The 4 steps in the search process are: 
1) In Image 1, feature selection by use of interest operator 
  
This step deals with the question which point or feature should be 
selected for measurement in image 1, while the other steps attempt to 
find it in the image 2. 
2) Using existing neighbourhood parallaxes, approximation to the 
search area in image 2 using weighted interpolation 
  
  
The objective here is to limit the search area in 3), but insure that 
the correct point still lies inside this area. A good approximation 
increases speed and reliability, since the chances of gross errors are 
reduced. The search area is computed by assuming a parallax for the 
current point, which is derived from parallaxes at points already 
fixed in the neighbourhood. These existing points may come from the 
current grid and / or the previous one. In the example given this 
search area generally lay within +/- 7 pixels of the final value. 
This depends on grid spacing variation of elevation. 
3) In image 2, coarse matching with cross correlation 
  
  
A standard cross correlation procedure with additional parabolic 
interpolation, is used to position the window within the search area 
at this stage. This step 3) is required in order to insure stable 
convergence in the following step 4). As already explained, epipolar 
geometry enables the search to be made in one direction only. 
Optimized techniques are then possible which more than double the 
search speed. Generally, positioning at this stage is better than 1 
pixel (often 0.1 - 0.2 pixel). 
4) In image 2, fine matching using least squares adjustment 
  
  
Final determination of position and slope at the chosen feature is 
carried out by least squares adjustment /1/. The following sum is 
minimized: 
(gtx) - g28,0 )* , 
where g1, g2 are grey levels in photo 1 and 2 respectivly. g2 is 
determined according to the following functional model: 
HO + H1 ( g1(u,y) ) 
AO + A1 x + A2 y 
g2(x,y) 
u 
where: HO, H1 
A0, M, A2 
unknowns of radiometric transformation 
unknowns of geometric transformation 
The geometric transformation is assumed to be affin in the x-direction 
only (no variation in y due to the epipolar condition), i.e. the 
surface is assumed to be flat around the feature selected. Iteration 
1s necessary because the system is non-linear (linearization by Taylor 
expansion). 
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