Master Thesis - Fachbereich Informatik

4.3. CAMERA CALIBRATION 59
4.2.7. Background Pattern
As introduced in Section 3.3, the measuring area is illuminated by a back light setup below
the conveyor belt. This setup emphasizes the structure of the belt which can be seen as a
characteristic pattern in the image. This pattern may differ between different belt types.
Depending on the light intensity it is possible to eliminate the background completely. If
the light source is bright enough, the background appears uniform white even with a short
shutter. For black tubes such an overexposed image would lead to an almost binary image.
Transparent tubes, however, do also disappear under too bright illumination. Hence, there
will be always a certain amount of background structure visible in the image in practice.
The strength of the background pattern increases with lower light intensity.
In the following, it is generally assumed that the illumination is adjusted to allow for
distinguishing between a tube edge and edges in the background. Larger amounts of dirt
or other particles than heat shrink tubes on the conveyor must be prevented.
4.3. Camera Calibration
In the previous section several assumptions regarding the camera position and the image
content have been presented. With respect to accurate measurements it is important that
an object is imaged as reliably as possible, this means, straight lines should appear straight
and not curved in the image, parallelism should be preserved, and objects of the same size
should be mapped to the same size in the image. Unfortunately, the later properties do
not hold in the perspective camera model as introduced before. However, under certain
constraintsitispossibletominimizetheperspectiveeffects.
If the internal camera parameters are known including the radial and tangential distortion
coefficients, it is possible to compute an undistorted version of an image. After
undistorting, straight lines in the world will appear as straight lines in the image. Furthermore,
if one can arrange the camera in way that objects of equal size are projected
onto the same size in the image within the camera’s field of view at a constant depth, one
can assume that the image plane is approximately parallel to the conveyor.
In the following the calibration method used to receive the intrinsic camera parameters
as well as a method to arrange the camera in a way that perspective effects are minimized
is presented.
4.3.1. Compensating Radial Distortion
To compensate for the radial distortion of an optical system, one needs to compute the
intrinsic camera parameters. Since the intrinsic parameters can be assumed to be constant
if the focal length is not changed, the calibration procedure does not have to be repeated
every time the system is started and therefore can be precomputed offline.
The common Camera Calibration Toolbox for Matlab of Jean-Yves Bouguet [9] is used
for this purpose. It is closely related to the calibration method proposed in [74] and [31].
The calibration pattern required in this method is a planar chessboard of known grid size.
The calibration procedure has to be performed for each lens separately. The camera is
placed at a working distance of approximately 250mm over the measuring area with a
16mm fix-focal lens. It is adjusted to bring tubes with a diameter of 8mm at this distance
intofocus(inthemeasuringplaneΠM).