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The TV scanning raster is probably the most familiar pattern and has been 
used for the electronic viewing of photographic transparencies and for several 
automatic plotting instruments. As can be seen from the figure, the scanning 
spot of a TV pattern covers each point in the image only once during a complete 
scan and the velocity of the spot remains constant except for the brief fly- 
bach periods at which time the spot is extinguished. These characteristics 
make the TV pattern ideal for the presentation of images to be viewed by the 
operator. Since the spot velocity is constant, the entire image area appears 
to be of uniform brightness from edge to edge. 
An instrument such as ARES employing a TV scanning raster could derive 
parallax information from the left and right video signals in a simple manner 
by measuring the time difference between corresponding points in these signals. 
If the scanning direction were arranged to be parallel to the X axis, then 
X parallaxes could be derived by this method simply and directly. Unfortu 
nately, under these conditions it would not be possible to measure Y paral 
laxes directly since there is not a simple relationship between time and 
displacements in the Y direction. By rotating the scanning raster periodi 
cally by 90° it would be possible to measure X parallaxes and Y parallaxes 
successively, and instruments have been designed to operate in this manner for 
some purposes. 
Another difficulty with the TV scanning raster arises out of errors in 
troduced by any variation in the time of arrival of the video signals arising 
out of conditions other than image parallax. Such conditions may be a drift 
in delay or transit time of the video signals in the video amplifiers and 
associated circuitry. Likewise, variation in the delay of scanning signals 
applied to either the viewer or the scanning cathode ray tube would be effec 
tive in producing an image offset indistinguishable from parallax. Scanning 
patterns in which the spot velocity is periodically reversed are free from 
errors of this type. 
The random scanning pattern shown in Figure 7(b) has qualities almost 
completely complementary to the characteristics of the TV scanning raster. 
The scanning spot in the random pattern has no preferred direction of scan 
either vertically or horizontally and moves, say, from left to right as fre 
quently as it moves from right to left. This characteristic eliminates the 
displacement error inherent in the TV scan and permits the detection of X 
and Y parallax without pattern reorientation. 
Unfortunately, the random scanning pattern is not suitable for the pro 
duction of viewed images. The pattern is bright in the center, which area 
is frequently scanned, and becomes dimmer and irregular toward the margin 
owing to the relatively infrequent scanning. Also, since the spot velocity 
is not constant but varies over wide limits, the resolution of the picture 
is impaired, being made up of the superposition of high resolution elements 
produced when the spot is moving at low velocity and low resolution elements 
produced when the spot is moving at relatively high velocity. 
Other scanning patterns have been proposed from time to time for use in 
photogrammetric image matching operations. The rosette pattern (Fig. 7(c)) 
is one of these and offers the advantage of constant spot velocity. Unfor 
tunately, the pattern has a strong radial symmetry and, therefore, treats 
image detail differently depending upon where in the scanned area it may 
appear. Also, the rosette pattern is not suitable for image viewing since, 
like the random pattern, a vignetted image with a bright center is produced.