SIP與RTP綜合應用5-RTP解包過程

RTP接收部分比較簡單(不用考慮jitterbuffer等)，先從這里入手。

其實主要就3步：

1 創建一個udp，監聽一個端口，比如5200。

2 收到RTP包，送到解包程序，繼續收第二個。

3 收齊一幀后，或保存文件，或解碼去播放。

下面詳細說一下具體過程：

1 創建UDP，非常非常地簡單(這里只是簡單地模擬RTP接收，雖然能正常工作，但是沒有處理RTCP部分，會影響發送端):

lass CUDPSocket : public CAsyncSocket
{
public:
??? CUDPSocket();
??? virtual ~CUDPSocket();
???
??? virtual void OnReceive(int nErrorCode);

};

調用者：CUDPSocket m_udp; m_udp.Create(...);這樣就可以了。注意端口，如果指定端口創建不成功，就端口+1或+2重試一下。

重寫OnReceive:

void CUDPSocket::OnReceive(int nErrorCode)
{
??? char szBuffer[1500];

??? SOCKADDR_IN sockAddr;
??? memset(&sockAddr, 0, sizeof(sockAddr));
??? int nSockAddrLen = sizeof(sockAddr);

??? int nResult = ReceiveFrom(szBuffer, 1500, (SOCKADDR*)&sockAddr, &nSockAddrLen, 0);
??? if(nResult == SOCKET_ERROR)
??? {
??? ??? return;
??? }

//如果必要可以處理對方IP端口

?? USHORT unPort = ntohs(sockAddr.sin_port);
??? ULONG ulIP = sockAddr.sin_addr.s_addr;

//收到的數據送去解碼

??? Decode((BYTE*)szBuffer, nResult);

}

2 收到了數據，開始Decode，一般通過RTP傳輸的視頻主要有h263 (old,1998,2000),h264,mpeg4-es。mpeg4-es格式最簡單，就從它入手。

如果了解RFC3160，直接分析格式寫就是了。如果想偷懶，用現成的，也找的到：在opal項目下，有個plugins目錄，視頻中包含了h261,h263,h264,mpeg4等多種解包，解碼的源碼，稍加改動就可以拿來用。

首先看：video/common下的rtpframe.h這個文件，這是對RTP包頭的數據和操作的封裝：

/*****************************************************************************/
/* The contents of this file are subject to the Mozilla Public License?????? */
/* Version 1.0 (the "License"); you may not use this file except in????????? */
/* compliance with the License.? You may obtain a copy of the License at???? */
/* http://www.mozilla.org/MPL/?????????????????????????????????????????????? */
/*?????????????????????????????????????????????????????????????????????????? */
/* Software distributed under the License is distributed on an "AS IS"?????? */
/* basis, WITHOUT WARRANTY OF ANY KIND, either express or implied.? See the? */
/* License for the specific language governing rights and limitations under? */
/* the License.????????????????????????????????????????????????????????????? */
/*?????????????????????????????????????????????????????????????????????????? */
/* The Original Code is the Open H323 Library.?????????????????????????????? */
/*?????????????????????????????????????????????????????????????????????????? */
/* The Initial Developer of the Original Code is Matthias Schneider????????? */
/* Copyright (C) 2007 Matthias Schneider, All Rights Reserved.?????????????? */
/*?????????????????????????????????????????????????????????????????????????? */
/* Contributor(s): Matthias Schneider (ma30002000@yahoo.de)????????????????? */
/*?????????????????????????????????????????????????????????????????????????? */
/* Alternatively, the contents of this file may be used under the terms of?? */
/* the GNU General Public License Version 2 or later (the "GPL"), in which?? */
/* case the provisions of the GPL are applicable instead of those above.? If */
/* you wish to allow use of your version of this file only under the terms?? */
/* of the GPL and not to allow others to use your version of this file under */
/* the MPL, indicate your decision by deleting the provisions above and????? */
/* replace them with the notice and other provisions required by the GPL.??? */
/* If you do not delete the provisions above, a recipient may use your?????? */
/* version of this file under either the MPL or the GPL.???????????????????? */
/*?????????????????????????????????????????????????????????????????????????? */
/* The Original Code was written by Matthias Schneider <ma30002000@yahoo.de> */
/*****************************************************************************/

#ifndef __RTPFRAME_H__
#define __RTPFRAME_H__ 1

#ifdef _MSC_VER
#pragma warning(disable:4800)? // disable performance warning
#endif

class RTPFrame {
public:
? RTPFrame(const unsigned char * frame, int frameLen) {
??? _frame = (unsigned char*) frame;
??? _frameLen = frameLen;
? };

