-------------------------------------------
variable: $cmid -- red/green curvature midpoint
-------------------------------------------
default: 0.0

Adjust position (in curvature units) where where
color changes from red to green (gray point in
curvature display), or from lightgray to darkgray
(overlay display).  Settings above 0.0 emphasize
convex (green/lightgray, negative curvature)
while settings below 0.0 emphasize concave
(red/darkgray) positive curvature).

The second unlabeled entry is $dimredcurvfact,
which dims red (sulcus) in curvature display.
This provides (e.g., at 0.8) better contrast for
full color illustrations, better conversion to
B/W, and better visibility for red-green
colorblind (N.B.: no effect on contrast of
light/dark gray curvature with data overlay; use
$cvfact to adjust that).
