Triangulation-Based Approaches to Three-Dimensional Scene Reconstruction - 3D Computer Vision: Efficient Methods and Applications - page 37

Fig. 1.8 Typical rig extraction results for images acquired with the Point Grey Digiclops wide-an-
gle camera system. Strong lens distortion effects are apparent

Corner Enumeration The corner enumeration algorithm assigns a number to
each corner candidate that can be used to assign rig coordinates to a corner candi-
date. It performs multiple runs starting at different assumed rig corners and neglects
the image if the enumerations are not identical.

Notch Direction Detector The direction marks resolve the 180 ◦ ambiguity of the
rig orientation. In order to detect the direction of each notch, the grey values along
two lines are extracted. These lines start in the middle of one square border and
end in the middle of the opposite square border. The grey levels on the horizontal
line lead to a higher standard deviation than those on the vertical line if the rig
is horizontal, as shown in Fig. 1.6 . Additionally, the weighted mean of these grey
values yields a position that clearly detects them to be off-centre. It is possible to
threshold the standard deviation to detect missing notch marks if the processing
options require them. In this case the image is discarded because expected notch
marks cannot be found.

Rig Direction The direction of the rig is determined by computing the mean di-
rection vector from the intersection points of the two lines to the centre of gravity.
The direction of the mean vector is quantised into four directions (left, right, up,
down), and the corner identifiers obtained in the previous step are adjusted accord-
ingly based on the square counts.
A typical rig extraction result is shown in Fig. 1.8 for the trinocular Point Grey
Digiclops wide-angle camera system with a field of view of 70 ◦ . The size of each
square amounts to 30 30 mm 2 . The rig was printed out at 600 dpi and attached to

×

a wooden board.

1.4.7.4 Discussion

The proposed method for calibration rig extraction successfully performs the diffi-
cult task of automatically assigning the control points on the rig to their counterparts