? RTPFrame(unsigned char * frame, int frameLen, unsigned char payloadType) {
??? _frame = frame;
??? _frameLen = frameLen;
??? if (_frameLen > 0)
????? _frame [0] = 0x80;
??? SetPayloadType(payloadType);
? }

? unsigned GetPayloadSize() const {
??? return (_frameLen - GetHeaderSize());
? }

? void SetPayloadSize(int size) {
??? _frameLen = size + GetHeaderSize();
? }

? int GetFrameLen () const {
??? return (_frameLen);
? }

? unsigned char * GetPayloadPtr() const {
??? return (_frame + GetHeaderSize());
? }

? int GetHeaderSize() const {
??? int size;
??? size = 12;
??? if (_frameLen < 12)
????? return 0;
??? size += (_frame[0] & 0x0f) * 4;
??? if (!(_frame[0] & 0x10))
????? return size;
??? if ((size + 4) < _frameLen)
????? return (size + 4 + (_frame[size + 2] << 8) + _frame[size + 3]);
??? return 0;
? }

? bool GetMarker() const {
??? if (_frameLen < 2)
????? return false;
??? return (_frame[1] & 0x80);
? }

? unsigned GetSequenceNumber() const {
??? if (_frameLen < 4)
????? return 0;
??? return (_frame[2] << 8) + _frame[3];
? }

? void SetMarker(bool set) {
??? if (_frameLen < 2)
????? return;
??? _frame[1] = _frame[1] & 0x7f;
??? if (set) _frame[1] = _frame[1] | 0x80;
? }

? void SetPayloadType(unsigned char type) {
??? if (_frameLen < 2)
????? return;
??? _frame[1] = _frame [1] & 0x80;
??? _frame[1] = _frame [1] | (type & 0x7f);
? }

? unsigned char GetPayloadType() const
? {
??? if (_frameLen < 1)
????? return 0xff;
??? return _frame[1] & 0x7f;
? }

? unsigned long GetTimestamp() const {
??? if (_frameLen < 8)
????? return 0;
??? return ((_frame[4] << 24) + (_frame[5] << 16) + (_frame[6] << 8) + _frame[7]);
? }

? void SetTimestamp(unsigned long timestamp) {
???? if (_frameLen < 8)
?????? return;
???? _frame[4] = (unsigned char) ((timestamp >> 24) & 0xff);
???? _frame[5] = (unsigned char) ((timestamp >> 16) & 0xff);
???? _frame[6] = (unsigned char) ((timestamp >> 8) & 0xff);
???? _frame[7] = (unsigned char) (timestamp & 0xff);
? };

protected:
? unsigned char* _frame;
? int _frameLen;
};

struct frameHeader {
? unsigned int? x;
? unsigned int? y;
? unsigned int? width;
? unsigned int? height;
};
???
#endif /* __RTPFRAME_H__ */

原封不動，可以直接拿來使用。當然，自己寫一個也不麻煩。很多人寫不好估計是卡在位運算上了。

然后，進入video/MPEG4-ffmpeg目錄下看mpeg4.cxx，這里包含了完整的RFC解包重組及MPEG4解碼的源碼。直接編譯可能通不過，好在代碼寫的非常整齊，提取出來就是了。解包解碼只要看這一個函數：

bool MPEG4DecoderContext::DecodeFrames(const BYTE * src, unsigned & srcLen,
?????????????????????????????????????? BYTE * dst, unsigned & dstLen,
?????????????????????????????????????? unsigned int & flags)

{
??? if (!FFMPEGLibraryInstance.IsLoaded())
??????? return 0;

??? // Creates our frames
??? RTPFrame srcRTP(src, srcLen);
??? RTPFrame dstRTP(dst, dstLen, RTP_DYNAMIC_PAYLOAD);
??? dstLen = 0;
??? flags = 0;
???
??? int srcPayloadSize = srcRTP.GetPayloadSize();
??? SetDynamicDecodingParams(true); // Adjust dynamic settings, restart allowed
???
??? // Don't exceed buffer limits.? _encFrameLen set by ResizeDecodingFrame
??? if(_lastPktOffset + srcPayloadSize < _encFrameLen)
??? {
??????? // Copy the payload data into the buffer and update the offset
??????? memcpy(_encFrameBuffer + _lastPktOffset, srcRTP.GetPayloadPtr(),
?????????????? srcPayloadSize);
??????? _lastPktOffset += srcPayloadSize;
??? }
??? else {

??????? // Likely we dropped the marker packet, so at this point we have a
??????? // full buffer with some of the frame we wanted and some of the next
??????? // frame.?

