How to convert from world coordinate

[henokyen]

I have some objects in the physics simulation and i want to know the local coordinates of these objects ?
How can i do that ?

[c0der]

Converting from world to local just involves multiplying the world transform matrix of the object by the inverse to get the identity matrix.

Normally you want to transform a different object B into the local frame of object A, which just involves multiplying B's world transform matrix by A's inverse world transform matrix, meaning that A will now be at the origin and B's position will be relative to this origin

[henokyen]

Thanks ,
basically i want to create a joint between two legs for my robot.
and i am using the hinge constraint. and this class requires the local coordinates of the legs to be joined.
I do not know who to get the local coordinated now.
can just provide a piece of codes that do this ?
thank
you

[c0der]

For my hinge joint, I have one anchor given in world coords and a world hinge axis.

From the world anchor, you build two local anchors as follows for each body

Code: Select all

	wt1T.transposeOf(m_pRigidBody1->m_matWorldTransform);
	wt2T.transposeOf(m_pRigidBody2->m_matWorldTransform);

	m_vLocalAnchor1 = (vAnchor - m_pRigidBody1->m_vPosition)*wt1T;
	m_vLocalAnchor2 = (vAnchor - m_pRigidBody2->m_vPosition)*wt2T;

Basically these are vectors ra and rb (of the relative position) in local coords of each body. Perhaps you can paste some code or show your approach

[shogun22]

I am sorry for reviving this thread but I am completely lost and already wasted countless hours on this simple function.
So I know I have to provide the HingeConstraint constructor with local coordinates for axis and anchor point. So I wanted to create two little helper functions to translate a vector from world to local and to translate a point from world to local (since both of them are needed for the axis and the anchor).
What I came up with so far is this:

Code: Select all

btVector3 pointWorldToLocal(int bodyIndex, btVector3 point)
{
	btVector3 localPoint;
	btTransform xform = arrayOfBtRigBodies[bodyIndex]->getCenterOfMassTransform();
	
	// The local coordinates should be [(worldCoords - localOrigin) * inverse(localRotation)] or am I wrong?
	localPoint = (point - xform.getOrigin()) * xform.getBasis().inverse();
	return localPoint;
}


btVector3 axisWorldToLocal(int bodyIndex, btVector3 axis)
{
	btTransform xform = arrayOfBtRigBodies[bodyIndex]->getCenterOfMassTransform();
	// This works quite well but only if I do not inverse the result of getBasis()... ?
	btVector3 localAxis = axis * xform.getBasis();
	return localAxis;
}

Which should be equivalent to c0der's suggestion:

wt1T.transposeOf(arrayOfBtRigBodies[bodyIndex]->m_matWorldTransform);
m_vLocalAnchor1 = (point - arrayOfBtRigBodies[bodyIndex]->m_vPosition)*wt1T;
return m_vLocalAnchor1

But when I run it, the axis is translated from world to local correctly while pointWorldToLocal produces strange results.
I basically wasted two complete days of my life on this so please release me from my pain

[shogun22]

Alright folks,

after I froze my project for a week I finally found the solution for my coordinate translation problem today. Might be trivial for most of you but for those who are stuck like I was, this is my final function:

Code: Select all

btVector3 RagdollDemo::pointWorldToLocal(int bodyIndex, btVector3 worldPoint)
{
        return body[bodyIndex]->getCenterOfMassTransform().inverse()(worldPoint);
}

yes guys it's that easy. Don't know why it took me so long to understand how getCenterOfMassTransform can be used to translate points. Maybe it's not in the manual or I completely overlooked it. Hope this helps someone else.

Best

[benfocus55]

Shogun! Thank you! Saved me countless hours and actually clarified these concepts.