Thanks to Raphael for pointing out that rlux is different from rillum -- I was mistaken in thinking them the same. I really need to think more carefully before I type, and especially before I press "send!" I realized after I'd sent that message that I had made another mistake, as the following is the correct way to execute rad to create an options file without running any renderings:
% rad -n -s scene.rif OPTFILE=render.opt
Without the -n option (as I had put in the last message), rad will run whatever renderings are indicated in the scene.rif file, which was not the point. The -s option simply prevents it from echoing the commands it _would_ have executed, which you probably don't want to see...
There are a number of other fun ways to execute rad. The following command will fill in ambient values for all the views in the scene.rif file at once (provided the AMBFILE variable is set):
% rad -v 0 scene.rif OPTFILE=render.opt
% rad -n -s -V scene.rif | rpict @render.opt -S 1 -x 64 -y 64 scene.oct > /dev/null
The -v 0 option causes rad to generate the octree without rendering any views. The -V option causes rad to print out all the views for this scene in the standard "VIEW=" specification that rpict accepts for animated sequences. These views are then rendered by rpict at very low resolution and discarded, filling in the ambient file as a side-effect.
I used a trick like this in the "compamb" script to compute a good -av setting for a particular scene. (See the man page for compamb, which comes with 3.4.)
I hope the above is correct, because I'm pressing "send," now!