Helge Maus Covers Creating and Working With Cryptomatte in Side FX Houdini.
Cryptomatte is a tool created by Jonah Friedman and Andy Jones at Psyop that can automatically generate ID mattes. It has support for things like motion blur, transparency, and depth of field. It does this with information that is already available at render time, using scene organizational information like names, object namespaces, and material names.
Maus shows how you can set up ID Mattes with Crytomatte along with the Mantra renderer in Houdini, ultimately creating OpenEXR files. Also covered is building custom properties and how you can use the Houdini COP context and the new Crypto-node to pull mattes after the rendering has finished automatically.