I'm am trying to get an LPE for a material in my scene. Is this possible in Karma? I basically want to be able to tag a material and then use that tag in an LPE to get just that material's contribution to the beauty. I've tried all sorts of stuff but I can't get it working. Has anybody managed to do this?

I have also been looking for this. I would like to use this to recreate something similar to the material ID passes in Vray. Any clues would be a great help

There is no LPE material tag, but there is an object tag. The render geometry settings node can add the tag to one or more objects.

I'm trying to figure out the best way to do this too -

I managed to make a new AOV with the Additional Render Vars LOP, just by copying settings from another AOV and inserting my object LPE tag. For some reason though if I put that downstream of my Karma Render Settings LOP it doesn't render, whereas if it's upstream it does. I can see the Render Var in the Scene Graph in both cases and I can't see any difference between the 2...

Can anyone advise?

Damian Fulmer
I would like to use this to recreate something similar to the material ID passes in Vray

you should be able to use Karma Cryptomatte LOP and turn on Materials

I understand that I can use a Cryptomatte setup as by material. This is really great for getting arbitrary mattes for things. From my workflow it's also good to have a predefined set of mattes that compositors can rely on. I've found something similar in Renderman and in Arnold:

https://rmanwiki.pixar.com/display/REN23/PxrMatteID [rmanwiki.pixar.com]

https://docs.arnoldrenderer.com/display/A5AFHUG/Passthrough [docs.arnoldrenderer.com]

I hope karma can support something like this in the future.

j00ey
For some reason though if I put that downstream of my Karma Render Settings LOP it doesn't render, whereas if it's upstream it does. I can see the Render Var in the Scene Graph in both cases and I can't see any difference between the 2...

In order for a render var to be "acknowledged" it needs to be added to a render product which needs to be added to the render settings.
While in both cases the render vars will be created, when creating them upstream of a Karma Render Settings LOP, the Karma Render Settings LOP will automatically add them to the default render product (/Render/Products/renderproduct) which is added to the render settings (/Render/rendersettings). Creating them downstream will only create the render vars.

Check the /Render/Products/renderproduct's "orderedVars" relationship to see which render vars are associated to it.
Check the /Render/rendersettings' "products" relationship to see which render products are associated to it.