-------------------------------------------
entry variable: $bwmid -- midpoint adj sigmoid slope
-------------------------------------------
default: 0.35

The "midpt:" entry sets the inflection point of
the contrast sigmoid.

A higher number increases midpoint of inflection
point of sigmoid -- that is, darkens the image.
For linear map, set linearflag TRUE

                         1.0
display =  -----------------------------------
            1.0 + exp(-bwslope*(image-bwmid))

To invert contrast, use a negative number for
$bwslope ("contr:" entry).
