# how to convert a rotation matrix to a rotation vector

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leo on 14 Feb 2020
Answered: James Tursa on 10 May 2020
In matlab function rodriguesMatrixToVector. There are some confusion I had about the code.
1. in comment: " % r = theta * v / |v|, where (w, v) is a unit quaternion". Where is the variable "w" ? 2. what does this equation from and mean? I didn't get it. James Tursa on 10 May 2020
I can't find the computeRotationVectorForAnglesCloseToPi( ) function. Where is this from?
1. The "(w,v) is a unit quaternion" statement is basically telling you that w is the scalar part of the quaternion and v is the vector part of the quaternion, so v / |v| is the rotation Eigen axis. I.e., the statement is generic in nature to tell you what v is in relation to a quaternion and there is no actual w in the code.
2. Without seeing the entire code, I cannot be sure what this is. What is t? How did r get calculated? If I were to hazard a guess, I might expect it to be a small angle limiting formula to handle the cases near 0.