ORBSLAM2中的关键帧插入判定

如果是仅追踪模式:mbOnlyTracking
>>判定失败
如果[mpLocalMapper->mbStopped]和[mpLocalMapper->mbStoppedmbStopRequested]都是flase
>>判定失败
如果已有[关键帧数量>mMaxFrames]且[重定位后还 没有 度过mMaxFrames个帧]
>>判定失败

如果经过以上三个判断后还未判定失败,考察一下几个条件:
c1a: 上次插入关键帧后已经过去mMaxFrames个帧
c1b: [上次插入关键帧后已经过去mMinFrames个帧]且[mpLocalMapper-> mbAcceptKeyFrames ==true]
c1c:[不是单目系统]且[mnMatchesInliers<nRefMatches*0.25 || bNeedToInsertClose ]
//含义是:当前帧track的内点相比参考关键帧来说很少,并且本身track的close特征点很少,但是可以被三角化的close特征点数量足够.c1c成立说明track比较弱,有lost的风险.但通过三角化能够通过该帧生成足够的新地图点
//其中:mnMatchesInliers是其中track局部地图时当前帧中 内点中 nObs>0的特征点的数量
//其中:nRefMatches参考关键帧的mvpMapPoints中 obn大于某阈值nMinObs 的mappoint数量
//bNeedToInsertClose = (nTrackedClose<100) && (nNonTrackedClose>70);
//nMinObs = 3;但如果已有关键帧数量不超过2个,则nMinObs=2;
//nNonTrackedClose 是当前帧close特征点中 不是tack局部地图的内点 的数量
//nTrackedClose    是当前帧close特征点中 是tack局部地图的内点 的数量

c2 = [mnMatchesInliers<nRefMatches*thRefRatio|| bNeedToInsertClose]且[ mnMatchesInliers>15]
//含义是:当前帧track的内点和参考关键帧的地图点的重合度要低于一定阈值,或者本身track的close特征点不能很多,但是当前帧track的内点要多余15个以保证当前帧的位姿精度.c2是可以被判定为关键帧的所必须满足的条件
//其中,如果是双目系统thRefRatio = 0.75f(但如果已有关键帧数量不超过2个,则thRefRatio = 0.4f)
//	  如果是单目系统thRefRatio = 0.9f

如果[c1a,c1b,c1c三个条件至少要满足一个]且[c2被满足]
>>如果mpLocalMapper-> mbAcceptKeyFrames==true
	>>判定成功
>>如果mpLocalMapper-> mbAcceptKeyFrames==false
	>>将mpLocalMapper->mbAbortBA设置为true(这一步不是判断)
	>>如果[不是单目系统]且[mpLocalMapper->mlNewKeyFrames.size()<3]
		>>判定成功
	
```c
//判断是否插入关键帧
bool Tracking::NeedNewKeyFrame()
{
    if(mbOnlyTracking)//仅追踪不添加关键帧
        return false;

    // If Local Mapping is freezed by a Loop Closure do not insert keyframes
    //如果建图模块由于回环检测被暂停
    if(mpLocalMapper->isStopped() || mpLocalMapper->stopRequested())//分别返回mbStopped和mbStopRequested
        return false;

    const int nKFs = mpMap->KeyFramesInMap();//返回mpMap->mspKeyFrames.size(),即地图中已有关键帧的数量;mspKeyFrames是set

    // Do not insert keyframes if not enough frames have passed from last relocalisation
    //(已有关键帧数量>mMaxFrames时)上次重定位后必须经过了>=mMaxFrames个帧
    if(mCurrentFrame.mnId<mnLastRelocFrameId+mMaxFrames && nKFs>mMaxFrames)
        return false;

    // Tracked MapPoints in the reference keyframe
    int nMinObs = 3;//地图点的nObs的阈值
    if(nKFs<=2)
        nMinObs=2;
    //参考关键帧的mvpMapPoints中obn大于nMinObs的mappoint数量
    int nRefMatches = mpReferenceKF->TrackedMapPoints(nMinObs);

    // Local Mapping accept keyframes?
    bool bLocalMappingIdle = mpLocalMapper->AcceptKeyFrames();//return mpLocalMapper-> mbAcceptKeyFrames,表示localmap是否接受关键帧,初始化为true,可以被 LocalMapping::SetAcceptKeyFrames修改

    // Check how many "close" points are being tracked and how many could be potentially created.
    int nNonTrackedClose = 0;//close特征点中 不是tack局部地图的内点 的数量
    int nTrackedClose= 0;    //close特征点中 是tack局部地图的内点 的数量
    if(mSensor!=System::MONOCULAR)
    {
        for(int i =0; i<mCurrentFrame.N; i++)
        {
            if(mCurrentFrame.mvDepth[i]>0 && mCurrentFrame.mvDepth[i]<mThDepth)//是closepoint
            {
                if(mCurrentFrame.mvpMapPoints[i] && !mCurrentFrame.mvbOutlier[i])//track局部地图中的内点
                    nTrackedClose++;
                else
                    nNonTrackedClose++;
            }
        }
    }

    bool bNeedToInsertClose = (nTrackedClose<100) && (nNonTrackedClose>70);

    // Thresholds
    float thRefRatio = 0.75f;
    if(nKFs<2)//地图中已有关键帧不超过2个,则降低阈值
        thRefRatio = 0.4f;

    if(mSensor==System::MONOCULAR)
        thRefRatio = 0.9f;

    // Condition 1a: More than "MaxFrames" have passed from last keyframe insertion
    const bool c1a = mCurrentFrame.mnId>=mnLastKeyFrameId+mMaxFrames;
    // Condition 1b: More than "MinFrames" have passed and Local Mapping is idle
    const bool c1b = (mCurrentFrame.mnId>=mnLastKeyFrameId+mMinFrames && bLocalMappingIdle);
    //Condition 1c: tracking is weak
    const bool c1c =  mSensor!=System::MONOCULAR && (mnMatchesInliers<nRefMatches*0.25 || bNeedToInsertClose) ;
    // Condition 2: Few tracked points compared to reference keyframe. Lots of visual odometry compared to map matches.
    const bool c2 = ((mnMatchesInliers<nRefMatches*thRefRatio|| bNeedToInsertClose) && mnMatchesInliers>15);

    if((c1a||c1b||c1c)&&c2)
    {
        // If the mapping accepts keyframes, insert keyframe.
        // Otherwise send a signal to interrupt BA
        if(bLocalMappingIdle)//表示localmap是否接受关键帧
        {
            return true;
        }
        else
        {
            mpLocalMapper->InterruptBA();//设置mpLocalMapper->mbAbortBA = true;
            if(mSensor!=System::MONOCULAR)
            {
                if(mpLocalMapper->KeyframesInQueue()<3)//返回mlNewKeyFrames.size()
                    return true;
                else
                    return false;
            }
            else
                return false;
        }
    }
    else
        return false;
}