how to render an object with transmissive materials and get the alpha to respect the transparency in karma XPU?

by default the transmissive areas just renders black!

I think you can do this with the unlit surface material

I assume you should be able to use background B event in LPE
You obviously not get the refraction information but you should get the background contribution for the chosen lightpaths

tamte
I assume you should be able to use background B event in LPE
You obviously not get the refraction information but you should get the background contribution for the chosen lightpaths

Hello @tamte, Thanks for this but don't quite understand how to set this up.

I've added two images here, the one with black shows what I get If I just render the object with nothing in the background and the one with the white bg and white showing in transparent areas is me putting a grid object behind it it the render.

But I want to be able to render with nothing behind it and get the transparent areas instead of black to use this image on a website.

I've also added a hip file if you have the time to show how to set this up.

Thanks much!

here is an example
1. added transmission_bg_mask AOV using LPE: CT.*B
2. combined it with Alpha in a slapcomp over custom BG

display slapcomp in the viewport to see the result

tamte
here is an example
1. added transmission_bg_mask AOV using LPE: CT.*B
2. combined it with Alpha in a slapcomp over custom BG

display slapcomp in the viewport to see the result

Thanks so much for this @tamte!!!

tamte
here is an example
1. added transmission_bg_mask AOV using LPE: CT.*B
2. combined it with Alpha in a slapcomp over custom BG

display slapcomp in the viewport to see the result

Hello, where can I get some more information regarding the LPE tags that you've used (CT.*B)?
I've been trying to find this many times but got nothing.
Thx!

razCHS
Hello, where can I get some more information regarding the LPE tags that you've used (CT.*B)?

its not the tag itself, but LPE expression on Render Var
https://www.sidefx.com/docs/houdini/render/lpe.html [www.sidefx.com]

tamte

razCHS
Hello, where can I get some more information regarding the LPE tags that you've used (CT.*B)?

its not the tag itself, but LPE expression on Render Var
https://www.sidefx.com/docs/houdini/render/lpe.html [www.sidefx.com]

Never thought searching under Mantra chapters, always looked for it in Solaris or Karma.
Thank you!