//FLOOR CASTING
for(int y = 0; y < h; y++)
{
  // rayDir for leftmost ray (x = 0) and rightmost ray (x = w)
  float rayDirX0 = dirX - planeX;
  float rayDirY0 = dirY - planeY;
  float rayDirX1 = dirX + planeX;
  float rayDirY1 = dirY + planeY;

  // Current y position compared to the center of the screen (the horizon)
  int p = y - screenHeight / 2;

  // Vertical position of the camera.
  float posZ = 0.5 * screenHeight;

  // Horizontal distance from the camera to the floor for the current row.
  // 0.5 is the z position exactly in the middle between floor and ceiling.
  float rowDistance = posZ / p;

  // calculate the real world step vector we have to add for each x (parallel to camera plane)
  // adding step by step avoids multiplications with a weight in the inner loop
  float floorStepX = rowDistance * (rayDirX1 - rayDirX0) / screenWidth;
  float floorStepY = rowDistance * (rayDirY1 - rayDirY0) / screenWidth;

  // real world coordinates of the leftmost column. This will be updated as we step to the right.
  float floorX = posX + rowDistance * rayDirX0;
  float floorY = posY + rowDistance * rayDirY0;

  for(int x = 0; x < screenWidth; ++x)
  {
    // the cell coord is simply got from the integer parts of floorX and floorY
    int cellX = (int)(floorX);
    int cellY = (int)(floorY);

    // get the texture coordinate from the fractional part
    int tx = (int)(texWidth * (floorX - cellX)) & (texWidth - 1);
    int ty = (int)(texHeight * (floorY - cellY)) & (texHeight - 1);

    floorX += floorStepX;
    floorY += floorStepY;

    // choose texture and draw the pixel
    int floorTexture = 3;
    int ceilingTexture = 6;
    Uint32 color;

    // floor
    color = texture[floorTexture][texWidth * ty + tx];
    color = (color >> 1) & 8355711; // make a bit darker
    buffer[y][x] = color;

    //ceiling (symmetrical, at screenHeight - y - 1 instead of y)
    color = texture[ceilingTexture][texWidth * ty + tx];
    color = (color >> 1) & 8355711; // make a bit darker
    buffer[screenHeight - y - 1][x] = color;
  }
}