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5.11. ROAD SCENE 113<br />
any case. All operators should work on encapsulated data structures. To allow optimization for<br />
performance, data structures have to be used that support transparent and efficient programming.<br />
The example segmentation of a road scene demonstrates how HALCON helps to achieve this<br />
goal.<br />
Here the task is to find the middle and border road markings of a motorway. The program is<br />
performed by a normal workstation with a processing time of maximum 20 ms per half image<br />
(video frequency) at a resolution of 512 ¢ 512 pixels. In figure 5.18 you see an image of such<br />
a road sequence on the left side.<br />
Figure 5.18: Part of an image sequence (left) and search grid for marking band (right).<br />
Assume that there is no specialized operator for this task. Thus, you have to make use of<br />
standard methods. The data structure used consists of a gray value image with a covering mask,<br />
i.e., the definition range. 3 All operators work only on those parts of the image data that lie<br />
within the definition range. This can be exploited to reduce computation time.<br />
The following assumptions on the image data help to specify a region as a search mask:<br />
1. Road markings remain in a certain image part only.<br />
2. Road markings have a certain minimum length in y-direction.<br />
3. Road markings are separated by an edge from their environment.<br />
The first two assumptions can restrict the search area enormously. To make use of this, we<br />
create a region as a grid whose line distance is determined by the minimum size of the road<br />
marking. Figure 5.18 shows the corresponding region (= line grid) on the right side.<br />
While performing an edge filter within the grid all pixels with a high gradient are candidates<br />
on the contour of a road marking. By enlarging these pixels by the minimum diameter of the<br />
markings (dilation) with rectangle, you will get the search window shown in figure 5.19 on the<br />
left side.<br />
Now the road markings can be easily extracted by a thresholding within the search windows.<br />
The segmentation result is shown on the right side of figure 5.19. The corresponding <strong>HDevelop</strong><br />
program looks as follows:<br />
3 See the manual Getting Started for a short introduction to the data structures used by <strong>HDevelop</strong>.<br />
HALCON 6.0