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function: smooth_val_sparse $steps
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keyboard equivalent: none

The "sp" radio button (lower left) causes the
"SMOOTH" button to perform a fixed-value
interpolation (sparse smoothing).

This operates on real or complex data
(vertex[i].val and maybe vertex[i].val2).

In contrast to "val" (nearset neighbor) or "w"
(weighted) smoothing, vertices with data are not
changed, but vertices that are initially
undefined are interpolated (at the 'speed of
light' -- one vertex per smoothstep) to fill in
between defined vertices.

N.B.: this will have no effect on full rank data
where all vertices have defined values.
