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variable: $fillpix -- 3d fill pixel size
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default: 1.0

If this is set to the current pixel size (1.0 or
0.5), a 3D fill (FILL3d button, to right) will be
performed with native-sized pixels resulting in a
volume similar (but not pixelwise identical) to
that produced by fill.

If fillpix is set larger than native, this will
result in a smaller number of total voxels in the
fill.  The effect is equivalent to scaling down
the surface by 1.0/(fillpix/nativepix) (e.g.,
with the function really_scale_brain) and filling
with native-sized voxels.

Setting fillpix larger than native this way can
be used to downsample a surface.  The resulting
fill.$hemi.mgz can be directly used as an input
to make a surface, which will have fewer vertices
than the original surface but be the same shape.
It will be smaller in linear dimensions by the
fraction: fillpix/nativepix.

The effect is reversed if fillpix is set smaller
than native (larger filled volume surface).
