Damping in XPBD

[korzen]

Hi Yuki,

great repo, thanks!

I was wondering whether you could add a proper damping support to Distance Constraints as described in the original XPBD paper (Eq. 26)

Thanks!

[yuki-koyama]

Thanks!

Yes, I recognize supporting the constraint damping in XPBD would be an important future TODO.

However, I'm not sure I can work on it shortly because many other higher-priority features are missing such as collisions.

[korzen]

I have come up with the following code but something is still not right. The motion is dampened also when cloth rotates, similarly to viscous dampening:

`DistanceConstraint spring = springsBuffer[id.x];

	int idA = spring.idA;
	int idB = spring.idB;

//pos at start of the sub-step
float4 posA = posBuffer[idA];
float4 posB = posBuffer[idB];

	float4 particelA = predPosBuffer[idA];
	float4 particelB = predPosBuffer[idB];

//predicted position during constraints solve
float3 predPosA = particelA.xyz;
float3 predPosB = particelB.xyz;

//inv mass
float wA = particelA.w;
float wB = particelB.w;

	float restLength = spring.restLength;
	float stiff = spring.stiffness;

	float3 dir = predPosB - predPosA;

	float dist = length(dir);

	if (dist > EPS)
	{
		float wSum = wA + wB;
		if (wSum == 0)
			return;

		float C = dist - restLength;
		float3 N = dir / dist;

		float3 gradA = -N;
		float3 gradB =  N;

		float3 velA = predPosA - posA;
		float3 velB = predPosB - posB;

		float alphaHat = _compliance/ (dt * dt);
		float betaHat = _damping * (dt * dt);
		float gamma = (alphaHat * betaHat) / dt;

		float vA = dot(gradA, velA);
		float vB = dot(gradB, velB);

		float numA = -C - gamma * vA;
		float numB = -C - gamma * vB;

		float den = (1.0 + gamma) * wSum + alphaHat;

		float lambdaA = (numA / den);
		float lambdaB = (numB / den);

		predPosBuffer[idA] += float4(lambdaA * gradA * wA, 0);
		predPosBuffer[idB] += float4(lambdaB * gradB * wB, 0);

	}`

Have a try if you can.

Cheers