RTP发送和接收(有图为证)

RTP协议接收和播放

前面讲过RTP协议的重要性，说过发送RTP协议，RTP协议重要性下面我们开始写发送和接收程序，不依赖于jrtplib等RTP库，自行接收。这样，有利于以后将RTP直接转到Webrtc上

RTP接收

首先定义RTPFrame结构体，一般来讲，我们都是12字节头部，但不是一定的，自行扩展是允许的，具体看RTP的头部字节含义。maker是代表结束，为1，没有结束为0，还有一个更简单的方法，当一帧结束时，timstamp一定会改变，也可以这样来断定，具体来说，也没有一帧的概念，或者称为一个nalu视频的切换。ok，show me the code，注意，像时间戳，seq no，以及ssrc等都是网络字节序，是大端格式，读者必须将这些字节序理解。seq no 为16位两字节，达到最大值又从零开始。

struct RTPFrame {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;
};

下面分为两种包解码h264，h265是同理的，后面会贴上来

//单时间聚合包(STAP)包含的是同一帧的数据
//实际测试时ffmpeg发送的RTP就是有这种包
bool c_h264frame::DeencapsulateSTAP (RTPFrame & frame)
{uint8_t* curSTAP = frame.GetPayloadPtr() + 1;uint32_t curSTAPLen = frame.GetPayloadSize() - 1; while (curSTAPLen > 0){// first, theres a 2 byte length fielduint32_t len = (curSTAP[0] << 8) | curSTAP[1];curSTAP += 2;//then the header, followed by the body.  We'll add the header//in the AddDataToEncodedFrame - that's why the nal body is dptr + 1AddDataToEncodedFrame(curSTAP + 1,  len - 1, *curSTAP, 1);curSTAP += len;if ((len + 2) > curSTAPLen){curSTAPLen = 0;return false;}else{curSTAPLen -= (len + 2);}}return true;
}

这种包其实很简单，一般像ffmpeg发送rtp的时候就是放入sps，pps信息，很好，每个单包前面有两个字节的长度代表单包长度，顺序读取就好，逻辑很简单。

接下来是FU包解包


bool c_h264frame::DeencapsulateFU (RTPFrame & frame)
{//FU指示字节有以下格式：//     +---------------+//     |0|1|2|3|4|5|6|7|//     +-+-+-+-+-+-+-+-+//     |F|NRI|  Type   |//     +---------------+//  FU指示字节的类型域的28，29表示FU-A和FU-B。F的使用在5。3描述。NRI域的值必须根据分片NAL单元的NRI域的值设置。//  FU头的格式如下：//     +---------------+//     |0|1|2|3|4|5|6|7|//     +-+-+-+-+-+-+-+-+//     |S|E|R|  Type   |//     +---------------+//  S: 1 bit//     当设置成1,开始位指示分片NAL单元的开始。当跟随的FU荷载不是分片NAL单元荷载的开始，开始位设为0。//  E: 1 bit//     当设置成1, 结束位指示分片NAL单元的结束，即, 荷载的最后字节也是分片NAL单元的最后一个字节。当跟随的//     FU荷载不是分片NAL单元的最后分片,结束位设置为0。//  R: 1 bit//     保留位必须设置为0，接收者必须忽略该位。//     //  Type: 5 bits//     NAL单元荷载类型定义在[1]的表7-1.uint8_t* curFUPtr = frame.GetPayloadPtr();uint32_t curFULen = frame.GetPayloadSize(); uint8_t header;//S为1 E不为1 为开始第一个分片if ((curFUPtr[1] & 0x80) && !(curFUPtr[1] & 0x40)){if (_currentFU) {_currentFU=1;}else{_currentFU++;//0xe0 11100000 0x1f 00011111header = (curFUPtr[0] & 0xe0) | (curFUPtr[1] & 0x1f);AddDataToEncodedFrame(curFUPtr + 2, curFULen - 2, header,  1);}} else if (!(curFUPtr[1] & 0x80) && !(curFUPtr[1] & 0x40)){if (_currentFU){_currentFU++;AddDataToEncodedFrame(curFUPtr + 2, curFULen - 2,  0, 0);}else{_currentFU=0;return false;}} else if (!(curFUPtr[1] & 0x80) && (curFUPtr[1] & 0x40)){if (_currentFU) {_currentFU=0;AddDataToEncodedFrame( curFUPtr + 2, curFULen - 2, 0, 0);}else{_currentFU=0;return false;}} else if ((curFUPtr[1] & 0x80) && (curFUPtr[1] & 0x40)){_currentFU=0;return false;} return true;
}void c_h264frame::AddDataToEncodedFrame (uint8_t *data, uint32_t dataLen, uint8_t header, bool addHeader) {uint8_t headerLen= addHeader ? 5 : 0;uint8_t* currentPositionInFrame = _encodedFrame + _encodedFrameLen;// add 00 00 00 01 [headerbyte] headerif (addHeader){*currentPositionInFrame++ = 0;*currentPositionInFrame++ = 0;*currentPositionInFrame++ = 0;*currentPositionInFrame++ = 1;if (_numberOfNALsInFrame + 1 >(_numberOfNALsReserved)){_NALs = (h264_nal_t *)realloc(_NALs, (_numberOfNALsReserved + 1) * sizeof(h264_nal_t));_numberOfNALsReserved++;}if (_NALs){_NALs[_numberOfNALsInFrame].offset = _encodedFrameLen + 4;_NALs[_numberOfNALsInFrame].length = dataLen + 1;_NALs[_numberOfNALsInFrame].type = header & 0x1f;_numberOfNALsInFrame++;}*currentPositionInFrame++ = header;}else{if (_NALs) _NALs[_numberOfNALsInFrame - 1].length += dataLen;}memcpy(currentPositionInFrame, data, dataLen);_encodedFrameLen += dataLen + headerLen;
}bool c_h264frame::IsStartCode (const uint8_t *positionInFrame)
{if (positionInFrame[0] == 0 &&positionInFrame[1] == 0 &&((positionInFrame[2] == 1) ||((positionInFrame[2] == 0) && positionInFrame[3] == 1))) {return true;}return false;
}

图一发送RTP
RTP发送遵循前面所说的方式，编码h264发送

图二接收RTP

至于绘图，可以使用很多种方法，这里介绍opengl方式，而且，我们可以用三维方式绘制出来。

把视频绘制在三维空间，空间里可以放很多东西，这个空间足够大。

后面会讲解如何制作这种程序。甚至在web里面也可以做到，在三维世界里播放我们的视频和视频会议。