JP4994158B2 - Image correction device - Google Patents

Description

  The present invention relates to an image correction apparatus that corrects shading in an imaging apparatus.

  The image pickup apparatus converts an image formed through a lens into an image signal by an image pickup device and takes out the image. However, a phenomenon called shading in which the peripheral portion becomes darker than the central portion appears due to a decrease in peripheral light amount. This is also called vignetting, and the incident angle of light incident on the imaging surface increases as it goes to the periphery, and the amount of light per unit area decreases, and when an image sensor such as a CCD or CMOS is used. This is because the incident angle to the microlens per unit pixel used for increasing the light collection efficiency is increased. In addition, since the lens is lowered and the characteristics are deteriorated due to the demand for miniaturization and thinning of the digital camera, the shading characteristics are asymmetric and distorted with respect to the center of the optical axis.

  Conventionally, as a method of correcting this asymmetric shading characteristic, an image is divided into blocks of different sizes in the central part and the peripheral part, and the gain data of each pixel is calculated and corrected using reference gain data at the boundary between the blocks. There is a technique for performing (see Patent Document 1, for example). There is also a technique for correcting shading characteristics in the case of asymmetry by performing coordinate conversion from the difference between the average values of regions equidistant from the center in the horizontal and vertical directions and obtaining a correction coefficient from a correction function. (For example, refer to Patent Document 2). Further, as a shading correction method, there is a technique for correcting shading characteristics in the case of asymmetry by having on-axis coefficients for the X and Y axes for correcting asymmetric sensor shading and a correction coefficient for correcting symmetric lens shading. Yes (see, for example, Patent Document 3).

JP 2006-121612 A JP 2007-134903 A JP-A-2005-341033

  Conventional shading correction is performed as described above. However, in the case of the technique disclosed in Patent Document 1, since an image is processed by being divided into blocks, the memory capacity increases, and the optical zoom position depends on the optical zoom position. The gain data needs to be changed. Further, in the case of the technique disclosed in Patent Document 2, it is possible to deal with only when the symmetric lens shading characteristic is tilted, and the characteristic when distorted cannot be corrected. In the case of the technique disclosed in Patent Document 3, asymmetric lens shading is not considered.

  The present invention has been made to solve the above-described problems, and an object thereof is to obtain an image correction apparatus capable of correcting asymmetric and distorted shading with a small memory capacity.

The image correction apparatus according to the present invention includes a pixel count means for calculating the coordinate position of each pixel constituting the image by counting the horizontal synchronization signal and the vertical synchronization signal, respectively,
Distance calculating means for calculating the distance from the optical axis center position to the coordinate position of each pixel in the image, angle calculating means for calculating the angle of the coordinate position of each pixel from the optical axis center, and the coordinates of each pixel A modulation coefficient is calculated according to the position angle value, and a distance modulation means for modulating the distance calculated by the distance calculation means based on the calculated modulation coefficient, and shading according to each modulated distance value Correction coefficient calculation means for calculating the correction coefficient for each, and image signal correction means for performing correction by multiplying the corresponding correction signal by the calculated correction coefficient , and the distance modulation means is a light divided by reference point intervals. The modulation coefficient is calculated using a distance modulation table in which the modulation coefficient is associated with the angle from the axis center.

  According to the present invention, the distance from the optical axis center of the image is modulated according to the angle from the optical axis center, and the shading correction coefficient corresponding to the modulated distance is obtained to correct the target portion of the image. As a result, asymmetric and distorted shading characteristics can be corrected. Further, since the image is not divided into blocks and processed, it can be configured with a small memory capacity.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a functional configuration of an image correction apparatus according to Embodiment 1 of the present invention.
In the figure, the image correction apparatus includes a horizontal pixel count unit 10, a vertical pixel count unit 20, a distance calculation unit 30, an angle calculation unit 40, a distance modulation unit 50, a distance modulation table 60, a correction coefficient calculation unit 70, and a correction coefficient calculation table. 80 and an image signal correction unit 90.
The horizontal pixel counting unit 10 and the vertical pixel counting unit 20 are means for calculating the coordinate position of each pixel constituting the image by counting the horizontal synchronizing signal and the vertical synchronizing signal, respectively. The distance calculation unit 30 is a means for calculating the distance from the optical axis center position in the image to the coordinate position of each pixel. The angle calculation unit 40 is a means for calculating the angle formed by the coordinate position of each pixel from the optical axis center. The distance modulation unit 50 is a unit that calculates a modulation coefficient corresponding to the angle value of the coordinate position of each pixel and modulates the distance based on the calculated modulation coefficient. The correction coefficient calculation unit 70 is a means for calculating a correction coefficient for shading in accordance with each modulated distance value. The image signal correction unit 90 is a unit that performs correction by multiplying the corresponding image signal by each calculated correction coefficient.

