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function: read_roi -- read 3D ROI/label
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keyboard equivalent: none

The "R" button reads a 256x256x256 AFNI BRIK
containing a 3D ROI using the filename in the
entry to the left.  The read-in ROI is initially
displayed in white.  The "roi" check (auto-ticked
on reading an ROI) can be used to toggle the
visibility of the white ROI.

Reading an ROI replaces any currently displayed
ROI or previously read-in ROI.

To mask displayed overlay data using an ROI,
untick "roi" and tick the neighboring "m".  The
overlay data will then only be visible in regions
where the 3D ROI mask is *not* equal to 0.

N.B.: if "roi" is also ticked, the white display
of the ROI will occlude the overlay data.
