[NeXus-code-tickets] [NeXusCode] #113: Add control of HDF5 compression level to NXcompmakedata()

[NeXus Data Format Library and Applications]

#113: Add control of HDF5 compression level to NXcompmakedata()
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 Reporter:  Mlesha  |       Owner:  Unassigned
     Type:  defect  |      Status:  new       
 Priority:  minor   |   Milestone:            
Component:  napi    |     Version:  trunk     
 Keywords:          |  
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 Hi everyone,
    There are three levels of compression defined in the NeXus API:
 NXcompmakedata() has a compress_type argument which can be NX_COMP_LZW
 (Lempel-Ziv as used in gzip), NX_COMP_HUF (Huffman) or NX_COMP_RLE
 (Runlength encoded).  These work when HDF4 is written out by NeXus,
 however for HDF5 output only NX_COMP_LZW has been implemented in napi5.c.
 I suspect the reason is that HDF5 does not have any equivalents for
 Huffman or run-length encoding.  What it does have instead is a level 0-9
 which specifies the compression level.  This is passed in via the call to
 H5Pset_deflate() in napi5.c.  The default value of 6 provides the
 equivalent performance to the HDF4 implementation.
    It would potentially be useful for us to control the compression level
 to suit the data, for some fast experiments the data set may be
 large/sparse and there may be advantages to applying less compression.  My
 suggestion is to create compress_type constants NX_COMP_HDF5_LVL0 thru
 NX_COMP_HDF5_LVL9, which can set the level via H5Pset_deflate() in
 napi5.c.  I would be happy to assist in implementation if desired.

 Mark Lesha
 Scientific Programmer
 Australian Nuclear Science and Technology Organisation
 PMB 1, Menai NSW 2234
 T 02 9717 7026
 M 0406 077 929
 E mle at ansto.gov.au
 www.ansto.gov.au/bragg

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Ticket URL: <http://trac.nexusformat.org/code/ticket/113>
NeXus Data Format Library and Applications <http://www.nexusformat.org/>
NeXus Data Format Library and Applications