As of Arnold 5.3, this flag is set automatically by changing the opacity or transmission on a material.

Determines whether the object is considered opaque (transparent). By default, Arnold built-in shaders set the opaque flag automatically, based on whether or not the shader settings would require disabling the opaque flag on the object to render correctly. For example, it's no longer necessary to manually disable the opaque flag to get transparent shadows for a glass shader.

Exceptions are curves and points when min_pixel_width is in use, and OSL shaders.  


This attribute, when enabled, turns an object or group into a holdout matte. It affects primary rays only. The matte color is always black, and the opacity of the object is preserved. Shaders will not run on the object anymore and it will output all-black (including the alpha), except if the opaque parameter is off, in which case shaders will be run just to compute the opacity. Note that even AOVs output by its shaders, in this case, will be black.


Trace sets this object is a member of.  Shaders will in some cases trace against specific trace sets (either including or excluding them).


When this object has subsurface scattering, if multiple objects share the same set name, they will bleed sub-surface scattering across their boundaries rather than only bleed within each single object.


For polymesh/subdmesh objects, this controls which types of rays can see the back sides of polygons in the object.


This controls which types of rays can intersect the object.


This controls when autobump is calculated for certain types of rays. Because autobump can be expensive to compute, often times it is advantageous to not bother with it for many types of indirect rays because the high frequency displacement detail will not contribute anything to an image when encountered indirectly.  It matters much more for camera rays, in general.


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