This page gives information on the Cryptomatte Render Element.
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The parameters for this render element appear in the Attribute Editor under Extra VRay Attributes.
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This example shows how to set the V-Ray user attribute ID type in the Cryptomatte render element options. The V-Ray user attribute ID type is useful to create masks for objects based on a specific user attribute name.
In the example, User Attributes are assigned to the wrenches in the scene. Each wrench has an attribute called wrench with a numeric value that was originally used to create variation for how glossy the reflections would be.
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Based on the attribute name, a Cryptomatte layer can be created to mask only the objects that have an attribute with the same name.
This is done by setting the ID Type to V-Ray User Attribute in the Cryptomatte render element. The User attribute name is set to wrench to match the user attribute names on our objects.
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The below images show the resulting render and the Cryptomatte preview with masks for only the objects that have the wrench attribute.
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After producing a multichannel exr image with the Cryptomatte layer, we can use it in a compositing application, such as Nuke, to mask out and color correct the objects in our image.
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