mirror of
https://github.com/ZLMediaKit/ZLMediaKit.git
synced 2024-11-23 19:20:53 +08:00
175 lines
5.3 KiB
C++
175 lines
5.3 KiB
C++
/*
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* Copyright (c) 2016 The ZLMediaKit project authors. All Rights Reserved.
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*
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* This file is part of ZLMediaKit(https://github.com/xiongziliang/ZLMediaKit).
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*
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* Use of this source code is governed by MIT license that can be found in the
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* LICENSE file in the root of the source tree. All contributing project authors
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* may be found in the AUTHORS file in the root of the source tree.
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*/
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#include "Stamp.h"
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#define MAX_DELTA_STAMP 1000
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#define MAX_CTS 500
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#define ABS(x) ((x) > 0 ? (x) : (-x))
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namespace mediakit {
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int64_t DeltaStamp::deltaStamp(int64_t stamp) {
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if(!_last_stamp){
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//第一次计算时间戳增量,时间戳增量为0
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if(stamp){
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_last_stamp = stamp;
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}
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return 0;
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}
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int64_t ret = stamp - _last_stamp;
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if(ret >= 0){
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//时间戳增量为正，返回之
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_last_stamp = stamp;
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//在直播情况下，时间戳增量不得大于MAX_DELTA_STAMP
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return ret < MAX_DELTA_STAMP ? ret : 0;
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}
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//时间戳增量为负，说明时间戳回环了或回退了
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_last_stamp = stamp;
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return 0;
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}
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void Stamp::setPlayBack(bool playback) {
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_playback = playback;
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}
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void Stamp::revise(int64_t dts, int64_t pts, int64_t &dts_out, int64_t &pts_out,bool modifyStamp) {
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if(!pts){
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//没有播放时间戳,使其赋值为解码时间戳
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pts = dts;
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}
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if(_playback){
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//这是点播
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dts_out = dts;
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pts_out = pts;
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_relativeStamp = dts_out;
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return;
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}
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//pts和dts的差值
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int pts_dts_diff = pts - dts;
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if(_last_dts != dts){
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//时间戳发生变更
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if(modifyStamp){
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_relativeStamp = _ticker.elapsedTime();
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}else{
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_relativeStamp += deltaStamp(dts);
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}
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_last_dts = dts;
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}
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dts_out = _relativeStamp;
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//////////////以下是播放时间戳的计算//////////////////
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if(ABS(pts_dts_diff) > MAX_CTS){
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//如果差值太大，则认为由于回环导致时间戳错乱了
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pts_dts_diff = 0;
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}
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pts_out = dts_out + pts_dts_diff;
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if(pts_out < 0){
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//时间戳不能小于0
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pts_out = 0;
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}
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}
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void Stamp::setRelativeStamp(int64_t relativeStamp) {
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_relativeStamp = relativeStamp;
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}
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int64_t Stamp::getRelativeStamp() const {
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return _relativeStamp;
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}
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bool DtsGenerator::getDts(uint32_t pts, uint32_t &dts){
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bool ret = false;
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if (pts == _last_pts) {
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//pts未变，说明dts也不会变，返回上次dts
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if (_last_dts) {
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dts = _last_dts;
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ret = true;
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}
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} else {
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//pts变了，尝试计算dts
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ret = getDts_l(pts, dts);
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if (ret) {
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//获取到了dts，保存本次结果
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_last_dts = dts;
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}
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}
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if (!ret) {
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//pts排序列队长度还不知道，也就是不知道有没有B帧，
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//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
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dts = pts;
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}
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//记录上次pts
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_last_pts = pts;
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return ret;
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}
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//该算法核心思想是对pts进行排序，排序好的pts就是dts。
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//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
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bool DtsGenerator::getDts_l(uint32_t pts, uint32_t &dts){
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if(_sorter_max_size == 1){
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//没有B帧，dts就等于pts
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dts = pts;
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return true;
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}
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if(!_sorter_max_size){
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//尚未计算出pts排序列队长度(也就是P帧间B帧个数)
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if(pts > _last_max_pts){
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//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)
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if(_frames_since_last_max_pts && _count_sorter_max_size++ > 0){
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//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
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_sorter_max_size = _frames_since_last_max_pts;
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//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)
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_dts_pts_offset = (pts - _last_max_pts) / 2;
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}
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//遇到P帧或关键帧，连续B帧计数清零
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_frames_since_last_max_pts = 0;
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//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
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_last_max_pts = pts;
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}
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//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
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++_frames_since_last_max_pts;
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}
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//pts放入排序缓存列队，缓存列队最大等于连续B帧个数
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_pts_sorter.emplace(pts);
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if(_sorter_max_size && _pts_sorter.size() > _sorter_max_size){
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//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，
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//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
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auto it = _pts_sorter.begin();
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//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),
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//那么我们加上时间戳偏移量，基本等于该帧的dts
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dts = *it + _dts_pts_offset;
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if(dts > pts){
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//dts不能大于pts(基本不可能到达这个逻辑)
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dts = pts;
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}
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//pts排序缓存出列
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_pts_sorter.erase(it);
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return true;
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}
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//排序缓存尚未满
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return false;
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}
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}//namespace mediakit
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