be described later together with its operation.

In FIG. 3A, a value obtained by A / D conversion of an image sensor output when the image sensor scans the white reference is white reference data Wn, where n is an address indicating a pixel of the image sensor. is there. Difference data ΔWn
Is the white reference data of the current pixel and the addition data F (W
n ₋₁ ), that is, ΔWn = Wn−F (Wn ₋₁ ). The rounding difference data f (ΔWn) is as shown in FIG. In FIG. 2, the absolute value of the difference data ΔWn
If the value is less than or equal to "3", it is coded with high precision and "3"
If it is larger than this, it is coded with lower precision. further,
Here, the absolute value is larger than “7” and equal to or smaller than “11”.
Sometimes they are coded with lower precision.

The addition data F (Wn) is a value obtained by restoring the white reference data from the rounded difference data f (ΔWn), that is, F (Wn) = F (Wn ₋₁ ) + f (ΔWn). The accuracy indicates one plus and minus one of the rounded difference data.

FIG. 3B shows that when the signal data is 50% of the white reference data, the added data F (W) restored in FIG.
FIG. 11 is a diagram comparing a corrected image signal A subjected to shading correction from n) with an original corrected image signal. Since the original correction image signal has a maximum value of 255 for the image sensor, it is 255
/ Wn, which is when the signal data is 50% of the white reference data, and is thus normalized to 128. On the other hand, the corrected image signal A obtained by performing shading correction from the addition data F (Wn) is 255 / F (Wn).

[0020] Here, the signal data x _n shading correction correction image signal Xn and one pixel before the significance determination corrected image signal Xn _-1 difference Xn-Xn _-1 correction if more precision image signal A Xn-Xn
_-1 is a significant difference determining the corrected image signal one pixel significance determined corrected image signal Xn _-1 before is less than accurate. FIG. 3 (b)
In, the corrected image signal after the significant difference determination of the addresses 3 and 6 is the corrected image signal Xn _-1 after the significant difference determination of one pixel before.

FIG. 3C shows an example in which the corrected image signal shown in FIG. 3B is recorded in halftone. Here the address is
It corresponds to the address of (b). Thus, the density of each address is set to a constant density. This is also apparent from the corrected image signal after the significant difference determination in FIG. However, although the corrected image signal after the significant difference determination at the address 7 is +1 compared to the original image signal, it is hardly noticeable when actually applied.

Next, the configuration and operation of the shading correction device of FIG. 1 will be described in detail. First, an operation in which the image sensor scans the white reference and detects a light correction value will be described. The image sensor scans the white reference, and the first pixel data W ₀ of the white reference data Wn, which is a value obtained by A / D converting the image sensor output at that time, is stored in the reference value memory 21. Also, rounding difference data f (ΔW ₀ ) output from a difference data rounding unit 12 described later is reset to 0.

Then, at the timing when the white reference data W ₁ one pixel after the head pixel data W ₀ is input to the subtractor 11, the difference data f (ΔW ₀ ) is reset from the difference data round 12 to 0. Is output after being delayed by one timing in the delay circuit 14, and is added to the output of the selector 17 in the adder 16. The selector 17 selects and outputs the white reference data W ₀ of the first pixel, which is the output from the reference value memory 21. Therefore, the addition data (rounded pixel data) F (W ₀ ) output from the adder 16 is F (W ₀ ) = W ₀ + f (ΔW ₀ ) = W ₀ .

Therefore, in the subtracter 11, the difference ΔW ₁ between the white reference data W ₁ and the added data F (W ₀ ) is ΔW ₁ = WF (W ₀ ) = W ₁ −W ₀ , and the difference ΔW ₁ Is input to the difference data rounding 12, and the output of the difference data rounding 12 is f (ΔW ₁ ). The rounding operation of the difference data is executed according to FIG. The calculated rounded difference data f (ΔW ₁ ) is coded by the coder 13 as shown in FIG.
2 is stored.

