I’m trying to create a tilemap in one of my games and I would like to split the tilemap up into grids. By making grids containing a fixed amount of tiles, I only need to render the portion(s) of the tilemap that’s visible inside the camera (i.e, the grid(s) that’s currently inside the camera viewport).

The code I’m having at the moment:

struct STileGrid {     std::vector<STile*> TileList;      // Upper left corner of the grid     CVector2D StartPosition;      // Lower right corner of the grid     CVector2D EndPosition; };    //////////////////////////////////////////////////////////////////////////  // Hardcoded during testing  const CVector2D WindowSize = CVector2D(1280.0f, 720.0f);  // Number of horizontal/vertical tiles in the map const CVector2D MapSize = CVector2D(60.0f, 36.0f);  const CVector2D TileSize = CVector2D(64.0f, 64.0f);  // (20.0f, 12.0f) in this case (number of horizontal/vertical tiles in each grid) const CVector2D GridSize = CVector2D(ceil(WindowSize.x / TileSize.x), ceil(WindowSize.y / TileSize.y));  // 3 horizontal, 3 vertical in this case const unsigned int NumHorizontalGrids   = (int)ceil(MapSize.x / GridSize.x); const unsigned int NumVerticalGrids     = (int)ceil(MapSize.y / GridSize.y);  //////////////////////////////////////////////////////////////////////////  std::vector<STileGrid*> TileGridList; m_TileGridList.resize(NumHorizontalGrids * NumVerticalGrids); CreateTileGridList();  for(unsigned int i = 0; i < m_pTmxMap->GetNumTileLayers(); ++i) {     const CTmxTileLayer* pTmxTileLayer = m_pTmxMap->GetTileLayer(i);      if(!pTmxTileLayer->GetIsVisible())         continue;      const unsigned int LayerWidth   = (int)MapSize.x;     const unsigned int LayerHeight  = (int)MapSize.y;      for(unsigned int y = 0; y < LayerHeight; ++y)     {         for(unsigned int x = 0; x < LayerWidth; ++x)         {             const CTmxTile& rTmxTile = pTmxTileLayer->GetTile(x, y);              if(rTmxTile.GetIsEmpty())                 continue;              STile* pTile = new STile;              // Upper left corner of the tile             pTile->Position = CVector2D(TileSize.x * x, TileSize.y * y);              pTile->ID = rTmxTile.GetID();              // Setting some other tile data              // Place the tile inside the first grid             TileGridList[0]->TileList.push_back(pTile);         }     } } 

This code works fine so far. However, I would like to place each tile in the correct grid. At the moment, I’m doing TileGridList[0]->TileList.push_back(pTile); to place each tile inside the first grid, but instead of placing each tile in the first grid, I would like to, in some way, calculate which grid each tile should be placed inside, based on the grid’s- and the tile’s position.

The following image shows what I would like to achieve:

The red squares symbolizes the grids with some example tiles placed inside.

How can I, in each iteration of the x and y for loop, calculate an index into the tilegrid list which tells which grid to place each tile in?