mjbWorld

Hi Martin,

I was looking at the matrixToQuaternion page, and I think the "Better code" and "C++ code" sections may have a problem with numerical stability as Tr + 1 may approach 0 in some cases. The test for greater-than-epsilon helps, but a better solution may be found in the implementation from the Wild Magic package by Dave Eberly:
http://www.geometrictools.com/LibFoundation/Mathematics/Wm4Quaternion.inl
(See "FromRotationMatrix" function)

There, they compare Tr > 0 (instead of Tr+1 > 0) for the initial conditional, thus avoiding getting close to 0 in the 0.5 / sqrt(Tr + 1) which follows it.

Do you have any thoughts, does this make sense to you? The implications of falling into the 'else' clause are the main concern here, but that seems safe, and appears to work in practice.

Thanks!