Page 1180 - Kitab3DsMax

Part XI: Advanced Lighting and Rendering
Using mental ray Sky Portal
If you look closely at the Photometric light category, you’ll notice another light type sandwiched between
the mr Sky and mr Sun lights. The mr Sky Portal provides a way to focus light streaming into an interior
space from an external source using a designated portal. This can be thought of as a sky light that causes all
Final Gather light rays to be focused on a specific area. It results in a better Final Gather result with fewer
light rays. The Sky Portal is direct light giving you one bounce for free if Final Gather is disabled.
To use a Sky Portal, you must have mental ray enabled, include a skylight or a Sun & Sky system, and have
Final Gather turned on. Then simply drag over the area where the window is located, and all light rays
entering the interior space are focused on those areas defined by the Sky Portal. The gizmo for the Sky
Portal is a simple rectangular box with an arrow pointing in the direction of the light rays. If the light rays
are pointing outward, you can use the Flip Light Flux Direction option to change their direction.
Tip
For better results for interior scenes using exterior lighting, enable the mr Photographic Exposure Control and use
the Physically Based Lighting, Indoor Daylight preset. n
Within the mr Skylight Portal Parameters rollout, you can enable or disable the Sky Portal. A Multiplier
value and a Filter Color work just like other lights. If the image appears grainy, you can increase the
Shadow Samples value to remove any splotchy effect on the walls, but this increases the render time.
Within the Advanced Parameters rollout, the Visible to Renderer makes the Sky Portal visible in the ren-
dered image, causing exterior objects to be blotted out.
You also can set the Color Source to Use the Scene Environment if you want light to be pulled from the
environment map. Sky Portals also support HDRI maps as lighting sources. Figure 47.5 shows a house
interior lighted using mental ray, Final Gather, and two Sky Portals.

FIGURE 47.5
Sky Portals can focus all rays coming from an external light source to speed up Final Gather passes.

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