Very strange lighting bug with glass windows in Iray

Matt_Castle

I've got a very strange lighting bug where if the camera is moved around, from some angles light can no longer shine through some of the windows in the scene. And one of these windows is only continuing to transmit light because I've turned on Thin Walled. Once that's turned off, the remaining window also becomes opaque. (That is not however a workaround - all of these windows have Thin Walled on, but it only works to keep one of them permitting light).

As this is best illustrated by a screen capture, I have provided one:

And the problem is, I want the camera in one of these angles where it stops working. And no, rotating the scene in world space does not solve the issue, nor does translating it. The problem appears to be in relation to the position of the camera relative to the windows/set somehow.

What is going on here, and how can I resolve it.

barbult

I've had similar experiences and don't know why, either. I ended up setting the glass surface Cutout Opacity to 0.

prixat

Does adding Caustics have any effect?

docbother

You usually want thin walled turned on for things like window panes because glass surfaces are often represented by a single plane (a 2-dimensional object) which has a thickness of zero. This tells iray not to try calculate the light path using the refraction index (which would be around 1.5 for glass). If those windows are planes (2D) and not really thin cubes (3d) then turning off thin walled will tell IRAY to treat everything on the other side of the window as being inside a huge (infinite?) block of glass.

Thin walled is best turned off only for object that have fully enclosed geometry like a glass sculpture or a volume of clear liquid in a glass.

Note this is calculated separately from caustics even though the two are related in the real world.

Matt_Castle

Following the various advice here, I've combined some options.

I've hidden all the original glass (which I had assumed to be solid, as that's how the presets had set it up, but apparently not) with Cutout Opacity, then created cube primitives with a solid glass shader to do the job instead (as I do want the reflections off them, rather than just having nothing there), which seems to have resolved the issue (at least, for the camera angle I actually want to use, and I'm not too fussed about checking every other angle).

Steel Rat

I have similar issues with light coming through "glass" messing with transmaps and shells with a lot of transparency. 

I just tried the above, turning cutout opacity of a window surface to zero but still have problems with transmaps. That's also with refraction index at 1. Only hiding the wall object removed the artifacts.

So I tried the cube option, and applying the Iray solid glass shader in the DAZ Uber folder. It's hard to tell if it's any better or not.

I also often get weird terminator artfacts, the point where shadows start on figures, when hdri, or any light, is coming through a glass surface. Sometimes I can compensate by placing a mesh light (like a Ghost Light Kit 3 simple mesh light, just inside the window. But it doesn't always work.

Extremely frustrating.