Camera calibration

Every camera has several internal parameters (so called intrinsic paramaters) defined by the optical system (e.g. camera chip, camera lens, etc.). For getting a high quality and accurate tracking and augmentation a camera calibration is needed and recommended. As the calibration is only valid for exactly one camera system it theoertically has to be performed for every physical camera system (which is done in industrial high accuracy applications). But even for Unifeye Design related use cases using a camera calibration is recommended when doing e.g. Kiosk/Terminal systems. This will usually improve tracking stability. When no camera calibration is available Unifeye Design tries to guess the camera intrinsic parameters.
There are several camera intrinsic parameters:

1. The focal length describes the distance between the optical center of the lens and the image plane/sensor and also determines the field of view. If it is not set correctly tracking values won't be and you might get strange especially the rendered 3D model is relativly far away from the tracking coordinate system it is bound to.
   
2. The principal point defines the center of projection on the camera image plane.
3. Several distortion parameters describe the distortion of the camera lens (e.g. radial and tangential distortion). In a distorted camera image, straight do not appear straight in the camera image until you apply the according undistortion (provided by the appropriate camera calibration):
   
   distorted (left) and undistorted image (right)

Fortunately Unifeye Design ships with a tool giving you the ability to calculate the camera intrinsic parameters (for both live cameras as well as still image cameras) called the Sextant camera calibration tool. You can start the tool either from the Windows start menu ("Calibration" -> "Standard camera calibration - Sextant") or from Unifeye Design GUI menu ("Tools" -> "Sextant"). Before doing the calibration you need to print out a calibration pattern which you can find in the Windows start menu ("Calibration" -> "Calibration Pattern A4 18x12"). Print the PDF and put (better stick it) to a planar surface. Now follow the process described here. After that you can load the camera calibration file into the Unifeye Design GUI or the Workflow Authoring GUI. In the Unifeye Design GUI use the tool "Camera Calibration" provided in the "Configuration" category. The calibration will be applied directly to the just loaded image source (e.g. webcam) and every image source loaded/activated afterwards. It will also be stored within a Unifeye scene file (a link to the calibration file) and loaded automatically once you load the scene again.

Notes

• A camera calibration is only valid for exactly one camera and one setting of the camera (resolution, shutter values, zoom/focus level).
• Using a camera calibration adds some additional load to your machine when doing Augmented Reality as it performs undistortion operations on the image. You can however switch off undistortion while keeping the general camera calibration. You will have to do this manually by editing the resulting file of the Sextant camera calibration tool (usually called CalibrationResult.xml) and setting all subtags of <Distortion> to 0.0. See also the Sextant camera calibration tool documentation.

-- SupportMetaio - 2010-02-23