Jedidiah Mitchell

EJaworenko
Finally, someone else encountered this! I've submitted the bug twice and it's killing me. The workaround is just using Houdini 19.0 for saving the HDA because it doesn't have that problem.

Oof, that’s a rough workaround.

A lot of my high-level tools rely heavily on multiparms and those break the hardest due to this bug. It has driven me to start developing a Python build system for all the high level HDAs because they are unmaintainable by hand. That’s probably better in the long run anyway, but it’s been a lot of work to set up and really slows down the prototyping.

Playing around some more I found a solution that almost works -- instead of disabling the refracted light in the beauty pass, I've left the background in the beauty and then done a subtractive comp operation with a custom LPE that only contains Camera-->Transmission-->Light events with a tag like CT+'env'(the 'env' tag is on the Dome Light):


I still need the mask generated by the blue channel of the refract pass, but I can modify it to correctly represent the transmission-only path and then subtract the Dome Light background from the beauty. Basically it works:


The results are still very noisy though (this is after the minus operation):


I was able to get the noise down to a reasonable level but only by massively oversampling:


Denoising helps in some areas but introduces more artifacts elsewhere, as usual -- and generally we can't denoise sequences anyway. Are there any other tricks I'm not thinking of here? I'm sure I could optimize the sampling settings to some extent but the jump in render time to get a clean comp was so great that it makes this approach basically unworkable for us.

tamte
custom LPE is the way to go if you want specific paths, because in just refraction AOV (Karma's Glossy Transmission) you will get lots of stuff refracted, but not everything makes sense as uv map doesn't work well with average values from diffuse or rough surfaces

Gotcha. I went back and enabled the default Glossy Transmission AOV in Karma and compared it to my own LPE-based approach and in this scenario they appear to be identical, which I guess makes sense since there is nothing else to refract in this scene.

Taking the difficulty up a notch, I'm trying to work this now with multiple transmissive planes -- like a car with an interior. This is a Karma reference render with all Dome Light contributions enabled:


It looks like the -refract setting on the Dome Light for Karma results in preventing illumination from entering the interior of the glass box even for the purpose of other glossy scattering events. This behavior is different from the equivalent settings in Arnold, which only removes the final transmission-->light event (Karma on the left, Arnold on the right):


So now it is necessary to create all that illumination in comp -- and the results are problematic in terms of both noise and the artifacts from internally reflected rays having nothing meaningful to sample from the st map:


This is a place where I feel the Arnold setup is just more practical. I understand it's not optically correct, but having those options is not preventing you from doing it the right way on hero elements, and having to work out a comp solution for this kind of transmission scenario isn't an appealing prospect for day-to-day renders of non-hero elements...

Maybe there's something else I'm missing though? Like is there an alternative to setting the Dome Light contributions to -refract to get it out of the beauty pass?