谁能给点意见，怎么做碰撞检测？
部分程序如下：
   procedure buildmain;
   begin
   setlinestyle(0,0,2);setcolor(green);
   line(150,0,150,479);line(450,0,450,479);
   setcolor(red);rectangle(5,5,105,55);
   setcolor(4);rectangle(455,5,555,55);
   setfillstyle(1,white);
   circle(300,429,9);
   rectangle(151,439,449,479);
   floodfill(152,440,red);
   setcolor(black);
   outtextxy(152,450,'Use the "END" key to exit.');
   outtextxy(152,462,'Use the direction key to move.');
   end;
   procedure lightgraph;
    begin
    for use[2]:=0 to 15 do
         begin
         setbkcolor(use[2]);
         delay(15);
         end;
    setbkcolor(black);
    end;
    procedure planemove(x,y:integer);
    begin
    setcolor(black);circle(pointx,pointy,9);
    if ((pointx<439)and(x>0)) then pointx:=pointx+x;
    if ((pointx>161)and(x<0)) then pointx:=pointx+x;
    if ((pointy<429)and(y>0)) then pointy:=pointy+y;
    if ((pointy>11)and(y<0)) then pointy:=pointy+y;
    setcolor(red);circle(pointx,pointy,9);
    end;

 procedure thunder;
 begin
 if enter2<>'thunder'then exit;
    gd:=detect;
     initgraph(gd,gm,' ');
     pointx:=300;pointy:=429;
     setcolor(3);
     SetLineStyle(3,2,1);
     circle(320,240,236);
     line(0,0,639,479);
     line(639,0,0,479);
     setcolor(8);delay(1000);lightgraph;soundmax(512,1024);
     outtextxy(250,88,'Thunderbolt flight');
     outtextxy(250,108,'This is a Demo');
     outtextxy(250,128,'Created by Lrc007');
     outtextxy(100,208,'The idea is form duxi');
     outtextxy(100,228,'Thanks for his idea');
     outtextxy(100,248,'Same to all members');
     outtextxy(385,208,'Provide by internet');
     outtextxy(385,228,'By Turbo Pascal 7.0');
     outtextxy(385,248,'Copyright(c)1991-2006');
     outtextxy(250,368,'TMD system 8.501 plus');
     outtextxy(230,388,'Waiting for the now system');
     delay(1000);
     cleardevice;lightgraph;soundmax(786,1429);setcolor(white);
     outtextxy(230,220,'TMD SYSTEM 8.501 PLUS');
     delay(1000);cleardevice;
     outtextxy(230,220,'WE WILL DO IT BETTER');
     delay(1000);
     soundmax(429,1340);soundmax(712,589);soundmax(100,1100);
     cleardevice;
     buildmain;{初始化图像}
     repeat{输入}
     Case Upcase(Readkey) Of
           #0:
           case readkey of
              #71:begin
                  setcolor(black);
                  circle(pointx,pointy,9);
                  pointx:=300;
                  pointy:=429;
                  setcolor(red);
                  circle(pointx,pointy,9);
                  end;
              #79:begin cleardevice;closegraph;use[1]:=0;exit;end;
              #72:planemove(0,-5);{移动飞机(一个圆圈)}
              #80:planemove(0,5);
              #75:planemove(-5,0);
              #77:planemove(5,0);
              end;
     end;