netcdf-c/docs/filters.md

NetCDF-4 Filter Support

NetCDF-4 Filter Support

[TOC]

Introduction

The HDF5 library (1.8.11 and later) supports a general filter mechanism to apply various kinds of filters to datasets before reading or writing. The netCDF enhanced (aka netCDF-4) library inherits this capability since it depends on the HDF5 library.

Filters assume that a variable has chunking defined and each chunk is filtered before writing and "unfiltered" after reading and before passing the data to the user.

The most common kind of filter is a compression-decompression filter, and that is the focus of this document.

HDF5 supports dynamic loading of compression filters using the following process for reading of compressed data.

1. Assume that we have a dataset with one or more variables that were compressed using some algorithm. How the dataset was compressed will be discussed subsequently.

2. Shared libraries or DLLs exist that implement the compress/decompress algorithm. These libraries have a specific API so that the HDF5 library can locate, load, and utilize the compressor. These libraries are expected to installed in a specific directory.

Enabling A Compression Filter

In order to compress a variable, the netcdf-c library must be given three pieces of information: (1) some unique identifier for the filter to be used, (2) a vector of parameters for controlling the action of the compression filter, and (3) a shared library implementation of the filter.

The meaning of the parameters is, of course, completely filter dependent and the filter description [3] needs to be consulted. For bzip2, for example, a single parameter is provided representing the compression level. It is legal to provide a zero-length set of parameters. Defaults are not provided, so this assumes that the filter can operate with zero parameters.

Filter ids are assigned by the HDF group. See [4] for a current list of assigned filter ids. Note that ids above 32767 can be used for testing without registration.

The first two pieces of information can be provided in one of three ways: using ncgen, via an API call, or via command line parameters to nccopy. In any case, remember that filtering also requires setting chunking, so the variable must also be marked with chunking information.

Using The API

The necessary API methods are included in netcdf.h by default. One API method is for setting the filter to be used when writing a variable. The relevant signature is as follows.

int nc_def_var_filter(int ncid, int varid, unsigned int id, size_t nparams, const unsigned int* parms);

This must be invoked after the variable has been created and before nc_enddef is invoked.

A second API methods makes it possible to query a variable to obtain information about any associated filter using this signature.

int nc_inq_var_filter(int ncid, int varid, unsigned int* idp, size_t* nparams, unsigned int* params);

The filter id will be returned in the idp argument (if non-NULL), the number of parameters in nparamsp and the actual parameters in params. As is usual with the netcdf API, one is expected to call this function twice. The first time to get nparams and the second to get the parameters in client-allocated memory.

Using ncgen

In a CDL file, compression of a variable can be specified by annotating it with the following attribute:

• ''_Filter'' -- a string containing a comma separated list of constants specifying (1) the filter id to apply, and (2) a vector of constants representing the parameters for controlling the operation of the specified filter. See the section on the parameter encoding syntax for the details on the allowable kinds of constants.

This is a "special" attribute, which means that it will normally be invisible when using ncdump unless the -s flag is specified.

Example CDL File (Data elided)

netcdf bzip2 {
dimensions:
  dim0 = 4 ; dim1 = 4 ; dim2 = 4 ; dim3 = 4 ;
variables:
  float var(dim0, dim1, dim2, dim3) ;
    var:_Filter = "307,9" ;
    var:_Storage = "chunked" ;
    var:_ChunkSizes = 4, 4, 4, 4 ;
data:
...
}

Using nccopy

When copying a netcdf file using nccopy it is possible to specify filter information for any output variable by using the "-F" option on the command line; for example:

nccopy -F "var,307,9" unfiltered.nc filtered.nc

Assume that unfiltered.nc has a chunked but not bzip2 compressed variable named "var". This command will create that variable in the filtered.nc output file but using filter with id 307 (i.e. bzip2) and with parameter(s) 9 indicating the compression level. See the section on the parameter encoding syntax for the details on the allowable kinds of constants.

The "-F" option can be used repeatedly as long as the variable name part is different. A different filter id and parameters can be specified for each occurrence.

