Порылся в документации к BMRT и нашёл прямой ответ. Стыдитесь.
> А bmrt при просчете glob illum умеет сохранять фотонные ( или
> что там у него карты)?
file:///D:/bmrt/doc/html/node4.html#sec:indirectoptions
Option "indirect" "string savefile" ["indirect.dat"]
If you specify this option, when rendering is done the contents of the irradiance data cache will be written out to disk in a file with the name you specify. This is useful mainly if the next time you render the scene, you use the following option:
Option "indirect" "string seedfile" ["indirect.dat"]
This option causes the irradiance data cache to start out with all the irradiance data in the file specified. Without this, it starts with nothing and must sample for all values it needs. If you read a data file to start with, it will still sample for points that aren't sufficiently close or have too much error. But it can greatly save computation by using the samples that were computed and saved from the prior run.
> Тоесть, например елси у меня есть
> шарик с источником света и я хочу вокруг него полетать, будет
> ли он пересчитывать освещение для каждого кадра?
И тут есть прямой ответ в документации:
file:///D:/bmrt/doc/html/node4.html#SECTION00440000000000000000
You shouldn't use the "seedfile" option if the objects have moved around. But if the objects are static, either because you have only moved the camera, or because you are rerendering the same frame, the combination of "seedfile" and "savefile" can tremendously speed up computation.
Here's another way they can be used. Say you can't afford to set the other quality options as nice as you would like, because it would take too long to render each frame. So you could render the environment from several typical viewpoints, with only the static objects, and save all the results to a single shared seed file. Then for main frames, always read this seed file (but don't save!) and most of the sampling is already done for you, though it will redo sampling on the objects that move around. Does this make sense?