ZLMediaKit/src/Common/Stamp.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 "Stamp.h"

// 时间戳最大允许跳变3秒，主要是防止网络抖动导致的跳变
#define MAX_DELTA_STAMP (3 * 1000)
#define STAMP_LOOP_DELTA (60 * 1000)
#define MAX_CTS 500
#define ABS(x) ((x) > 0 ? (x) : (-x))

using namespace toolkit;

namespace mediakit {

DeltaStamp::DeltaStamp() {
    // 时间戳最大允许跳跃300ms
    _max_delta = 300;
}

int64_t DeltaStamp::relativeStamp(int64_t stamp, bool enable_rollback) {
    _relative_stamp += deltaStamp(stamp, enable_rollback);
    return _relative_stamp;
}

int64_t DeltaStamp::relativeStamp() {
    return _relative_stamp;
}

int64_t DeltaStamp::deltaStamp(int64_t stamp, bool enable_rollback) {
    if (!_last_stamp) {
        // 第一次计算时间戳增量,时间戳增量为0
        if (stamp) {
            _last_stamp = stamp;
        }
        return 0;
    }

    int64_t ret = stamp - _last_stamp;
    if (ret >= 0) {
        // 时间戳增量为正，返回之
        _last_stamp = stamp;
        // 在直播情况下，时间戳增量不得大于MAX_DELTA_STAMP，否则强制相对时间戳加1
        if (ret > _max_delta) {
            needSync();
            return 1;
        }
        return ret;
    }

    // 时间戳增量为负，说明时间戳回环了或回退了
    _last_stamp = stamp;
    if (!enable_rollback || -ret > _max_delta) {
        // 不允许回退或者回退太多了, 强制时间戳加1
        needSync();
        return 1;
    }
    return ret;
}

void DeltaStamp::setMaxDelta(size_t max_delta) {
    _max_delta = max_delta;
}

void Stamp::setPlayBack(bool playback) {
    _playback = playback;
}

void Stamp::syncTo(Stamp &other) {
    _need_sync = true;
    _sync_master = &other;
}

void Stamp::needSync() {
    _need_sync = true;
}

void Stamp::enableRollback(bool flag) {
    _enable_rollback = flag;
}

// 限制dts回退
void Stamp::revise(int64_t dts, int64_t pts, int64_t &dts_out, int64_t &pts_out, bool modifyStamp) {
    revise_l(dts, pts, dts_out, pts_out, modifyStamp);
    if (_playback) {
        // 回放允许时间戳回退
        return;
    }

    if (dts_out < _last_dts_out) {
        // WarnL << "dts回退:" << dts_out << " < " << _last_dts_out;
        dts_out = _last_dts_out;
        pts_out = _last_pts_out;
        return;
    }
    _last_dts_out = dts_out;
    _last_pts_out = pts_out;
}

// 音视频时间戳同步
void Stamp::revise_l(int64_t dts, int64_t pts, int64_t &dts_out, int64_t &pts_out, bool modifyStamp) {
    revise_l2(dts, pts, dts_out, pts_out, modifyStamp);
    if (!_sync_master || modifyStamp || _playback) {
        // 自动生成时间戳或回放或同步完毕
        return;
    }

    // 需要同步时间戳
    if (_sync_master && _sync_master->_last_dts_in && (_need_sync || _sync_master->_need_sync)) {
        // 音视频dts当前时间差
        int64_t dts_diff = _last_dts_in - _sync_master->_last_dts_in;
        if (ABS(dts_diff) < 5000) {
            // 如果绝对时间戳小于5秒，那么说明他们的起始时间戳是一致的，那么强制同步
            auto target_stamp = _sync_master->_relative_stamp + dts_diff;
            if (target_stamp > _relative_stamp || _enable_rollback) {
                // 强制同步后，时间戳增加跳跃了，或允许回退
                TraceL << "Relative stamp changed: " << _relative_stamp << " -> " << target_stamp;
                _relative_stamp = target_stamp;
            } else {
                // 不允许回退, 则让另外一个Track的时间戳增长
                target_stamp = _relative_stamp - dts_diff;
                TraceL << "Relative stamp changed: " << _sync_master->_relative_stamp << " -> " << target_stamp;
                _sync_master->_relative_stamp = target_stamp;
            }
        }
        _need_sync = false;
        _sync_master->_need_sync = false;
    }
}

