AS3 - Collision detection using tween, frame gap - actionscript-3

Question

Here's the thing : I have a ball bouncing using Tweens, and I want to detect collisions between the ball and platforms (only when the ball is falling down).
But, my solution is testing for collisions in an ENTER_FRAME loop, so when the ball speed is to high, at a x frame the ball is above the plateform, and at a x+1 frame the ball is below the platform, so my loop never detects collision (because the ball and the platform never really collide).
Here's the jump method of my ball :
public function jump():void
{
TweenLite.killTweensOf(this);
TweenLite.to(this, jumpSpeed, {y:250, ease:Cubic.easeOut});
TweenLite.to(this, jumpSpeed, {delay:jumpSpeed, y:stage.stageHeight-this.height, ease:Cubic.easeIn, onComplete:jump});
}
And here's what's executed in my ENTER_FRAME loop :
for each (var platform:Platform in platforms)
{
if (ball.hitTestObject(platform) && ballPreviousY < ball.y)
ball.jump();
}
ballPreviousY = ball.y;
I already started to work on a solution using the physics engine Box2D, but I would like to know if there was a simpler solution to that problem.

Answer 1

This is a common problem in collision detection - small, fast objects moving through thin obstacles. What your looking for is sweep testing
Instead of testing for intersection between two static shapes, we can
instead create new shapes by sweeping the original shapes along their
trajectory, and testing for overlap between these swept shapes.
From N Tutorial A - Fast Moving Objects, (I'd recommend reading all their stuff if your interested in more detail in collision algorithms).
For example, if you had a 1px by 1px square and it was travelling at 5px per frame along the x axis, you'll want to distort your square so that becomes a 6px by 1px rectangle for collision testing. Catching anything that it would hit next frame, but not anything more.
Circles are a little more difficult because when you stretch them they become ellipses (which are far harder to check collisions for). See this answer if you want to go the ellipse route.