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authorMathis Rosenhauer <rosenhauer@dkrz.de>2016-04-14 08:48:43 (GMT)
committerMathis Rosenhauer <rosenhauer@dkrz.de>2016-04-14 08:48:43 (GMT)
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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.
+
+Libaec implements
+"Golomb-Rice":http://en.wikipedia.org/wiki/Golomb_coding coding as
+defined in the Space Data System Standard documents [121.0-B-2][1] and
+[120.0-G-2][2].
+
+## Patents
+
+In [doc/license.txt] a clarification on potentially applying
+intellectual property rights is given.
+
+## Installation
+
+See [INSTALL] for details.
+
+## SZIP Compatibility
+
+[Libaec can replace SZIP][README.SZIP].
+
+## Encoding
+
+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.
+
+```C++
+#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. 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.
+
+### Flags:
+
+* 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 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 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.
+
+### Data size:
+
+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
+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.
+
+### Flushing:
+
+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.
+
+### Output:
+
+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.
+
+
+## Decoding
+
+Using decoding is very similar to encoding, only the meaning of input
+and output is reversed.
+
+```C++
+#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.
+
+
+## References
+
+[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.][1]
+[1] http://public.ccsds.org/publications/archive/121x0b2.pdf
+
+[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.][2]
+[2] http://public.ccsds.org/publications/archive/120x0g3.pdf