Table Of Contents
Intro
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| Scene with all lights affecting all objects | Same scene demonstrating light linking | Same scene demonstrating shadow linking |
By default every light in a scene illuminates and casts a shadow of every object in a scene. However there are times when you want specific lights to only affect certain objects. Even further than that you might want a certain light to illuminate an object without casting a shadow.
Light linking lets you control the affect a light has on your scene objects on a per-object basis.
Shadow linking allows you to break the casting of shadows from lights on a per-object basis.
Each of these methods is covered in detail below.
Light Linking
The example scene below has a simple lighting setup with only 3 lights: a visible red, green, and blue light that are currently affecting all the objects in the scene.
Example scene with all lights affecting all objects
This all white scene is helpful in interpreting what is happening further down the road when we start introducing some light linking into the scene. As you can see in the image above every object is white due to the red, green, and blue light mixing together. The slight color shifts here and there are simply due to the slightly different light positions. It is important to remember that in all the examples below all 3 lights are always linked to the ground object.
In Houdini you can easily link lights to objects by using the "Objects" section of your Redshift light settings and then enable "Enable Light-Mesh Associations" as pictured below:
Houdini "Objects" section for light linking
- Light Enabled For: This field controls which objects are lit by the light. Any objects excluded from this list will not recieve illumination by the light.
- Shadows Enabled For: This field controls which objects cast shadows by the light. Any objects excluded from this list will not cast shadows by the light. For more information on shadow linking please refer to this section.
By default all lights use the * which means the lights are linked with every object in a scene. To customize and light linking on a per-object basis you can either write in the name of the objects in your scene or click on the object picker button 
situation next to the field. This brings up a list of all the objects in the scene and any object highlighted in orange will be included in the light or shadow linking as pictured below:
Example image showing 3 objects being included for light linking
The example scene has now been linked to the 3 different lights as pictured below, please note that for the purposes of these examples we are using the same object linking for lights and shadows:
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| Red light linking setup |
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| Green light linking setup |
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| Blue light linking setup |
Example scene with the light linking setup above
As you can see above, the rocks have been split up into thirds each with their own light exclusively affecting each third. The ground object remains affected by all 3 lights which is why the shadow color of each rock section is the additive result of the remaining lights still casting in this area. For example, the red rocks at the top of the image are receiving light and casting shadows from the red light alone, however, the blue and green lights are still casting light in this shadowed region resulting in cyan colored shadows.
For another example, the inner circle of rocks and outer circle of rocks will be linked to completely separate lights to demonstrate another potential side effect of light linking. The inner circle of rocks is linked to the red light and the outer circle is linked to the green light while the blue light excludes all the rocks as pictured below:
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| Red light linking setup |
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| Green light linking setup |
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| Blue light linking setup |
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Example scene with the light linking setup above
Now with this light linking setup you can see we are getting double shadows and surreal lighting with these rocks. This is due to the fact that the green light is completely ignoring the inner circle of rocks and only illuminating and casting shadows of the outer circle of rocks. You can see that the green light hitting the outer rocks is even bouncing onto the backside of the inner rocks due to global illumination. The double shadows occur when the shadows from the inner red rocks and the shadows cast by the green light coincide with one another, resulting in the darker blue shadow seen in the picture above.
Shadow Linking
Shadow linking takes light linking a step further by being able to decouple light links from shadow links, allowing for even more custom control over your scenes. Shadow links are controlled by the "Shadows Enabled For" option discussed in the light linking section here. Since you can have different objects listed in the "Shadows Enabled For" section compared to the "Light Enabled For" section so you can get some very interesting and custom results.
In the example scene below the shadow links have been broken for only the light that provides illumination for that group of rocks. Therefore the light and shadow linking for each group of rocks is as follows:
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| Red light linking setup |
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| Green light linking setup |
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| Blue light linking setup |
Example scene using shadow linking mode "Use Shadow Links"
Since the primary light source for these rocks has had its shadow link broken the shadow color result for these objects is the same color as its diffuse lighting.
For example, the top group of rocks is being lit just by the red light. However the red light has had its shadow link broken with the red light and since we are using the "Use Shadow Links" Shadow Linking Mode then any light linking is completely ignored when it comes to shadow contribution, this means that both the green and blue lights are still casting shadows for these rocks even though they are not illuminating them. Since the shadow color of a green light is magenta and the shadow color of a blue light is yellow (as scene in the baseline example) they are added together, resulting in the red shadow above.