It can be convenient to specify that the same compression is to be applied to more than one variable. To support this, two additional -F cases are defined.

1. ````-F *,...``` means apply the filter to all variables in the dataset.
2. ````-F v1|v2|..,...``` means apply the filter to a multiple variables.

Note that the characters '*' and '|' are bash reserved characters, so you will probably need to escape or quote the filter spec in that environment.

As a rule, any input filter on an input variable will be applied to the equivalent output variable -- assuming the output file type is netcdf-4. It is, however, sometimes convenient to suppress output compression either totally or on a per-variable basis. Total suppression of output filters can be accomplished by specifying a special case of "-F", namely this.

nccopy -F none input.nc output.nc

The expression -F *,none is equivalent to -F none.

Suppression of output filtering for a specific set of variables can be accomplished using these formats.

nccopy -F "var,none" input.nc output.nc
nccopy -F "v1|v2|...,none" input.nc output.nc

where "var" and the "vi" are the fully qualified name of a variable.

The rules for all possible cases of the "-F none" flag are defined by this table.

-F none	-Fvar,...	Input Filter	Applied Output Filter
true	unspecified	NA	unfiltered
true	-Fvar,none	NA	unfiltered
true	-Fvar,...	NA	use output filter
false	unspecified	defined	use input filter
false	-Fvar,none	NA	unfiltered
false	-Fvar,...	NA	use output filter
false	unspecified	none	unfiltered

Parameter Encode/Decode

The parameters passed to a filter are encoded internally as a vector of 32-bit unsigned integers. It may be that the parameters required by a filter can naturally be encoded as unsigned integers. The bzip2 compression filter, for example, expects a single integer value from zero thru nine. This encodes naturally as a single unsigned integer.

Note that signed integers and single-precision (32-bit) float values also can easily be represented as 32 bit unsigned integers by proper casting to an unsigned integer so that the bit pattern is preserved. Simple integer values of type short or char (or the unsigned versions) can also be mapped to an unsigned integer by truncating to 16 or 8 bits respectively and then zero extending.

Machine byte order (aka endian-ness) is an issue for passing some kinds of parameters. You might define the parameters when compressing on a little endian machine, but later do the decompression on a big endian machine. Byte order is not an issue for 32-bit values because HDF5 takes care of converting them between the local machine byte order and network byte order.

Parameters whose size is larger than 32-bits present a byte order problem. This specifically includes double precision floats and (signed or unsigned) 64-bit integers. For these cases, the machine byte order issue must be handled, in part, by the compression code. This is because HDF5 will treat, for example, an unsigned long long as two 32-bit unsigned integers and will convert each to network order separately. This means that on a machine whose byte order is different than the machine in which the parameters were initially created, the two integers will be separately endian converted. But this will be incorrect for 64-bit values.

So, we have this situation:

1. the 8 bytes come in as native machine order for the machine doing the call to nc_def_var_filter.
2. HDF5 divides the 8 bytes into 2 four byte pieces and ensures that each piece is in network (big) endian order.
3. When the filter is called, the two pieces are returned in the same order but with the bytes in each piece consistent with the native machine order for the machine executing the filter.

Encoding Algorithms

In order to properly extract the correct 8-byte value, we need to ensure that the values stored in the HDF5 file have a known format independent of the native format of the creating machine.

The idea is to do sufficient manipulation so that HDF5 will store the 8-byte value as a little endian value divided into two 4-byte integers. Note that little-endian is used as the standard because it is the most common machine format. When read, the filter code needs to be aware of this convention and do the appropriate conversions.

This leads to the following set of rules.

Encoding

1. Encode on little endian (LE) machine: no special action is required. The 8-byte value is passed to HDF5 as two 4-byte integers. HDF5 byte swaps each integer and stores it in the file.

2. Encode on a big endian (BE) machine: several steps are required:

   1. Do an 8-byte byte swap to convert the original value to little-endian format.
   2. Since the encoding machine is BE, HDF5 will just store the value. So it is necessary to simulate little endian encoding by byte-swapping each 4-byte integer separately.
   3. This doubly swapped pair of integers is then passed to HDF5 and is stored unchanged.

