3ds Max 2012 Bible - Kelly L. Murdock [495]
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.
FIGURE 47.7
The Sun & Sky system can be viewed and interactively updated in the viewport.
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. •
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 rendered 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 47.8 shows a house interior lighted using mental ray, Final Gather, and two Sky Portals.
FIGURE 47.8
Sky Portals can focus all rays coming from an external light source to speed up Final Gather passes.
Understanding Caustics and Photons
Light properties for the mental ray renderer include four unique properties: Energy, Decay, Caustic Photons, and Global Illumination (GI) Photons. You can find the settings for these properties in the mental ray Indirect Illumination rollout. Before learning about these properties, you need to understand what caustics and photons are.
Caustics are those strange glowing lines that you see at the bottom of an indoor swimming pool caused by the light refracting through the water. Caustics are common in nature, and now with mental ray you can add these effects to your scenes. Photons are small bundles of light energy, and like the raytracing rays, they are emitted from a light source with a given amount of energy. This energy is lost as the photon travels and as it hits objects in the scene.
The Energy value is the amount of light energy that each photon starts out with, and the Decay value specifies how quickly that energy dissipates. The number of Caustic and GI Photons determines the resulting accuracy of the lighting. More photons yield a better solution, but the greater number also increases the render time substantially. The Multiplier and color swatch lets you set the intensity and color of the caustics.
Figure 47.9 shows a swimming pool with caustics glowing on the side of the wall. The left image shows the pool scene with caustics disabled, and the right image shows them enabled.
To get caustics to work in your scene, you need to add a Raytrace, Flat Mirror, or Reflect/Refract map to the Reflection map channel for the material that you want to generate caustics.
FIGURE 47.9
This indoor swimming pool scene is rendered without caustics (left) and with caustics (right).
Tutorial: Using Caustic Photons to create a disco ball
When using the mental ray renderer, you can see the caustic photons as they are reflected around the room to help you determine the correct settings you need, but these photons themselves can be used to make a good disco ball effect.
Tip
A common way to get a good, crisp caustic effect is to