When the white reference data W ₂ two pixels after the head pixel data W ₀ is input to the subtractor 11, the difference
The rounded difference data f (ΔW ₁ ) is output from the data rounder 12 with a delay of one timing by the delay circuit 14, and is added to the output of the selector 17 by the adder 16. At this time, the selector 17 selects and outputs the added data (rounded pixel data) F (W ₀ ) delayed by one timing in the delay circuit 15. Therefore, the added data (rounded pixel data) F (W ₁ ) output from the adder 16 is as follows: F (W ₁ ) = F (W ₀ ) + F (ΔW ₁ ) = ΔW ₀ + f (ΔW ₁ ). Therefore, the subtractor 11 calculates the difference ΔW ₂ between the white reference data W ₂ and the addition data F (W ₁ ) as follows. ΔW ₂ = W ₂ −F (W ₁ )

The difference data ΔW ₂ is the difference data round 12
And the output of the difference data rounder 12 is f (Δ
W ₂ ). The rounding operation of the difference data is executed according to FIG. Calculated rounding difference data f (ΔW ₂ )
Are coded by the coder 13 as shown in FIG.

In the same manner, the round difference data f (ΔWn) of the white reference data Wn for one line is coded and stored in the brightness correction value memory 22. in this way,
White reference data Wn, which is a value obtained by A / D conversion of an image sensor output when the image sensor scans the white reference, is used as the first pixel data W ₀ and the rounded difference data f (ΔWn).
Detected as

Next, the top value stored in the reference value memory 21 is stored.
Pixel data W₀, Stored in the bright correction value memory 22
Image from code data of difference data f (ΔWn)
Signal data x when scanning the original with the sensor_nOne lie
A description will now be given of the brightness correction operation of the camera. One line of signal data
Ta x_nThe first pixel x of₀Is input to the correction calculator 39.
The selector 32 that has selected the reference value memory 21 by the timing
From the first pixel data W of the white reference data₀Is output and
It is input to the calculator 34. At this time, the output of the decoder 31 is
Since it has been reset to 0, the adder 34
Adder output and the output of selector 32 are added.
The addition data F (W₀) Is W ₀Becomes Supplement
The signal data x₀And the output of the adder 34
Addition data F (W₀) Is input and the normalization operation
(X₀/ F (W₀)) × 255 and the corrected image signal X
_OGet. The selector 35 outputs the corrected image signal X_OSelect and
The corrected image signal X is included in the image signal after the difference determination.₀Is output.

The signal data x ₁ one pixel after the head pixel x ₀
Is input to the correction calculator 39, the adder 3
The decoder 31 decodes the output of the selector 32, which has selected the output obtained by delaying the added data F (W ₀ ) by one timing by the delay circuit 33, and the round difference data f (ΔW ₁ ) of the bright correction value memory 22. Addition operation of the decoded data f (ΔW ₁ ) is performed, and the addition data F
(W ₁ ) is obtained. The signal data x ₁ and the added data F (W ₁ ) output from the adder 34 are input to the correction calculator 39, and a normalization operation (x ₁ / F (W ₁ )) × 255 is executed to perform correction. get the Eshingo X _1. The subtractor 37 executes a difference operation between the corrected image signal X ₀ delayed by one timing by the delay circuit 36 and the corrected image signal X _1, and the operation result X ₁ −X ₀
And the decoded data f (ΔW ₁ ) of the decoder 31 are input to the significant difference determination circuit 38, and it is determined whether there is a significant difference in the operation result X ₁ −X _0.
A correction image signal X ₁ at 5, if there is no significant difference delay circuit 36
In selecting the correct image signal X ₀ which is 1 timing delay.
The significant difference is determined from the decoded data f (ΔW ₁ ). In this embodiment, when the calculation results X ₁ and −X ₀ are less than the accuracy shown in FIG. Thus, selector 3
5 significance determined after correction signal of the signal data x ₁ is output from. Hereinafter, similarly, signal data x _{n for} one line
Is performed, and a corrected image signal after significant difference determination is obtained.

In the above description, only the bright correction is performed, but the dark correction can be performed in the same manner.

FIG. 4 is an explanatory diagram of a second embodiment of the present invention. In FIG. 4A, as in the first embodiment, a value obtained by A / D conversion of the image sensor output when the image sensor scans the white reference is white reference data Wn, where n is the image sensor output. This is an address indicating a pixel. However, here, the difference data ΔWn is a difference between the white reference data of the current pixel and the white reference data of one pixel before, that is, ΔWn = Wn−Wn ₋₁ . The rounding difference data f (ΔWn) is as shown in FIG. The addition data F (Wn) is a value obtained by restoring the white reference data from the rounded difference data f (ΔWn), that is, F (Wn) = F (Wn ₋₁ ) + f (ΔWn). The accuracy indicates one plus and minus one of the rounded difference data.

