Use Cryptomatte to extracts mattes out of Cryptomatte data passes.
To access the Cryptomatte, use:
Batch, then select a node from the Node bin.
Timeline, then use Batch FX (see Creating Batch FX).
The Modular Keyer, then select the node from the Node bin.
This node has seven inputs: one Front and three RGB-A pairs for cryptomatte data passes. All seven nodes must be connected or data will be missing and will prevent you from picking some objects.
To add an objet to an output:
The mouse changes to a picker.
This adds the object to the Layers list. Click Active to display an overlay that highlights the picked object.
The Layers list displays the contents that will be part of one of the Matte output on one of the four Matte outputs.
In the Objects list, you can: