Here is some research I’ve done on the lighting: As the scene is such a large indoor scene, there is a problem with not using FG as there will be lots of lights providing as bounce lights. That in it self will eventually increase the render times. Therefore I’m still not sure if that is the way to go. I’ve found a new approach, were FG and portal lights are being used.
Portal lights is a way of lighting interior scenes when the main light source comes through a window. This setup generally creates problem when using FG as there is such a limited number of information to calculate the rays on. FG only calculates on the direct light so in a such case: the settings needs to both have a high FG ray count and a high FG point density, which will increase the render times. Even then, the result may still not be very convincing. So the solution is to use a shader called portal light. The portal light is an area light placed at the window, which is using the mia_physicalsky (which you’ll have to create) attributes to give the correct intensity and color from the outside sky.
What the portal light actually is doing is to block the FG rays, and converts the light from beyond the window to direct light. Which will give high quality area shadows with no interpolation issues. The portal ligh is concentrating the FG rays, so it doesn’t have to send lots of FG rays around the room. As the FG rays will see a well lit room rather than a black, it can be performed with lower FG rays. It will also give an extra bounce of light as the light from the window is direct. The portal light can also be used when using GI as it can shoot photons as well.
Area Light Shape –> Custom Shaders –> Light Shader –> mia_portal_light =for FG
Area Light Shape –> Custom Shaders –> Photon Emitter –> mia_portal_light =for GI
Another Problem I’ve been struggling with is using the light fog in mental ray, apparently mental ray doesn’t work very well with light fog. When they are rendered in mental ray the light fog just goes through the wall. So the only solution I’ve found is to render the light fog as a separate layer in maya software.
Below is a fast light test of the environment when I was using no fg with my own bounce lights, and the pictures with the purple ball is showing lighting with portal lights and with no portal lights. The last one is a picture showing the environment with a final gather test. With the portal lights you can achieve good results with fg even though the environment is dark.