Verlet integration

[Scott Ayers]

There are ways to store the value of an object's position in both xpresso and coffee. And then use the distance node or vlen() function to subtract them. And then do something with that value later on.
But the way you want to carry this all out is kind of important to know in order to give you a more specific answer.

Perhaps if you explained how you want to do the animation rather than the mathematical formula. Someone might reply?

-ScottA

[Darter]

Here's a quick translation of the formula into COFFEE code for a single position. Arrays could be used for multiple values. The acceleration variable has been initialized with an arbitrary value but for meaningful application you'll need something like the force accumulation function in the Gamasutra example.


var framePrev;
var posPrev;

main(doc,op)
{
// allow pos to be reset at current frame
var frame = doc->GetTime()->GetFrame(doc->GetFps());
if(!framePrev) framePrev = 0;
var pos = op->GetPosition();
if(frame == framePrev) posPrev = pos;

// calculate new position
var accel = vector(0, 1, 0);
var timeStep = 0.5;
var temp = pos;
if(!posPrev) posPrev = pos;
pos += pos - posPrev + accel * timeStep * timeStep;
doc->StartUndo();
doc->AddUndo(UNDO_CHANGE, op);
op->SetPosition(pos);
doc->EndUndo();

framePrev = frame;
posPrev = temp;
}

[Benek]

Thanks Darter, I'll look into this a let you know how I get on.

Scott, at the moment I have no specific ideas on how I plan to use this... I was just experimenting, this came up and I couldn't resolve it... and sometimes I just can't let things go until I find an answer.

For now, I'm just trying to perform the function on any object, like a cube or vertex. I've set up a quick Xpresso file which seems to function. The thing that confuses me is taking an object's current position and feeding it a new position in the same expression – my Xpresso lags by 1 frame/step, but I don't think I can help that... It's also quite unstable

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