ffmpeg播放rtp流，为了降低首开延迟，需要在SDP文件中指定PPS、SPS、VPS信息。抓包后发现wireshark无法解析AP包。需要自己进行AP包解析。RTP协议AP包格式如下：
根据如上信息，我们可以解析AP包，效果如下
- 40 01，type=32,VPS(视频参数集)
- 42 01，type=33,SPS(序列参数集)
- 44 01，type=34,PPS(图像参数集)
- 4E 01, type=39,SEI(补充增强信息)
- 26 01，type=19,可能有RADL图像的IDR图像的SS编码数据 IDR
- 02 01, type=01,被参考的后置图像，且非TSA、非STSA的SS编码数据
- 46 01，type=35,分隔符，没用。
下图中红色部分是分隔符，橙色是VPS，黑色是SPS，黄色为PPS
SDP中的VPS等信息需要转换成base64。使用下面的函数可转换
#include <stdio.h>
#include <inttypes.h>
#include <math.h>
#include <limits.h>
#include <signal.h>
#include <stdint.h>
#include <string>
#include <string.h>
#include <iostream>
#define AV_BASE64_SIZE(x) (((x)+2) / 3 * 4 + 1)
define AV_RB32(x) \
(((uint32_t)((const uint8_t*)(x))[0] << 24) | \
(((const uint8_t*)(x))[1] << 16) | \
(((const uint8_t*)(x))[2] << 8) | \
((const uint8_t*)(x))[3])
using namespace std;
char *av_base64_encode(char *out, int out_size, const uint8_t *in, int in_size)
{
static const char b64[] =
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
char *ret, *dst;
unsigned i_bits = 0;
int i_shift = 0;
int bytes_remaining = in_size;
if (in_size >= UINT_MAX / 4 ||
out_size < AV_BASE64_SIZE(in_size))
return NULL;
ret = dst = out;
while (bytes_remaining > 3) {
i_bits = AV_RB32(in);
in += 3; bytes_remaining -= 3;
*dst++ = b64[ i_bits>>26 ];
*dst++ = b64[(i_bits>>20) & 0x3F];
*dst++ = b64[(i_bits>>14) & 0x3F];
*dst++ = b64[(i_bits>>8 ) & 0x3F];
}
i_bits = 0;
while (bytes_remaining) {
i_bits = (i_bits << 8) + *in++;
bytes_remaining--;
i_shift += 8;
}
while (i_shift > 0) {
*dst++ = b64[(i_bits << 6 >> i_shift) & 0x3f];
i_shift -= 6;
}
while ((dst - ret) & 3)
*dst++ = '=';
*dst = '\0';
return ret;
}
int main() {
uint8_t in[]={0x67, 0x42,0xc0,0x14,0xf4,0x0b,0x04,0xb4,0x20,0x00,0x00,0x03,0x00,0x20,0x00,0x00,0x03,0x03,0xd1,0xe2,0x85,0x54};
char str_sps[100]={0};
av_base64_encode(str_sps,100,in,22);
cout<<"sps'sbasecode:"<<str_sps<<endl;
uint8_t pps[]={0x68 ,0xce ,0x04 ,0xf2};
char str_sps2[100]={0};
av_base64_encode(str_sps2,100,pps,4);
cout<<"pps'sbasecode:"<<str_sps2<<endl;
return 0;
}