FIG. 4B shows the sum data F (W) restored in FIG. 4A when the signal data is 50% of the white reference data.
FIG. 11 is a diagram comparing a corrected image signal A subjected to shading correction from n) with an original corrected image signal. Since the original correction image signal has a maximum value of 255 for the image sensor, it is 255
/ Wn, which is when the signal data is 50% of the white reference data, and is thus normalized to 128. On the other hand, the corrected image signal A obtained by performing shading correction from the addition data F (Wn) is 255 / F (Wn), which is a value obtained from the addition data F (Wn) in FIG.

However, in this method, the rounded difference data and the restored added data are as follows. F (Wn) = F (Wn ₋₁ ) + f (ΔWn) = F (W
n ₋₂ ) + f (ΔWn ₋₁ ) + f (ΔWn) = F (Wn ₋₃ )
+ F (ΔWn ₋₂ ) + f (ΔWn ₋₁ ) + f (ΔWn) = ·
In this manner, since the difference data ΔWn is a difference between the white reference data of the current pixel and the white reference data of one pixel before, the rounding error is accumulated in the added data which is the white reference data restored from the rounding difference data. There is.

Further, as shown in FIG. 4B, when the signal data is constant at the same density, an error occurs between the corrected image signal A subjected to shading correction from the added data F (Wn) and the original corrected image signal. In such a case, the density difference tends to be conspicuous at the boundary where the error occurs. In particular, if the signal data has the same intermediate density over several lines and an error occurs in the same pixel of the image sensor, the density difference may be visually conspicuous at the boundary where the error has occurred. FIG. 4C shows an example when the corrected image signal of FIG. 4B is recorded in halftone. Here the address is
It corresponds to the address of (b). Address 3, 4, 5
Is -2 compared to the corrected image signal of the surrounding address, the density becomes slightly blacker than the surrounding, and it may be visually conspicuous when the boundary is clear.

FIG. 5A is an explanatory view of a third embodiment of the present invention, and FIG. 5B is a graph thereof. When the absolute value of the difference data is small, the encoding is performed with high accuracy, and when the absolute value of the difference data is large, the encoding is performed with low accuracy, so that the 6-bit data is compressed into 4 bits. In accordance with the difference code of FIG. 5A, the image sensor output obtained by scanning the image sensor with the white reference by the shading correction device of FIG.
Wn, rounding difference data f (ΔWn), addition data F
FIG. 6A shows (Wn) and the rounding accuracy. This corresponds to FIG. 3A of the first embodiment. FIG. 6 (b)
Corresponds to FIG. 3B of the first embodiment. When the signal data is 50% of the white reference data, the corrected image signal A obtained by shading correction from the added data F (Wn) restored in FIG. FIG. 9 is a diagram comparing original corrected image signals.

FIG. 7 is a block diagram of a fourth embodiment of the present invention. This image sensor includes a contact image sensor 50 including a DC regeneration circuit and a contact image sensor 50.
A dark correction value detection / correction circuit 51 for correcting the data read by the black and white levels and a light correction value detection /
And a correction circuit 52. Further, these circuits include a light-dark correction memory 53 for storing light-dark correction values to be described later, an address counter 54 for designating an address of the light-dark correction memory 53, and a timing control unit for the contact image sensor 50 and other control circuits. A timing circuit 55 for supplying a signal is connected. The bright correction value detection / correction circuit 5
2 includes the same configuration as the bright correction value detection / correction circuit shown in the first embodiment of FIG. Further, the dark correction value detection / correction circuit 51 includes the same configuration although the signals handled are different.

In the present embodiment, the corrected signal processing system is constituted by a tracking comparison type analog-to-digital converter.
a peak detector circuit 56 for detecting the _peak, and ABC gain control circuit 57 for controlling the peak value from the history of the white level peak voltage, the ABC gain peak DA converting the back white density in document reading detected by the control circuit 57 Hold 58 and finally the analog input signal after the light / dark correction
7-bit image signal A for analog-to-digital conversion
DC59, the output of the peak hold 45 as the light / dark reference voltage of the image signal ADC, and the crank level V of the dark level output
2-bit DAC 6 for generating relative voltage from _CLP
0, 61.

Next, the operation of the image sensor shown in FIG. 7 will be described. A light source for document illumination (not shown) is turned off, and the output of the contact image sensor 50 at this time is AD-converted by a 6-bit tracking / comparison analog-to-digital converter of a dark correction value detection / correction circuit 51. A dark correction value for each element is obtained and stored in the light / dark correction memory 53.

