重读ORB_SLAM之LoopClosing线程难点

1. DetectLoop

这里有个ConsistenGroup概念，比较难懂。这里是最让人迷惑的地方。一旦vbConsistentGroup为真，其他帧的spCanditateGroup就进不来了。

if(!vbConsistentGroup[iG])
                {
                    ConsistentGroup cg = make_pair(spCandidateGroup,nCurrentConsistency);
                    vCurrentConsistentGroups.push_back(cg);
                    vbConsistentGroup[iG]=true; //this avoid to include the same group more than once
                }

2. ComputeSim3

不太明白为什么都使用Sim3去计算相对位姿。在双目和RGBD情况下，地图点深度已知，可以不优化Scale的啊。关于sim3的优化，请参考此博客与paper：sim3优化，paper。这里面用到了三种不同的匹配方法：

 int nmatches = matcher.SearchByBoW(mpCurrentKF,pKF,vvpMapPointMatches[i]);

//为了找到更多的匹配点，用于优化
   matcher.SearchBySim3(mpCurrentKF,pKF,vpMapPointMatches,s,R,t,7.5);

    //从回环帧和它的邻域帧里，得到更多地图点，并与当前帧匹配，如果内点多于某个阈值，则接受这个loop
    // Retrieve MapPoints seen in Loop Keyframe and neighbors
    vector<KeyFrame*> vpLoopConnectedKFs = mpMatchedKF->GetVectorCovisibleKeyFrames();
    vpLoopConnectedKFs.push_back(mpMatchedKF);
    mvpLoopMapPoints.clear();
    for(vector<KeyFrame*>::iterator vit=vpLoopConnectedKFs.begin(); vit!=vpLoopConnectedKFs.end(); vit++)
    {
        KeyFrame* pKF = *vit;
        vector<MapPoint*> vpMapPoints = pKF->GetMapPointMatches();
        for(size_t i=0, iend=vpMapPoints.size(); i<iend; i++)
        {
            MapPoint* pMP = vpMapPoints[i];
            if(pMP)
            {
                if(!pMP->isBad() && pMP->mnLoopPointForKF!=mpCurrentKF->mnId)
                {
                    mvpLoopMapPoints.push_back(pMP);
                    pMP->mnLoopPointForKF=mpCurrentKF->mnId;
                }
            }
        }
    }

    // Find more matches projecting with the computed Sim3
    matcher.SearchByProjection(mpCurrentKF, mScw, mvpLoopMapPoints, mvpCurrentMatchedPoints,10);

3. EssentialGraph 的意义

  Optimizer::OptimizeEssentialGraph(mpMap, mpMatchedKF, mpCurrentKF, NonCorrectedSim3, CorrectedSim3, LoopConnections, mbFixScale);

只优化位姿，里面包含当前回环边信息，所有帧与父边信息，它的回环边信息，以及共视边信息。优化所有帧位姿。