mr Sky Portal

The mr (mental ray) Sky Portal object provides an efficient method of “gathering” existing sky lighting in interior scenes without requiring high final gather or global illumination settings that would result in excessively long render times. In effect, a portal acts as an area light that derives its brightness and coloring from the environment.

Attention: For mr Sky Portal to work correctly, the scene must contain a Skylight component. This can be an IES Sky light, an mr Sky light, or a Skylight.

Procedures

Example: To use the mr Sky Portal object:

  1. Make sure mental ray is the active production renderer.
  2. Create a scene with a windowed interior. Set up a camera in the interior and set a viewport to show the camera view.
  3. Add a Daylight system to the scene:
    1. Create menu Systems Daylight System. When prompted to use the mr Photographic Exposure Control, click Yes.
    2. On the mr Photographic Exposure Control rollout, set Preset to Physically Based Lighting, Indoor Daylight.

      This changes the exposure value to 10.0.

    3. Change the Sunlight object to mr Sun and the Skylight object to mr Sky. (see mental ray Sun & Sky)

    For best results, position the sun so it’s not shining directly into the interior, or turn it off. Otherwise, the direct lighting could overwhelm the indirect lighting from the portal, especially when you use final gathering or global illumination.

  4. For each window, add an mr Skylight Portal object. The portal object takes the form of a wireframe rectangle with a central, perpendicular arrow showing the direction of light flow, or flux. Make each portal slightly larger than its respective opening, and position it immediately outside or inside the opening.
    Tip: To help place the portal object as close as possible to the outside surface, use AutoGrid when you add the portal. Also, make sure portals do not overlap; this would cause the illumination from the overlapping area to be doubled.

    Adding the Sky Portal object with AutoGrid on. Note the Light Flux Direction arrow pointing outward.

  5. Make sure all the portals’ arrows are pointing inside. If a portal’s arrow points outside, toggle its Flip Light Flux Direction checkbox on the mr Skylight Portal Parameters rollout.

    With Flip Light Flux Direction on, the arrow points inward.

  6. Turn on Final Gather and render the scene. If the image looks grainy, increase the Shadow Samples setting on the mr Skylight Portal Parameters rollout.

    Scene lit by mr Sky Portal with final gather at Draft preset, no diffuse bounces

Interface

mr Skylight Portal Parameters rollout

On
Toggles the illumination from the portal. When off, the portal has no effect on scene lighting.
Multiplier
Amplifies the power of the light. For example, if you set the value to 2.0, the light will be twice as bright.
Filter Color
Tints the coloring coming in from the outside.
Shadows group
On
Toggles shadow casting by the light from the portal.

By default, the portal casts shadows only from objects inside the portal; that is, on the arrow side.

From “Outdoors”
When on, casts shadows from objects outside the portal; that is, on the side away from the arrow icon. This is off by default, because turning it on can significantly increase render times.
Shadow Samples
The overall quality of shadows cast by the portal. If the rendered image is grainy, increase this value.
Dimensions group
Length and Width
Set the Length and Width using these spinners.
Tip: To change the arrow size, use the Preferences Viewports panel Viewport Parameters group Non-scaling object size setting.
Flip Light Flux Direction

Determines the direction in which light flows through the portal. The arrow must point toward the interior for the portal to cast light from the sky or environment. If it points outside, toggle this setting.

Advanced Parameters rollout

Visible to Renderer
When on, the mr Sky Portal Object appears in the rendered image. Turn this on to prevent outside objects from appearing in the window.
Transparency
Filters the view outside the window. Changing this color doesn’t change the light coming in, but has the effect of darkening outside objects, which can help if they’re overexposed. To avoid recoloring the outside view, use a shade of gray, such as R=G=B=0.5.
Color Source group

Sets the source of the light from which the mr Sky Portal derives its illumination.

  • Use existing Skylight Uses the skylight. By default, with the mr Sky light using the mr Physical Sky environment map at their default values, this tends to give a bluish illumination, as with real-world skylight.
  • Use Scene Environment Uses the environment map for illumination color. Use this if your sky light and environment map are different colors, and you wish to use the latter for the interior illumination.
  • Custom Lets you use any map for the illumination coloring. Choose Custom, and then click the button (“None”) to open the Material Map Browser. Choose a map and click OK.
Parent topic: mental ray Sun & Sky