//输入为关键帧,当前帧以及地图点vector
//关键帧与当前帧进行特征点匹配
//返回匹配点数量
int ORBmatcher::SearchByBoW(KeyFrame* pKF,Frame &F, vector<MapPoint*> &vpMapPointMatches)
{
const vector<MapPoint*> vpMapPointsKF = pKF->GetMapPointMatches();
//获取关键帧的mvpMapPoints
vpMapPointMatches = vector<MapPoint*>(F.N,static_cast<MapPoint*>(NULL));
//当前帧的地图点初始化为NULL
//引用输入vpMapPointMatches,并将其初始化为空,数量为当前帧的特征点个数
const DBoW2::FeatureVector &vFeatVecKF = pKF->mFeatVec;
//获取关键帧的单词向量 正向索引
int nmatches=0;
//cout << "SearchByBoW: CP1 nmatches = " << nmatches << endl;
vector<int> rotHist[HISTO_LENGTH];
for(int i=0;i<HISTO_LENGTH;i++)
rotHist[i].reserve(500);
const float factor = 1.0f/HISTO_LENGTH;
//这个参数是什么意思??
// We perform the matching over ORB that belong to the same voc