I still don't understand what direction "tangent" is?
There are two dimensions on a surface so a tangent emitter has to pick a single direction lying in the tangent plane for each emitting face. But I can't work out what that direction is.
I tried looking at the face normal axes to see if that made sense, but even for the basic cube with tangent emission some faces emit in the direction of the face y-axis and others emit along the negative face y-axis.
