From the official site for FDS:
Fire Dynamics Simulator (FDS) is a large-eddy simulation (LES) code for low-speed flows, with an emphasis on smoke and heat transport from fires.
Smokeview (SMV) is a visualization program used to display the output of FDS and CFASTsimulations.
FDS+Evac is the evacuation simulation module for FDS.
These tools are open source and freely available.
One use of FDS is simulation of visibility through smoke. The most realistic way to visualize smoke is volume rendering, which the default post processor Smokeview can do.
EnSight volume rendering color and transparency are customizable by the user, so they cannot be relied upon to accurately represent the visibility through smoke. The attached document and python script present a method which sets the volume rendering opacity in EnSight to accurately represent visibility. With minor modification the script also applies to any data that has a light extinction coefficient or optical density variable.