the Robot Construction Kit

Frame Tree

Frame Tree

The frame tree stores information about cartesian reference frames that are available in the environment. Every environment has a root frame, which can be retrieved from an environment object using the getRootNode() method.

Additional frames can be added to the frame tree using one of the two methods

FrameNode *childFrame = new FrameNode(
    Eigen::Transform3d( Eigen::Translation3d( 0, 0, 1.0 ) ) );

env->addChild( env->getRootNode(), childFrame )

or alternatively

env->getRootNode()->addChild( childFrame );

The second method obviously only works if the parent framenode is already attached to the environment.


The transformation given in the constructor of the FrameNode denotes the transformation from the given frame to the parent frame. For the above example, a point with (0, 0, 0) in childFrame would be (0, 0, 1.0) in the parentFrame (in this case the root Frame).

The transformation can be accessed using the getTransform and setTransform methods of the FrameNode.

In order to obtain the transformation between two arbitrary frames (both of which need to be connected to the same frame tree) the relativeTransform method can be used.

Transform3d C_a2b = env->relativeTransform( frameA, frameB );

Will return the transform C_a2b, which given a vector v in frame A would provide v’ = C_a2b * v, the same vector in frame B.


The frame tree also supports uncertainty, but special methods have to be used for that. Using the setTransformWithUncertainty method of the frame allows you to provide a gaussian uncertainty of the transformation. The class for that is TransformWithUncertainty, which additionaly to the transformation stores the covariance of the position and rotation (as a scaled axis rotation) as a 6x6 matrix.

Chaining of transformations with uncertainty is also available, so you can see how the uncertainty propages through the frames. A simple example would be the following

// create a point with uncertainty.
// we provide sigma values of 0.01 m, which have to be squared, 
// since the class is expecting a covariance
PointWithUncertainty p(
	Eigen::Vector3d( 0.0, 0, 0 ),
	Eigen::Vector3d( 0.01, 0.01, 0.01 )
	    .array().square().matrix().asDiagonal() );

// create a 6x6 covariance matrix
Eigen::Matrix<double,6,1> lt1; 
lt1 <<
    0.0, 0.0, 0.0, 0.0, 0.0, 0.0;

// create a transform with uncertainty from the given 
// diagonal of sigma values
TransformWithUncertainty t1(
	lt1.array().square().matrix().asDiagonal() );

// and a second one 
Eigen::Matrix<double,6,1> lt2; 
lt2 << 
    0.0, 0.0, 0.1, 0.0, 0.0, 0.0;

TransformWithUncertainty t2(
	lt2.array().square().matrix().asDiagonal );

// The point transformation is the same as without uncertainty, 
// but the uncertainty information is also propagated
PointWithUncertainty pp = t2 * t1 * p;

The same works when storing the TransformationWithUncertainty in the frame tree. Instead of relativeTransform, the relativeTransformWithUncertainty can be used.