Hi David,
On Aug 13, 2015, at 12:30 AM, Schneider, David A. <[email protected]>
wrote:
> I have a question about the virtual view feature that the hdf5 group is
> developing. My understanding is that I can build a view dataset out of
> elements from different datasets of the same type. I'm wondering if I can
> also build a view dataset that has a different but compatible type. For
> instance, if I had
>
> datasetA
> type = Compound { float x, int a}
>
> could I make a view dataset that is just of float, that pulls out the float x
> in the compound from in datasetA?
>
HDF5 virtual dataset uses only spacial information for setting a view (or
defining mappings), i.e., there is no way to define the self-describing mapping
as above. Also, the source datasets have to have compatible datatypes (one type
can be converted to another by the HDF5 library). There is no restriction that
they have the same datatype.
In HDF5, atomic type and a field of a compound type of the same atomic type are
not compatible. This is a current limitation and it can be probably addressed
in the future.
Said this, it is possible to pull data out as you described ( we still need to
test it!), but it is clunky.
If all source datasets in the mapping use compatible compound datatypes, the
library should be able to read or write virtual dataset data by a field. It
will be similar to reading a field from a compound dataset.
If the source datasets have non-compatible datatypes, then one can do I/O only
on a selection in the virtual dataset that maps to the elements of compatible
datatypes. It will be application’s responsibility to make sure that all
elements in the selection have compatible types, otherwise I/O will fail - not
fun ;-)
Elena
> best,
>
> David
>
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