Refractive object set to Not Cast Shadows blocks light specular.

[mitchgates]

In Vray Next I'm finding that if I set an object with refractive shader Not Cast Shadows, any light behind or inside this geo doesn't cast specular on other objects' surfaces that have reflective vrayMtls. Reflection channel is unchanged. Can anyone else confirm this issue?

[Morbid Angel]

yup looks like a bug, even with affect all channels in refraction, specular in particular does not show up - works fine if the cast shadows is on.

[Morbid Angel]

here is a simple scene (maya 2018.5) but same issue.

[^Lele^]

behaviour confirmed.
As to why, who knows.

[Morbid Angel]

its probably when the cast shadow is off the specular ray is not traced either they could be connected some how?

[vlado]

Thanks for the scene, looking into it.

Best regards,
Vlado

[vlado]

So when a shadow or reflection ray passes through an object with inconsistent specular visibility (visible to reflections, but not visible to shadows) this breaks the physical validity of the scene and messes up with the multiple importance sampling of light sources and may cause artifacts with algorithms like the adaptive dome light; in that case the reflection of the light will be computed only through reflections and not through direct illumination, which is why the reflections of the light end up in the reflections render element and not in the specular.

What is the reason for making the glass object not cast shadows in that case? If it's for performance reasons, it is not really necessary anymore, V-Ray knows enough about this situation to do the right thing.

Best regards,
Vlado

[staddon]

I know this is an old thread, but it seems like the issue is still a thing.

We ran into this issue while trying to render a metallic product. Our usual workflow is to use reflection cards to control the reflections in addition to an hdri in a dome light. The dome light environment gives us the basics, and then we fine-tune with the cards. What we've always done in the past was set the cards to be a color, add a gradient to the opacity, and make the object invisible to the camera, GI, and lights. That allows us to place the cards wherever we want and get really smooth gradient reflections that allow the reflection environment through in some spots for interest. But now (maybe since specular and reflection was split?) if shadows are turned on for the object, they completely block what's in the hdri/specular regardless of the opacity. When the shadows are turned on, the specular issue is solved, but then they become visible to the GI even though generate and receive GI is turned off for those objects and they're excluded from all lights. I know we're trying to make an object that is physically impossible, but so are matte objects. These are the three workarounds we were able to come up with:

1. Turn off GI. Since we're working with individual reflective objects on a white background, the difference isn't huge.
2. Give up on using opacity maps for the reflection cards. It's a little more work to set up since you need more cards, but it works
3. Render every shot twice--with and without the reflection cards.

Anyway, I know that's a really specific situation, but hopefully, that's helpful to someone.