Method for finding Orientation Error using Axis-Angle

asked 2018-06-12 21:05:54 -0500

malharjajoo gravatar image

updated 2018-06-13 09:31:35 -0500

Hi,

I have a reference value for Roll, pitch and yaw (Euler Angles) and my estimate for the same. I want to find the error between the two. If I convert each of the RPY values to a Rotation matrix, I see some possible ways (see below) of finding the orientation error.

I recently came across this openCV function in the calib3d module: get_rotation_error that uses Rodrigues/Axis-Angle (I think they mean the same) for finding the error between 2 rotation matrices.

I have 2 questions -

1) In the method given in get_rotation_error, it seems to "subtract" the two rotation matrices by transposing one (not sure what the negative sign is about)

error_mat = R_ref * R.transpose() * -1
error = cv::norm( cv::Rodrigues ( error_mat ))

How are we supposed to interpret the output ( I believe the output of the cv::norm( rodrigues_vector) is the angle of the rodrigues vector according to openCV convention. Does this mean I simply need to convert it to degrees to find the angle error (between reference and my estimates) in degrees ?

I would also like to mention that this method keeps returning 3.14159 even for wildly different values of the reference and my estimates. Is there something that I''m missing ?

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2) I thought of another method, slightly different from the above , what if I do the following -

my_angle = cv::norm (cv::Rodrigues ( R ))

reference_angle = cv::norm (cv::Rodrigues ( R_ref ))

error = reference_angle - my_angle

Is there something wrong with method 2) ? I have tried it and it gives a different output compared to method 1).

I would be very grateful if someone can answer the above queries or even point me in the right direction.

Thanks!

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Comments

Quick answer:

  • inverse of rotation is rotation transpose (no idea about why the minus sign but it should not matter much)
  • if the two rotations are identical, R_ref . R^T is identity, otherwise you get the rotation transformation / error
  • axis-angle formalism represents rotation as a combination of a vector and a magnitude, in your second case you are comparing only the angle error but not the vector error

You should look at a course about rotation, axis-angle, quaternion, formalisms and homogeneous transformation in robotics or in computer graphics if you want more information.

Eduardo gravatar imageEduardo ( 2018-06-14 16:06:34 -0500 )edit

In my second case I am not comparing the direction vector of the axis angle. Yes, but that is not being done in method 1) either (and it's a part of the openCV library).

malharjajoo gravatar imagemalharjajoo ( 2018-06-14 16:57:22 -0500 )edit

Computing R_ref . R^T basically computes the rotation difference / error.

Eduardo gravatar imageEduardo ( 2018-06-15 16:47:04 -0500 )edit

But then we only find the norm of the error vector ? What about the direction then

malharjajoo gravatar imagemalharjajoo ( 2018-06-15 17:25:32 -0500 )edit

What is bothering you? R_ref . R^T is a 3x3 rotation matrix. Norm of the Axis-Angle rotation representation is the amount of rotation. The direction doesn't matter if you are looking for quantifying (more/less rotation error?) the rotation error, at least in most of the cases.

Eduardo gravatar imageEduardo ( 2018-06-18 15:28:04 -0500 )edit

So why doesn't the direction of cv::Rodrigues ( error_mat ) matter ? ( If I have an angle around an axis, and then the same angle around a different axis, then shoulnd't they be considered as different ? )

malharjajoo gravatar imagemalharjajoo ( 2018-06-18 16:18:18 -0500 )edit