The seventh step builds a two-dimensional map of the world that indicates which areas are 
empty (also see [Bridwell 1983]). This construction is demonstrated for a few points in Fig. 
ure 18. The principle here is that if a feature is seen continuously over some interval by a 
moving camera, then during that motion nothing occludes it. Since nothing occludes it, noth- 
ing lies in front of it, and the triangle in the scene defined by the feature and its first and last 
points of observation is empty space. We build a map of this free space by constructing one 
of these triangular regions for each line segment found in the EPI, and then ORing these all 
together. Notice in Figure 18 that one of the features is viewed once while the other is seen 
in three distinct intervals, and so gives rise to three free-space triangles. Figure 19a shows the 
full free-space map constructed for the EPI features of Figure 17, with Figure 19b showing 
a good view of the actual scene. To take the INTERSECTION of the free-space maps from 
these individual EPIs, perhaps over some vertical interval, would give us the known free-space 
volume in that interval. This would be useful for navigation, as we know we could move freely 
in that volume without running into obstacles. 
  
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Fig. 19a. Free space 
Figure 20 is a stereo (crossed-eye) display, showing some preliminary results in the eighth step 
of our analysis - combining the spatial data from the individual EPIs. For spatial continuity, 
we link points between the various EPIs (nearest neighbors in overlapping error ellipses). This 
displays those features whose total baseline is greater than 3 inches, and whose connected extent 
vertically is greater than 2 scanlines. 
        
   
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126