Parent Topic: DETAILS

FUNCTIONS

Various types of functions can be obtained by varying the value of the exponent:

 EXPO = 0.5     Square root function (default)
 EXPO = 1       Linear ramp function
 EXPO = 2       Squaring function

In general, the user has total control of the parameters which make up the contrast stretch function. All default minimum and maximum input and output values can be overridden by user- specified values (ISTR,OSTR). If ISTR is specified, DBIC is ignored.

 EXPO           Power exponent (must be positive value)
                (default = 0.5)

 ISTR(1)        Minimum input grey-level value (must be <= ISTR(2))
                (default = 0)

 ISTR(2)        Maximum input grey-level value (must be >= ISTR(1))
                (default = 255)

 OSTR(1)        Minimum output grey-level value
                (default = 0)

 OSTR(2)        Maximum output grey-level value
                (default = 255)

If OSTR(1) > OSTR(2), the result is a `negative' image.

LCTR and HCTR control the mapping function for input values that are outside the range specified by ISTR:

 For X < ISTR(1)        F(X) = OSTR(1)  if LCTR = "MIN" (default)
                        F(X) = OSTR(2)  if LCTR = "MAX"
                        F(X) unchanged  if LCTR = "OFF"

 For X > ISTR(2)        F(X) = OSTR(2)  if HCTR = "MAX" (default)
                        F(X) = OSTR(1)  if HCTR = "MIN"
                        F(X) unchanged  if HCTR = "OFF"

LCTR or HCTR should only be set to "OFF" when modifying an existing lookup table segment (DBLUT is specified), so that the table values outside ISTR are not modified.

The area under which the image is sampled to determine the minimum and maximum input values can be restricted to a specified database window or under a specified database (MASK). The low and high ends of this sample can be trimmed (TRIM) by specified percentages to eliminate outliers or noise.

Parent Topic: DETAILS
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