The image correction apparatus is supplied with a horizontal synchronization signal, a vertical synchronization signal, and an image signal from a well-known imaging unit (not shown). The horizontal pixel counting unit 10 calculates horizontal coordinates X of pixels constituting the image by counting input horizontal synchronization signals. Further, the vertical pixel count unit 20 calculates the vertical coordinate Y of the pixels constituting the image by counting the vertical synchronization signal input from the imaging unit. In the distance calculation unit 30, the distance R between the coordinates (X, Y) calculated by the horizontal pixel counting unit 10 and the vertical pixel counting unit 20 and the optical axis center coordinates (x _c , y _c ) is set as the processing of equation (1). And the calculated value is output to the distance modulation unit 50.

距離変調部５０では、各画素の座標位置の角度θの値に応じて変調係数を算出し、算出した変調係数に基づいて距離Ｒを変調する。この例では、図２に示されるような距離変調テーブル６０を用いて距離変調係数を算出する。距離変調テーブル６０では、基準点間隔Ｔ、テーブル数Ｎ、テーブル番号ｉの時の変調係数をｍ_i として設定している。ここで、角度θが（３）式を満たす場合（ｎは整数）、変調係数ｍは、（４）式を用いて、線形補間により求められる。なお、ここでは角度θの両端２点から線形補間により変調係数を算出したが、両端の４点を多項式で近似し、近似式から、角度θにおける変調係数を求めてもよい。
ｎT≦θ＜（ｎ+１）T （３）

変調後の距離をＲ’とすると、（５）式を用いて求められる。
Ｒ’＝Ｒ×ｍ （５） The angle calculation unit 40 calculates the angle θ of the coordinate position of each pixel from the center of the optical axis according to the processing of equation (2), and outputs the calculated value to the distance modulation unit 50.

The distance modulation unit 50 calculates a modulation coefficient according to the value of the angle θ of the coordinate position of each pixel, and modulates the distance R based on the calculated modulation coefficient. In this example, a distance modulation coefficient is calculated using a distance modulation table 60 as shown in FIG. In the distance modulation table 60, the modulation coefficient for the reference point interval T, the number of tables N, and the table number _i is set as mi. Here, when the angle θ satisfies the equation (3) (n is an integer), the modulation coefficient m is obtained by linear interpolation using the equation (4). Here, the modulation coefficient is calculated by linear interpolation from two points at both ends of the angle θ, but the four points at both ends may be approximated by a polynomial, and the modulation coefficient at the angle θ may be obtained from the approximate expression.
nT ≦ θ <(n + 1) T (3)

If the distance after modulation is R ′, the distance can be calculated using equation (5).
R ′ = R × m (5)

なお、ここでは変調後の距離Ｒ’の両端２点から線形補間により補正係数を算出したが、距離Ｒ’の両端の４点を多項式で近似し、近似式から、変調後の距離Ｒ’における補正係数を求めるようにしてもよい。 The correction coefficient calculation unit 70 calculates a correction coefficient for shading according to the value of the distance R ′ after modulation. The correction coefficient calculation table 80 is used for this process. In the correction coefficient calculation table 80, as shown in FIG. 3, the correction coefficient for the reference point interval S, the number of tables M, and the table number _j is set as k _j . Here, when the distance R ′ after modulation satisfies the equation (6) (n ′ is an integer), the correction coefficient k is obtained by linear interpolation using the equation (7).
n ′S ≦ R ′ <(n ′ + 1) S (6)

Here, the correction coefficient is calculated by linear interpolation from two points at both ends of the distance R ′ after modulation. However, the four points at both ends of the distance R ′ are approximated by a polynomial, and the distance R ′ after modulation is calculated from the approximation formula. A correction coefficient may be obtained.

The image signal correction unit 90 corrects the image signal I by multiplying the correction coefficient k obtained by the correction coefficient calculation unit 70. That is, the luminance value of the pixel is corrected. If the corrected signal is O, it can be obtained using equation (8).
O = I × k (8)
Note that the correction coefficient calculation unit 70 may calculate the correction coefficient for each of R, G, and B using the correction coefficient calculation table 80 in which the correction coefficient is provided for each of R, G, and B. In that case, the image signal correction unit 90 multiplies correction coefficients corresponding to the R, G, and B signal components of each pixel of the image.

