void zoom() {
         
        if( getTilesize()==50) 
            {
             setTilesize(25); 
          //   offx = 0;
         //    offy = 0;
            } 
        else
            {
             setTilesize(50);   
            }
        
    if(this.offy<0) offy=0;
    if(this.offx<0) offx = 0;
     
    if(offx>(tilenumx-600/tilesize)) offx=tilenumx-600/tilesize;
    if(offy>(tilenumy-600/tilesize)) offy=tilenumy-600/tilesize;
       
    }