Emanuele Feronato

Talking about Sling game, Flash and Users contributions.

Only 2 days. That’s how much time Kevin Ward needed to solve and publish the solution of the problem posted in Throw a ball with a sling physics Flash tutorial.

You may know Kevin for his excellent post about a different approach to create a game like Line Rider, and now he come with the second part of the sling tutorial.

Take your honour, Kevin, you deserve it!!

This is the movie Kevin created:

And this is the actionscript, with comments in some critical parts:

var ball:MovieClip = _root.attachMovie("ball", "ball", 2, {_x:250, _y:100});
var elastic:MovieClip = _root.createEmptyMovieClip("elastic", 1);
var point1:MovieClip = _root.attachMovie("sling", "point1", 3, {_x:50, _y:200});
var point2:MovieClip = _root.attachMovie("sling", "point2", 4, {_x:450, _y:200});
var gravity = 0.1;
var angle1:Number = 0;
var angle2:Number = 0;
var radius:Number = 13.0;
// Or whatever half the width of your ball is.
var elasticCoefficient:Number = 0.001;
// This number will affect the stretchiness of the sling.  The greater the number
// the tighter the elastic will be.					
var released:Boolean = true;
var forced:Boolean = false;
var acc:Object = {x:0, y:0};
var vel:Object = {x:0, y:0};
_root.onMouseDown = function() {
	ball._x = _root._xmouse;
	ball._y = _root._ymouse;
	ball.startDrag();
	_root.released = false;
	vel.x = 0;
	vel.y = 0;
};
_root.onMouseUp = function() {
	ball.stopDrag();
	_root.released = true;
};
ball.onPress = function() {
	ball.startDrag();
	_root.released = false;
	vel.x = 0;
	vel.y = 0;
};
ball.onRelease = function() {
	ball.stopDrag();
	_root.released = true;
};
_root.onEnterFrame = function() {
	elastic.clear();
	elastic.lineStyle(3, 0xFF0000);
	if (released) {
		ball._x += vel.x;
		ball._y += vel.y;
	}
	if (ball._y>=187) {
		/* This area differs from the code in the tutorial.
		The reason for that is I didn't read the code, I just looked at the examples.
		I try to gain experience with problem-solving by doing things as close to by myself as possible. */
		forced = true;
		var x1:Number = ball._x-point1._x;
		var y1:Number = ball._y-point1._y;
		var x2:Number = point2._x-ball._x;
		var y2:Number = point2._y-ball._y;
		var distance1:Number = Math.sqrt(x1*x1+y1*y1);
		var distance2:Number = Math.sqrt(x2*x2+y2*y2);
		_root.angle1 = Math.atan2(y1, x1);
		_root.angle2 = Math.atan2(y2, x2);
		var xOffset:Number = Math.cos(angle1+Math.PI/2)*radius;
		var yOffset:Number = Math.sin(angle1+Math.PI/2)*radius;
		var xOffset2:Number = Math.cos(angle2+Math.PI/2)*radius;
		var yOffset2:Number = Math.sin(angle2+Math.PI/2)*radius;
		angle1 += Math.sin(radius/distance1);
		angle2 += Math.sin(radius/distance2)*-1;
		elastic.moveTo(point1._x, point1._y);
		elastic.lineTo(ball._x+xOffset, ball._y+yOffset);
		elastic.moveTo(point2._x, point2._y);
		elastic.lineTo(ball._x+xOffset2, ball._y+yOffset2);
	} else {
		forced = false;
		elastic.moveTo(point1._x, point1._y);
		elastic.lineTo(point2._x, point2._y);
	}
	acc.x = 0;
	acc.y = 0;
	acc.y = gravity;
	if (released && forced) {
		/* This section applies the force of the sling to the ball in terms of acceleration based on the stretching of
		the sling, in the direction of the displacement, scaled by a coefficient that also encapsulates the mass of 
		the ball. */
		acc.x += distance1*Math.sin(angle2)*elasticCoefficient;
		acc.y += -distance1*Math.cos(angle1)*elasticCoefficient;
		acc.x += distance2*Math.sin(angle1)*elasticCoefficient;
		acc.y += -distance2*Math.cos(angle2)*elasticCoefficient;
	}
	if (released) {
		vel.x += acc.x;
		vel.y += acc.y;
	}
};

This is an excellent solution to the problem, and I think this should be an inspiration for the ball physics Flash game design competition.

Download the source and say “good work Kevin!”.