Light intensity severely drops with refractive objects

[Alex_M]

There's something strange happening with lights and refractive objects. If a light passes through a refractive object with glossy refractions even slightly lower than 1.00, the light's intensity severely drops. Even if I set refraction glossiness to 0.999 it makes a huge difference in how Vray renders the lights. It's as if 99% of the light's energy is suddenly lost or cut off for some reason. Please check the screenshot below. The 2 images should be pretty much identical and look like the one on the left, but the one with refraction glossiness of 0.999 is drastically different visually. I am attempting to do a milky glass effect for the glass pieces in front of the cylinders like here, but as soon as I decrease the refraction glossiness, the lights dim significantly and I can't achieve that effect.

[vladimir_krastev]

Hi are you using visible lights behind the glass? Why not just increase the light's intensity?

[Alex_M]

Yes, the lights are visible and their intensity is 1000000 in "Default (image)" units. Please see the attached scene.

[^Lele^]

It works fine for me.
What behaviour would you expect?

[Alex_M]

Hi Lele,
I would expect the light not to dim at all with refraction glossiness of the glass set to 0.999. Did you test with the attached scene from my previous post?

[^Lele^]

Did you test with the attached scene from my previous post?

I have, it doesn't misbehave at all between 1.0 and 0.999, that's why i am asking.

[Alex_M]

That's strange. Please see attached a video I recorded. I'm using Max 2023.1 and Vray 6.00.04 build 00000 (the version before hotfix 1).

[^Lele^]

Mh, i see what you mean now.

[seandunderdale]

Isnt it because of the max ray intensity? Bump that up from default 20 to 200 and youll see a difference and possibly lots of fireflies...

[^Lele^]

Indeed, that's what it is.
I was wondering why i had never seen this behaviour before, and it's because i have MRI always off.

[Alex_M]

Increasing MRI to 100 and above gives me a ton of fireflies that are very hard to resolve so that's not a solution for me. I still think there's a bug somewhere and that this should not happen even with MRI at 20 because there's just 0.1% difference between glossiness of 0.999 and 1.00. So why does the light dim hundreds of times?

[^Lele^]

It's due to the way a ray is considered when behind rough glass.
No bug, i'm afraid.
Fireflies, where you want to light with exclusively caustic paths (i.e. light through glass), are par for the course: you'll need to sample much more for them to converge, if at all.