Bouncing Ball Game - delphi

Question

Working on a school project and need some help please... I have built a game which purpose it to avoid your primary ball from crashing into the random balls that generate every 30 seconds. My problem is that the random balls are not staying within the frame and I am not sure what I am doing wrong. Any help would be much 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 the shape is outside.
if bAbove=true or bBelow=true or bFarLeft=true or bFarRight=true then
Begin
bOutside:=true;
End
Else
begin
bOutside:=False;
end;
// if shape is outside swop relavent direction
if bOutside=true then
Begin
Begin
if bAbove=true then
begin
iyDirec:=1;
end;
if bBelow=true then
begin
iyDirec:=-1;
end;
if bFarRight then
begin
ixDirec:=-1;
end;
if bFarLeft then
begin
ixDirec:=1;
end;
End;
End;
shpEnemy.Top := shpEnemy.Top + iyMove * iyDirec;
shpEnemy.Left := shpEnemy.Left + ixMove * ixDirec; // Change pos of enemy shapes
end;

Answer 1

You've made two main mistakes:
You are not calculating correctly the conditions (bAbove, bBelow etc.) responsible for the change of ball direction.
The important thing to know here is that the ball position is relative to its parent (pnlArena in this case). To explain it in different words: ball doesn't know anything about the outside world, pnlArena is the whole world for ball. So if your window coordinate system origin is top left then the far most left of the pnlArena equals to 0 (pnlArena.Left = 0) and the far most top is also 0 (pnlArena.Top = 0).
Knowing this you could probably guess now that ball will cross the left border of its world when shpEnemy.Left < 0. I will not go onto details about other directions, try to understand the code which I've provided.
The second mistake is more subtle. Currently your shpEnemy will not bounce back correctly from walls of your arena. The thing is that shpEnemy does not remember its last direction. You need to change the direction only when it is necessery, not each time.
Here is a fully working example:
unit Unit144;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, ExtCtrls;
// Our shape needs to remeber its direction, otherwise it 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 the shape is outside.
if bAbove or bBelow or bFarLeft or bFarRight then
Begin
bOutside:=true;
End
Else
begin
bOutside:=False;
end;
// if shape is outside swop relavent direction
if bOutside=true then
Begin
Begin
if bAbove=true then
begin
shpEnemy.yDirec:=1;
end;
if bBelow=true then
begin
shpEnemy.yDirec:=-1;
end;
if bFarRight then
begin
shpEnemy.xDirec:=-1;
end;
if bFarLeft then
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;
// We chose random direction of our ball.
Self.Parent := AParent;
LDirection := Random(1);
if LDirection = 0 then
xDirec := - 1
else
xDirec := 1;
LDirection := Random(1);
if LDirection = 0 then
yDirec := - 1
else
yDirec := 1;
// We must place our ball somewhere on the parent.
Left := Random(AParent.Width) - Self.Width;
if Left < 0 then
Left := 0;
Top := Random(AParent.Height) - Self.Height;
if Top < 0 then
Top := 0;
end;
end.
Hope this helps.