bouncing ball expression

[Trent Armstrong]

Ahh. The old browser auto-crop of expressions trick. They get me with that all the time!

I loaded it in Firefox and it works. Thanks for the heads-up on browser issues.

Trent

Trent Armstrong – Creative Cow Leader
https://www.dallasaeug.com

[David Cabestany]

I’m getting the same timeout error and I’m adding the expression to a null, the thing is I can’t replace it with a solid, because it’s an Element 3D Group Null, so it’s being referenced by the 3d objects, any suggestions on how to fix this error?

[David Cabestany]

Update: I’m using this expression instead, it works perfectly, no timeout errors or anything like. Just change where it says LAYER NAME HERE to the floor layer you want to use.
Thanks to Marcus Geduld and his excellent book After Effects Expressions for it.

var bouncesPerSecond=1.5;
var bounceStrength=200;
var decayRate=.5;
var floor=thisComp.layer("LAYER NAME HERE").transform.position[1]
var bounceOffset= Math.abs(Math.cos(bouncesPerSecond*time*2*Math.PI));
var y=bounceStrength*bounceOffset/Math.exp(decayRate*time);
floor=floor-(height/2);
[position[0], floor]-[0,y]

[Peter Menich]

Hey Dan,

Is there anyway to define a ‘start time’ for this expression?

Elements start to drop on frame 0, I tried dragging/staggering the footage in the timeline but they all still start dropping on the first frame.

I need them to start dropping on their ‘inPoint’ or after a defined amount of time, if thats possible.

Cheers

Pete

[Dan Ebberts]

You just need to replace all references to time with a variable that represents the time elapsed since the bounce start time. Like this for the position expression:

tStart = inPoint; // or whatever time you want
tt = Math.max(time-tStart,0); // I used tt because t is already used

Vy0 = 500; //initial y velocity (pixels/second)
Vx0 = 300; // initial x velocity (pixels/second)
g = 2500; // gravity (pixels/second/second)
floor = 400;
e = .85; //elasticity

b = floor – position[1];
h = b + Vy0*Vy0/(2*g);
T = Vy0/g + Math.sqrt(2*h/g);

if (tt < T){
y = Vy0*tt – g*tt*tt/2 + b;
}else{
Vy = -(Vy0 – g*T);
while (true){
Vy *= e;
t = T;
T += 2*Vy/g;
if (tt < T){
t = tt – t;
y = Vy*t – g*t*t/2;
break;
}else if (T – t < thisComp.frameDuration){
y = 0;
break;
}
}
}
[position[0] + Vx0*tt, floor – y]

Dan