expression with springs

[Dan Ebberts]

George Polevoy posted a good solution to this back in 2003 I think. Search the archives and you should find it.

Dan

[David Miller]

Thanks Dan

I did find that and have been trying to understand it. It is a little wild although giving it some preroll time seems to help and changing some of the parameters. But do you remember the motion math spring script. It was so perfect for doing little projects like this and it would add a dampening effect to the spring so it didn’t just bounce wildly. Do you know if I can repurpose that script? I suppose I could reload 5.5 and try to work with that or try to figure this one out but I don’t really know the quantitative differences between expressions and motion math.

anyway thanks for the follow up.

David

Here was the script

// One spring – version 1.2

// This script attaches a spring between the 2 layers shown in the popups.
// Layer 1 keeps its original motion, while layer 2 is attached by a spring to it.

// LAYER PROPERTY CHANNEL
// —— ———- ——–
// 1: Layer that moves doesn’t matter doesn’t matter
// 2: Layer attached to #1 by a spring doesn’t matter doesn’t matter

if (time() == start_time) {
rest_length = 10; // rest length of spring in pixels
damp = 0.95; // Damping (0 = infinite friction, 1 = none)

p1 = value(pop_layer(1), position);
p2 = value(pop_layer(2), position);

last_p1 = tmap(time() – step_time, value(pop_layer(1), position));
last_p2 = tmap(time() – step_time, value(pop_layer(2), position));

v1 = (p1 – last_p1);
v2 = (p2 – last_p2);
} else {
p1 = value(pop_layer(1), position);

delta = p2 – p1;
n_delta = normalize(delta);

a = 2 * n_delta * (length(delta) – rest_length) * step_time;

v2 = (v2 – a) * damp;
v1 = (v1 + a) * damp;

p1 = p1 + v1;
p2 = p2 + v2;
}
value(pop_layer(2), position) = p2;

[Dan Ebberts]

That particular MM script is one of the things that doesn’t translate well to expressions. It’s because expressions can’t have persistant variables so the expression has no idea what its value was on the previous frame. And the spring calculation really needs to know where everything was on the previous frame. With MM, the calculation for the entire timeline is done all at once, it’s not an issue.

Dan

[David Miller]

I finally understand. That must be why it lays down keyframes instead of dynamically updating as the parmeters change. Thanks for not letting me head down that direction. I wanted to play with some type following a tracking point but have it have action like the connections in that beautiful site https://www.visualthesaurus.com. In reality I can hand animate the whole thing which I may end up doing but this is one of those things that I just didn’t want to give up on. I know I could use it in the future and I know the community would take it into some amazing directions.

Do you know if there is at least a dampening function that could be added to this?

Thanks Dan

David

[Dan Ebberts]

You know, I just remebered that I had a brute-force method of doing the MM spring thing with expressions. It has to loop through the entire time line on every frame so it will undoubtedly bog down in long comps, but here it is:

restLength = 20; //rest length (pixels)
damp = .95;
leader = thisComp.layer(“leader”);

fDur = thisComp.frameDuration;
currFrame = Math.round(time/fDur);

p2 = position.valueAtTime(0);
v2 = 0;
for (f = 0; f <= currFrame; f++){ t = f*fDur; p1 = leader.position.valueAtTime(t); delta = p2 - p1; nDelta = normalize(delta); a = 2*nDelta*(length(delta) - restLength)*fDur; v2 = (v2 - a)*damp; p2 += v2; } p2 Dan

[David Miller]

Thanks Dan

I will give this a try on some short footage and report back from the front lines…

david