// 求取相对时间戳
void Stamp::revise_l2(int64_t dts, int64_t pts, int64_t &dts_out, int64_t &pts_out, bool modifyStamp) {
    if (!pts) {
        // 没有播放时间戳,使其赋值为解码时间戳
        pts = dts;
    }

    if (_playback) {
        // 这是点播
        dts_out = dts;
        pts_out = pts;
        _relative_stamp = dts_out;
        _last_dts_in = dts;
        return;
    }

    // pts和dts的差值
    int64_t pts_dts_diff = pts - dts;

    if (_last_dts_in != dts) {
        // 时间戳发生变更
        if (modifyStamp) {
            // 内部自己生产时间戳
            _relative_stamp = _ticker.elapsedTime();
        } else {
            _relative_stamp += deltaStamp(dts, _enable_rollback);
        }
        _last_dts_in = dts;
    }
    dts_out = _relative_stamp;

    //////////////以下是播放时间戳的计算//////////////////
    if (ABS(pts_dts_diff) > MAX_CTS) {
        // 如果差值太大，则认为由于回环导致时间戳错乱了
        pts_dts_diff = 0;
    }

    pts_out = dts_out + pts_dts_diff;
}

void Stamp::setRelativeStamp(int64_t relativeStamp) {
    _relative_stamp = relativeStamp;
}

int64_t Stamp::getRelativeStamp() const {
    return _relative_stamp;
}

bool DtsGenerator::getDts(uint64_t pts, uint64_t &dts) {
    bool ret = false;
    if (pts == _last_pts) {
        // pts未变，说明dts也不会变，返回上次dts
        if (_last_dts) {
            dts = _last_dts;
            ret = true;
        }
    } else {
        // pts变了，尝试计算dts
        ret = getDts_l(pts, dts);
        if (ret) {
            // 获取到了dts，保存本次结果
            _last_dts = dts;
        }
    }

    if (!ret) {
        // pts排序列队长度还不知道，也就是不知道有没有B帧，
        // 那么先强制dts == pts，这样可能导致有B帧的情况下，起始画面有几帧回退
        dts = pts;
    }

    // 记录上次pts
    _last_pts = pts;
    return ret;
}

// 该算法核心思想是对pts进行排序，排序好的pts就是dts。
// 排序有一定的滞后性，那么需要加上排序导致的时间戳偏移量
bool DtsGenerator::getDts_l(uint64_t pts, uint64_t &dts) {
    if (_sorter_max_size == 1) {
        // 没有B帧，dts就等于pts
        dts = pts;
        return true;
    }

    if (!_sorter_max_size) {
        // 尚未计算出pts排序列队长度(也就是P帧间B帧个数)
        if (pts > _last_max_pts) {
            // pts时间戳增加了，那么说明这帧画面不是B帧(说明是P帧或关键帧)
            if (_frames_since_last_max_pts && _count_sorter_max_size++ > 0) {
                // 已经出现多次非B帧的情况，那么我们就能知道P帧间B帧的个数
                _sorter_max_size = _frames_since_last_max_pts;
                // 我们记录P帧间时间间隔(也就是多个B帧时间戳增量累计)
                _dts_pts_offset = (pts - _last_max_pts);
                // 除以2，防止dts大于pts
                _dts_pts_offset /= 2;
            }
            // 遇到P帧或关键帧，连续B帧计数清零
            _frames_since_last_max_pts = 0;
            // 记录上次非B帧的pts时间戳(同时也是dts)，用于统计连续B帧时间戳增量
            _last_max_pts = pts;
        }
        // 如果pts时间戳小于上一个P帧，那么断定这个是B帧,我们记录B帧连续个数
        ++_frames_since_last_max_pts;
    }

    // pts放入排序缓存列队，缓存列队最大等于连续B帧个数
    _pts_sorter.emplace(pts);

    if (_sorter_max_size && _pts_sorter.size() > _sorter_max_size) {
        // 如果启用了pts排序(意味着存在B帧)，并且pts排序缓存列队长度大于连续B帧个数，
        // 意味着后续的pts都会比最早的pts大，那么说明可以取出最早的pts了，这个pts将当做该帧的dts基准
        auto it = _pts_sorter.begin();

