How to create a distance constraint?

[Julio Jerez]

Oh I undestand now, Joe is right it onlt the one dof.

However that make unstanble because by trying to inforce only the distance there side velocity can become very large for the
joint to is one step. It may need the calculation of the giorosccopy effect to make up for the side veliocity.

make the video to see how it looks.

[JoeJ]

Code: Select all

p0 -= dir * distance * 0.5;
      p1 += dir * distance * 0.5;
      
      sQuat rot;
      rot.FromVecToVec (m_js.XAxis(), dir);      
      m_js = rot * m_js;
      m_js.Normalize ();
      matrix0.Rotation()->FromQuat (m_js);

      NewtonUserJointAddLinearRow (joint, (float*)&p0, (float*)&p1, (float*)&matrix0[0]);
      NewtonUserJointAddLinearRow (joint, (float*)&p0, (float*)&p1, (float*)&matrix0[1]);
      NewtonUserJointAddLinearRow (joint, (float*)&p0, (float*)&p1, (float*)&matrix0[2]);


To add the stabilizing side rows we could use Gramm Schmidt or store a quat and adjust it to distance direction by axis / angle rotation from old to new directon. This avoids the bad discontinuous changes from Gramm Schmidt. Tested and works very good

[jamesm6162]

Hi
Apologies for reviving this thread. I am stuck with the exact same problem now.

Code: Select all

p0 -= dir * distance * 0.5;
      p1 += dir * distance * 0.5;
      
      sQuat rot;
      rot.FromVecToVec (m_js.XAxis(), dir);      
      m_js = rot * m_js;
      m_js.Normalize ();
      matrix0.Rotation()->FromQuat (m_js);

      NewtonUserJointAddLinearRow (joint, (float*)&p0, (float*)&p1, (float*)&matrix0[0]);
      NewtonUserJointAddLinearRow (joint, (float*)&p0, (float*)&p1, (float*)&matrix0[1]);
      NewtonUserJointAddLinearRow (joint, (float*)&p0, (float*)&p1, (float*)&matrix0[2]);


To add the stabilizing side rows we could use Gramm Schmidt or store a quat and adjust it to distance direction by axis / angle rotation from old to new directon. This avoids the bad discontinuous changes from Gramm Schmidt. Tested and works very good

Could you please explain how you store the m_js quaternion for this distance constraint?

[JoeJ]

the quaternion m_js must be available from the joint callback, so i store it in the class of my joint, like other joint data (distance value in this case).
Does that answer the question?

It might be worth to add a direction check to minimize rotation, while still keeping final orientation aligned to error direction:

Code: Select all

sQuat rot;
if (m_js.XAxis().Dot(dir)) > 0
      rot.FromVecToVec (m_js.XAxis(), dir);
else   
       rot.FromVecToVec (m_js.XAxis(), -dir);


Usually i also use this quat relative to one of the joint bodies to make it completely independent from worldspace.

The advantage from this over simpler gramm schmidt is small but it's there
Distance joints work best if they are as close to the bodys COM as possible.

[jamesm6162]

Hi

Thanks I got it working now.

Regards

James