8 
The fifth correlator output in the list above is the degree of 
correlation. This output is the result of a conventional cross-corre 
lation operation on the two normalized video inputs. In general, it 
exhibits a maximum when the two images are aligned as well as possi 
ble, and decreases toward zero as the images are displaced signifi 
cantly in any direction. It is used by the computer to recognize situ 
ations in which the degree of image correlation is too low for the 
system to produce meaningful parallax measurements. It is also 
used in determining the size of the scan pattern. 
FUNCTIONS OF THE COMPUTER 
The correlator outputs, as just described, are used by the AP-2 
computer to control over-all operation of the system and thus generate 
the required plotting motions. As in the basic system, the computer 
determines photo coordinates for the servos in the viewing unit and 
controls image magnification and rotation. It also supplies model 
coordinates defining current position on a contour or profile to the 
coordinatograph servos. 
In addition, as an integral part of the image correlation system, 
the computer furnishes data to the scan generator to control the size 
and shape of the CRT scan patterns. Over-all size of the scan patterns 
is determined by the computer on the basis of a number of factors. In 
general, a small diameter is desirable in order to obtain maximum 
resolution in what ideally would be a point matching process. However, 
sufficient image detail must be encompassed within the scan pattern to 
obtain an acceptable level of correlation. Adjustment of scan size is 
thus a compromise, which the computer handles continuously on the 
basis of semi-empirical relationships involving several parameters. 
The resulting size command, as passed on to the correlator, controls 
the effective gains of the X and Y scan-generation channels. 
Scan shape information is provided by the computer in the form 
of eight coefficients, four for each photograph. These coefficients in 
effect are the partial derivatives of the required scanning motions in 
photo coordinates, with respect to corresponding motions in an X-Y 
model coordinate system centered at the scan center: