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I need to convert coordinates and rotations from left-handed coordinate system (used by Unity) to right-handed (used by camera calib. toolbox in MATLAB\Octave)

enter image description here enter image description here

While converting point coordinates may be easy, I cannot say the same for rotations. I need rotations in form of 3x3 rotation matrices, not quaternions.

For each object I have

$ t_{left} = \begin{bmatrix} x \\ y \\ z \end{bmatrix} $ and $ R_{left} = \begin{bmatrix} r11 & r12 & r13 \\ r21 & r22 & r23 \\ r31 & r32 & r33 \end{bmatrix} $

defined in coordinate frame from the left image. Which transformation should I apply to get $ t_{right} $ and $ R_{right} $ in the right coordinate frame?

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  • $\begingroup$ Would it help to say that the transformation is given by reflecting across $y=z$? $\endgroup$ Commented Oct 20, 2018 at 9:25
  • $\begingroup$ @TobyMak, as far as I suppose, this would help for point coordinates, but not for rotations. Correct me please if I'm not true $\endgroup$ Commented Oct 20, 2018 at 9:27
  • $\begingroup$ The way you've labed your axis makes it look like they are both right handed systems and they treat z as up instead of y in one. You'll need to label axis correctly with respect to sign if you want proper help. $\endgroup$ Commented Apr 19, 2019 at 21:03
  • $\begingroup$ @opa, they are labelled absolutely correctly, they come from different software. What I need is a conversion from system from the left to the system on the right. $\endgroup$ Commented Apr 20, 2019 at 5:57
  • $\begingroup$ @Simon Well they are both the same handed-ness as they are labeled now, so either the labels are wrong with respect to your question or you don't need handed-ness conversions. $\endgroup$ Commented Apr 20, 2019 at 18:56

2 Answers 2

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Flip $y\leftrightarrow z$, apply the original rotation, flip $y\leftrightarrow z$ again. So $$\begin{pmatrix}r11&r12&r13\\r31&r33&r32\\r21&r23&r22\end{pmatrix} $$

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  • $\begingroup$ Unfortunately this does not work for me... This is what I have as a source: i.sstatic.net/fh2bO.png and this is what I get as a result of this conversion: i.sstatic.net/G9vGw.png The objects are rotated in a strange way. Maybe I've implemented something incorrectly? Could you please explain your answer a bit more?.. $\endgroup$ Commented Oct 25, 2018 at 16:01
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Finally I managed to solve this by saving all data in Euler angles in Unity and converting it to Matlab/Octave coordinate system.

$R_X = rotX(-a_x);$

$R_Y = rotY(-a_y);$

$R_Z = rotZ(-a_z);$

$R = R_Z*R_X*R_Y;$

$coords = [x; z; y];$

The code is here: https://gist.github.com/tushev/b5c162c059b5e1c0011809ae0e871015

You will need Camera Calibration Toolbox by Jean-Yves Bouguet to get it working.

Thanks to everyone for help!

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