Light Linking and Shadow Linking

You can choose which objects are illuminated by a light, and which objects cast shadows from a light. For example, you can assign individual lights to different characters. The light mask on an object, and the shadow mask on a light control the association of lights and objects.

You can link lights and shadows interactively using the Light Linker pane, individually for each object and light directly in the parameter editor, or by specifying categories.

Linking Lights and Shadows with the Light Linker

To use the light linker, you must create a Light Linker tab. You can do this by clicking the new tab icon in the network editor and selecting New Pane Tab Type > Light Linker. You can also change an existing pane to a light linker pane by clicking RMB on an existing pane and selecting Light Linker.

To...	Do this
Illuminate objects	Choose Light Mask from the Link Type drop-down menu. Choose a mode from the Mode drop-down menu. Select the lights from the Light Sources pane. Select the objects to light from the Lit Objects pane.
Cast shadows from objects	Choose Shadow Mask from the Link Type drop-down menu. Choose a mode from the Mode drop-down menu. Select the lights from the Light Sources pane. Select the objects which will cast shadows from the Shadow Casters pane.
Reflect objects in other objects	Choose Reflection Mask from the Link Type drop-down menu. Choose a mode from the Mode drop-down menu. Select the objects that are reflected from the Reflection Sources pane. Select the objects to reflect from the Reflector Objects pane.

Linking Lights and Shadows Directly

To...	Do this
Illuminate objects	On the Render tab of an object, set the Light Mask on the Shading sub-tab in the parameter editor to determine which lights will illuminate your object.
Cast shadows from objects	On the Shadows tab of a light, choose the Shadow Type from the drop-down menu. The default is No Shadows. Set the Shadow Mask to determine which objects will cast shadows from your light.
Reflect objects in other objects	On the Render tab of an object, set the Reflection Mask on the Shading tab in the parameter editor to determine which objects will be reflected by your object.

Linking Lights and Shadows using Categories

Overview

You can specify categories on an object using the parameter editor (Render tab, Shading sub-tab, Categories parameter). Objects can belong to more than one category – separate multiple categories with commas and/or spaces. Categories are similar to tags in Web applications; you can “tag” lights and objects, and use the unions and intersections of tags to control light linking.

For example, a light might have Categories set to keylight, shadower, distant. This makes the light part of the keylight set, the shadower set, and the distant set. You can then use these categories.

Note

Categories will probably supplant masks as the preferred method for light/shadow/reflection linking in a future version of Houdini, but support for categories in the UI is currently limited.

You must manually add the Houdini 9.1/Light selection, Houdini 9.1/Shadow selection, and/or Houdini 9.1/Reflection selection properties to an object to use categories for linking.

You can also use categories as an optional argument in an illuminance loop in VEX to loop over the lights based on categories.

Specifying objects by categories

Category expressions are one or more category names. When specifying multiple name, join them with the & (and) or | (or) symbols. For example, keylight|distant would specify all objects which have either the keylight or distant categories, while keylight|distant would specify only objects that have both keylight and distant categories.

Category expressions currently do not support grouping with parentheses. Category expressions are processed left to right with & (and) at a higher precedence than | (or). So:

a & b & c | d & e | f | g & h

…is equivalent to:

(a & b & c) | (d & e) | f | (g & h)

Symbol	Meaning
`name`	Matches objects tagged with the `name` category.
`-name`	Matches objects which are not tagged with the `name` category.
`*`	Matches all objects.
`-*`	Matches no objects.
`+`	Matches objects which have any categories defined.
`-`	Matches objects which don’t have any categories defined.
`a\|b`	Matches objects which match `a` or `b`.
`a&b`	Matches objects which match `a` and `b`.