Enclosed moving ball

[Padu]

This beginner here is strugling to solve some math problems:

Imagine a ball moving in a room. When the ball hits a wall, its movement vector is reflected with the same incident angle. (disconsider friction and loss of energy).

Now, in my game, I have an object where I know it's position (x,y) and it's direction angle (0 to 360). How to reproduce the above cited behavior?

I remember one of the first virus around for PC-XT's, the pingpong virus that do exactly what I'm describing.

Any hints?

[Stevie56]

Well, luckily, you've kept the problem simple and are only concerned with orthogonal collisions (the floor, ceiling and walls are flat, horizontal and vertical).

First, I believe you would find it easier to control the balls by defining a type

type ball=record
	X,Y:integer;
	dX,dY:real;
end;

X, Y are the current location.
dX and dY are the speeds (positive or negative) along X and Y.

they might be kept in an array:-

var balls:array[0..16] of ball;

And initialised at random:-

procedure randomisethem(width,height:integer);
//width, height are the area where we want the balls to stay from 0,0
var t:integer;
begin
	for t:=0 to 16 do
	begin
		balls[t].X:=random(width);
		balls[t].Y:=random(height);
		balls[t].dX:=random-0.5;  //random speeds
		balls[t].dY:=random-0.5;
	end;
end;

and I'd move them....

Procedure movethem(width,height:integer);
var t:integer;

begin
	for t:=0 to 16 do
	begin
		with balls[t] do
		begin
			X:=round(X+dX);
			Y:=round(Y+dY);
			//and test for out of bounds
			if X<0 or X>width then  begin
	                dX&#58;=-dX;   //reverse direction
	                X;=round&#40;X+2*dX&#41;;   //bring back inside arena
         end;

			if Y<0 or Y>height then begin
	                dY&#58;=-dY;   //reverse direction
	                Y;=round&#40;Y+2*dY&#41;;   //bring back inside arena
         end;
      end;
   end;
end;

Hope this helps a little.