Help for GRADREM

PURPOSE:
Wide angle aircraft scanners often exhibit large, systematic, and non-linear
artificial brightness gradients along scan lines.  GRADREM seeks to remove
these gradients by ratioing the input picture with an estimate of the gradient
upon a flat field.

EXECUTION:

Example 
	GRADREM INP=A OUT=B SIZE=(50,100,500,500) LINC=10 PERCENT=2.0

	In this example, every tenth line of the entire region specified by the
	SIZE parameter is used to compute the gradient function.  The output
	picture will be about 2.0 percent saturated (1.0 high, 1.0 low).

	GRADREM INP=A OUT=B LENGTH=300 GAIN=150.0

	In this example, the first 300 lines are used to compute the
	gradient function.  DNout = 150.0 * (DNin / gradient).

	GRADREM INP=A OUT=B 'NOSAT

	In this example, the entire region is used to compute the gradient
	function; the program sets the offset (OFF) to 0 and chooses a
	gain such that no unsaturated input pixel becomes saturated in the
	output image.  


OPERATION:
The user selects a region of the picture whose differences in brightness
are believed to be predominantly artificial.  The lines in this region are
averaged to produce a 'standard' DN level at each sample.  If requested, a
box filter is used to smooth the standard DN function.  Each pixel is then
ratioed with its appropriate standard DN, and the specified gains and
offsets applied.  Floating point arithmetic is used throughout.


WRITTEN BY:  Ron Alley, 7 June 1979
COGNIZANT PROGRAMMER:  Steve Pohorsky