??????? //I'm on the fence about whether to send the data to the
??????? // decoder and hope for the best, or to throw it all away and start
??????? // again.

??????? // throw the data away and ask for an IFrame
??????? TRACE(1, "MPEG4/tDecoder/tWaiting for an I-Frame");
??????? _lastPktOffset = 0;
??????? flags = (_gotAGoodFrame ? PluginCodec_ReturnCoderRequestIFrame : 0);
??????? _gotAGoodFrame = false;
??????? return 1;
??? }

??? // decode the frame if we got the marker packet
??? int got_picture = 0;
??? if (srcRTP.GetMarker()) {
??????? _frameNum++;
??????? int len = FFMPEGLibraryInstance.AvcodecDecodeVideo
??????????????????????? (_avcontext, _avpicture, &got_picture,
???????????????????????? _encFrameBuffer, _lastPktOffset);

??????? if (len >= 0 && got_picture) {
#ifdef LIBAVCODEC_HAVE_SOURCE_DIR
??????????? if (DecoderError(_keyRefreshThresh)) {
??????????????? // ask for an IFrame update, but still show what we've got
??????????????? flags = (_gotAGoodFrame ? PluginCodec_ReturnCoderRequestIFrame : 0);
??????????????? _gotAGoodFrame = false;
??????????? }
#endif
??????????? TRACE_UP(4, "MPEG4/tDecoder/tDecoded " << len << " bytes" << ", Resolution: " << _avcontext->width << "x" << _avcontext->height);
??????????? // If the decoding size changes on us, we can catch it and resize
??????????? if (!_disableResize
??????????????? && (_frameWidth != (unsigned)_avcontext->width
?????????????????? || _frameHeight != (unsigned)_avcontext->height))
??????????? {
??????????????? // Set the decoding width to what avcodec says it is
??????????????? _frameWidth? = _avcontext->width;
??????????????? _frameHeight = _avcontext->height;
??????????????? // Set dynamic settings (framesize), restart as needed
??????????????? SetDynamicDecodingParams(true);
??????????????? return true;
??????????? }

??????????? // it's stride time
??????????? int frameBytes = (_frameWidth * _frameHeight * 3) / 2;
??????????? PluginCodec_Video_FrameHeader * header
??????????????? = (PluginCodec_Video_FrameHeader *)dstRTP.GetPayloadPtr();
??????????? header->x = header->y = 0;
??????????? header->width = _frameWidth;
??????????? header->height = _frameHeight;
??????????? unsigned char *dstData = OPAL_VIDEO_FRAME_DATA_PTR(header);
??????????? for (int i=0; i<3; i ++) {
??????????????? unsigned char *srcData = _avpicture->data[i];
??????????????? int dst_stride = i ? _frameWidth >> 1 : _frameWidth;
??????????????? int src_stride = _avpicture->linesize[i];
??????????????? int h = i ? _frameHeight >> 1 : _frameHeight;
??????????????? if (src_stride==dst_stride) {
??????????????????? memcpy(dstData, srcData, dst_stride*h);
??????????????????? dstData += dst_stride*h;
??????????????? }
??????????????? else
??????????????? {
??????????????????? while (h--) {
??????????????????????? memcpy(dstData, srcData, dst_stride);
??????????????????????? dstData += dst_stride;
??????????????????????? srcData += src_stride;
??????????????????? }
??????????????? }
??????????? }
??????????? // Treating the screen as an RTP is weird
??????????? dstRTP.SetPayloadSize(sizeof(PluginCodec_Video_FrameHeader)
????????????????????????????????? + frameBytes);
??????????? dstRTP.SetPayloadType(RTP_DYNAMIC_PAYLOAD);
??????????? dstRTP.SetTimestamp(srcRTP.GetTimestamp());
??????????? dstRTP.SetMarker(true);
??????????? dstLen = dstRTP.GetFrameLen();
??????????? flags = PluginCodec_ReturnCoderLastFrame;
??????????? _gotAGoodFrame = true;
??????? }
??????? else {
??????????? TRACE(1, "MPEG4/tDecoder/tDecoded "<< len << " bytes without getting a Picture...");
??????????? // decoding error, ask for an IFrame update
??????????? flags = (_gotAGoodFrame ? PluginCodec_ReturnCoderRequestIFrame : 0);
??????????? _gotAGoodFrame = false;
??????? }
??????? _lastPktOffset = 0;
??? }
??? return true;
}