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 */ }