Decoding

1. Decode on LE machine: no special action is required. HDF5 will get the two 4-bytes values from the file and byte-swap each separately. The concatenation of those two integers will be the expected LE value.

2. Decode on a big endian (BE) machine: the inverse of the encode case must be implemented.

   1. HDF5 sends the two 4-byte values to the filter.
   2. The filter must then byte-swap each 4-byte value independently.
   3. The filter then must concatenate the two 4-byte values into a single 8-byte value. Because of the encoding rules, this 8-byte value will be in LE format.
   4. The filter must finally do an 8-byte byte-swap on that 8-byte value to convert it to desired BE format.

To support these rules, some utility programs exist and are discussed in Appendix A.

Filter Specification Syntax

Both of the utilities ncgen and nccopy allow the specification of filter parameters in text format. These specifications consist of a sequence of comma separated constants. The constants are converted within the utility to a proper set of unsigned int constants (see the parameter encoding section).

To simplify things, various kinds of constants can be specified rather than just simple unsigned integers. The utilities will encode them properly using the rules specified in the section on parameter encode/decode.

The currently supported constants are as follows.

Example	Type	Format Tag	Notes
-17b	signed 8-bit byte	b|B	Truncated to 8 bits and zero extended to 32 bits
23ub	unsigned 8-bit byte	u|U b|B	Truncated to 8 bits and zero extended to 32 bits
-25S	signed 16-bit short	s|S	Truncated to 16 bits and zero extended to 32 bits
27US	unsigned 16-bit short	u|U s|S	Truncated to 16 bits and zero extended to 32 bits
-77	implicit signed 32-bit integer	Leading minus sign and no tag
77	implicit unsigned 32-bit integer	No tag
93U	explicit unsigned 32-bit integer	u|U
789f	32-bit float	f|F
12345678.12345678d	64-bit double	d|D	LE encoding
-9223372036854775807L	64-bit signed long long	l|L	LE encoding
18446744073709551615UL	64-bit unsigned long long	u|U l|L	LE encoding

Some things to note.

1. In all cases, except for an untagged positive integer, the format tag is required and determines how the constant is converted to one or two unsigned int values. The positive integer case is for backward compatibility.
2. For signed byte and short, the value is sign extended to 32 bits and then treated as an unsigned int value.
3. For double, and signed|unsigned long long, they are converted as specified in the section on parameter encode/decode.

Dynamic Loading Process

The documentation[1,2] for the HDF5 dynamic loading was (at the time this was written) out-of-date with respect to the actual HDF5 code (see HDF5PL.c). So, the following discussion is largely derived from looking at the actual code. This means that it is subject to change.

Plugin directory

The HDF5 loader expects plugins to be in a specified plugin directory. The default directory is:

• "/usr/local/hdf5/lib/plugin” for linux/unix operating systems (including Cygwin)
• “%ALLUSERSPROFILE%\hdf5\lib\plugin” for Windows systems, although the code does not appear to explicitly use this path.

The default may be overridden using the environment variable HDF5_PLUGIN_PATH.

Plugin Library Naming

Given a plugin directory, HDF5 examines every file in that directory that conforms to a specified name pattern as determined by the platform on which the library is being executed.

Platform	Basename	Extension
Linux	lib*	.so*
OSX	lib*	.so*
Cygwin	cyg*	.dll*
Windows	*	.dll

Plugin Verification

For each dynamic library located using the previous patterns, HDF5 attempts to load the library and attempts to obtain information from it. Specifically, It looks for two functions with the following signatures.

1. H5PL_type_t H5PLget_plugin_type(void) -- This function is expected to return the constant value H5PL_TYPE_FILTER to indicate that this is a filter library.
2. const void* H5PLget_plugin_info(void) -- This function returns a pointer to a table of type H5Z_class2_t. This table contains the necessary information needed to utilize the filter both for reading and for writing. In particular, it specifies the filter id implemented by the library and if must match that id specified for the variable in nc_def_var_filter in order to be used.

