JP2006135621A - Camera calibrating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a camera calibrating device that can estimate an object parameter without a large error with a marker whose position relative to an object to be imaged, e.g. a vehicle is not known, in other words, without knowing world coordinates, and then can calibrate a camera nearly accurately when monitoring, object detection, etc., by the camera is carried out. <P>SOLUTION: The camera calibrating device has a linear marker 1 where a figure including a linear part is drawn, the camera 2 imaging the linear marker 1, and a linear marker camera calibrating means 3 of finding a camera internal parameter of the camera 2 from an image of the linear marker 1 that the camera 2 images. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a camera calibration apparatus.

  2. Description of the Related Art In recent years, in various industrial fields, camera calibration devices that determine the distance and direction from a camera to an object to be imaged have been developed in order to perform monitoring and object detection using a camera. For example, when a camera is mounted on the rear side of a vehicle and a guideline that is an expected trajectory corresponding to the steering angle of the steering wheel of the vehicle is drawn on the image, a camera calibration operation is required to accurately determine the positional relationship between the road surface and the camera. .

Here, camera parameters obtained by camera calibration will be described.
FIG. 4 is an explanatory diagram of a coordinate system used in camera calibration. α, the camera an orthogonal coordinate system of the three-dimensional space for capturing a world coordinate system to the origin and _{O W (x w, y w} , z w). On the other hand, β is a camera coordinate system (x, y, z) expressed by a pinhole camera model (ideal camera model without distortion aberration). The relationship between the world coordinate system (x _w , y _w , z _w ) and the camera coordinate system (x, y, z) can be expressed by the following equation by combining the coordinate rotation transformation matrix and the coordinate movement transformation matrix.

なお、Ｒは回転を表す３行３列の行列、Ｔは移動を表す３行１列の行列である。

Note that R is a 3 × 3 matrix representing rotation, and T is a 3 × 1 matrix representing movement.

An arbitrary point P in the camera coordinate system (x, y, z) expressed by the above-described pinhole camera model is projected from the camera origin O onto the projection plane S at the focal length f. This projection plane S is referred to as a first image coordinate system (x _u , y _u ) without distortion, and has the following relationship with the camera coordinate system (x, y, z).

In the camera actually used, since the lens has distortion, the second image coordinate system (x _d , y _d ) that is a distorted projection plane in consideration of this lens distortion and image coordinates that are a distortion-free projection plane. The system (x _u , y _u ) has the following relationships (3) and (4).

  Also,

Here, an actual image coordinate system (x _f , y _f ) with the origin at the left corner of the image is introduced in a two-dimensional coordinate system corresponding to the image on the monitor. The actual image coordinate system (x _f , y _f ) and the second image coordinate system (x _d , y _d ) having the aforementioned distortion have the following relationship.

The parameters obtained by camera calibration are R, T, f, κ, Sx, Cx, and Cy. Among these, R and T (both refer to equation (1)) are parameters relating to the camera posture and are external parameters, and the rest are parameters relating to the camera and are called internal parameters.
Parameters to be obtained by the camera calibration device of the present invention are Sx, Cx, and Cy. The parameters f and κ are assumed to be known from the camera specifications. Of these, the parameter κ is given not only as a constant but also as a distortion ratio corresponding to the distance from the image center.

  FIG. 5 is a configuration diagram of a conventional camera calibration apparatus. In FIG. 5, the camera calibration apparatus includes a world coordinate known marker 10 including a feature point whose world coordinates are known, a camera 20 that is a calibration target for imaging the world coordinate known marker 10, and the camera 20 images. World coordinate known marker calibration means 30 for calculating camera parameters from an image of the world coordinate known marker 10 is provided. FIG. 6 is a three-dimensional view showing the positional relationship between the world coordinate known marker 10 and the camera 20 in the conventional camera calibration apparatus.

Here, a conventional camera calibration apparatus will be described with reference to FIGS. 5 and 6.
First, as shown in FIG. 6, the world coordinate known marker 10 is imaged by the camera 20. For example, each black square vertex of the world coordinate known marker 10 is used as a feature point, and the world coordinates of the feature point are measured. Next, the world coordinate known marker calibration means 30 calculates camera parameters from the world coordinates of a plurality of feature points and the actual image coordinates of the feature points on the captured image, and obtains target parameters Sx, Cx, and Cy. Details of this calculation method are described in Non-Patent Document 1, for example.
"A Versatile Camera Calibration Technique for High-Accuracy 3D Machine Vision Metrology Using Off-the-Shelf TV Cameras and Lenses", Roger Y. Tsai, IEEE Journal of Robotics and automation Vol. RA-3. No.4 August 1987, pp323 -344 (Roger Y. Tsai, "Multi-purpose camera calibration technology using high-precision three-dimensional measurement method using general TV camera and lens" IEEE Journal on Robotics and Automation, Volume RA-3, No. 4 August 1987, pp. 323-344)

  However, in the above camera calibration device, the world coordinates of the feature point of the world coordinate known marker must be known, and camera parameters other than the target camera parameters (Sx, Cx, Cy), that is, R, T , F, and κ are also obtained, and there is a problem that an estimation error becomes large.

  The present invention has been made in view of the above circumstances, and by using a marker whose relative position to an object to be imaged, for example, a vehicle, is unknown, in other words, even if the world coordinates are not known, only the target parameter is obtained. It is an object of the present invention to provide a camera calibration apparatus that can perform estimation without a large error, and as a result, can perform camera calibration almost accurately when performing monitoring or object detection using a camera.

