EVEN, Philippe 
  
6 SEMI-AUTOMATED EDGE SEGMENT SPECIFICATION 
The automatic edge segment attraction function has been integrated within a semi-automatic mode that was called segment 
magnetisation. It is summarized in the flow chart of figure 5. The operator first draws an initial segment using the mouse. 
A press on the mouse button defines the first segment end. As long as the button is pressed, the segment is continuously 
displayed so that its second end follows the cursor. At the release of the button, an initial segment is provided, and the 
automatic attraction is immediately run. If the segment is attracted towards a spurious edge segment, the operator can 
click at one of the segment ends and drag it towards the desired edge. The automatic attraction is run again as soon as 
the segment is released. After any automatic attraction, the operator can come back to the manual solution with a simple 
undoing action. 
Undo - 
IN Segment - Automatic » "9 OUT 
specification " A attraction 
e : Operator's choice Segment  . 
modification 
Figure 5: Flow chart of the semi-automated edge segment specification. 
The semi-automated edge segment specification was evaluated using real sites images. Both the performance of the 
automatic attraction function and the one of the semi-automatic mode have been evaluated. In the latter case, we recorded 
the time spent to model some objects with and without the assistance. 
Because of the automatic function robustness, the desired segment edge is often detected at the first go. An initial solution 
is very quickly drawn. As most of segments length is less than 300 pixels, the average response time is less than one 
second on a Silicon Graphics Indigo workstation. The worst case is observed when the initial segment is one of the 
image diagonal. The computation time may then rise to about eight seconds. But such segments are few and the waiting 
situations are rather seldom so that their impact on the semi-automatic assistance remains low. 
Most frequent failures are due to the presence of a better quality edge in the neighbourhood of the desired segment. When 
it happens, a simple modification of the segment to drag it away from the disturbing edge is most of the time sufficient. 
Such an example is shown in figure 6. In case of repeated failures, only a manual accurate specification can solve the 
problem. 
  
Figure 6: Example of a two steps edge segment specification : a) initial image (the desired edge contour is the left side of 
the pipe), b) manual coarse specification of a line segment, c) extracted segments around the initial segment, d) automatic 
attraction towards the wrong edge (right side of the screw), e) manual modification to drag the segment towards the desired 
edge, f) successful attraction on the correct edge. 
The operating mode of the semi-automatic edge segment specification is very intuitive and requires no learning. It is 
quite flexible. For instance for the virtual edges of figure (3), the assistance could be deactivated in order to avoid any 
disturbance of the operators actions. Up to now, the only images where the assistance had to be deactivated because of 
multiple failures are very difficult ones, featuring many piping lines made of highly specular stainless steel which produce 
plenty of reflects. Using all the other images from realistic sites, the time spent to the modelling tasks is greatly reduced 
thanks to the semi-automatic assistance. The few failures are quickly recovered, so that their influence on the over-all 
performance is quite low. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000. 227