If plugin verification fails, then that plugin is ignored and the search continues for another, matching plugin.

Debugging

Debugging plugins can be very difficult. You will probably need to use the old printf approach for debugging the filter itself.

One case worth mentioning is when you have a dataset that is using an unknown filter. For this situation, you need to identify what filter(s) are used in the dataset. This can be accomplished using this command.

ncdump -s -h <dataset filename>

Since ncdump is not being asked to access the data (the -h flag), it can obtain the filter information without failures. Then it can print out the filter id and the parameters (the -s flag).

Test Case

Within the netcdf-c source tree, the directory netcdf-c/nc_test4 contains a test case (test_filter.c) for testing dynamic filter writing and reading using bzip2. Another test (test_filter_misc.c) validates parameter passing. These tests are disabled if --enable-shared is not set or if --enable-netcdf-4 is not set.

Example

A slightly simplified version of the filter test case is also available as an example within the netcdf-c source tree directory __netcdf-c/examples/C. The test is called filter_example.c and it is executed as part of the run_examples4.sh shell script. The test case demonstrates dynamic filter writing and reading.

The files example/C/hdf5plugins/Makefile.am and example/C/hdf5plugins/CMakeLists.txt demonstrate how to build the hdf5 plugin for bzip2.

Notes

Memory Allocation Issues

Starting with HDF5 version 1.10.x, the plugin code MUST be careful when using the standard malloc(), realloc(), and free() function.

In the event that the code is allocating, reallocating, for free'ing memory that either came from or will be exported to the calling HDF5 library, then one MUST use the corresponding HDF5 functions H5allocate_memory(), H5resize_memory(), H5free_memory() [5] to avoid memory failures.

Additionally, if your filter code leaks memory, then the HDF5 library generates a failure something like this.

H5MM.c:232: H5MM_final_sanity_check: Assertion `0 == H5MM_curr_alloc_bytes_s' failed.

One can look at the the code in plugins/H5Zbzip2.c and H5Zmisc.c to see this.

SZIP Issues

The current szip plugin code in the HDF5 library has some behaviors that can catch the unwary. Specifically, this filter may do two things.

1. Add extra parameters to the filter parameters: going from the two parameters provided by the user to four parameters for internal use. It turns out that the two parameters provided when calling nc_def_var_filter correspond to the first two parameters of the four parameters returned by nc_inq_var_filter.
2. Change the values of some parameters: the value of the options_mask argument is known to add additional flag bits, and the pixels_per_block parameter may be modified.

The reason for these changes is has to do with the fact that the szip API provided by the underlying H5Pset_szip function is actually a subset of the capabilities of the real szip implementation. Presumably this is for historical reasons.

In any case, if the caller uses the nc_inq_var_szip, then the values returned may differ from those originally specified. If one used the nc_inq_var_filter API calls, it may be the case that both the number of parameters and the values will differ from the original call to nc_def_var_filter.

Supported Systems

The current matrix of OS X build systems known to work is as follows.

Build System	Supported OS
Automake	Linux, Cygwin
Cmake	Linux, Cygwin, Visual Studio

Generic Plugin Build

If you do not want to use Automake or Cmake, the following has been known to work.

gcc -g -O0 -shared -o libbzip2.so <plugin source files>  -L${HDF5LIBDIR} -lhdf5_hl -lhdf5 -L${ZLIBDIR} -lz

Appendix A. Support Utilities

Two functions are exported from the netcdf-c library for use by client programs and by filter implementations.

1. int NC_parsefilterspec(const char* spec, unsigned int* idp, size_t* nparamsp, unsigned int** paramsp);

   • idp will contain the filter id value from the spec.
   • nparamsp will contain the number of 4-byte parameters
   • paramsp will contain a pointer to the parsed parameters -- the caller must free. This function can parse filter spec strings as defined in the section on Filter Specification Syntax. This function parses the first argument and returns several values.

