Is it possible to query/calculate (irradiance)gradient vectors through any of the Radiance programs?
If you generate an ambient file (through the -af option) while running rpict or rtrace, then you can query the position and directional info about the gradients through lookamb. There is already a tool that creates visualization-friendly Radiance definitions for the ambient data (description). You might have to download this from the repository and call it through a compatible perl installation if you are on Windows.
I dont think irradiance gradients are relevant for pure monte-carlo simulations performed with rcontrib/rfluxmtx as there is no ambient cache.
There is a newer version of genambpos in the distribution that handles the more recent format and can show both direction and rotation gradients along with (elliplical) regions of influence and corral test directions. (See presentation slides 15-31 for details.)
Thanks for the answers.