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tcl funct: writeupsampmask -- write upsamp mask
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keyboard equivalent: none

The "W" button writes out the current current
data in the registered, upsampled, stat mask
buffer to a 1mm^3 or 0.5mm^3 float BRIK using the
file name in the entry to the left.

Note that this will be a much larger file
than the native resolution stat mask.
