ZLMediaKit/src/Extension/H264Rtp.cpp

/*
 * Copyright (c) 2016 The ZLMediaKit project authors. All Rights Reserved.
 *
 * This file is part of ZLMediaKit(https://github.com/xia-chu/ZLMediaKit).
 *
 * Use of this source code is governed by MIT license that can be found in the
 * LICENSE file in the root of the source tree. All contributing project authors
 * may be found in the AUTHORS file in the root of the source tree.
 */

#include "H264Rtp.h"

namespace mediakit{

#if defined(_WIN32)
#pragma pack(push, 1)
#endif // defined(_WIN32)

class FuFlags {
public:
#if __BYTE_ORDER == __BIG_ENDIAN
    unsigned start_bit: 1;
    unsigned end_bit: 1;
    unsigned reserved: 1;
    unsigned nal_type: 5;
#else
    unsigned nal_type: 5;
    unsigned reserved: 1;
    unsigned end_bit: 1;
    unsigned start_bit: 1;
#endif
} PACKED;

#if defined(_WIN32)
#pragma pack(pop)
#endif // defined(_WIN32)

H264RtpDecoder::H264RtpDecoder() {
    _frame = obtainFrame();
}

H264Frame::Ptr H264RtpDecoder::obtainFrame() {
    auto frame = FrameImp::create<H264Frame>();
    frame->_prefix_size = 4;
    return frame;
}

bool H264RtpDecoder::inputRtp(const RtpPacket::Ptr &rtp, bool key_pos) {
    return decodeRtp(rtp);
}

/*
RTF3984 5.2节  Common Structure of the RTP Payload Format
Table 1.  Summary of NAL unit types and their payload structures

   Type   Packet    Type name                        Section
   ---------------------------------------------------------
   0      undefined                                    -
   1-23   NAL unit  Single NAL unit packet per H.264   5.6
   24     STAP-A    Single-time aggregation packet     5.7.1
   25     STAP-B    Single-time aggregation packet     5.7.1
   26     MTAP16    Multi-time aggregation packet      5.7.2
   27     MTAP24    Multi-time aggregation packet      5.7.2
   28     FU-A      Fragmentation unit                 5.8
   29     FU-B      Fragmentation unit                 5.8
   30-31  undefined                                    -
*/

bool H264RtpDecoder::decodeRtp(const RtpPacket::Ptr &rtp) {
    auto frame = rtp->getPayload();
    auto length = rtp->getPayloadSize();
    auto stamp = rtp->getStampMS();
    auto seq = rtp->getSeq();
    auto nal_type = *frame & 0x1F;
    auto nal_suffix = *frame & (~0x1F);

    if (nal_type >= 0 && nal_type < 24) {
        //a full frame
        _frame->_buffer.assign("\x00\x00\x00\x01", 4);
        _frame->_buffer.append((char *) frame, length);
        _frame->_pts = stamp;
        auto key = _frame->keyFrame();
        onGetH264(_frame);
        return (key); //i frame
    }

    switch (nal_type) {
        case 24: {
            // 24 STAP-A   单一时间的组合包
            bool haveIDR = false;
            auto ptr = frame + 1;
            while (true) {
                size_t off = ptr - frame;
                if (off >= length) {
                    break;
                }
                //获取当前nalu的大小
                uint16_t len = *ptr++;
                len <<= 8;
                len |= *ptr++;
                if (off + len > length) {
                    break;
                }
                if (len > 0) {
                    //有有效数据
                    _frame->_buffer.assign("\x00\x00\x00\x01", 4);
                    _frame->_buffer.append((char *) ptr, len);
                    _frame->_pts = stamp;
                    if ((ptr[0] & 0x1F) == H264Frame::NAL_IDR) {
                        haveIDR = true;
                    }
                    onGetH264(_frame);
                }
                ptr += len;
            }
            return haveIDR;
        }

        case 28: {
            //FU-A
            FuFlags *fu = (FuFlags *) (frame + 1);
            if (fu->start_bit) {
                //该帧的第一个rtp包  FU-A start
                //预留空间，防止频繁扩容拷贝
                _frame->_buffer.reserve(_max_frame_size);
                _frame->_buffer.assign("\x00\x00\x00\x01", 4);
                _frame->_buffer.push_back(nal_suffix | fu->nal_type);
                _frame->_buffer.append((char *) frame + 2, length - 2);
                _frame->_pts = stamp;
                //该函数return时，保存下当前sequence,以便下次对比seq是否连续
                _last_seq = seq;
                return _frame->keyFrame();
            }

            if (seq != (uint16_t) (_last_seq + 1)) {
                //中间的或末尾的rtp包，其seq必须连续(如果回环了则判定为连续)，否则说明rtp丢包，那么该帧不完整，必须得丢弃
                _frame->_buffer.clear();
                WarnL << "rtp丢包: " << seq << " != " << _last_seq << " + 1,该帧被废弃";
                return false;
            }

            if (!fu->end_bit) {
                //该帧的中间rtp包  FU-A mid
                _frame->_buffer.append((char *) frame + 2, length - 2);
                //该函数return时，保存下当前sequence,以便下次对比seq是否连续
                _last_seq = seq;
                return false;
            }

