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entry variable: $fillrad -- max fill radius
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default: 0.0 (off)

The unmarked entry to the right of the FILL
button controls the maximum radius (in mm) of an
ROI generated by FILL, if set above zero.

When FILL is clicked, fthresh (or WMTRUNC limits
if "overlay" is unclicked) are always respected,
so to fill a complete sphere, first set fthresh
to 0.0 (or WMTRUNC limits to 0 to 255).
