Expression Needed: Rotation based on Position!!!

[Filip Vandueren]

I’ve done something like this before but don’t have time to dig up the code right now.
I can work on it tonight (CET timezone), and post a project and the expressions.

Basically it is looking at the velocity by which the rack moves, applying inertia to this and figuring out to what angles the difference in position between where the shirt is now and where it ‘wants’ to be by lagging behind.
Inertia of course can only be achieved by calculating every frame that has gone before the current time. As a bonus you get a springy swingy motion.
The ‘collision detection’ should be either not necessary depending on the spacing of the shirts, or a bit difficult to code.

You can see a little testrender I did here:

https://forums.creativecow.net/readpost/227/7118?

I’ll get back to you with the source files.

[Shirak Agresta]

Great, thanks so much. I look forward to the code! The shirts are supposed to be hard, basically like swinging cards or cardboard, so I’m not worried about deformation or anything like that. I just don’t want them to intersect. When I get home I’ll post a still (if I can) of the image I’m using.

I couldn’t see the example you have, not sure if it’s my flash player or what.

Thanks again!

Julius Caesar and the Roman Empire couldn’t conquer the blue sky.

[Filip Vandueren]

Hi,

here’s the project:

https://www.vandueren.be/forumstuff/inertia/inertia-shirts.aep

You can easily control the stiffness and springiness and amount by which the shirts rotate with a few sliders I placed on a control Null.

Here’s the code (note that my solids have orientation [270,0,0]:

for X rotation:

inertia = thisComp.layer("CONTROLLER").effect("inertia")("Slider");
stiffness = thisComp.layer("CONTROLLER").effect("Stiffness")("Slider");
mass=thisComp.layer("CONTROLLER").effect("mass")("Slider");

lagValue = calcValue(0);
f= thisComp.frameDuration;
will=0;

for (t=0; t<=time; t+=f) { delta = calcValue(t) - lagValue; will = will * inertia + delta*stiffness ; lagValue +=will } lagValue; function calcValue(t) { return parent.position.velocityAtTime(t)[2]/-mass; }

for Z rotation:

inertia = thisComp.layer("CONTROLLER").effect("inertia")("Slider");
stiffness = thisComp.layer("CONTROLLER").effect("Stiffness")("Slider");
mass=thisComp.layer("CONTROLLER").effect("mass")("Slider");

lagValue = calcValue(0);
f= thisComp.frameDuration;
will=0;

for (t=0; t<=time; t+=f) { delta = calcValue(t) - lagValue; will = will * inertia + delta*stiffness ; lagValue +=will } lagValue; function calcValue(t) { return parent.position.velocityAtTime(t)[0]/mass; }

This is what it looks like: