Author:	Peter Stierlen (peter.stierlen@t-online.de)
History:	2008/08/21 (v1.0): first version
Source:	Optic_Calib.java
Installation:	Download Optic_Calib.java to the plugins folder and compile it. You must also download the Zhang calibration tool EasyCalib.exe from http://research.microsoft.com/research/downloads and copy it into the same folder as the Optic_Calib Plugin files.
Description:	This plugin allows you to automatically calculate the intrinsic camera parameters. You can find more details about the Zhang calibration method at http://research.microsoft.com/%7Ezhang/Calib/

Determination of intrinsic camera parameters

Optical lense systems have more or less imperfections which lead to optical distortions. This distortions must be taken into account if one wants to use images for measuring. There are several methods available to calculate the optical distortions and finally undistort the images. The developed plugin will use the Zhang calibration tool EasyCalib.exe for the calculation of the calibration parameters. This parameters will be used in following plugins for the undistortion of images taken with the same camera.

The plugin Optic_Calib will take the thresholded images of the calibration boards and detect the centers of the black calibration dots. This image coordinates of the centers are sorted in the sequence needed for the export into EasyCalib.exe.

EasyCalib.exe will then calculate the intrinsic camera parameters and returns the values back to the Optic_Calib plugin.

Open your calibration image stack

Start the plugin d__Optic_Calib

Start the calibration plugin

The plugin will first make a copy of the stack and then shows the Analyse Particle dialog box. You must define the min and max size and the allowed circularity of the dots which should be detected. You should input a min size roughly the size of your smallest calibration dot in the stack. Please keep in mind that your images must be thresholded accurately and must only contain the calibration dots. Starting with values shown in the following images may work in most cases.

Input of the detection parameters

Finish the input by clicking OK. Process the calculation for all images in the stack by click the OK button.

In the following dialog box you will be asked for the grid space and the grid tolerance. The grid space is the distance between the calibration dots on the calibration board. The distance between the dots must be equal in horizontal and vertical direction. The grid tolerance describes the allowed deviation of the inner points of the matrix from the straight connection lines of the surrounding points. A tolerance value of 5 pixels will work in most cases.

If the plugin can not automatically detect the calibration dots you must change the calibration tolerance. Up to now I found the right tolerance value for all my calibration images and realized complete calibration pattern detections. The complete detection of the patterns can be controlled with the numbering of the dots in all slices of the stack.

Marking of the detected calibration dots

The values calculated by EasyCalib.exe will be saved in the folder of your source calibration stack as Params.txt

You can open it with an editor for controlling the plausibility of the results.

Listing of the calibration results (EasyCalib)

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