The camera calibration device of the present invention obtains camera internal parameters of the camera from a linear marker on which a graphic including a straight line portion is drawn, a camera that captures the linear marker, and an image of the linear marker that is captured by the camera. And a linear marker camera calibration means.
According to this configuration, the camera can be calibrated with a marker whose relative position to the vehicle is unknown, and only the target parameter can be estimated.

から算出するカメラパラメータ（Ｓｘ，Ｃｘ，Ｃｙ）である構成のものである。 Further, in the camera calibration apparatus of the present invention, the camera internal parameter is represented by the following arithmetic expression:

The camera parameters (Sx, Cx, Cy) calculated from

  According to the present invention, the camera can be calibrated by a marker whose relative position to the object to be imaged, for example, the vehicle is unknown, in other words, even if the world coordinates are not known, only the target parameter has a large error. It is possible to provide a camera calibration device that can be estimated without any problem.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a block diagram showing a configuration of a camera calibration apparatus according to an embodiment of the present invention. The camera calibration apparatus of this embodiment includes a linear marker 1, a camera 2, and a linear marker calibration unit 3. Among these, the straight line marker 1 is drawn with a graphic including a straight line as shown in FIG. On the other hand, the linear marker calibration means 3 calculates camera parameters (Sx, Cx, Cy) of the camera 2 based on the image of the camera 2 that captures the linear marker 1. For these camera parameters (Sx, Cx, Cy), refer to equation (5) in the [Background Art] column.

FIG. 2 is a perspective view showing a positional relationship between the linear marker 1 and the camera 2 in the camera calibration apparatus according to the embodiment of the present invention.
First, as shown in FIG. 2, the linear marker 1 and the camera 2 are arranged. Next, the straight line marker 1 on which a graphic including a straight line is drawn by the camera 2 is imaged. Thereafter, the linear marker calibration means 3 mathematically calculates the camera parameters (Sx, Cx, Cy) of the camera 2 from the image of the camera 2 that captures the linear marker 1.

FIG. 3 is an explanatory diagram of the linear marker calibration means 3 in the camera calibration apparatus according to the embodiment of the present invention.
Since the straight line in the world coordinate system (x _w , y _w , z _w ) is also a straight line in the camera coordinate system (x, y, z), it is a straight line even in a coordinate system without distortion. However, in actuality, the image of the linear marker 1 looks like a distorted image like the coordinate system 1A of FIG. This is because the lens is distorted and does not become a straight line due to distortion caused by the equations (3) and (4).
If the camera parameters (Sx, Cx, Cy) are correct values, using the equations (3) to (5), in the coordinate system without distortion, as in the coordinate system 1B in FIG. Should return to a straight line. On the other hand, if the camera parameters (Sx, Cx, Cy) are not correct values, the camera parameters (Sx, Cx, Cy) do not return to a straight line as in the coordinate system 1C of FIG.
Using this property, the camera parameters (Sx, Cx, Cy) are optimized so as to be a straight line in a coordinate system without distortion.

First, the actual image coordinates of a pixel for calibrating a straight line on the image are set as a point sequence (X _fi , Y _fi ) i = 1, n. Then, (Sx = 1, Cx = image width / 2, Cy = image height / 2) is used as an initial value, and there is no distortion of the point sequence constituting the straight line using the equations (3) to (5). Image coordinates (X _ui , Y _ui ) are obtained.
Next, a straight line expression is obtained from the image coordinates without distortion of the point sequence by the least square method, and the sum of the distances from the straight line to each point sequence is used as the evaluation value. Then, the camera parameters (Sx, Cx, Cy) are optimized so that the evaluation value becomes low, and the camera parameters are calculated.
Thus, according to the camera calibration device of the present embodiment, the camera can be calibrated with a marker whose relative position to the vehicle is unknown.

  The present invention is not limited to the above-described embodiment, and can be implemented in various forms without departing from the spirit of the invention.

  The camera calibration apparatus according to the present invention can detect only the target parameter without a large error even if the world coordinate is not known by a marker whose relative position to the object to be imaged, for example, the vehicle is unknown. It has an effect that can be estimated, and camera calibration for obtaining the distance and direction from the camera to the object to be imaged can be performed almost accurately, which is useful when performing monitoring or object detection by the camera.

1 is a schematic block diagram showing the configuration of a camera calibration device according to an embodiment of the present invention. The perspective view which shows the arrangement | positioning relationship of the linear marker and camera which concern on embodiment of this invention (A)-(C) is explanatory drawing which shows the effect | action of the linear marker calibration means which concerns on embodiment of this invention. Explanatory diagram showing the relationship between the world coordinate system and the camera coordinate system Schematic block diagram showing the configuration of a conventional camera calibration device The perspective view which shows the arrangement | positioning relationship of the world coordinate known marker and camera used with the conventional camera calibration apparatus.

Explanation of symbols

1 linear marker 2 camera 3 linear marker calibration means 10 world coordinate known marker 30 world coordinate known marker calibration means

Claims (2)

A straight line marker on which a figure including a straight line part is drawn, and
A camera for imaging the linear marker;
And a linear marker camera calibration unit that obtains and optimizes camera internal parameters of the camera from an image of the linear marker captured by the camera. The camera internal parameters are the following arithmetic expressions:

The camera calibration device according to claim 1, wherein the camera parameters are calculated from (Sx, Cx, Cy).

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