How to obtain the Alpha channel of the virtual object in ARkit, similar to the character mask effect.

How do the virtual objects added in ARkit obtain their alpha channel for image processing, for example, when I use the painting transfer function, I want to process the virtual object part separately, because each virtual object has a collision plane, so I obtained before All channels are channels with this collision plane, but this is wrong. Is there any good way to get the Alpha channel of the virtual object? Thanks!

How to obtain the Alpha channel of the virtual object in ARkit, similar to the character mask effect.
 
 
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