ObiParticlePicker for any object [might be helpful for everyone]

[josemendez]

Update: this is a sample and it somehow works, but not precise yet as there is a spring thing happening. It would be great if we can make the springing thing disappear. 

Code:

void FixedUpdate()
    {
        Ray ray = new Ray(this.transform.position, transform.forward);

        if (triggerOnce)
        {
            Debug.Log("Tigger once");
            triggerOnce = false;
            RenderOnce();
        }
        else if (pickedParticleIndex >= 0)
        {
            float invmass = solver.invMasses[pickedParticleIndex];
            Vector4 position = solver.transform.TransformPoint(solver.positions[pickedParticleIndex]);
            Vector4 velocity = solver.velocities[pickedParticleIndex];
            Vector4 targetposition = this.transform.position;

            //// calculate picking position in solver space:
            solver.externalForces[pickedParticleIndex] = ((targetposition - position) * springStiffness - velocity * springDamping) / invmass;

        }

    }

    void RenderOnce()
    {
        Ray ray = new Ray(transform.position, transform.forward);

        float closestMu = float.MaxValue;
        float closestDistance = float.MaxValue;

        for (int i = 0; i < solver.renderablePositions.count; ++i)
        {
            float mu;
            Vector3 projected = ObiUtils.ProjectPointLine(solver.transform.TransformPoint(solver.renderablePositions[i]), ray.origin, ray.origin + ray.direction, out mu, false);
            float distanceToRay = Vector3.SqrMagnitude((Vector3)solver.transform.TransformPoint(solver.renderablePositions[i]) - projected);
            
            mu = Mathf.Max(0, mu);

            float radius = solver.principalRadii[i][0] * radiusScale;

            if (distanceToRay <= radius * radius && distanceToRay < closestDistance && mu < closestMu)
            {
                closestMu = mu;
                closestDistance = distanceToRay;
                pickedParticleIndex = i;
            }
        }
    }

Hi,

Instead of calculating a spring force, fix the particle (by setting its inverse mass and velocity to zero) and set the particle position directly. That will completely get rid of the "spring thing":

solver.velocities[pickedParticleIndex] = Vector4.zero;
solver.invMasses[pickedParticleIndex] = 0;
solver.positions[pickedParticleIndex] = solver.transform.InverseTransformPoint(this.transform.position);

Note that you should revert the inverse mass to whatever it was when you want the particle to start behaving normally again.