To accelerate rendering, FumeFX uses an illumination map, which is a per-voxel, pre-computed light map that includes shadows. It is created at rendering time, before any actual rendering takes place.
Caching of illumination maps to disk is an option that will accelerate subsequent renderings. Cache files are named just like simulation output files, but with the extension .fim. You invalidate a cache by deleting it or starting the simulation again. In the latter case, only the frames that have been simulated will be invalidated.
Illumination Map - If you turn this checkbox off, the rendering process will be much slower. It is provided for the rare cases in which you need extreme accuracy of the shadows received by FumeFX. It does not influence shadows cast by FumeFX on other objects.
Multiplier - This is a convenient way to affect all lights at once. A value of 0 creates no illumination, 2 creates twice the illumination, etc.
Undresample/Threshold - To speed up the illumination map calculation, you can instruct FumeFX to evaluate every 2nd voxel over each axis. This means it would only have to process 1/8 of the total number of voxels. Other voxels will only be evaluated if the difference in neighboring voxels??? illumination is greater than the Threshold parameter value. This will not reduce the illumination map file size and will have no effect if the illumination map is loaded from disk. So long as you do not notice rendering artifacts, you can set this value as high as you like.
Read/Write to Disk - During rendering, the illumination map is saved to disk as illumination is pre-computed for each frame. The saved map is unaffected by the Multiplier parameter. While this option is selected, the map cache is automatically used for subsequent renderings, regardless of any changes to light settings. The only exception to this is that the cache map will not be used if the grid dimensions have been changed.
This data will be saved to the folder you have selected for output and will overwrite any existing file of the same name
Clear Cache - Select this to delete all cache files with names corresponding to current output file names.