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6 WHY FEEDBACK MATCHING AND BANDS? 
1} To 
2) To 
3) To 
4) To 
facilitate the coarse-to-fine iteration process. 
promote the spread of correlation. 
locate automatically all homologous pixels in the window. 
eliminate automatically detail present only in one image. 
Stereo correlation involves multiplication of input data followed by integra- 
tion or the summing of products over an area. The larger the area that can be 
integrated the less the noise or roughness in the Z signal. We define the rel- 
ative coherence area as the largest circular portion of the stereo image areas 
that can be regarded as identical and over which dx is constant. Integrating 
beyond the limits of the coherence area sharply increases Z noise and reduces 
the reliability of the Z data. 
A major problem in the design of stereo correlators is the small coherence area 
of stereo imagery before transformation, typically about one video cycle in 
both X and Y. Therefore the low frequency video components, representing large 
image detail, have a larger 
coherence area than higher 
frequency video components 
representing small image 
detail. 
Dividing the video spectrum 
into frequency bands allows 
the integrating area to be 
separately matched to the 
coherence area for each band 
and also provides a basis for 
progressive feedback matching. 
Figure 2 illustrates the pro- 
gressive clearance of disparity 
for an object in the form of a 
layered pyramid. A large B/H 
ratio of 1/1 has been used to 
exaggerate the differences 
between the stereo images. The 
cameras are placed at infinity 
to simplify the drawing. 
Unmatched parts of the images 
are shown diagonally hatched. 
Matched parts are shown clear 
and parts seen by only one i 
camera are shown black. Four 
band iterations are shown and 
are sufficient to illustrate 
correlative matching on the 
simple object chosen. Six or 
seven bands are required to 
resolve fully the imagery of a 
typical 500 by 500 pixel patch. 
  
  
  
  
  
OBJECT 
  
  
  
  
  
  
  
  
  
  
STEREO 
IMAGES 
AFTER BAND 4 
TRANSFORMAT ION 
  
  
  
  
  
  
  
    
  
  
AFTER BAND 3 
TRANSFORMATION 
    
  
AFTER BAND 2 
TRANSFORMATION 
    
   
  
AFTER BAND | 
TRANSFORMATION 
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Figure 2 
PROGRESSIVE: DISPARITY ‘CLEARANCE