Commit 9438be93 authored by Mathis Rosenhauer's avatar Mathis Rosenhauer Committed by Thomas Jahns

clarifications

parent 8ce474cf
......@@ -3,13 +3,15 @@
**********************************************************************
libaec provides fast lossless compression of 1 up to 32 bit wide
signed or unsigned integers. 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.
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 a Space Data System Standard [1], [2].
libaec implements Golomb Rice coding as defined in the Space Data
System Standard document 121.0-B-2 [1], [2].
**********************************************************************
......@@ -24,7 +26,7 @@ intellectual property rights is given.
Installation
**********************************************************************
See INSTALL for details.
See INSTALL for details.
**********************************************************************
......@@ -45,7 +47,7 @@ output goes into *dest.
int32_t *source;
unsigned char *dest;
/* input data ist 32 bits wide */
/* input data is 32 bits wide */
strm.bits_per_sample = 32;
/* define a block size of 16 */
......@@ -83,9 +85,10 @@ output goes into *dest.
aec_encode_end(&strm);
...
block_size can vary from 8 to 64. Smaller blocks allow the compression
to adapt to rapid changes in entropy. Larger blocks create less
overhead but can be less efficient.
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 requiremens
......@@ -98,9 +101,10 @@ Flags:
AEC_DATA_SIGNED: input data are signed integers. Specifying this
correctly increases compression efficiency. Default is unsigned.
AEC_DATA_PREPROCESS: preprocessing input will almost always improve
compression efficiency. It may only cost performance for no gain in
efficiency if the data is already uncorrelated.
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
......@@ -112,28 +116,36 @@ AEC_DATA_3BYTE: the 24 bit input data is stored in three bytes.
Data size:
Except for the AEC_DATA_3BYTE case for 24 bit data, the following
rules apply:
rules apply for deducing storage size from sample size
(bits_per_sample):
data size storage size
sample size storage size
1 - 8 bit 1 byte
9 - 16 bit 2 bytes
17 - 32 bit 4 bytes (also for 24bit if AEC_DATA_3BYTE is not set)
If you use 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.
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 occcurred. aec.c
is an example of streaming usage of encoding and decoding.
function can check avail_in and avail_out to see what occcurred. 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.
**********************************************************************
Decoding
**********************************************************************
Usage for decoding is very similar to encoding, only the meaning of
input and output is reversed.
Using decoding is very similar to encoding, only the meaning of input
and output is reversed.
#include <libaec.h>
......
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