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Frame transformations (Python)

Description: This tutorials gets you started with KDL Frames, Vectors, Rotations, etc

Tutorial Level:

Data types

Creating a Frame, Vector and Rotation

   1 import PyKDL
   2 
   3 # create a vector
   4 v = PyKDL.Vector(1,3,5)
   5 
   6 # create a rotation from Roll Pitch, Yaw angles
   7 r1 = PyKDL.Rotation.RPY(1.2, 3.4, 0)
   8 
   9 # create a rotation from XYZ Euler angles
  10 r2 = PyKDL.Rotation.EulerZYX(0, 1, 0)
  11 
  12 # create a rotation from a rotation matrix
  13 r3 = PyKDL.Rotation(1,0,0, 0,1,0, 0,0,1)
  14 
  15 # create a frame from a vector and a rotation
  16 f = PyKDL.Frame(r2, v)

Extracting information from a Frame, Vector and Rotation

   1 import PyKDL
   2 
   3 # frame
   4 f = PyKDL.Frame(PyKDL.Rotation.RPY(0,1,0),
   5                 PyKDL.Vector(3,2,4))
   6 
   7 # get the origin (a Vector) of the frame
   8 origin = f.p
   9 
  10 # get the x component of the origin
  11 x = origin.x()
  12 x = origin[0]
  13 
  14 # get the rotation of the frame
  15 rot = f.M
  16 
  17 # get ZYX Euler angles from the rotation
  18 [Rz, Ry, Rx] = rot.GetEulerZYX()
  19 
  20 # get the RPY (fixed axis) from the rotation
  21 [R, P, Y] = rot.GetRPY()

Creating from ROS types

   1 from tf_conversions import posemath
   2 
   3 # you have a Pose message
   4 pose = Pose()
   5 
   6 # convert the pose into a kdl frame
   7 f1 = posemath.fromMsg(pose)
   8 
   9 # create another kdl frame
  10 f2 = PyKDL.Frame(PyKDL.Rotation.RPY(0,1,0),
  11                  PyKDL.Vector(3,2,4))
  12 
  13 # Combine the two frames
  14 f = f1 * f2
  15 
  16 # and convert the result back to a pose message
  17 pose = toMsg(f)

And it wouldn't be Python if you couldn't have done all this in a single line:

   1 from tf_conversions import posemath
   2 
   3 pose = posemath.toMsg(posemath.fromMsg(pose) * PyKDL.Frame(PyKDL.Rotation.RPY(0,1,0), PyKDL.Vector(3,2,4)))

Transformations

Transforming a point

   1 import PyKDL
   2 
   3 # define a frame
   4 f = PyKDL.Frame(PyKDL.Rotation.RPY(0,1,0),
   5                 PyKDL.Vector(3,2,4))
   6 
   7 # define a point
   8 p = PyKDL.Vector(1, 0, 0)
   9 print p
  10 
  11 # transform this point with f
  12 p = f * p
  13 print p


2019-11-09 12:52