I am playing with the billboards again now the opacity problem is gone (since 1.30). I am using them to display simple clouds and they now look lovely. However, they rotate along with the camera roll, which makes sense for Cesium but does not look too good when it comes to clouds viewed from a plane.
My question is then, is there a cheap way to fix their up direction to the world up direction. Stated otherwise: I would like the billboard to keep facing the camera but not rotate around their Z axis (vector going from the billboard to the camera)