In rgl, you can set up camera direction with rgl.viewpoint. It accepts theta, phi: polar coordinates. They specify the position of the camera looking at the origin. However, there is yet another one degree of freedom: angle of rotation of the camera along "eye" vector. I.e. one can imagine two vectors associated with camera: "eye" vector and "up" vector; theta and phi allow one to adjust "eye" vector, but I want then to adjust "up" vector after it. Is it possible to do it?
I guess that it can be possible to do it with userMatrix parameter («4x4 matrix specifying user point of view»), but I found no information how to use it.
The ?par3d help topic documents the rendering process in the "Rendering" section. It's often tricky to accomplish what you're asking for, but in this case it's not too hard:
par3d(userMatrix = rotationMatrix(20*pi/180, 0,0,1)
%*% par3d("userMatrix"))
will rotate by 20 degrees around the user's z-axis, i.e. line of sight.