Image analysis – Applications – Range or distance measuring
Reexamination Certificate
1998-07-21
2001-03-13
Johns, Andrew W. (Department: 2721)
Image analysis
Applications
Range or distance measuring
Reexamination Certificate
active
06201882
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a camera calibration apparatus, and more particularly to a camera calibration apparatus which estimates the position and the direction of a camera from the positions of characteristic points of an image imaged.
2. Description of the Related Art
In a conventional camera calibration apparatus of the type mentioned, an object having three or more characteristic points (or characteristic lines) whose relative positions are known is imaged, and calibration wherein the position and the direction of the camera are estimated from the positions of the characteristic points in the thus imaged image is performed.
Conversely to the calibration described above, as an object posture estimation method, a tetrahedron arrangement estimation method which uses four or more characteristic points, a three-line segment and two-point arrangement estimation method which uses three or more line segments and two or more characteristic points, and so forth have been proposed. While any of the object posture estimation methods estimates the posture of an object, since the relationship between the camera and the object is relative, it is also possible to estimate the position or the direction of the camera using the method.
The tetrahedron arrangement estimation method is disclosed in Japanese Patent Laid-Open Application No. Heisei 1-232484, and the three-line segment and two-point arrangement estimation method is disclosed in Japanese Patent Laid-Open Application No. Heisei 2-51008.
In the conventional camera calibration described above, processing of mapping between characteristic points originating from an image and characteristic points of an original object imaged is required. In this instance, in order to raise the accuracy in calibration, it is necessary to increase the number of characteristic points. However, as the number of characteristic points increases, the mapping processing becomes less stable and more complicated.
Further, with the tetrahedron arrangement estimation method, the three-line segment and two-point estimation method and so forth as an object posture estimation method, it is difficult to stably extract four or more characteristic points (characteristic lines) depending upon imaging conditions. As a result, even if an error occurs with the mapping between characteristic points, no countermeasure to correct the error can be taken.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a camera calibration apparatus which eliminates the necessity to effect mapping making a distinction among a plurality of characteristic points and can prevent complication in mapping processing even if the number of characteristic points increases.
In order to attain the object described above, according to an aspect of the present invention, there is provided a camera calibration apparatus, comprising object imaging means for imaging a sphere whose magnitude and position in a three-dimensional coordinate system are known, magnitude/position detection means for determining a magnitude and a position of the sphere on a screen from an image imaged by the object imaging means, center position estimation means for estimating a three-dimensional position of the center of the sphere from the magnitude and the position of the sphere on the screen determined by the magnitude/position detection means, and means for calculating a position of the object imaging means in the three-dimensional coordinate system based on the three-dimensional position of the center of the sphere estimated by the center position estimation means.
In the camera calibration apparatus, a sphere whose magnitude and position in a three-dimensional coordinate system are known is imaged, and from the thus imaged image, the magnitude and the position of the sphere on the screen are determined. Then, from the magnitude and the position of the sphere on the screen thus determined, the three-dimensional position of the center of the sphere is estimated, and the position of the camera in the three-dimensional coordinate system is calculated based on the thus estimated three-dimensional position of the center of the sphere. Consequently, the camera calibration apparatus is advantageous in that the necessity to perform mapping of a plurality of characteristic points making a distinction among them can be eliminated and complication of mapping processing can be prevented even if the number of characteristic points is increased.
The camera calibration apparatus may further comprise zenith detection means for determining a position of the zenith of the sphere on the screen from the image imaged by the object imaging means, zenith position estimation means for estimating a three-dimensional position of the zenith from the position of the zenith on the screen determined by the zenith detection means and the three dimensional position of the center of the sphere, and means for calculating a direction of the object imaging means in the three-dimensional coordinate system based on the three-dimensional position of the zenith estimated by the zenith position estimation means and the three-dimensional position of the center of the sphere estimated by the center position estimation means.
As an alternative, the camera calibration apparatus may further comprise equator characteristic point detection means for determining a position of a characteristic point on the equator of the sphere on the screen from the image imaged by the object imaging means, orthogonal vector calculation means for determining, from the position of the characteristic point on the equator determined by the equator characteristic point detection means and the three-dimensional position of the center of the sphere, a line segment orthogonal to a straight line interconnecting the characteristic point on the equator and the center of the sphere, and means for calculating a direction of the object imaging means in the three-dimensional coordinate system based on the three-dimensional position of the zenith estimated based on the line segment determined by the orthogonal vector calculation means and the three-dimensional position of the center of the sphere estimated by the center position estimation means.
In each of the camera calibration apparatus which comprises the additional elements described above, the position of a characteristic point at the zenith or on the equator of a sphere on the screen is determined from an imaged image, and from the thus determined position of the characteristic at the zenith or on the equator on the screen and the three-dimensional position of the center of the sphere, a line segment orthogonal to a straight line interconnecting the three-dimensional position of the zenith or the characteristic point on the equator and the center of the sphere is estimated. Then, based on the thus estimated line segment orthogonal to the straight line interconnecting the three-dimensional position of the zenith or the characteristic point on the equator and the center of the sphere and the three-dimensional position of the center of the sphere, the position and the direction of the camera in the three-dimensional coordinate system are calculated. Consequently, the camera calibration apparatus is advantageous in that the necessity to perform mapping of a plurality of characteristic points making a distinction among them can be eliminated and complication of mapping processing can be prevented even if the number of characteristic points is increased.
Alternatively, the camera calibration apparatus may further comprise equator characteristic point detection means for determining positions of a plurality of characteristic points on the equator of the sphere on the screen from the image imaged by the object imaging means, equator plane calculation means for determining an equator plane of the equator from the positions of the plurality of characteristic points on the equator determined by the equator characteristic point detection means and the three-dimensional posi
Johns Andrew W.
McGuireWoods LLP
Nakhjavan Shervin
NEC Corporation
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