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Calibration

Calibration refers to the process of determing the extrinsic and intrinsic camera parameters.

Most users will only need to follow the calibration instructions given in the manual. The following information is for advanced users who e.g. want to use a different calibration body or pattern.

For DAVID, the world coordinate system is the same as the calibration pattern coordinate system.

DAVID can determine the following camera parameters:

Extrinsic camera parameter:

  • Tx, Ty, Tz - Position of the world coordinate system w.r.t. the camera coordinate system (x,y,z-coordinates)
  • Rx, Ry, Rz - Rotation from the camera to the world system (rotations around the x,y,z-Axes)

The camera coordinate system lies in its focal point, with the following axes:

  • x-axis = rightwards in the image plane
  • y-axis = downwards in the image plane
  • z-axis = viewing direction (from camera towards scene)

To obtain the camera pose w.r.t. the world, you have to invert the transformation given by (Tx, Ty, Tz, Rx, Ry, Rz).

Intrinsic camera parameter:

  • f - focal length
  • aspect ratio of a physical pixel (on the CCD-Chip)
  • Cx, Cy - image coordinates of the center of radial lens distortions
  • kappa1 - radial lens distortion

To calibrate the camera, DAVID needs to know the 2d coordinates of all 25 calibration markers in the image and their 3d coordinates (coordinates with respect to the coordinate system of the calibration corner, which are stored in the file calibpoints.dat).

How to calibrate

Before clicking on “calibrate” you have to put in the Scale-value, if you want to store the correct size of the scanned object too.

David needs to see just 26 points in the cam´s picture (see Camsettings) taken from the Calibrationpattern fixed on the Calibrationcorner:

  • 6 Inner points (white points in black circles)
  • 20 normal points (black on white)

The middle of the distance between the 2 inner points (double point down,right) is calculated as point number one. So DAVID gets 25 calculated points out of 26 visible points.


There are two modes to do calibration

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There is one tool to clean the cam´s picture to help the automatic mode

—-

The basic coordinates of these 25 points are stored in the calibpoints.dat-file in your DAVID folder.

If you change the position of the printed pattern (in X or Z direction) on the calibration corner in reality, you have to change these values adequately. However you do it, for scanning it is necessary that

  • The plane in the left part of the camera image is the YZ-plane (x coordinate = 0)
  • The plane in the right part of the camera image is the XY-plane (z coordinate = 0)

Calibration fails???

calibration.txt · Last modified: 2014/06/06 18:38 (external edit)