Use Background

You can create custom shadow and reflection passes with the useBackground material which catches shadow and, or reflections.

When the shadow pass is rendered, a black image is created with mask channel that contains the shadow information. A compositor can work with this channel to blur, lighten, darken, and so on, the look of the shadows. When the reflection pass is rendered, an RGB image is created with a white mask in the mask channel.

For more information about the Use Background material as a catcher, see Shadow catching in the Lighting guide.

Note

Shadow color has no effect on surfaces assigned to a useBackground shader.
When raytracing refractions through a useBackground shader, artifacts may appear on the useBackground shaded surface. To workaround, set the “refractions” attribute of the refracted object’s shader to off.

Use Background Attributes

Specular Color: Defines the material’s specular color. If you change this color or assign a texture to it, the reflections in the scene display your changes. For instance, if you map a checker texture to the Specular Color, the reflection appears in a checkered pattern. Drag the slider to set the Specular Color to white if you want to see very clear reflections in the rendered result. The default is gray.
Reflectivity: Determines how much of the object’s texture reflects on the Use Background material. For instance, if the object is very shiny, increase this value. The default is 0.5.
Reflection Limit: Determines the reflection’s distance. The default is 2.
Shadow Mask: Determines the density of the material’s shadow mask. If you change this value, the shadow masks becomes darker or lighter, allowing you to better verify the shadows and reflections in the scene when viewing the mask channels in Render View.

The default value is 1.0. If set to 0.0, the shadow does not display.

Matte Opacity

For best results when adjusting these attributes, select the Display Alpha Channel icon in the Render View to view the mask channels when re-rendering the scene.

Matte Opacity Mode

Select one of the following options:

Opacity Gain: Displays both reflections and shadows.
Solid Matte: Adjusts the density of the Use Background material.

Matte Opacity: Use the slider or enter a value to adjust the density of the mask channels.