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libaec - Adaptive Entropy Coding library¶
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.
Source code and binary installer can be downloaded here.
In license.txt a clarification on potentially applying intellectual property rights is given.
See INSTALL 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 to rapid changes in entropy. Larger blocks create less overhead but can be less efficient if entropy changes across the block.
rsi sets the reference sample interval. 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 each RSI will be packetized and possible error propagation 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. You have to specify AEC_DATA_MSB even if your host architecture is big endian. Default is little endian on all architectures.
- AEC_DATA_3BYTE: the 24 bit input data is stored in three bytes.
- AEC_RESTRICTED: use a restricted set of code options. This option is only valid for bits_per_sample <= 4.
- AEC_PAD_RSI: assume that the encoded RSI is padded to the next byte boundary while decoding. The preprocessor macro ENABLE_RSI_PADDING needs to be defined while compiling for the encoder to honour this flag.
Except for the AEC_DATA_3BYTE case for 24 bit data, 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 - 32 bits||4 bytes (also for 24bit 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 check 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 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.
1 Consultative Committee for Space Data Systems. Lossless Data Compression. Recommendation for Space Data System Standards, CCSDS 121.0-B-2. Blue Book. Issue 2. Washington, D.C.: CCSDS, May 2012.
2 Consultative Committee for Space Data Systems. Lossless Data Compression. Recommendation for Space Data System Standards, CCSDS 120.0-G-3. Green Book. Issue 3. Washington, D.C.: CCSDS, April 2013.