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tickbox variable: $phasecontoursflag -- show
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default: 0 (FALSE)

When the "phc" tickbox (middle right) is ticked
($phasecontoursflag is 1), white polygon-wise
isophase contours are shown (no effect if not
complex-valued data).

Number and width of phase contours are controlled
by "n" and "w" entries below.

The sharp edge of each phasecontour points in
direction of phase gradient (steepest uphill
direction).  The opposite edge fades into the
current phase color (downhill direction).
