A typical problem with architectural scenes is that environment light can sometimes only enter through few small openings, such as small windows. In cases like this the convergence can often be improved by giving the renderer a hint where these openings are in the scene.
For Iray Photoreal this can be done by placing light geometry (e.g. rectangle lights) over the openings and marking them as light portals using a attribute on the mi::neuraylib::IAttribute_set class:
- bool light_portal = false
If set to true, the light portal attribute flags a light to be used as a light portal.
Technically, these lights are no longer light sources (they are invisible and do not need to have a light shader or emitting material), but merely a hint to Iray Photoreal where to expect environment light from.
Using light portals should not change the rendering result, only convergence speed. However, it should be stated that they do not always help, in fact they can cause lower performance if placed incorrectly. For an architectural scene a rule of thumb for using portals is:
A significant amount of illumination comes through the windows.
The illumination coming through the windows is not dominated by direct sun light or other very bright regions in the environment.
The windows are relatively small.
The caustic sampler is enabled.