parallax-free regions, electrical signals are generated which cause the 
scan raster to automatically locate, lock on, and follow stereomode.1 
contours. 
The electrical signals which cause the rosette scan raster to 
null to the contour are purposely derived from a correlation system inde 
pendent of the one producing the electrical signals which allow the scan 
raster to move along the contour. Such an arrangement is desirable since 
the gains and time constants of each feedback loop may be optimized with 
a minimum of interaction. In addition, marginal detection of one or more 
stereomodel parameters in areas of poor diapositive detail generally re 
sults in degraded performance of only a single feedback system and not 
necessarily both. 
The primary feedback system, the nulling system, enables the 
scan pattern to lock onto the contours existing within the area intercepted 
by the flying spot scanner. Correlation existing in the contour region is 
sensed by an even-function instrumentation of the correlation equation; 
that is, an instrumentation whose output maximizes in regions free of para 
llax. X-axis and Y-axis nulling signals are developed by multiplying the 
correlator output by the appropriate rosette scan deflection current wave 
form. Physically, this process may be considered as a comparison of the 
correlation existing in each of the four quadrants occupied by the rosette 
scan pattern. The X-axis null signal compares the correlation existing on 
both sides of the Y-axis, while the Y-axis null signal compares the cor 
relation existing on both sides of the X-axis . The scan pattern moves to 
cause the resultant correlation in both axes to be reduced to zero, there 
by achieving a balance along both axes . 
To obtain the necessary forcing voltages to move the scan pattern 
along the contour, the correlation existing throughout each petal of the 
rosette scan pattern is measured. The process may be visualized by con 
sidering Figure 5. 
The contour makes an angle 0 with the deflection axis of the 
rosette pattern. It is helpful to consider the rosette scan pattern as being 
comprised of a low-frequency circle, perturbed at a high-frequency rate. 
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