        // 由于该pts是前面偏移了个_sorter_max_size帧的pts(也就是那帧画面的dts),
        // 那么我们加上时间戳偏移量，基本等于该帧的dts
        dts = *it + _dts_pts_offset;
        if (dts > pts) {
            // dts不能大于pts(基本不可能到达这个逻辑)
            dts = pts;
        }

        // pts排序缓存出列
        _pts_sorter.erase(it);
        return true;
    }

    // 排序缓存尚未满
    return false;
}

void NtpStamp::setNtpStamp(uint32_t rtp_stamp, uint64_t ntp_stamp_ms) {
    if (!ntp_stamp_ms || !rtp_stamp) {
        // 实测发现有些rtsp服务器发送的rtp时间戳和ntp时间戳一直为0
        WarnL << "Invalid sender report rtcp, ntp_stamp_ms = " << ntp_stamp_ms << ", rtp_stamp = " << rtp_stamp;
        return;
    }
    update(rtp_stamp, ntp_stamp_ms * 1000);
}

void NtpStamp::update(uint32_t rtp_stamp, uint64_t ntp_stamp_us) {
    _last_rtp_stamp = rtp_stamp;
    _last_ntp_stamp_us = ntp_stamp_us;
}

uint64_t NtpStamp::getNtpStamp(uint32_t rtp_stamp, uint32_t sample_rate) {
    if (rtp_stamp == _last_rtp_stamp) {
        return _last_ntp_stamp_us / 1000;
    }
    return getNtpStampUS(rtp_stamp, sample_rate) / 1000;
}

uint64_t NtpStamp::getNtpStampUS(uint32_t rtp_stamp, uint32_t sample_rate) {
    if (!_last_ntp_stamp_us) {
        // 尚未收到sender report rtcp包，那么赋值为本地系统时间戳吧
        update(rtp_stamp, getCurrentMicrosecond(true));
    }

    // rtp时间戳正增长
    if (rtp_stamp >= _last_rtp_stamp) {
        auto diff_us = static_cast<int64_t>((rtp_stamp - _last_rtp_stamp) / (sample_rate / 1000000.0f));
        if (diff_us < MAX_DELTA_STAMP * 1000) {
            // 时间戳正常增长
            update(rtp_stamp, _last_ntp_stamp_us + diff_us);
            return _last_ntp_stamp_us;
        }

        // 时间戳大幅跳跃
        uint64_t loop_delta_hz = STAMP_LOOP_DELTA * sample_rate / 1000;
        if (_last_rtp_stamp < loop_delta_hz && rtp_stamp > UINT32_MAX - loop_delta_hz) {
            // 应该是rtp时间戳溢出+乱序
            uint64_t max_rtp_us = uint64_t(UINT32_MAX) * 1000000 / sample_rate;
            return _last_ntp_stamp_us + diff_us - max_rtp_us;
        }
        // 不明原因的时间戳大幅跳跃，直接返回上次值
        WarnL << "rtp stamp abnormal increased:" << _last_rtp_stamp << " -> " << rtp_stamp;
        update(rtp_stamp, _last_ntp_stamp_us);
        return _last_ntp_stamp_us;
    }

    // rtp时间戳负增长
    auto diff_us = static_cast<int64_t>((_last_rtp_stamp - rtp_stamp) / (sample_rate / 1000000.0f));
    if (diff_us < MAX_DELTA_STAMP * 1000) {
        // 正常范围的时间戳回退，说明收到rtp乱序了
        return _last_ntp_stamp_us - diff_us;
    }

    // 时间戳大幅度回退
    uint64_t loop_delta_hz = STAMP_LOOP_DELTA * sample_rate / 1000;
    if (rtp_stamp < loop_delta_hz && _last_rtp_stamp > UINT32_MAX - loop_delta_hz) {
        // 确定是时间戳溢出
        uint64_t max_rtp_us = uint64_t(UINT32_MAX) * 1000000 / sample_rate;
        update(rtp_stamp, _last_ntp_stamp_us + (max_rtp_us - diff_us));
        return _last_ntp_stamp_us;
    }
    // 不明原因的时间戳回退，直接返回上次值
    WarnL << "rtp stamp abnormal reduced:" << _last_rtp_stamp << " -> " << rtp_stamp;
    update(rtp_stamp, _last_ntp_stamp_us);
    return _last_ntp_stamp_us;
}

} // namespace mediakit