I'm trying to follow this tutorial http://www.tonypa.pri.ee/tbw/tut05.html to implement real-time collisions in a tile-based world.

I find the center coordinates of my entities thanks to these properties:

public float CenterX
{
    get { return X + Width / 2f; }
    set { X = value - Width / 2f; }
}
public float CenterY
{
    get { return Y + Height / 2f; }
    set { Y = value - Height / 2f; }
}

Then in my update method, in the player class, which is called every frame, I have this code:

public override void Update()
{
    base.Update();

    int downY = (int)Math.Floor((CenterY + Height / 2f - 1) / 16f);
    int upY = (int)Math.Floor((CenterY - Height / 2f) / 16f);
    int leftX =     (int)Math.Floor((CenterX + Speed * NextX - Width / 2f) / 16f);
    int rightX = (int)Math.Floor((CenterX + Speed * NextX + Width / 2f - 1) / 16f);

    bool upleft = Game.CurrentMap[leftX, upY] != 1;
    bool downleft = Game.CurrentMap[leftX, downY] != 1;
    bool upright = Game.CurrentMap[rightX, upY] != 1;
    bool downright = Game.CurrentMap[rightX, downY] != 1;

    if(NextX == 1)
    {
        if (upright && downright)
           CenterX += Speed;
        else
           CenterX = (Game.GetCellX(CenterX) + 1)*16 - Width / 2f;
    }
 }

downY, upY, leftX and rightX should respectively find the lowest Y position, the highest Y position, the leftmost X position and the rightmost X position. I add + Speed * NextX because in the tutorial the getMyCorners function is called with these parameters:

getMyCorners (ob.x+ob.speed*dirx, ob.y, ob);

The GetCellX and GetCellY methods:

public int GetCellX(float mX)
{
    return (int)Math.Floor(mX / SGame.Camera.TileSize); 
}
public int GetCellY(float mY)
{
    return (int)Math.Floor(mY / SGame.Camera.TileSize); 
}

The problem is that the player "flickers" while hitting a wall, and the corner detection doesn't even work correctly since it can overlap walls that only hit one of the corners. I do not understand what is wrong. In the tutorial the ob.x and ob.y fields should be the same as my CenterX and CenterY properties, and the ob.width and ob.height should be the same as Width / 2f and Height / 2f. However it still doesn't work.

Thanks for your help.