[yt-dev] "faking" away variable-by-level refinement

[Chris Malone]

Hi all,

I've been out of the yt-loop for awhile, but am now getting back into the
game.  In particular I work with the BoxLib-based codes, Maestro and
Castro.  I'm glad Matt has taken the time to consolidate the BoxLib
frontends - this is something I've wanted to do for some time!

One issue that has come up is that (at least) Castro supports jumps in
refinement that can vary between levels.  In other words, Level 1 might be
twice as refined as Level 0, but Level 2 might be four times as refined as
Level 1.

This causes problems in yt as there is only a single global refine_by
parameter, which assumes all levels are refined by the same ratio.  This
probably isn't a major concern as I think I'm the only one so far who has
used this variable-by-level  refinement with Castro, and it may not be
something useful in the future.

So, for the time being, I want to move forward with testing out the
consolidated BoxLib frontend on a real dataset.  I could, of course, write
a script offline that would take my old datafile, create intermediate-level
grids and populate the data by averaging or interpolating the real data so
that all levels have the same refinement ratio, but I've just "faked" the
data for the levels that I have added.

Another option would be to do this within yt when it loads a plotfile, if
it notices the refinement ratio isn't the same at all levels.  I'm not sure
exactly if/how this could/should be done.  One thing that pops to mind is
use the child_mask that is set to all zeros.

For instance, the data file contains

Level 0
Level 1 (refined by 2 w.r.t. Level 0)
Level 2 (refined by 4 w.r.t. Level 1)

then when the data is actually needed to make a plot or slice or
whathaveyou, yt could do something like this

Level 0
Level 1 (refined by 2 w.r.t. Level 0)
Level 2 (refined by 2 w.r.t. Level 1; same bounding grid as Level 2 from
data file; child_mask = 0)
Level 3 (refined by 2 w.r.t Level 2; same data/grid as Level 2 from data
file)

I'm scouring the code for "child_mask" to get a feel for when it is
actually used in generating plots/data containers, but I thought I'd ask
some basic questions first:

When the mask is set, do all operations ignore coarse data?  Would having
an entire level that is masked be problematic?

Chris
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