mfhdf(3) mfhdf(3)
NAME
GRsetchunk, GRgetchunkinfo, GRetchunkcache - GR Chunking Routines
SYNOPSIS
#include <hdf.h>
extern intn GRsetchunk
(
int32 riid,
HDF_CHUNK_DEF chunk_def,
int32 flags
);
extern intn GRgetchunkinfo
(
int32 riid,
HDF_CHUNK_DEF *chunk_def,
int32 *flags
);
extern intn GRsetchunkcache
(
int32 riid,
int32 maxcache,
int32 flags
);
PARAMETERS
int32 riid
IN: raseter access id.
HDF_CHUNK_DEF chunk_def
IN: chunk definition.
int32 flags (GRsetchunk)
IN: flags.
HDF_CHUNK_DEF *chunk_def
IN/OUT: chunk definition.
int32 *flags
IN/OUT: flags.
int32 maxcache
IN: max number of chunks to cache.
int32 flags (GRsetchunkcache)
IN: flags = 0, HDF_CACHEALL.
DESCRIPTION
GRsetchunk
This routine makes the GR a chunked GR according to the chunk defini-
tion passed in.
The dataset currently cannot be special already i.e. NBIT, COMPRESSED,
or EXTERNAL. This is an Error.
The defintion of the "HDF_CHUNK_DEF" union with relvant fields is:
typedef union hdf_chunk_def_u
{
int32 chunk_lengths[2]; Chunk lengths along each dimension.
{
int32 chunk_lengths[2]; Chunk lengths along each dimension.
int32 comp_type; Compression type
comp_info cinfo; Compression info struct
}comp;
} HDF_CHUNK_DEF
The variable agruement 'flags' is a bit-or'd value which can currently
be 'HDF_CHUNK' or 'HDF_CHUNK | HDF_COMP'.
The simplist is the 'chunk_lengths' array specifiying chunk lengths for
each dimension where the 'flags' argument set to
COMPRESSION is set by using the 'HDF_CHUNK_DEF' union to set the appro-
priate compression information along with the required chunk lengths
for each dimension. The compression information is the same as that set
in 'GRsetcompress()'. The bit-or'd 'flags' argument' is set to
See the example in pseudo-C below for further usage.
The maximum number of Chunks in an HDF file is 65,535.
The performance of the GRxxx interface with chunking is greatly
affected by the users access pattern over the image and by the maximum
number of chunks set in the chunk cache. The cache contains the Least
Recently Used(LRU cache replacment policy) chunks. See the routine
GRsetchunkcache() for further info on the chunk cache and how to set
the maximum number of chunks in the chunk cache. A default chunk cache
is always created.
The following example shows the organization of chunks for a 2D array.
e.g. 4x4 array with 2x2 chunks. The array shows the layout of chunks in
the chunk array.
4 ---------------------
| | |
Y | (0,1) | (1,1) |
^ | | |
| 2 ---------------------
| | | |
| | (0,0) | (1,0) |
| | | |
| ---------------------
| 0 2 4
---------------> X
--Without compression--:
{
HDF_CHUNK_DEF chunk_def;
.......
-- Set chunk lengths --
chunk_def.chunk_lengths[0]= 2;
chunk_def.chunk_lengths[1]= 2;
-- Set Chunking --
GRsetchunk(riid, chunk_def, HDF_CHUNK);
......
}
--With compression--:
{
HDF_CHUNK_DEF chunk_def;
.......
-- Set chunk lengths first --
chunk_def.chunk_lengths[0]= 2;
chunk_def.chunk_lengths[1]= 2;
-- Set compression --
chunk_def.comp.cinfo.deflate.level = 9;
chunk_def.comp.comp_type = COMP_CODE_DEFLATE;
-- Set Chunking with Compression --
GRsetchunk(riid, chunk_def, HDF_CHUNK | HDF_COMP);
......
}.
GRgetchunkinfo
This routine gets any special information on the GR. If its chunked,
chunked and compressed or just a regular GR. Currently it will only
fill the array of chunk lengths for each dimension as specified in the
"HDF_CHUNK_DEF" union. It does not tell you the type of compression or
the compression parameters used. You can pass in a NULL for "chunk_def"
if don't want the chunk lengths for each dimension. Additionaly if
successfull it will return a bit-or'd value in "flags" indicating if
the GR is:
Chunked -> flags = HDF_CHUNK
Chunked and compressed -> flags = HDF_CHUNK | HDF_COMP
Non-chunked -> flags = HDF_NONE
e.g. 4x4 array - Pseudo-C
{
int32 rcdims[3];
HDF_CHUNK_DEF rchunk_def;
int32 cflags;
...
rchunk_def.chunk_lengths = rcdims;
GRgetchunkinfo(sdsid, &rchunk_def, &cflags);
...
}.
GRsetchunkcache
Set the maximum number of chunks to cache.
The cache contains the Least Recently Used(LRU cache replacment policy)
chunks. This routine allows the setting of maximum number of chunks
that can be cached, "maxcache".
The performance of the GRxxx interface with chunking is greatly
affected by the users access pattern over the dataset and by the maxi-
mum number of chunks set in the chunk cache. The number chunks that can
be set in the cache is process memory limited. It is a good idea to
always set the maximum number of chunks in the cache as the default
heuristic does not take into account the memory available for the
application.
By default when the GR is created as a chunked element the maximum num-
ber of chunks in the cache "maxcache" is set to the number of chunks
along the last dimension.
The values set here affects the current object's caching behaviour.
If the chunk cache is full and "maxcache" is greater then the current
"maxcache" value, then the chunk cache is reset to the new "maxcache"
value, else the chunk cache remains at the current "maxcache" value.
If the chunk cache is not full, then the chunk cache is set to the new
"maxcache" value only if the new "maxcache" value is greater than the
current number of chunks in the cache.
Use flags argument of "HDF_CACHEALL" if the whole object is to be
cached in memory, otherwise pass in zero(0). Currently you can only
pass in zero.
See GRsetchunk() for a description of the organization of chunks in a
GR .
RETURNS
GRsetchunk
SUCCEED/FAIL.
GRgetchunkinfo
SUCCEED/FAIL.
GRsetchunkcache
Returns the 'maxcache' value for the chunk cache if successful and FAIL
otherwise.
NAME
GRsetchunk
GRsetchunk -- make GR a chunked GR.
GRgetchunkinfo
GRgetchunkinfo -- get Info on GR.
GRsetchunkcache
GRsetchunkcache -- maximum number of chunks to cache.
c2man hproto.h 16 October 1997 mfhdf(3)
hdf 4.2r4 - Generated Fri Feb 6 07:38:27 CST 2009