Beer bottle bug

[jimmay]

Hi Houdini folks,

I'm trying to build a beer bottle product shot in Houdini 16 to learn the new materials system.  I've run into a weird issue with the principled shader implementation of the brown glass.  It renders the absorption of the glass incorrectly when in the Mantra physically based renderer, but looks right in RayTracing mode.

In PBR mode, the first two "hits" of rays in tinted glass don't get tinted.  So, for a tinted glass tumbler, light that passes side-to-side through the thick bottom, without going through the empty part of the glass, *should* have the deepest color, having travelled through the most glass.  However, in PBE mode it renders perfectly clear.  Light passing through the middle of the glass should have less tinting, since it passed through the air inside the glass - but since it has four hits, two for each side, it does show tint.

In raytracing mode, it renders correctly.

First: What settings do you think I should check before wondering if this is a bug?  I'm relatively new to Houdini, but not to rendering in general.  The limits are at default settings.

Second, may I submit bugs as an Apprentice user?

 

Asking here b/c the SideFX forums are currently broken for me - first posts need to be moderated first, supposedly, but that can't happen b/c it never submits anything - just keeps kicking it back for editing.

[jimmay]

Hi all,

Just in case anyone else runs into this:  

1. Yes, rookie Apprentice users can submit bugs.  SideFX is great, they were quick to respond with a workaround.

2. Nested Dielectrics must be turned on for the transmission tinting to work in PBR, even when not nesting, in this build.  I take this as meaning, just turn it on right away and leave it there.

 

Editing to Add:  Here's the first test render of the shot I was building.  

Would anyone have suggestions on what to tweak to fix the "shimmering" in the beer bottle backlight (and apparently the area light reflections)?  That's done via a luminous sphere using incident-angle shading.  Should I bump up the Reflection and Refraction quality sliders?  Those were both set to 8, and Stochastic Transparency also set at 8, with all else at defaults.

Possibly relevant: the backlight sphere is using a Material Builder shader, while everything else is Principled Shaders, and it's actually behind the set - I've set up render flags so the set is unseen in secondary rays.  Mantra PBR.

 

 

 

Edited March 27, 2017 by jimmay

[Matt_K]

Hi Jim,
Just a quick note - is it possibly something to do with shadowing that's causing the shimmer?

I ran into something similar a while back and it was either fixed by tweaking the "Raytracing Bias" setting on the Mantra ROP > Rendering > Shading tab (I think I had to set it to something like 0.1 for that particular scene scale), or by excluding the inner liquid from the environment light shadow. Was a while back and can't quite remember which one it was!

Now that I think about it, it might also have been fixed by upping the light's "Sampling Quality" to 4 as well...

Hope you are able to sort it out,
Matt.

 

[jimmay]

 

Hi Matt,

Raytracing Bias might be worth looking at.  I have it set very small (.0001) in order to keep small water droplets on the label from "punching through" it.  When I Googled around, I couldn't find any information on what the consequences were for going too far in that direction.  

This scene is real-world scale, using meters. I've not tried switching it to centimeters.

Light quality is set to 8 for the softboxes, and they have HDR textures on them.  There is a "self shadow" option for the light that is on by default, I could try that too.

But beyond that, the bottle "backlight" is emissive, so it should't be catching any shadows at all.  I'll have to set a camera back there to look at it and see if it does anything.  It might be a while before I do, though, as I'm knee-deep in an RBD sim at the moment.