-------------------------------------------
variable: $cvfact -- background gray contrast
-------------------------------------------
default: 1.3

The second unlabeled entry under the RESTORE
button (just to left of "intp" tickbox) sets the
contrast of the background light/dark gray
curvature underlay (def=1.3).

Darker gray is set by entry to left ($offset,
def=0.3).  Lighter gray is set by $offset *
$cvfact.  Using a $cvfact less than 1 reverses
contrast.

For ease of blink comparison between conditions,
it is best to keep this constant.
