Hi Charlie,
> A user wants to uncompress their files with NCO and setting the
> deflation level to zero works yet the variables are still listed
> as having the deflation filter set. Now I make NCO manually turn-off
> the deflate flag when the deflation level is zero yet I'm wondering
> why the netCDF library does not do this automatically when I call
> var_deflate() with level 0. This makes me think I don't fully
> understand what deflation level equals zero means. And I'm wary
> of introducing bugs through naÃvete.
I'm CC:ing hdf5 support, in case they have a correction for my
understanding of why deflation level 0 is distinguished from
uncompressed.
I think netCDF-4 is following HDF5's meaning for deflate level 0. The
HDF5 reference documentation for H5P_set_deflate() says:
Note that setting the compression level to 0 (zero) does not turn
off use of the gzip filter; it simply sets the filter to perform no
compression as it processes the data.
> Is there any distinction between uncompressed variables and variables
> with compression set to level zero?
I think this means that the variable must still use chunked rather than
contiguous layout, whereas an uncompressed variable could use contiguous
layout.
> Are such variables fully uncompressed?
Yes.
> Perhaps this could be indicated in the manual.
Right, if hdf5 support confirms my understanding, I'll make this
difference explicit in the documentation.
> Is there a reason not to unset the deflation flag when deflation
> level is zero?
I'll pass the buck to HDF5 support, in case they know a good reason
why the deflation property is set for data "compressed" with deflation
level 0.
--Russ
Russ Rew UCAR Unidata Program
address@hidden http://www.unidata.ucar.edu
Ticket Details
===================
Ticket ID: WNT-807985
Department: Support netCDF
Priority: Normal
Status: Closed
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