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。

巴特西

重读ORB_SLAM之LoopClosing线程难点

1. DetectLoop

2. ComputeSim3

3. EssentialGraph 的意义

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