reduced field of view (about 256 x 256 pixels), and tilting 
the stereoscope downwards to rectify the image convergence. 
XYZ Model Control 
The I?S system includes a trackball unit, but three 
dimensional model control requires a third continuous coordinate 
device. This limitation has been overcome in two ways. Firstly a 
button on the trackball unit is used to switch the trackball to Z 
control and back again to XY control. This is somewhat awkward to 
use, as plan and height cannot be altered simultaneously. Secondly, 
real-time image correlation is provided to automatically adjust the 
height values when feature following. As the correlation must work 
in real-time, the function has to be very simple. Basically, the 
trackball defines the x, y, image coordinate on the left image, and 
the corresponding x, y. right hand image coordinate is predicted 
using the ground Z value of the previous loop. One dimensional 
arrays of pixels are read from the images and the x, position of best 
match is found. The correlation function used is the Error Function: 
Error Function (i) = § | A(x) - B(x + i) | 
This works reasonably well when following distinct features such as 
roads, and the operator is always able to override the correlation 
and place the mark on the ground. 
The lack of an XYZ control remains a problem, however. 
Either a more powerful processor is required to perform a more 
sophisticated real-time correlation, or the trackball interface should 
be altered to accept a three dimensional control. 
Image Handling 
The digital ploter must have the ability of roaming over 
large areas. The I?S system is limited in the number of 512 x 512 
pixel sub-scenes it can hold in refresh memory, so this requirement 
concerns the automatic reading of images from disk. This problem 
has been overcome by simply reading 2 new 512 x 512 pixel 
sub-scenes into memory when the edge of the model is approached. 
Reforming the model takes about 5 seconds, but does not seem a 
major limitation. A future enhancement to the system would invovle 
background processing to load reserve refresh memories dependent 
on the direction of movement of the model. 
Sub-Pixel Accuracy 
The standard zoom function of the I?S works by instructing 
the refresh memory to send each pixel to the display 2, 4, or 8 times 
in the rows and columns directions. This precludes sub-pixel 
movements, as the zoom is applied to the image after scrolling and 
not before. However, sub-pixel movement is possible by feeding back 
the zoomed image into a new refresh memory so that each original 
pixel now occupies 4 pixels, and displaying the new image instead. 
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