Results

For the experiments a standard, hand-held digital camera (KODAK DC 120) with an image resolution of $720 \times 576$ was used. For the process of vanishing point detection the parameters were set as: $t_{a} = 5^o$, $w_1 = 0.3$ and $w_2 = 0.7$. The maximal difference between the principal point and the midpoint of the image was set to $400$ pixel, i.e. $0.43$% of the image diagonal. The acceptable range of the focal length was not limited, i.e. $f \in [0,\infty)$.

Figure 5 shows four images where the three orthogonal vanishing points were correctly identified. The line segments which correspond to the three mutually orthogonal directions are colored in red, green and dark blue respectively. The remaining line segments which were not assigned to one of the three vanishing points are displayed in yellow with black dots. The principal point of the camera is drawn as a black cross.

Figure 5: Correct results

Figure 6 (left) depicts a result where one of the vanishing points (blue) was determined falsely. After restricting the range of accepted focal lengths to $f \in (700,1200)$ (this range was verified in a separate camera calibration process) we obtained a better result (see figure 6 (right)). This shows that additional knowledge about the camera geometry can improve the results.

Figure 6: Incorrect result (left) and improved result (right)