在RTSP协议的交互过程中，第二步客户端发送DESCRIBE请求之后，服务端会返回SDP内容，该SDP内容中有关于媒体和会话的描述，本篇文章主要给出如何从SDP字符串中得到H264视频信息中的sps、pps的二进制数据。
我们知道，在RTSP协议中DESCRIBE请求回复内容的SDP部分中，如果服务端的直播流的视频是H264的编码格式的话，那么在SDP中会将H264的sps、pps信息通过Base64编码成字符串发送给客户端(也就是解码器端)，sps称为序列参数集，pps称为图形参数集。这两个参数中包含了初始化H.264解码器所需要的信息参数，包括编码所用的profile，level，图像的宽和高，deblock滤波器等。这样解码器就可以在DESCRIBE阶段，利用这些参数初始化解码器的设置了。那么如何将SDP中的字符串还原成sps、pps的二进制数据呢。下面的部分代码是从live555项目中取出来的，可以作为小功能独立使用，如果大家有用的着，可以直接拿去使用在项目中：
//main.cpp的内容#include#include "Base64.h"int main(){ /* RTSP 响应的SDP的内容中sprop-parameter-sets键值： sprop-parameter-sets=Z2QAKq2wpDBSAgFxQWKQPQRWFIYKQEAuKCxSB6CKwpDBSAgFxQWKQPQRTDoUKQNC4oJHMGIemHQoUgaFxQSOYMQ9MOhQpA0LigkcwYh6xEQmIVilsQRWUURJsogxOU4QITKUIEVlCCTYQVhBMJQhMIjGggWQJFaIGBJZBAaEnaMIDwsSWQQKCwsrRBQYOWQweO0YEBZASNAogszlAUAW7/wcFBwMQAABdwAAr8g4AAADAL68IAAAdzWU//+MAAADAF9eEAAAO5rKf//CgA==,aP48sA==; 其中逗号前面的内容是sps的二进制数据被base64之后的结果 而逗号后面的内容(不要分号,分号是sdp中键值对的分隔符),是pps的内容 使用live555中的base64Decode函数分别对这两部分进行反base64解码得到的二进制数据就是h264中的sps pps 的二进制内容 分别是以67 和 68 开头 */ char * sps_sdp = "Z2QAKq2wpDBSAgFxQWKQPQRWFIYKQEAuKCxSB6CKwpDBSAgFxQWKQPQRTDoUKQNC4oJHMGIemHQoUgaFxQSOYMQ9MOhQpA0LigkcwYh6xEQmIVilsQRWUURJsogxOU4QITKUIEVlCCTYQVhBMJQhMIjGggWQJFaIGBJZBAaEnaMIDwsSWQQKCwsrRBQYOWQweO0YEBZASNAogszlAUAW7/wcFBwMQAABdwAAr8g4AAADAL68IAAAdzWU//+MAAADAF9eEAAAO5rKf//CgA=="; char * pps_sdp = "aP48sA=="; unsigned int result_size=0; unsigned char * p = base64Decode(sps_sdp,result_size);//解码sps字符串 for(int i =0;i
其中用到的一个主要函数 base64Decode 的实现如下：
#include "Base64.h"#include "strDup.h"#includestatic char base64DecodeTable[256];static void initBase64DecodeTable() { int i; for (i = 0; i < 256; ++i) base64DecodeTable[i] = (char)0x80; // default value: invalid for (i = 'A'; i <= 'Z'; ++i) base64DecodeTable[i] = 0 + (i - 'A'); for (i = 'a'; i <= 'z'; ++i) base64DecodeTable[i] = 26 + (i - 'a'); for (i = '0'; i <= '9'; ++i) base64DecodeTable[i] = 52 + (i - '0'); base64DecodeTable[(unsigned char)'+'] = 62; base64DecodeTable[(unsigned char)'/'] = 63; base64DecodeTable[(unsigned char)'='] = 0;}unsigned char* base64Decode(char const* in, unsigned& resultSize, Boolean trimTrailingZeros) { static Boolean haveInitedBase64DecodeTable = False; if (!haveInitedBase64DecodeTable) { initBase64DecodeTable(); haveInitedBase64DecodeTable = True; } unsigned char* out = (unsigned char*)strDupSize(in); // ensures we have enough space int k = 0; int const jMax = strlen(in) - 3; // in case "in" is not a multiple of 4 bytes (although it should be) for (int j = 0; j < jMax; j += 4) { char inTmp[4], outTmp[4]; for (int i = 0; i < 4; ++i) { inTmp[i] = in[i+j]; outTmp[i] = base64DecodeTable[(unsigned char)inTmp[i]]; if ((outTmp[i]&0x80) != 0) outTmp[i] = 0; // pretend the input was 'A' } out[k++] = (outTmp[0]<<2) | (outTmp[1]>>4); out[k++] = (outTmp[1]<<4) | (outTmp[2]>>2); out[k++] = (outTmp[2]<<6) | outTmp[3]; } if (trimTrailingZeros) { while (k > 0 && out[k-1] == '\0') --k; } resultSize = k; unsigned char* result = new unsigned char[resultSize]; memmove(result, out, resultSize); delete[] out; return result;}static const char base64Char[] ="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";char* base64Encode(char const* origSigned, unsigned origLength) { unsigned char const* orig = (unsigned char const*)origSigned; // in case any input bytes have the MSB set if (orig == NULL) return NULL; unsigned const numOrig24BitValues = origLength/3; Boolean havePadding = origLength > numOrig24BitValues*3; Boolean havePadding2 = origLength == numOrig24BitValues*3 + 2; unsigned const numResultBytes = 4*(numOrig24BitValues + havePadding); char* result = new char[numResultBytes+1]; // allow for trailing '\0' // Map each full group of 3 input bytes into 4 output base-64 characters: unsigned i; for (i = 0; i < numOrig24BitValues; ++i) { result[4*i+0] = base64Char[(orig[3*i]>>2)&0x3F]; result[4*i+1] = base64Char[(((orig[3*i]&0x3)<<4) | (orig[3*i+1]>>4))&0x3F]; result[4*i+2] = base64Char[((orig[3*i+1]<<2) | (orig[3*i+2]>>6))&0x3F]; result[4*i+3] = base64Char[orig[3*i+2]&0x3F]; } // Now, take padding into account. (Note: i == numOrig24BitValues) if (havePadding) { result[4*i+0] = base64Char[(orig[3*i]>>2)&0x3F]; if (havePadding2) { result[4*i+1] = base64Char[(((orig[3*i]&0x3)<<4) | (orig[3*i+1]>>4))&0x3F]; result[4*i+2] = base64Char[(orig[3*i+1]<<2)&0x3F]; } else { result[4*i+1] = base64Char[((orig[3*i]&0x3)<<4)&0x3F]; result[4*i+2] = '='; } result[4*i+3] = '='; } result[numResultBytes] = '\0'; return result;}
(Ubuntu Linux下运行没问题)