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entry variable: $scalepercent -- scale brain
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keyboard equivalent: [cmd/alt]-plus/minus
direct manip. equiv.: Shift-right-click-drag-up/down

The "%:" entry (bottom middle) scales the brain
brain around the center of the display window.

Clicking in the slider troughs scales 1 percent.

In a tcl rendering script, this scaling can be
achieved with:

  scale_brain <fraction>   ;# use 1.01 for 1% increase
  redraw

If focus is on the brain surface window, the +/-
keys (no need for Shift) scale the brain up and
down by 1% (in the interface window, cmd-minus or
cmd-plus is required to avoid typing characters
in the current entry).
