Guehring, Jens 
  
3 DATA ACQUISITION AND PROCESSING 
3.1 Introduction to Coded Light Techniques 
In light sectioning using light stripe range finders, a single line is projected onto the object (Figure 1 (a)) and then 
observed by a camera from another viewpoint. Tracking the bright line appearing in the image will yield all parallaxes 
and thus object depth. To recover the object's 3-D geometry, many lines have to be projected under different angles 
(Figure 1 (b)), which can be accomplished either by projecting an array of parallel lines simultaneously or by projecting 
different lines in temporal succession. 
  
   
  
Light k^ —— 
ne, 
LL 
P 
Projector 
  
  
Projector Camera 
(a) (b) 
Figure 3. (a) Standard light sectioning with one light stripe. 
(b) Top view of light sectioning using more than one stripe. 
The first method has the disadvantage that the lines have to be projected closely for a dense reconstruction. In this case, 
a correspondence problem arises, especially at steep surfaces or jump edges. Projecting in temporal succession means 
that for n different light section angles n images have to be acquired, where n may be in the order of several 
hundreds to thousands. 
  
Projected 
Code Observed 
Sequence Code 
"a m mm C mb 
t, CC. X mt 
to CCLL NN * Db 
Projector Camera 
Figure 4. Light sectioning using structured light. 
This problem can be elegantly solved by the use of coded light techniques, which require only in the order of ldn 
images to resolve n different angles (see Figure 4 for n —8 ). Rather than projecting a single line per image, a binary 
pattern is used. This technique has been initially proposed by Altschuler (Altschuler et al. 1979, Stahs and Wahl 1990). 
32 Solutions to the Correspondence Problem — Phase Shift Processing 
3.2.4 Projection of Gray Code 
For structured light analysis, projecting a Gray Code is superior to a binary code projection (see Figure 5, top). On the 
one hand, successive numbers of the Gray Code vary exactly in one bit. Thus, wrong decoding which is most likely to 
occur at locations where one bit switches, introduces only a misplacement of at most one resolution unit. On the other 
hand, the width of bright and dark lines in the pattern with finest resolution is twice as wide compared to the binary 
code. This facilitates analysis especially at steep object surfaces where the code appears to be compressed. 
  
330 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000.