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variable: $offset -- brightness background gray
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default: 0.30

The first unlabeled entry under the RESTORE
button (just right of "curv" tickbox) sets the
intrinsic brightness of the darker (concave or
sulcus) background gray (1.0 is white).

The lighter gray is set by $offset * $cvfact.
The actual rendered brightness if affected by the
settings of $light0, $light1, $light2, and
$light3.  See also $blufact on large interface
([fn]-F3) to give the brain a bluish or yellowish
tint.
