qccentarithmeticgetcontext.3

spiht for linux this is used to decod and encode vedio i wich all enjoy

.TH QCCENTARITHMETICGETCONTEXT 3 "QCCPACK" ""
.SH NAME
QccENTArithmeticGetContext \- 
data type 
.B QccENTArithmeticGetContext
for functions for retrieving the current context
for symbols during arithmetic encoding and decoding
.SH SYNOPSIS
.B #include "libQccPack.h"
.sp
.BI "typedef int (*QccENTArithmeticGetContext)(const int *" symbol_stream ", int " current_symbol_index );
.SH DESCRIPTION
For multiple-context arithmetic, the QccPack arithmetic encoding
and decoding routines need the context of the current symbol in order
to encode the symbol and update the relevant probability models.
When an arithmetic-coder model
(type
.BR QccENTArithmeticModel (3))
is created via routines
.BR QccENTArithmeticEncodeStart (3)
or
.BR QccENTArithmeticDecodeStart (3),
one specifies a function of type
.B QccENTArithmeticGetContext
that serves as a callback function.
Within routines
.BR QccENTArithmeticEncode (3)
or
.BR QccENTArithmeticDecode (3),
this callback function is called for the coding/decoding of
each symbol in order to provide the context for the symbol.
.LP
In order to be a valid callback routine, functions to be used as such
must have a prototype as shown in the "SYNOPSIS" section above.
The callback routine must accept a pointer
.IR symbol_stream .
This pointer is the same pointer passed as the symbol stream to
.BR QccENTArithmeticEncode (3)
or
.BR QccENTArithmeticDecode (3).
Additionally, the callback function must accept
an integer,
.IR current_symbol_index ,
which gives the index into
.I symbol_stream
of the current symbol.
The callback routine can thus use
.I symbol_stream
and
.I current_symbol_index
to obtain a past history of the symbol stream relative to the current
symbol, and use this information in determining the current context.
For example, the previous symbol,
.IR symbol_stream [ current_symbol_index " - 1]"
could be used for an arithmetic coder that attempts to
code at the first-order entropy,
.IR "H(X[n] | X[n-1])" ,
of the source.
Of course, the
callback routine could ignore this information, and use
other means to determine the current context.
.SH "RETURN VALUE"
The callback routine must return an integer which indicates the
context of the current symbol. This context must be greater than or
equal to 0, and less than 
.IR num_contexts ,
the total number of contexts as setup initially when the arithmetic
model was created via the call to 
.BR QccENTArithmeticEncodeStart (3)
or
.BR QccENTArithmeticDecodeStart (3).
.SH "SEE ALSO"
.BR QccENTArithmeticModel (3),
.BR QccENTArithmeticEncode (3),
.BR QccENTArithmeticEncodeStart (3),
.BR QccENTArithmeticEncodeEnd (3),
.BR QccENTArithmeticDecode (3),
.BR QccENTArithmeticDecodeStart (3),
.BR QccENTArithmeticDecodeRestart (3),
.BR QccPackENT (3),
.BR QccPack (3)
.LP
I. H. Witten, R. M. Neal, and J. G. Cleary,
"Arithmetic Coding for Data Compression,"
.IR "Communications of the ACM" ,
vol. 30, no. 6, pp. 520-540, June 1987.
.SH AUTHOR
Copyright (C) 1997-2009  James E. Fowler
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