[Yt-dev] covering grids

[Matthew Turk]

Hey guys,

I have implemented the integer-based covering grid.  It's not really
much faster; the interpolated one should be, but the primary one is
not.  (This was disappointing to me.)

Anyway, this exposed a slight bug in the covering grid generation of
ghost zones.  I have attached an example plot.  Upper left is the new,
upper right is the old, lower left is the relative difference.
abs(new-old)/(new+old).

It looks to me like an off-by-one in the grid values.  This should not
change substantially any of the calculations, as it still gives
*correct* values, so contouring and whatnot should still work exactly
correctly.  We just get a little bit of bonus data one one side and a
little less on the other.  It's *possible* that if your contour join
was exclusively in the ghost zone of a single grid that you now have
too many; this seems kind of unlikely to me, but it still should be
addressed.  I'm going to fix it, but probably by inserting the new
routine rather than by fixing the old one.  The only thing remaining
(you can see for yourself at my bzr repo:
http://bzr.enzotools.org/integer_cgrid/ ) thing to do is to do
periodic domains and smoothing.  For smoothing I will be implementing
the (int math friendly) algorithm from Ralf Kaehler.

-Matt
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