Imagine I have three overlapping objects.

I would like to have an extra image plane for each that displays an attribute of the object without holdouts (i.e. the other objects are invisible).

Additionally, I would like an extra image plane that *adds* the values of an attribute (could be Cd, or whatever, doesn't matter) of the overlapping objects (instead of *over*-ing the samples, as it normally does). So if each object had their attribute at 1.0, the final pixel value where all three overlap would be 3.0.

Is this possible? I could swear I've done this before but I can't for the life of me remember how.

You can't have different opacity rules for different image planes, the image planes are simply information that was already available to record. If an object is obscured in the beauty pass, then no rays are hitting the surface, and there is nothing to record to the alternate image planes. Create a separate mantra rop for each set of visibility rules you need.

For your first request, you need to create a rop for each object that has primary visibility, and specify the remaining objects as phantom (if you want shadows/reflections.)

To create an additive pass, where every object has no opacity, export the color value to Ce, and export zero to Of.

Thank-you for the info!