Question

I am working with some code which outputs a 3×3 rotation matrix and a translation vector representing a cameras orientation and location.

However, the documentation states that to get the location of the camera one must multiply the transposed and inverted rotation matrix by the translation vector. Does that mean that the original vector is not the location of the camera? If not, what does that original vector represent?

Answer 1

I’m assuming that the R (rotation matrix) and t (the translation vector) you obtained were w.r.t a world coordinate system with (0,0,0) as the origin.

With R and t you can now move a point from the world coordinate system (WC) to the camera coordinate system (CC), i.e. X_c = RX + t where X is a 3D point in WC and X_c is X in CC (i.e. seen from the camera’s point of view). This is applicable assuming we’re dealing with rigid bodies so we just rotate the point and then translate it.

Now, you need to find the coordinates of the camera center which is the origin of CC, or when X_c = 0:

0 = RC + t where C is the 3D coordinates of the camera center in WC. By solving for C we get,

C = -R^-1t

And by the way,

A correction in your documentation

Transposing and multiplying the rotation matrix does not change the rotation matrix — a rotation matrix is orthogonal which means it’s transpose is equal to its inverse and therefore, (R^T)^-1 = R.