If the option Use as Skylight Portal is on, the entity will be used as a portal geometry for skylight luminances. The option is 'Off' by default. Portals are useful for interior scenes for example, where sky light is entering by small openings which are difficult to sample efficiently. By marking explicitely the openings using portals, the engine can render difficult situations at a much higher quality in less time. For all Luminance: If On the portal will be used for all luminances with skylight in scene, no matter how this sky light was created (drawing luminance or graphic luminance). If Off the portal will be used for this objects luminance only. The option is enabled if option only if Use as Skylight Portal is On. The option is 'On' by default.
Portals are objects (surfaces) that control the flow of photons from the light source. More precisely: the spatial configuration of the photon's flow. The simplified algorithm is the following: if the photon has not passed through the portals, it is not considered in the calculation of illumination and the CPU time is not spent on it. Instead of it the next photon is launched (generally at random). This continues until the number of photons passing through the portals will not be equal to the target parameters of the light source. In our case it is a sky light (RedSDK supports only sky lights for the portals). In the case of absence of portals all launched photons are involved in the calculation of the illumination. So, portals allow you to concentrate the quality and the intensity of the light in the right places of the scene. This facilitates the creation of a scene with proper lighting and improves the speed of calculation. The main purpose of the portals is using them for the openings through which the outer sky light penetrates. Portals need not to be rectangular necessarily. The portal may consist of any set of surfaces. At the same time you should not make the portal very complicated (for example as an ADT window), because of the additional time is required to clarify whether the photon passes through a portal or not. Ideally it should be 1 rectangular facet. In TurboCAD any 3D primitive can be a portal. This simplifies creation of portals. It should be noted that in this case the geometry of this primitive is not loaded into the scene. In other words an option 'Use as skylight portal' is similar to the option "Load to render luminance only". Below is shown the effect of the portal's size on the lighting quality and intensity. By increasing the size of the portal 2 times the quality worsens 4 times (at the square). Below is shown how it is possible to highlight the places in the scene by using the portal's position. Below is shown how it is possible to highlight the places in the scene by turning the option For all luminance Off.