h264 h265 nal type计算方式 关键帧判断

nal_type = (nalu[0] >> 1) & 0x3f;

// Rec. ITU-T H.265 v4 (12/2016) (p26)
// intra random access point (IRAP) picture: 
//   A coded picture for which each VCL NAL unit has nal_unit_type 
//   in the range of BLA_W_LP to RSV_IRAP_VCL23, inclusive.
static int mpeg_h265_find_keyframe(const uint8_t* p, size_t bytes)
{
  size_t i;
  uint8_t type;
  for (i = 2; i + 1 < bytes; i++)
  {
    if (0x01 == p[i] && 0x00 == p[i - 1] && 0x00 == p[i - 2])
    {
      type = (p[i + 1] >> 1) & 0x3f;
      if (type < 32)
        return (16 <= type && type <= 23) ? 1 : 0;
    }
  }
  return 0;
}