www.gusucode.com > 一个可以在局域网进行视频聊天的源代码 > 一个可以在局域网进行视频聊天的源代码/VoIP/decoder/Sac.cpp
////////////////////////////////////////////////////////////////////////////
//
//
// Project : VideoNet version 1.1.
// Description : Peer to Peer Video Conferencing over the LAN.
// Author : Nagareshwar Y Talekar ( nsry2002@yahoo.co.in)
// Date : 15-6-2004.
//
// This is the modified version of tmndecode (H.263 decoder)
// written by Karl & Robert.It was in ANSI C. I have converted into C++
// so that it can be integrated into any windows application. I have
// removed some of the files which had display and file storing
// functions.I have removed the unnecessary code and also added some
// new files..
// Original library dealt with files. Input & Output , both were files.
// I have done some major changes so that it can be used for real time
// decoding process. Now one can use this library for decoding H263 frames.
//
//
// File description :
// Name : sac.cpp
//
/////////////////////////////////////////////////////////////////////////////
/************************************************************************
*
* sac.c, part of tmndecode (H.263 decoder)
* Copyright (C) 1996 Telenor R&D, Norway
* Karl Olav Lillevold <Karl.Lillevold@nta.no>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*
* Karl Olav Lillevold <Karl.Lillevold@nta.no>
* Telenor Research and Development
* P.O.Box 83 tel.: +47 63 84 84 00
* N-2007 Kjeller, Norway fax.: +47 63 81 00 76
*
* Robert Danielsen e-mail: Robert.Danielsen@nta.no
* Telenor Research and Development www: http://www.nta.no/brukere/DVC/
* P.O.Box 83 tel.: +47 63 84 84 00
* N-2007 Kjeller, Norway fax.: +47 63 81 00 76
*
************************************************************************/
/*********************************************************************
* SAC Decoder Module
* Algorithm as Specified in H26P Annex -E
* (c) 1995 BT Labs
*
* Author: Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk>
*
*********************************************************************/
#include "Sac.h"
/*********************************************************************
* SAC Decoder Algorithm as Specified in H26P Annex -E
*
* Name: decode_a_symbol
*
* Description: Decodes an Aritmetically Encoded Symbol
*
* Input: array holding cumulative freq. data
* also uses static data for decoding endpoints
* and code_value variable
*
* Returns: Index to relevant symbol model
*
* Side Effects: Modifies low, high, length, cum and code_value
*
* Author: Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk>
*
*********************************************************************/
static long low, high, code_value, bit, length, sacindex, cum, zerorun=0;
int decode_a_symbol(int cumul_freq[ ])
{
length = high - low + 1;
cum = (-1 + (code_value - low + 1) * cumul_freq[0]) / length;
for (sacindex = 1; cumul_freq[sacindex] > cum; sacindex++);
high = low - 1 + (length * cumul_freq[sacindex-1]) / cumul_freq[0];
low += (length * cumul_freq[sacindex]) / cumul_freq[0];
for ( ; ; ) {
if (high < q2) ;
else if (low >= q2) {
code_value -= q2;
low -= q2;
high -= q2;
}
else if (low >= q1 && high < q3) {
code_value -= q1;
low -= q1;
high -= q1;
}
else {
break;
}
low *= 2;
high = 2*high + 1;
bit_out_psc_layer();
code_value = 2*code_value + bit;
}
return (sacindex-1);
}
/*********************************************************************
*
* Name: decoder_reset
*
* Description: Fills Decoder FIFO after a fixed word length
* string has been detected.
*
* Input: None
*
* Returns: Nothing
*
* Side Effects: Fills Arithmetic Decoder FIFO
*
* Author: Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk>
*
*********************************************************************/
void decoder_reset( )
{
int i;
zerorun = 0; /* clear consecutive zero's counter */
code_value = 0;
low = 0;
high = top;
for (i = 1; i <= 16; i++) {
bit_out_psc_layer();
code_value = 2 * code_value + bit;
}
if (trace)
fputs("Arithmetic Decoder Reset \n",dlog);
}
/*********************************************************************
*
* Name: bit_out_psc_layer
*
* Description: Gets a bit from the Encoded Stream, Checks for
* and removes any PSC emulation prevention bits
* inserted at the decoder, provides 'zeros' to the
* Arithmetic Decoder FIFO to allow it to finish
* data prior to the next PSC. (Garbage bits)
*
* Input: None
*
* Returns: Nothing
*
* Side Effects: Gets a bit from the Input Data Stream
*
* Author: Wayne Ellis <ellis_w_wayne@bt-web.bt.co.uk>
*
*********************************************************************/
void bit_out_psc_layer()
{
if (showbits(17)!=1) { /* check for startcode in Arithmetic Decoder FIFO */
bit = getbits(1);
if(zerorun > 13) { /* if number of consecutive zeros = 14 */
if (!bit) {
if (trace)
fputs("PSC/GBSC, Header Data, or Encoded Stream Error \n",dlog);
zerorun = 1;
}
else { /* if there is a 'stuffing bit present */
if (trace)
fputs("Removing Startcode Emulation Prevention bit \n",dlog);
bit = getbits(1); /* overwrite the last bit */
zerorun = !bit; /* zerorun=1 if bit is a '0' */
}
}
else { /* if consecutive zero's not exceeded 14 */
if (!bit)
zerorun++;
else
zerorun = 0;
}
} /* end of if !(showbits(17)) */
else {
bit = 0;
if (trace)
fputs("Startcode Found:Finishing Arithmetic Decoding using 'Garbage bits'\n",dlog);
}
/*
printf("lastbit = %ld bit = %ld zerorun = %ld \n", lastbit, bit, zerorun);
lastbit = bit;
*/
/* latent diagnostics */
}