How do I create object matte passes using Arnold in Cinema 4D?

Creating matte passes for individual objects or a specific group of objects is incredibly easy when using Arnold in Cinema 4D. Quite honestly, because of the nature of mattes and masks in After Effects, I tend to try and keep things simple. If I don’t need to have nested pre-comps within other pre-comps, I try and make do without them. Sometime I just want to simply drag a sequence into my timeline and immediately start using it to matte different layers and effects. Until After Effects gets some way to utilize Cryptomatte, this is the best way for me to generate flawless mattes.

Creating a custom AOV mask is as simple as adding an Arnold Object Mask tag to an object. What’s great is that you can add an Arnold Object mask tag to multiple objects and simply give them all the same AOV name if you want them to be included in the same mask. You can even add multiple AOV tags to a single object, so you can render a mask of a single object as well as grouped with multiple other objects. It’s very flexible and any object with a tag automatically renders pure white, while everything else renders black. You can even embed an alpha of the object/s if you wanted to, and objects not tag will cut out the tagged object if they occlude it. Here how you use Arnold Object Mask tags:

1. Select the object/s that you want a mask for and click Create/Tag/C4DtoA/Arnold Object Mask or right-click on the object and select the tag.
2. In the Arnold Object Mask Dialog, give the new AOV a unique name. You can always add more objects later by simply adding the Arnold Object Mask tag to them and using the same name.
3. Now assuming you already know how to set up standard AOVs, Click the ‘Setup AOVs…’ button either in the display driver settings or render settings. You’ll now see your (custom) mask listed in ‘Inactive AOVs’. Simply move it to Active AOVs and it will now be added to the list of outputs.
4. All you need to do now, is change the Data type to either RGB or RGBA (if you want an alpha of the object/s) and leave the filter mode set at ‘Use driver’s filter’. Set the render path, and when you hit render you’ll get perfectly separated greyscale mattes that work flawlessly in After Effects.

As this is a frequently asked question we have provided both the question and answer in this thread.