Next, the light source for document illumination is turned on, a document feed roller (not shown) is read on the basis of white, a dark correction value is read from the light / dark correction memory 53, and the output of the contact image sensor 50 at this time is read. The dark correction value is analog-subtracted by the dark correction value detection / correction circuit 51 for each sensor pixel. The result of the subtraction is input to the peak detection circuit 56, and the white level peak voltage _Vpeak is detected. Then, the white level peak voltage V _{peak is set} to 100%, and the clamp level V reproduced by the direct current reproduction circuit of the contact image sensor 50 is reproduced.
_{The CLP is set} to 0%, and the white reference value is detected by the bright correction value detection / correction circuit 52 for each sensor pixel, with the range of 0% to 100% being the full scale. This detailed operation is the same as the operation of the light correction value detection circuit 10 in FIG. At this time, the white level peak voltage V _peak is held by the peak hold 58.
The detected white reference value is stored in the brightness correction memory 53.

When an actual document is read, the dark correction value is analog-subtracted from the output of the close contact image sensor 50 for each sensor pixel by the dark correction value detection / correction circuit 51, and the dark correction value is detected by the light correction value detection / correction circuit 52. Then, the subtraction result is amplified using the correction value for normalizing the white reference value as gain data.
This detailed operation is the same as that of the light correction circuit 30 in FIG. The dark correction value and the light correction value at this time are stored in the light / dark correction memory 53.
Is read out. During the reading of the document, the white peak is detected by the peak detection circuit 56 for each line, and the ABC level control circuit 57 controls the white level peak voltage _Vpeak from the history of the white peak detection result. The peak hold 58 holds the white level peak voltage V _peak .

The analog image signal corrected for brightness is converted from analog to digital by a 7-bit image signal ADC59. As shown in FIG. 8, the light-dark reference voltage of the image signal ADC 59 is a 2-bit D for generating a relative voltage from the white level peak voltage V _peak and the dark level output clamp level V _CLP.
Determined from AC60 and AC61. That is, the bright reference voltage V
_{DAC2 is, V DAC2 = V CLP + β} × (V peak -V CLP) dark reference voltage V _{DAC1 is, V DAC1 = V CLP + α} × (V peak -V CLP) where, α = 0,0.08,0 .17, 0.25 β = 1, 0.92, 0.83, 0.75.

Generally, by setting the light reference voltage V _DAC2 lower than the white level peak voltage V _peak and setting the dark reference voltage higher than the clamp level V _CLP of the dark level output, the image signal ADC 59 is subjected to A / D conversion. Enhances contrast.

In the above description, an example in which the concept of the correction shown in FIG. 2 is applied to a bright correction value detection / correction circuit is shown. However, it is needless to say that the correction concept can be similarly applied to a dark correction value detection / correction circuit.

[0044]

As described above, the first shading correction method of the present invention calculates the difference between the sensor output following the first pixel of the image sensor output and the pixel data of one pixel before, and calculates the difference data value. Is preset
With high precision when the reference value or less, also the difference data values before
When greater than serial reference value stored in each coded with a low accuracy, on the basis of the code data Ime
Sensor when the image sensor scans the density reference
Since the shading correction of the image sensor output is performed so that the value after the shading correction of the output becomes almost constant ,
The correction data capacity can be reduced, and the memory capacity for storing the correction data can be reduced. In addition, since the accuracy at the time of encoding is made equal, the occurrence frequency of the absolute value of the difference data equal to or less than the reference value is high, and the occurrence frequency of the range where the absolute value of the difference data is larger than the reference value is small, As a whole, it is possible to suppress the deterioration of the image quality in spite of the great effect of reducing the correction data capacity.

According to the second shading correction method of the present invention, sensor output correction data corresponding to the image sensor output is calculated, and correction data following the first pixel of the correction data is calculated with correction data one pixel before. Calculate the difference and make this difference data value less than the preset reference value
Is a high degree of accuracy, the difference data value is the reference value when the
When the image sensor is larger than the image sensor, it is coded and stored with low precision, and based on the coded data,
Of the image sensor output when scanning the density reference
Since the shading correction of the image sensor output is performed so that the value after the coding correction becomes almost constant, the memory capacity of the correction data can be reduced, and the accumulation of rounding errors caused by coding can be prevented. The deterioration of the image quality of the pixel corresponding to the data having a large absolute value of the difference data can be visually improved.

