【ORB_SLAM2】void Frame::AssignFeaturesToGrid()

AssignFeaturesToGridThe main purpose of this function is based on https://blog.csdn.net/qq_21950671/article/details/126084120Calculated dewarped image bounds and grid size,Assign the distorted feature points to the corresponding grids,The purpose of this is to speed up the subsequent feature point matching.

// #define FRAME_GRID_ROWS 48
// #define FRAME_GRID_COLS 64
// Number of KeyPoints. int N;
void Frame::AssignFeaturesToGrid()
{
    
    // Step 1 Gives a grid array storing feature points Frame::mGrid 预分配空间
    // FRAME_GRID_COLS = 64,FRAME_GRID_ROWS=48
    int nReserve = 0.5f*N/(FRAME_GRID_COLS*FRAME_GRID_ROWS);// 64 * 48
    //开始对mGrideach of this two-dimensional arrayvectorElements are traversed and space is preallocated
    for(unsigned int i=0; i<FRAME_GRID_COLS;i++)
        for (unsigned int j=0; j<FRAME_GRID_ROWS;j++) {
    
            // reserve的作用是更改vector的容量capacity, 使vector至少可以容纳n个元素
            // std::vector<std::size_t> mGrid[FRAME_GRID_COLS][FRAME_GRID_ROWS]
            // std::cout << "nReserve " << nReserve << std::endl;
            // std::vector<std::size_t> mGrid[FRAME_GRID_COLS][FRAME_GRID_ROWS];
            mGrid[i][j].reserve(nReserve);
        }
    // Step 2 遍历每个特征点,Put each feature point inmvKeysUnThe index value in is placed in the corresponding gridmGrid中
    for(int i=0;i<N;i++)
    {
    
    	  //Get the feature points that have been distorted from the member variables of the class
    	  
        const cv::KeyPoint &kp = mvKeysUn[i];
        //Stores the grid coordinates of the grid where a feature point is located,nGridPosX范围：[0,FRAME_GRID_COLS], nGridPosY范围：[0,FRAME_GRID_ROWS]
        int nGridPosX, nGridPosY;
        // Calculate the grid coordinates of the grid where a feature point is located,If the grid coordinates where the feature points are located are found,记录在nGridPosX,nGridPosY里,返回true,没找到返回false
        if(PosInGrid(kp,nGridPosX,nGridPosY))
            //如果找到特征点所在网格坐标,将这个特征点的索引添加到对应网格的数组mGrid中
            mGrid[nGridPosX][nGridPosY].push_back(i);
    }
}