delphi - Bouncing Ball Game -

working on school project , need please... have built game purpose avoid primary ball crashing random balls generate every 30 seconds. problem random balls not staying within frame , not sure doing wrong. appreciated. thank you

procedure tfrmgamepage.enemyball(shpenemy: tshape);   var     boutside, babove, bbelow, bfarleft, bfarright : boolean;     ixmove, iymove, iydirec{negative = increase, positive = decrease}, ixdirec{positive = increase, negative = decrease} : integer;   begin      babove := pnlarena.height-shpenemy.top > pnlarena.height;     bbelow := pnlarena.height < shpenemy.top;      bfarleft := pnlarena.width-shpenemy.left > pnlarena.width;     bfarright := pnlarena.width < shpenemy.left;      ixmove:=random(3)+1;     iymove:=random(3)+1;      ixdirec:=1;     iydirec:=1;        //check if shape outside.         if babove=true or bbelow=true or bfarleft=true or bfarright=true           begin             boutside:=true;           end         else           begin             boutside:=false;           end;       // if shape outside swop relavent direction         if boutside=true           begin                begin                 if babove=true                   begin                     iydirec:=1;                   end;                 if bbelow=true                   begin                     iydirec:=-1;                   end;                  if bfarright                   begin                     ixdirec:=-1;                   end;                 if bfarleft                   begin                     ixdirec:=1;                   end;                end;           end;        shpenemy.top := shpenemy.top + iymove * iydirec;       shpenemy.left := shpenemy.left + ixmove * ixdirec;    // change pos of enemy shapes    end; 

you've made 2 main mistakes:

1. you not calculating correctly conditions (babove, bbelow etc.) responsible change of ball direction.

the important thing know here ball position relative parent (pnlarena in case). explain in different words: ball doesn't know outside world, pnlarena whole world ball. if window coordinate system origin top left far left of pnlarena equals 0 (pnlarena.left = 0) , far top 0 (pnlarena.top = 0).

knowing guess ball cross left border of world when shpenemy.left < 0. not go onto details other directions, try understand code i've provided.

2. the second mistake more subtle. shpenemy not bounce correctly walls of arena. thing shpenemy not remember last direction. need change direction when necessery, not each time.

here working example:

unit unit144;  interface  uses   windows, messages, sysutils, variants, classes, graphics, controls, forms,   dialogs, stdctrls, extctrls;  // our shape needs remeber direction, otherwise won't move correctly. type   tballshape = class(tshape)   public     xdirec, ydirec: integer;     constructor create(aowner: tcomponent; aparent: twincontrol); reintroduce;   end;   type   tform144 = class(tform)     pnlarena: tpanel;     button1: tbutton;     procedure button1click(sender: tobject);     procedure formcreate(sender: tobject);   private     ballshape: tballshape;   public     procedure enemyball(shpenemy: tballshape);   end;  var   form144: tform144;  implementation  {$r *.dfm}  { tform144 }  procedure tform144.button1click(sender: tobject); begin   enemyball(ballshape); end;  procedure tform144.enemyball(shpenemy: tballshape);  var     boutside, babove, bbelow, bfarleft, bfarright : boolean;     ixmove, iymove : integer;   begin      ixmove:=random(3)+1;     iymove:=random(3)+1;      babove := shpenemy.top < 0;     bbelow := shpenemy.top + shpenemy.height >  pnlarena.height;      bfarleft := shpenemy.left < 0;     bfarright := shpenemy.left + shpenemy.width >  pnlarena.width;        //check if shape outside.         if babove or bbelow or bfarleft or bfarright           begin             boutside:=true;           end         else           begin             boutside:=false;           end;       // if shape outside swop relavent direction         if boutside=true           begin               begin                 if babove=true                   begin                     shpenemy.ydirec:=1;                   end;                 if bbelow=true                   begin                     shpenemy.ydirec:=-1;                   end;                 if bfarright                   begin                     shpenemy.xdirec:=-1;                   end;                 if bfarleft                   begin                     shpenemy.xdirec:=1;                   end;                end;           end;        shpenemy.top := shpenemy.top + iymove * shpenemy.ydirec;       shpenemy.left := shpenemy.left + ixmove * shpenemy.xdirec;    // change pos of enemy shapes end;  procedure tform144.formcreate(sender: tobject); begin   randomize;   ballshape := tballshape.create(self, pnlarena);   ballshape.shape := stcircle; end;  { tballshape }  constructor tballshape.create(aowner: tcomponent; aparent: twincontrol); var   ldirection: integer; begin   inherited create(aowner);    width := 20;   height := 20;    // chose random direction of our ball.   self.parent := aparent;   ldirection := random(1);   if ldirection = 0     xdirec := - 1   else     xdirec := 1;   ldirection := random(1);   if ldirection = 0     ydirec := - 1   else     ydirec := 1;   // must place our ball somewhere on parent.   left := random(aparent.width) - self.width;   if left < 0     left := 0;   top := random(aparent.height) - self.height;   if top < 0     top := 0; end;  end. 

hope helps.