HDF compression and chunking

The HDF interfaces that support compression and/or chunking are in the following table

SD - Multifile Scientific Datasets

Compression

In the SDS interface, compression is done with the SDsetcompress routine. The syntax of the routine SDsetcompress is as follows:

status = SDsetcompress(sds_id, comp_type, &c_info);

The parameter comp_type specifies the compression type definition. Compression information is specified by the parameter c_info. The following table summarizes the available options

SDsetcompress writes the compressed data, in its entirety, to the data set. The data set is built in-core then written in a single write operation.

Chunking

The SDsetchunk function is called to make a SDS a chunked SDS. There are two restrictions that apply to chunked SDSs. The maximum number of chunks in a single HDF file is 65,535 and a chunked SDS cannot contain an unlimited dimension. SDsetchunk sets the chunk size and the compression method for a data set. The syntax of SDsetchunk is as follows:

status = SDsetchunk(sds_id, c_def, c_flag);

The chunking information is provided in the parameters c_def and c_flag. The parameter flag specifies the type of the data set, i.e., if the data set is chunked or chunked and compressed. The following table summarizes the available options

GR - Multifile General Raster Images

Compression

GR Images are compressed using the routine GRsetcompress. The syntax of the routine GRsetcompress is as follows:

status = GRsetcompress(ri_id, comp_type, c_info); 

The compression method is specified by the parameter comp_type. The parameter c_info has type comp_info and contains algorithm-specific information for the library compression routines. The following table summarizes the available options

Chunking

The GR interface also supports chunking in a manner similar to that of the SD interface. There is one restriction on a raster image: it must be created with MFGR_INTERLACE_PIXEL in the call to GRcreate. See why. The function GRsetchunk makes the raster image, identified by the parameter ri_id, a chunked raster image according to the provided chunking and compression information. The syntax of GRsetchunk is as follows:

status = GRsetchunk(ri_id, c_def, c_flags); 

The parameters c_def and c_flags provide the chunking and compression information and are discussed below

DFR8 - Single-file 8-Bit Raster Images

The compression type is determined by the tag passed as the fifth argument in calls to the DFR8putimage and DFR8addimage routines. DFR8setcompress provides a method for compressing the next raster image written. 

intn DFR8addimage(char *filename, VOIDP image, int32 width, int32 height, uint16 compress);
intn DFR8setcompress(int32 type, comp_info *cinfo);

The compress options are

The comp_info union contains algorithm-specific information for the library routines that perform the compression. It is only used by the COMP_JPEG compression type. A pointer to a valid comp_info union is required for all compression types other than COMP_JPEG, but the values in the union are not used.

DF24 - Single-file 24-Bit Raster Image Interface

To store a 24-bit raster image using compression, the calling program must contain the following function calls

intn DF24setcompress(int32 type, comp_info *cinfo);
intn DF24addimage(char *filename, VOIDP image, int32 width, int32 height);

The compress options are the same as for the DFR8 case. 

Compression and chunking

Compression done with the SDsetcompress (or GRsetcompress) and chunking done with SDsetchunk (or GRsetchunk) are mutually exclusive. This is, in a application it is not possible to chunk a dataset and compress it with SDsetcompress. The following table illustrates the valid and invalid options.