Libaec provides fast lossless compression of 1 up to 32 bit wide signed or unsigned integers (samples). The library achieves best results for low entropy data as often encountered in space imaging instrument data or numerical model output from weather or climate simulations. While floating point representations are not directly supported, they can also be efficiently coded by grouping exponents and mantissa.
Libaec implements extended Golomb-Rice coding as defined in the CCSDS recommended standard 121.0-B-3. The library covers the adaptive entropy coder and the preprocessor discussed in sections 1 to 5.2.6 of the standard.
Source code and binary installer can be downloaded here or here.
As stated in section A3 of the current standard
At time of publication, the specifications of this Recommended Standard are not known to be the subject of patent rights.
See INSTALL.md for details.
In this context efficiency refers to the size of the encoded data. Performance refers to the time it takes to encode data.
Suppose you have an array of 32 bit signed integers you want to
compress. The pointer pointing to the data shall be called source,
output goes into dest.
#include <libaec.h>
...
struct aec_stream strm;
int32_t *source;
unsigned char *dest;
/* input data is 32 bits wide */
strm.bits_per_sample = 32;
/* define a block size of 16 */
strm.block_size = 16;
/* the reference sample interval is set to 128 blocks */
strm.rsi = 128;
/* input data is signed and needs to be preprocessed */
strm.flags = AEC_DATA_SIGNED | AEC_DATA_PREPROCESS;
/* pointer to input */
strm.next_in = (unsigned char *)source;
/* length of input in bytes */
strm.avail_in = source_length * sizeof(int32_t);
/* pointer to output buffer */
strm.next_out = dest;
/* length of output buffer in bytes */
strm.avail_out = dest_length;
/* initialize encoding */
if (aec_encode_init(&strm) != AEC_OK)
return 1;
/* Perform encoding in one call and flush output. */
/* In this example you must be sure that the output */
/* buffer is large enough for all compressed output */
if (aec_encode(&strm, AEC_FLUSH) != AEC_OK)
return 1;
/* free all resources used by encoder */
aec_encode_end(&strm);
...block_size can vary from 8 to 64 samples. Smaller blocks allow the
compression to adapt more rapidly to changing source
statistics. Larger blocks create less overhead but can be less
efficient if source statistics change across the block.
rsi sets the reference sample interval in blocks. A large RSI will
improve performance and efficiency. It will also increase memory
requirements since internal buffering is based on RSI size. A smaller
RSI may be desirable in situations where errors could occur in the
transmission of encoded data and the resulting propagation of errors
in decoded data has to be minimized.
-
AEC_DATA_SIGNED: input data are signed integers. Specifying this correctly increases compression efficiency. Default is unsigned. -
AEC_DATA_PREPROCESS: preprocessing input will improve compression efficiency if data samples are correlated. It will only cost performance for no gain in efficiency if the data is already uncorrelated. -
AEC_DATA_MSB: input data is stored most significant byte first i.e. big endian. Default is little endian on all architectures. -
AEC_DATA_3BYTE: the 17 to 24 bit input data is stored in three bytes. This flag has no effect for other sample sizes. -
AEC_RESTRICTED: use a restricted set of code options. This option is only valid forbits_per_sample<= 4.
The following rules apply for deducing storage size from sample size
(bits_per_sample):
| sample size | storage size |
|---|---|
| 1 - 8 bits | 1 byte |
| 9 - 16 bits | 2 bytes |
| 17 - 24 bits | 3 bytes (only if AEC_DATA_3BYTE is set) |
| 25 - 32 bits | 4 bytes (if AEC_DATA_3BYTE is set) |
| 17 - 32 bits | 4 bytes (if AEC_DATA_3BYTE is not set) |
If a sample requires less bits than the storage size provides, then you have to make sure that unused bits are not set. Libaec does not enforce this for performance reasons and will produce undefined output if unused bits are set. All input data must be a multiple of the storage size in bytes. Remaining bytes which do not form a complete sample will be ignored.
Libaec accesses next_in and next_out buffers only bytewise. There
are no alignment requirements for these buffers.
aec_encode can be used in a streaming fashion by chunking input and
output and specifying AEC_NO_FLUSH. The function will return if either
the input runs empty or the output buffer is full. The calling
function can check avail_in and avail_out to see what occurred. The
last call to aec_encode() must set AEC_FLUSH to drain all
output. aec.c is an example of streaming usage of encoding and
decoding.
Encoded data will be written to the buffer submitted with
next_out. The length of the compressed data is total_out.
See libaec.h for a detailed description of all relevant structure members and constants.
Using decoding is very similar to encoding, only the meaning of input and output is reversed.
#include <libaec.h>
...
struct aec_stream strm;
/* this is now the compressed data */
unsigned char *source;
/* here goes the uncompressed result */
int32_t *dest;
strm.bits_per_sample = 32;
strm.block_size = 16;
strm.rsi = 128;
strm.flags = AEC_DATA_SIGNED | AEC_DATA_PREPROCESS;
strm.next_in = source;
strm.avail_in = source_length;
strm.next_out = (unsigned char *)dest;
strm.avail_out = dest_lenth * sizeof(int32_t);
if (aec_decode_init(&strm) != AEC_OK)
return 1;
if (aec_decode(&strm, AEC_FLUSH) != AEC_OK)
return 1;
aec_decode_end(&strm);
...It is strongly recommended that the size of the output buffer
(next_out) is a multiple of the storage size in bytes. If the buffer
is not a multiple of the storage size and the buffer gets filled to
the last sample, the error code AEC_MEM_ERROR is returned.
It is essential for decoding that parameters like bits_per_sample,
block_size, rsi, and flags are exactly the same as they were for
encoding. Libaec does not store these parameters in the coded stream
so it is up to the calling program to keep the correct parameters
between encoding and decoding.
The actual values of coding parameters are in fact only relevant for efficiency and performance. Data integrity only depends on consistency of the parameters.
The exact length of the original data is not preserved and must also be transmitted out of band. The decoder can produce additional output depending on whether the original data ended on a block boundary or on zero blocks. The output data must therefore be truncated to the correct length. This can also be achieved by providing an output buffer of just the correct length.