I was scanning some real old posts about Regrid and the new improved
version that VIS Inc. did but the posts don't seem to mention a bit
of unsavoriness I just discovered. I'm currently using an older SGI
version, so this is the standard Regrid module.
I have a field with a scalar component ("dot") and a vector component
("cross"). I need to regrid this field onto a particular grid. The
two components are bound to the same positions, then I send that
field to Regrid along with the desired output grid. Works fine, but...
If the input data field has "dot" as "data" and "cross" as "cross",
the output has "data" and "cross", regridded, as advertised in the
Regrid docs. But something seemed not quite right about the result,
so I tried reversing the input so that "cross" was "data" and "dot"
was "dot". Sure enough, the values are different in both arrays! Aack!
If I send a field with only "dot" as "data" through Regrid, I get the
same output values as if I send "dot" as "data" along with "cross" as
"cross". That is, the "dot" regridded values agree, since in both
cases they were "data" in and out.
Likewise, sending only "cross" as "data" gives the same output values
(3-vector) as when I send "cross" as "data" and "dot" as "dot", but
note that these "dot" outputs are not the same as those of the
previous paragraph, and these "data" ("cross") values are not the
same as those of the previous paragraph. They are similar in
magnitude, but different numbers. In particular, the vector field
looks quite different if glyphed.
So it appears that Regrid is doing something weird internally and the
short-term workaround for me will be to run 2 parallel Regrids and
unite the data components post-regridding.
David, Jeff, anyone? Did you spot this bug when recoding AutoRegrid?
--
Chris Pelkie
Managing Partner
Practical Video LLC
30 West Meadow Drive, Ithaca, NY 14850