A shading correction device according to the present invention comprises an image sensor and a sensor output correction unit which corrects the output of the image sensor when the image sensor scans the density reference or when the incident light quantity is scanned under the specific conditions. A correction calculator that calculates data, a first memory that stores a head pixel of the correction data, and a subtractor that calculates a difference between correction data one pixel before the correction data following the head pixel; the difference data value is in advance
A coder for coding with high accuracy when the difference data value is equal to or less than the set reference value , and with low accuracy when the difference data value is larger than the reference value; and a second coder for storing the coded data.
, The second shading correction method of the present invention can be particularly suitably implemented.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 2 is a diagram for explaining a basic concept of differential coding in the present invention.

FIG. 3 is a diagram for explaining the operation of the first embodiment of the present invention.

FIG. 4 is a diagram for explaining the operation of a second embodiment of the present invention.

FIG. 5 is a diagram illustrating a difference code according to a third embodiment of the present invention.

FIG. 6 is a diagram for explaining the operation of a third embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of a fourth embodiment of the present invention.

FIG. 8 is a diagram for explaining an operation in a fourth embodiment of the present invention.

FIG. 9 is a block diagram for explaining a data compression technique in the related art.

FIG. 10 is a diagram for explaining an operation in the configuration of FIG. 9;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Bright correction value detection circuit 20 Bright correction memory 30 Bright correction circuit 12 Difference data rounding 13 Coder 16 Adder 17 Selector 21 Reference value memory 22 Bright correction value memory 31 Decoder 32 Selector 38 Significant difference judgment circuit 39 Correction calculator

Claims (6)

(57) [Claims] 1. An image sensor output when the image sensor scans the density reference or when the density reference is scanned with the incident light amount being a specific condition. Is calculated, and the difference data value is equal to or smaller than a predetermined reference value.
When the value is below, the accuracy is high, and the difference data value is the reference value.
When it is larger than the above , each is coded and stored with low precision, and based on the coded data, the image sensor is stored.
Of the image sensor output when the sensor scans the density reference.
A shading correction method for an image sensor, wherein the shading correction of the output of the image sensor is performed so that the value after the shading correction becomes substantially constant . 2. A sensor output correction data corresponding to an output of the image sensor when the image sensor scans the density reference or when the density reference is scanned with the incident light amount being a specific condition, and the first pixel of the correction data is calculated. The difference between the correction data following 1 and the correction data one pixel before is calculated, and the difference data value is equal to or less than a predetermined reference value.
When the difference data value is higher than the reference value with high accuracy .
When the image sensor is larger than the image sensor, it is coded and stored with low precision, and based on the coded data,
Of the image sensor output when scanning the density reference
A shading correction method for an image sensor, wherein the shading correction of the output of the image sensor is performed so that a value after the shading correction becomes substantially constant . 3. An image sensor, and a correction operation for calculating sensor output correction data corresponding to an image sensor output when the image sensor scans a density reference or scans a density reference with an incident light amount as a specific condition. A first memory for storing a head pixel of the correction data, a subtractor for calculating a difference between the correction data following the head pixel and the correction data one pixel before, and a difference data value which is predetermined. A coder for encoding with high accuracy when the difference data value is equal to or less than the reference value , and with low accuracy when the difference data value is greater than the reference value, and a second memory for storing the encoded data. Characteristic image sensor shading correction device. 4. An adder for adding the rounded difference data corresponding to the coded data and the correction data after one pixel, and a difference between the addition data and the correction data after one pixel as the difference data; 4. The image sensor shading correction apparatus according to claim 3, wherein the code is coded by the coder and stored in the second memory. 5. A decoder for decoding coded data read from the second memory, and one of a head pixel of the correction data of the image sensor read from the first memory and one of added data one pixel before the selected data. 4. The image processing apparatus according to claim 3, further comprising a selector, an adder that adds the decoded data and the selector output, and a correction calculator that calculates a corrected image signal from an image signal obtained when the image sensor scans a document and the added data. 4 is a shading correction device for an image sensor. 6. A subtractor for calculating a difference between the corrected image signal and the corrected image signal one pixel before, a significant difference determination circuit for determining the result of the subtraction, and a difference between the corrected image signal and 1 based on the significant difference determination result. 6. The shading correction device for an image sensor according to claim 5, further comprising: a selector for selecting one of the correction image signals before the pixel.