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The automatic gain control feature together with the rapid transport and 
zoom facilities can be used to optimize the appearance and interpretability of 
individual portions of the imagery. For example, let us assume that the area 
being viewed contains elements of high contrast together with some areas of 
lesser contrast. Under these conditions the tonal range of the viewed image 
will adjust itself to accommodate the high contrast areas of the image and a 
relatively small fraction of the brightness range will be effective in the 
presentation of the lower contrast material. By adjusting the zoom control 
and by moving the transport in X and Y directions, it is often possible 
to exclude the high contrast portions of such an image so that the entire 
field consists of lower contrast material. Under these conditions the auto 
matic gain control immediately readjusts the signal levels so that the lower 
contrast material will be increased in apparent contrast until the brightness 
range of this material fills the full range of the viewing cathode ray tube. 
There is a limit to the amount of edge enhancement or high frequency 
emphasis that can be applied to the scanned image by means of video networks. 
This limit is set by the appearance of undesirable smear and overshoot effects 
such as are obtained in an incorrectly tuned television set. The crossed 
diagonal raster permits considerably more of this type of enhancement than 
does a conventional TV raster. This is because smearing or overshoot effects 
in a TV raster appear always to the right of a boundary being enhanced whilst 
in the crossed diagonal raster such effects appear symmetrically about such 
boundaries in the four diagonal directions. A dot when enhanced by video net 
works is stretched out into a line in the case of a TV raster and appears as a 
four-pointed star in the case of a crossed diagonal raster. Experience indi 
cates that enhancement to the extent of a two-to-one increase in amplitude over 
the highest video octave can be tolerated with a dual diagonal raster and that 
such enhancement is effective in increasing the apparent sharpness of the 
viewed image. 
Some thought has been given to the inclusion of a tone distribution cor 
rection, probably under manual control, as an additional image-enhancing fea 
ture. Such tone correction could compensate for the distortion of tonal val 
ues occurring when the exposure range of the original negative is confined 
chiefly to the toe or shoulder regions of the H and D curve. Tone dis 
tribution correction would also be effective in reducing similar errors 
arising in subsequent printing operations or in the characteristics of the 
cathode ray tubes employed in the ARES device. Doubtless for some applica 
tions enhancement by adjustment of tone distribution would be of value. It 
is felt, however, that the automatic gain control feature, combined with the 
zoom and rapid transport facility, gives sufficient flexibility in the con 
trol of tone range that the nonlinear circuitry required for tone distribution 
control is probably not warranted for photogrammetric or photointerpretive 
purposes.