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2 libaec - Adaptive Entropy Coding library
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5 Libaec provides fast lossless compression of 1 up to 32 bit wide
6 signed or unsigned integers (samples). The library achieves best
7 results for low entropy data as often encountered in space imaging
8 instrument data or numerical model output from weather or climate
9 simulations. While floating point representations are not directly
10 supported, they can also be efficiently coded by grouping exponents
13 Libaec implements Golomb Rice coding as defined in the Space Data
14 System Standard document 121.0-B-2 [1], [2].
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21 In doc/license.txt a clarification on potentially applying
22 intellectual property rights is given.
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29 See INSTALL for details.
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36 In this context efficiency refers to the size of the encoded
37 data. Performance refers to the time it takes to encode data.
39 Suppose you have an array of 32 bit signed integers you want to
40 compress. The pointer pointing to the data shall be called *source,
41 output goes into *dest.
46 struct aec_stream strm;
50 /* input data is 32 bits wide */
51 strm.bits_per_sample = 32;
53 /* define a block size of 16 */
56 /* the reference sample interval is set to 128 blocks */
59 /* input data is signed and needs to be preprocessed */
60 strm.flags = AEC_DATA_SIGNED | AEC_DATA_PREPROCESS;
62 /* pointer to input */
63 strm.next_in = (unsigned char *)source;
65 /* length of input in bytes */
66 strm.avail_in = source_length * sizeof(int32_t);
68 /* pointer to output buffer */
71 /* length of output buffer in bytes */
72 strm.avail_out = dest_length;
74 /* initialize encoding */
75 if (aec_encode_init(&strm) != AEC_OK)
78 /* Perform encoding in one call and flush output. */
79 /* In this example you must be sure that the output */
80 /* buffer is large enough for all compressed output */
81 if (aec_encode(&strm, AEC_FLUSH) != AEC_OK)
84 /* free all resources used by encoder */
85 aec_encode_end(&strm);
88 block_size can vary from 8 to 64 samples. Smaller blocks allow the
89 compression to adapt to rapid changes in entropy. Larger blocks create
90 less overhead but can be less efficient if entropy changes across the
93 rsi sets the reference sample interval. A large RSI will improve
94 performance and efficiency. It will also increase memory requirements
95 since internal buffering is based on RSI size. A smaller RSI may be
96 desirable in situations where each RSI will be packetized and possible
97 error propagation has to be minimized (e.g. on board a spacecraft[2]).
101 AEC_DATA_SIGNED: input data are signed integers. Specifying this
102 correctly increases compression efficiency. Default is unsigned.
104 AEC_DATA_PREPROCESS: preprocessing input will improve compression
105 efficiency if data samples are correlated. It will only cost
106 performance for no gain in efficiency if the data is already
109 AEC_DATA_MSB: input data is stored most significant byte first
110 i.e. big endian. You have to specify AEC_DATA_MSB even if your host
111 architecture is big endian. Default is little endian on all
114 AEC_DATA_3BYTE: the 24 bit input data is stored in three bytes.
118 Except for the AEC_DATA_3BYTE case for 24 bit data, the following
119 rules apply for deducing storage size from sample size
122 sample size storage size
125 17 - 32 bit 4 bytes (also for 24bit if AEC_DATA_3BYTE is not set)
127 If you use less bits than the storage size provides, then you have to
128 make sure that unused bits are not set. Libaec does not check this for
129 performance reasons and will produce undefined output if unused bits
132 Libaec accesses next_in and next_out buffers only bytewise. There are
133 no alignment requirements for these buffers.
137 aec_encode can be used in a streaming fashion by chunking input and
138 output and specifying AEC_NO_FLUSH. The function will return if either
139 the input runs empty or the output buffer is full. The calling
140 function can check avail_in and avail_out to see what occcurred. The
141 last call to aec_encode() must set AEC_FLUSH to drain all
142 output. aec.c is an example of streaming usage of encoding and
147 Encoded data will be written to the buffer submitted with
148 next_out. The length of the compressed data is total_out.
150 See libaec.h for a detailed description of all relevant structure
151 members and constants.
154 **********************************************************************
156 **********************************************************************
158 Using decoding is very similar to encoding, only the meaning of input
159 and output is reversed.
164 struct aec_stream strm;
165 /* this is now the compressed data */
166 unsigned char *source;
167 /* here goes the uncompressed result */
170 strm.bits_per_sample = 32;
171 strm.block_size = 16;
173 strm.flags = AEC_DATA_SIGNED | AEC_DATA_PREPROCESS;
174 strm.next_in = source;
175 strm.avail_in = source_length;
176 strm.next_out = (unsigned char *)dest;
177 strm.avail_out = dest_lenth * sizeof(int32_t);
178 if (aec_decode_init(&strm) != AEC_OK)
180 if (aec_decode(&strm, AEC_FLUSH) != AEC_OK)
182 aec_decode_end(&strm);
185 It is essential for decoding that parameters like bits_per_sample,
186 block_size, rsi, and flags are exactly the same as they were for
187 encoding. Libaec does not store these parameters in the coded stream
188 so it is up to the calling program to keep the correct parameters
189 between encoding and decoding.
191 The actual values of coding parameters are in fact only relevant for
192 efficiency and performance. Data integrity only depends on consistency
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200 [1] Consultative Committee for Space Data Systems. Lossless Data
201 Compression. Recommendation for Space Data System Standards, CCSDS
202 121.0-B-2. Blue Book. Issue 2. Washington, D.C.: CCSDS, May 2012.
203 http://public.ccsds.org/publications/archive/121x0b2.pdf
205 [2] Consultative Committee for Space Data Systems. Lossless Data
206 Compression. Recommendation for Space Data System Standards, CCSDS
207 120.0-G-2. Green Book. Issue 2. Washington, D.C.: CCSDS, December
209 http://public.ccsds.org/publications/archive/120x0g2.pdf