Function project

physx/source/physxextensions/src/ExtD6Joint.cpp:408–417

we decompose the quaternion as q1 * q2, where q1 is a rotation orthogonal to the unit axis, and q2 a rotation around it. (so for example if 'axis' is the twist axis, this is separateSwingTwist).

// we decompose the quaternion as q1 * q2, where q1 is a rotation orthogonal to the unit axis, and q2 a rotation around it.
// (so for example if 'axis' is the twist axis, this is separateSwingTwist).
static PxQuat project(const PxQuat& q, const PxVec3& axis, PxReal cosHalfTol, bool& truncated)
{
	const PxReal a = q.getImaginaryPart().dot(axis);
	const PxQuat q2 = PxAbs(a) >= 1e-6f ? PxQuat(a*axis.x, a*axis.y, a*axis.z, q.w).getNormalized() : PxQuat(PxIdentity);
	const PxQuat q1 = q * q2.getConjugate();

	PX_ASSERT(PxAbs(q1.getImaginaryPart().dot(q2.getImaginaryPart())) < 1e-6f);

	return truncate(q1, cosHalfTol, truncated) * q2;
}

// Here's how the angular part works:
// * if no DOFs are locked, there's nothing to do.