remove libcf?

[doutriaux1]

I don't think anybody uses or trust the libcf regrid implementation, and it just adds a useless dependency to cdms2 (and it also adds a dependency to uuid)

[durack1]

@doutriaux1 you have a 👍 from me, the TechX folks started on libCF and then quickly moved across to ESMF and the current integration in cdms2 so removing a partially implemented dependency is a good idea to me

[pletzer]

@doutriaux1 @durack1 Ok to remove libCF dependency. The main advantage of libCF interpolation is multidimensionality (no limits on the number of dimensions). The main shortcoming is that it does not support conservative interpolation.

[durack1]

@pletzer n-dimensional interpolation is certainly something that isn't provided by any other modules (at least that I am aware of).. Is there fairly thorough documentation available for the libCF regridding functionality? If so where would I find this?

[pletzer]

See
https://www.unidata.ucar.edu/software/libcf/html/group__gs__regrid__grp.html.
Care was taken to use only primitive data types in the API so all calls can
be accessed directly from Python using ctypes. In UV-CDAT, we tried to
implement the same API for all interpolation methods (
https://github.com/UV-CDAT/uvcdat/blob/65ca26c122d3939cdf406eafa3683ffec130315c/Packages/regrid2/Lib/mvLibCFRegrid.py),
making it easy for users to switch from one method to another. The methods
are documented in the python code and tested.

Another difference with ESMF is that LibCF uses a Newton algorithm to find
the location of the src cell that contains a target point whereas ESMF uses
an octtree. I don't remember the performance comparison but I suspect LibCF
is on average faster while being occasionally slower (some odd initial
guesses may lead Newton to enter a limit cycle). You need only one
iteration with LibCF if the grid is rectilinear (without having to specify
it as rectilinear). The performance of ESMF on the other hand is more
consistent.

ESMF supports unstructured grids and runs in parallel.

On Fri, Sep 2, 2016 at 3:50 AM, Paul J. Durack notifications@github.com
wrote:

@pletzer https://github.com/pletzer n-dimensional interpolation is
certainly something that isn't provided by any other modules (at least that
I am aware of).. Is there fairly thorough documentation available for the
libCF regridding functionality? If so where would I find this?

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[durack1]

@pletzer great thanks for this.. My statement above about removing libCF I think was misplaced.

@doutriaux1 I think the functionality available is not provided by any other integrated tools - so adding some tests to ensure that libCF continues to work with cdms changes would be good..

@dnadeau4 it would be useful to incorporate the documentation linked above into the cdms documentation tree too..

[pletzer]

@doutriaux1 I believe there are tests exercising both esmf and libcf and
comparing the results. testing/regrid/testAccuracy.py
https://github.com/UV-CDAT/uvcdat/blob/65ca26c122d3939cdf406eafa3683ffec130315c/testing/regrid/testAccuracy.py
is one of them.

On Fri, Sep 2, 2016 at 12:43 PM, Paul J. Durack notifications@github.com
wrote:

@pletzer https://github.com/pletzer great thanks for this.. My
statement above about removing libCF I think was misplaced.

@doutriaux1 https://github.com/doutriaux1 I think the functionality
available is not provided by any other integrated tools - so adding some
tests to ensure that libCF continues to work with cdms changes would be
good..

@dnadeau4 https://github.com/dnadeau4 it would be useful to incorporate
the documentation linked above into the cdms documentation tree too..

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[jypeter]

I confess I still use the old regrid2.Horizontal when going from regular to regular. I remember ESMF being terribly memory hungry (and therefore slow)

[pletzer]

I think it's important to keep a choice.

Interpolation on a regular, uniform grid does not require all the machinery
for structured or unstructured grids. ESMF can handle any grid and this
naturally comes with a performance penalty.

On Tue, Sep 6, 2016 at 2:47 AM, Jean-Yves Peterschmitt <
notifications@github.com> wrote:

I confess I still use the old regrid2.Horizontal when going from
regular to regular. I remember ESMF being terribly memory hungry (and
therefore slow)

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