Karma Shadow Matte

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I've also been struggling with this. We oftentimes want to render shadows completely separate (separate EXR), with a shadow catcher on relevant geometry to give comp the ability to mix the shadows / adjust them to their liking. Using Geometry properties and background plate I was able to create all of the necessary mattes / holdouts needed to individual objects & render layers - the one hang-up seems to be getting shadows alone on alpha.
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A noob question: I saw an LPE holdouts;shadow;CDL in the tutorial "Houdini Karma | Render Matte-Phantom-Shadow", and there are similar-looking LPEs in this thread (post of Feb. 2, 2023 6:57 P.M.)

I understand CDL is an LPE, but what about this holdouts;shadow; prefix? I haven't found any documentation mentioning these names, and I'm not sure where to look---is this an aspect of Karma only, or... ?

Thanks in advance for your help.
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I understand CDL is an LPE, but what about this holdouts;shadow; prefix? I haven't found any documentation mentioning these names, and I'm not sure where to look---is this an aspect of Karma only, or... ?

Yes, something like that. I think it's Karma specific. As interoperable as LPE's were meant to be, every renderer has their own syntax for renderer-specific stuff.
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Excellent information, thanks so much!

Now, my final question on the subject: is there any documentation on what LPE prefixes Karma supports? unoccluded is mentioned in the Karma Standard Render Vars documentation, and houdouts;shadow; was mentioned in a tutorial, but what about the others? I found a good explanation in the renderman documentation (https://rmanwiki.pixar.com/display/REN22/Using+LPE) but I'm not clear on which ones Karma supports.
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