Traditionally, to simulate the illumination that comes
through the window, area lights are placed in the window. This method is
enhanced by using portal light which obtains its intensity and color from the
environment outside the window.
The portal light helps in a great deal reducing Final Gathering's noise and
since it's a light source it can cast shadow.
Here's a scene with a
room. I have set up an area lights for each window and a portal light is
connected to each light.
This is the spherical environment texture used for IBL as a
primary environment for Mental ray in HDR format. It's a very high resolution
texture. ( 6250x3125)
Using the default setting, this is the initial render
result.
