def computeGroundFrame(self, robotModel):
        t1 = robotModel.getLinkFrame( self.ikPlanner.leftFootLink )
        t2 = robotModel.getLinkFrame( self.ikPlanner.rightFootLink )
        pelvisT = robotModel.getLinkFrame( self.ikPlanner.pelvisLink )

        xaxis = [1.0, 0.0, 0.0]
        pelvisT.TransformVector(xaxis, xaxis)
        xaxis = np.array(xaxis)
        zaxis = np.array([0.0, 0.0, 1.0])
        yaxis = np.cross(zaxis, xaxis)
        yaxis /= np.linalg.norm(yaxis)
        xaxis = np.cross(yaxis, zaxis)

        stancePosition = (np.array(t2.GetPosition()) + np.array(t1.GetPosition())) / 2.0

        footHeight = 0.0811

        t = transformUtils.getTransformFromAxes(xaxis, yaxis, zaxis)
        t.PostMultiply()
        t.Translate(stancePosition)
        t.Translate([0.0, 0.0, -footHeight])

        return t
Given a robol model, returns a vtkTransform at a position between
dataset/ETHPy150Open RobotLocomotion/director/src/python/director/bihandeddemo.py/BihandedPlannerDemo.computeGroundFrame