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How would I go about extracting the angle from a 2x2 rotational matrix? I'm using a matrix to track transformations in 2D space, but I'm struggling to figure out how to reverse this once I've got the rotation matrix so I can just see the angle that was applied.

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If it's a 2D rotation matrix, then it equals $$R(\theta)=\begin{pmatrix} \cos\theta & -\sin\theta\\ \sin\theta & \cos \theta \end{pmatrix}$$ where $\theta$ is the angle you are looking for. Therefore, you can simply take $\cos^{-1}$ of the first entry in your matrix.

Due to the periodicity of the cosine function though, you won't know the sign of $\theta$ (i.e., whether it is clockwise or anticlockwise). You can determine this by noting the signs of the sines (e.g. if the angle is $-30^\circ$, then the $\sin$ entry in the first column would be negative).

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  • $\begingroup$ Note that that's not the complete answer. You still need to distinguish between clockwise and anticlockwise. You can do that by looking at the sign of the off-diagonal entries. $\endgroup$ – Arthur Sep 9 at 16:11
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    $\begingroup$ I would use the atan2 function on the entries with $\cos$ and $\sin$, otherwise you can get the direction wrong: $$\cos\theta=\cos(-\theta)$$ $\endgroup$ – Andrei Sep 9 at 16:11
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    $\begingroup$ You're right, let me edit my answer. $\endgroup$ – Luke Collins Sep 9 at 16:12
  • $\begingroup$ This looks like I what I'm looking for, thanks. What does @Andrei mean by the entries with cos and sin for atan2? There are two of each in the matrix. $\endgroup$ – Incredidave Sep 9 at 16:17
  • $\begingroup$ The definition of tangent is $\tan\theta=\frac{\sin\theta}{\cos\theta}$. The atan2 function takes two inputs, the cosine and the sine. So, in this case, use the two values in the first column. $\endgroup$ – Andrei Sep 9 at 16:21

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