I'm rendering a product model on a cyc and the client is going to want the background to change (just a solid color), so I would like to be able to isolate the cyc's reflections so I can just tint those in comp and easily accommodate any color changes. How can this be done? Is there something like the LPE tag for objects, to break out their individual contributions?

On a possibly-related note: this product is a transparent bottle holding an almost-transparent liquid, but the alpha (even with the cyc set to phantom or turned off) is 100% opaque. How can I get the alpha to match the theoretical transparency of the object(s) so I can change backgrounds freely (notwithstanding reflection/lighting changes)? I even tried lowering the bottle's IOR to 1.5 and turning off the environment light, neither of which helped.

Thanks!

1. Yes, you can assign LPE tag to your object and then filter just for reflections hitting objects with that tag

2. You can try filtering for refractive paths ending with B event which stands for background, to get something like Alpha for areas that hit nothing

Both are described here
https://www.sidefx.com/docs/houdini/render/lpe.html [www.sidefx.com]

Alrighty, I've gone through the docs but there's still some context missing in my brain. This is all easy with lights, but trying to isolate the transmission/refractions of my background replies some syntax which I can't figure out based on the examples. I made an LPE tag for the cyc, and set up two extra render AOVs, both type color4f, with the names:
lpe:C+`cyc`L
lpe:C<T>+`cyc`L

The planes show up in the render, but they're black, so I'm clearly doing something wrong. What am I missing?

My simple dummy test project is attached.

Thanks!

Main issue is that you're typing the LPE's into the wrong parameter, secondary issue is using the wrong quotes.

Thanks for this. There's apparently another step involved somewhere--I replicated what you did in your image and those AOVs are blank. What am I missing?

Never mind--had to update the link in the geometry settings node that created the tag. Thanks so much! Now just trying to fight my way through the doc to figger this out.

Your prim path on `rendergeometrysettings1` is wrong, setting it to `/cyc` instead of `loplastmodifiedprims("../box")` fixes it

These answers helped me too!!