            //该帧最后一个rtp包  FU-A end
            _frame->_buffer.append((char *) frame + 2, length - 2);
            _frame->_pts = stamp;
            //计算最大的帧
            auto frame_size = _frame->size();
            if (frame_size > _max_frame_size) {
                _max_frame_size = frame_size;
            }
            onGetH264(_frame);
            return false;
        }

        default: {
            // 29 FU-B     单NAL单元B模式
            // 25 STAP-B   单一时间的组合包
            // 26 MTAP16   多个时间的组合包
            // 27 MTAP24   多个时间的组合包
            WarnL << "不支持的rtp类型:" << (int) nal_type << " " << seq;
            return false;
        }
    }
}

void H264RtpDecoder::onGetH264(const H264Frame::Ptr &frame) {
    //rtsp没有dts，那么根据pts排序算法生成dts
    _dts_generator.getDts(frame->_pts,frame->_dts);
    RtpCodec::inputFrame(frame);
    _frame = obtainFrame();
}


////////////////////////////////////////////////////////////////////////

H264RtpEncoder::H264RtpEncoder(uint32_t ssrc, uint32_t mtu, uint32_t sample_rate, uint8_t pt, uint8_t interleaved)
        : RtpInfo(ssrc, mtu, sample_rate, pt, interleaved) {
}

void H264RtpEncoder::insertConfigFrame(uint32_t pts){
    if (!_sps || !_pps) {
        return;
    }
    //gop缓存从sps开始，sps、pps后面还有时间戳相同的关键帧，所以mark bit为false
    packRtp(_sps->data() + _sps->prefixSize(), _sps->size() - _sps->prefixSize(), pts, false, true);
    packRtp(_pps->data() + _pps->prefixSize(), _pps->size() - _pps->prefixSize(), pts, false, false);
}

void H264RtpEncoder::packRtp(const char *ptr, size_t len, uint32_t pts, bool is_mark, bool gop_pos){
    if (len + 3 <= getMaxSize()) {
        //STAP-A模式打包小于MTU
        packRtpStapA(ptr, len, pts, is_mark, gop_pos);
    } else {
        //STAP-A模式打包会大于MTU,所以采用FU-A模式
        packRtpFu(ptr, len, pts, is_mark, gop_pos);
    }
}

void H264RtpEncoder::packRtpFu(const char *ptr, size_t len, uint32_t pts, bool is_mark, bool gop_pos){
    auto packet_size = getMaxSize() - 2;
    if (len <= packet_size + 1) {
        //小于FU-A打包最小字节长度要求，采用STAP-A模式
        packRtpStapA(ptr, len, pts, is_mark, gop_pos);
        return;
    }

    //末尾5bit为nalu type，固定为28(FU-A)
    auto fu_char_0 = (ptr[0] & (~0x1F)) | 28;
    auto fu_char_1 = H264_TYPE(ptr[0]);
    FuFlags *fu_flags = (FuFlags *) (&fu_char_1);
    fu_flags->start_bit = 1;

    size_t offset = 1;
    while (!fu_flags->end_bit) {
        if (!fu_flags->start_bit && len <= offset + packet_size) {
            //FU-A end
            packet_size = len - offset;
            fu_flags->end_bit = 1;
        }

        //传入nullptr先不做payload的内存拷贝
        auto rtp = makeRtp(getTrackType(), nullptr, packet_size + 2, fu_flags->end_bit && is_mark, pts);
        //rtp payload 负载部分
        uint8_t *payload = rtp->getPayload();
        //FU-A 第1个字节
        payload[0] = fu_char_0;
        //FU-A 第2个字节
        payload[1] = fu_char_1;
        //H264 数据
        memcpy(payload + 2, (uint8_t *) ptr + offset, packet_size);
        //输入到rtp环形缓存
        RtpCodec::inputRtp(rtp, gop_pos);

        offset += packet_size;
        fu_flags->start_bit = 0;
    }
}

void H264RtpEncoder::packRtpStapA(const char *ptr, size_t len, uint32_t pts, bool is_mark, bool gop_pos){
    //如果帧长度不超过mtu,为了兼容性 webrtc，采用STAP-A模式打包
    auto rtp = makeRtp(getTrackType(), nullptr, len + 3, is_mark, pts);
    uint8_t *payload = rtp->getPayload();
    //STAP-A
    payload[0] = (ptr[0] & (~0x1F)) | 24;
    payload[1] = (len >> 8) & 0xFF;
    payload[2] = len & 0xff;
    memcpy(payload + 3, (uint8_t *) ptr, len);

    RtpCodec::inputRtp(rtp, gop_pos);
}

void H264RtpEncoder::inputFrame(const Frame::Ptr &frame) {
    auto ptr = frame->data() + frame->prefixSize();
    switch (H264_TYPE(ptr[0])) {
        case H264Frame::NAL_AUD:
        case H264Frame::NAL_SEI : {
            return;
        }
        case H264Frame::NAL_SPS: {
            _sps = Frame::getCacheAbleFrame(frame);
            return;
        }
        case H264Frame::NAL_PPS: {
            _pps = Frame::getCacheAbleFrame(frame);
            return;
        }
        default: break;
    }

    if (_last_frame) {
        //如果时间戳发生了变化，那么markbit才置true
        inputFrame_l(_last_frame, _last_frame->pts() != frame->pts());
    }
    _last_frame = Frame::getCacheAbleFrame(frame);
}

void H264RtpEncoder::inputFrame_l(const Frame::Ptr &frame, bool is_mark){
    if (frame->keyFrame()) {
        //保证每一个关键帧前都有SPS与PPS
        insertConfigFrame(frame->pts());
    }
    packRtp(frame->data() + frame->prefixSize(), frame->size() - frame->prefixSize(), frame->pts(), is_mark, false);
}

}//namespace mediakit