2. int NC_filterfix8(unsigned char* mem8, int decode);

   • mem8 is a pointer to the 8-byte value either to fix.
   • decode is 1 if the function should apply the 8-byte decoding algorithm else apply the encoding algorithm. This function implements the 8-byte conversion algorithms. Before calling nc_def_var_filter (unless NC_parsefilterspec was used), the client must call this function with the decode argument set to 0. Inside the filter code, this function should be called with the decode argument set to 1.

Examples of the use of these functions can be seen in the test program nc_test4/tst_filterparser.c.

Appendix B. Programmatic Filter Definition

HDF5 provides an API [6] to allow for the programmatic definition of filters -- as opposed to using the HDF5_PLUGIN_PATH environment variable. The idea is that instead of using dynamic shared libraries, the filter code is compiled into the application and the relevant information (namely an instance of H5Z_class2_t) is passed to the HDF5 library API. Because it is anticipated that in the future, other plugin formats will be used, this netcdf-c API is deliberately more general than strictly required by HDF5.

API Concepts

Three concepts are used in this API.

1. Format - this is an integer defining the format of the plugin. Currently, only NC_FILTER_FORMAT_HDF5 is defined and corresponds to the existing HDF5 plugin format.

2. ID - this is an integer that is a unique identifier for the filter. This value is interpreted in the context of the format, so the same id might be assigned to different filters if the format is different.

3. The structure NC_FILTER_INFO that provides generic information to the API and has a placeholder for format-specific information.

    typedef struct NC_FILTER_INFO {
      int version; /* Of this structure */
      int format; /* Controls actual type of this structure */
      int id;     /* Must be unique WRT format */
      void* info; /* The filter info as defined by the format. */
    } NC_FILTER_INFO;
   

   When the format is the value NC_FILTER_FORMAT_HDF5, then the info field is a pointer to an instance of H5Z_class2_t as define in H5Zpublic.h. The use of void* is, of course, to allow for passing arbitrary objects.

NetCDF API

The following function signatures are provided (see netcdf_filter.h).

1. Register a filter

    int nc_filter_register(NC_FILTER_INFO* filter_info);
   

   Register a filter whose format and ID are specified in the 'filter_info' argument.

2. Unregister a filter

    int nc_filter_unregister(int format, int id);
   

   Unregister the filter specified by the id. Note that only filters registered using 'nc_filter_register' can be unregistered.

3. Inquire about a filter

    int nc_filter_inq(int format, int id, NC_FILTER_INFO* filter_info);
   

   Unregister the filter specified by the id. Note that only filters registered using 'nc_filter_register' can be inquired. The 'filter_info' is filled with a copy of the original argument to 'nc_filter_register'.

Example

static const H5Z_class2_t H5Z_REG[1] = {
    ...
};
...
NC_FILTER_INFO info;
...
info.version = NC_FILTER_INFO_VERSION;
info.format = NC_FILTER_FORMAT_HDF5;
info.id = FILTER_ID;
info.info = (void*)&H5Z_REG[0];
stat = nc_filter_register(&info);
...
memset(&info,0,sizeof(NC_FILTER_INFO));
stat = nc_filter_inq(NC_FILTER_FORMAT_HDF5, FILTER_ID, &info);
...
stat = nc_filter_unregister(NC_FILTER_FORMAT_HDF5, FILTER_ID);

References

1. https://support.hdfgroup.org/HDF5/doc/Advanced/DynamicallyLoadedFilters/HDF5DynamicallyLoadedFilters.pdf
2. https://support.hdfgroup.org/HDF5/doc/TechNotes/TechNote-HDF5-CompressionTroubleshooting.pdf
3. https://portal.hdfgroup.org/display/support/Contributions#Contributions-filters
4. https://support.hdfgroup.org/services/contributions.html#filters
5. https://support.hdfgroup.org/HDF5/doc/RM/RM_H5.html
6. https://confluence.hdfgroup.org/display/HDF5/Filters

Point of Contact

Author: Dennis Heimbigner
Email: dmh at ucar dot edu Initial Version: 1/10/2018
Last Revised: 2/5/2018