  As described above, according to the first embodiment, the distance from the optical axis center of the image is modulated according to the angle from the optical axis center, and the shading correction coefficient corresponding to the modulated distance is obtained to obtain the image. Since the target portion is corrected, asymmetric and distorted shading characteristics can be corrected. Further, since the image is not divided into blocks and processed, it can be configured with a small memory capacity.

Embodiment 2. FIG.
Here, the distance modulation table 60 used by the distance modulation unit 50 to calculate the distance modulation coefficient is configured by a plurality of tables divided for each distance range from the optical axis center.
The table for each distance range associates the angle from the optical axis center with the modulation coefficient as in the first embodiment, but the modulation coefficient is set to a value suitable for the distance range. For example, when there are two tables A and B divided by the distance range, the distance modulation unit 50 selects the table A when the distance R is equal to or less than the threshold value Rth. In other cases, the distance modulation table B is selected. Next, using the selected distance modulation table, the modulation coefficient corresponding to the angle from the optical axis center is calculated in the same manner as in the first embodiment. The distance modulation table may be divided into three or more.

  As described above, according to the second embodiment, the distance modulation unit 50 selects one from a plurality of distance modulation tables according to the distance from the optical axis center, and from the selected distance modulation table from the optical axis center. Since the distance is modulated by calculating the modulation coefficient according to the angle, correction can be performed even when the degree of distortion shows shading characteristics that are not uniform with respect to the distance from the center of the optical axis. It becomes like this. At this time, the reference point interval of the table may be changed depending on whether the distance R is equal to or less than Rth and exceeds Rth. By changing the number of reference points, an increase in the amount of memory for the table can be suppressed.

ここで、ｆ（ｘ）はｘに対する多項式であるが、この多項式を、レンズのズーム位置、フォーカス位置、絞り量の少なくとも１つの撮像条件に応じた式に自動的に変更するようにする。このことにより、撮像条件を変更した際に、その都度補正係数算出テーブル８０を変更することなく補正係数を算出することができるようになる。 Embodiment 3 FIG.
Here, the distance calculation unit 30 calculates the distance R from the center position of the optical axis in the image to the coordinate position of each pixel using the equation (9).

Here, f (x) is a polynomial for x, and this polynomial is automatically changed to an expression corresponding to at least one imaging condition of the zoom position, focus position, and aperture amount of the lens. This makes it possible to calculate the correction coefficient without changing the correction coefficient calculation table 80 each time the imaging condition is changed.

It is a block diagram which shows the function structure of the image correction apparatus by Embodiment 1 of this invention. It is explanatory drawing which shows the data structure of the distance modulation table which concerns on Embodiment 1 of this invention. It is explanatory drawing which shows the data structure of the correction coefficient calculation table which concerns on Embodiment 1 of this invention.

Explanation of symbols

  10 horizontal pixel count unit, 20 vertical pixel count unit, 30 distance calculation unit, 40 angle calculation unit, 50 distance modulation unit, 60 distance modulation table, 70 correction coefficient calculation unit, 80 correction coefficient calculation table, 90 image signal correction unit.

Claims (2)

In an image correction apparatus for correcting shading of an image generated when an image formed on an image sensor through a lens is extracted and reproduced as an image signal,
Pixel counting means for calculating the coordinate position of each pixel constituting the image by counting the horizontal synchronizing signal and the vertical synchronizing signal, respectively;
Distance calculating means for calculating the distance from the optical axis center position in the image to the coordinate position of each pixel;
Angle calculating means for calculating the angle of the coordinate position of each pixel from the optical axis center;
A distance modulation unit that calculates a modulation coefficient according to a value of an angle of a coordinate position of each pixel, and modulates the distance calculated by the distance calculation unit based on the calculated modulation coefficient;
Correction coefficient calculating means for calculating a correction coefficient for shading in accordance with the value of each modulated distance;
Image signal correction means for performing correction by multiplying the corresponding image signal by each calculated correction coefficient ,
The distance modulation means calculates a modulation coefficient using a distance modulation table in which a modulation coefficient is associated with an angle from an optical axis center divided by a reference point interval. The distance modulation table is a plurality of tables generated for each range of distance from the optical axis center,
Distance modulation means, after selecting the corresponding table to the distance from the distance calculating unit optical center computed in claim 1, characterized in that to calculate the modulation factor using the table the selected The image correction apparatus described.

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