Camera Calibration

Camera Calibration

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In AR or other machine vision applications, there is always faced with this question: To determine the relationship of the position on the location of objects in space and screen image.

[image]=[?][object]

The geometric model of the camera imaging, namely, camera parameters (including the rotation matrix R of the internal reference K and external parameters, translation matrix T).

•Extrinisic parameters define location and orientation of camera reference frame with respect to world frame

•Intrinsic parameters define pixel coordinates of image point with respect to coordinates in camera reference frame

Pinhole model

Intrinsic parameters

•Intrinsic Parameters:

–Focal Length f

–Pixel size sx , sy

–Image center ox , oy

–(Nonlinear radial distortion coefficients k1 , k2…)

•Calibration  = Determine the intrinsic parameters of a camera

Calibration: relates points in the image to rays in the scene

f: effective focal length:distance of image plane from O.

Extrinsic Parameters

The relation between camera and world coordinate frame

•Step 1: Transform into camera coordinates

•Step 2: Transform into image coordinates

Geometric Model

nWhich parameters need to be estimated.

nFocal length, image center, aspect ratio

nRadial distortions

nWhat kind of accuracy is needed.

nApplication dependent

nWhat kind of calibration object is used.

nOne plane, many planes

·Complicated three dimensional object 

Parameters can be calculated based on the above five equations.

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