What is the Center of the Image?
CARNEGIE-MELLON UNIV PITTSBURGH PA SCHOOL OF COMPUTER SCIENCE
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People need to calibrate camera systems in order to determine the relationship between the positions of features in object space and their corresponding positions in the image. Part of camera calibration is the determination of image center. But, what is the image center Ideally, the image center is considered to be the point of intersection of the cameras optical axis with the cameras sensing plane. In fact there are many possible definitions of image center, and in real lenses most do not have the same coordinates. In addition, the image centers move as lens parameters are changed. In this paper we examine why image centers are not necessarily the same for different image properties and why they vary with lens parameters. We then provide a taxonomy of 16 different image centers and describe procedures for measuring them. Finally we examine the calibration of image center for a variable parameter lens. Several techniques are applied to a precision automated zoom lens and experimental results are shown. We conclude that the accuracy of the image center can be an important factor in the accuracy of the overall camera calibration, and that the large variation in the position of the image center across different definitions and different lens settings makes the calibration problem much more complex than is conventionally believed. With proper modeling, by using correct definitions for all image centers in a system, we can improve the accuracy of our camera calibration.