Problem with fire flies in rendering a chrome reflector, catching light from sun HDR

[Alexx31]

Hello,

I have a scene, where I want to render a car's head lights and am trying since a day how to get rid of fireflies in my render:

There is a rippled chrome reflector inside each lamp (behind a thick glas), which is important to brighten up the "lamp interior" behind the glass.
The IOR of this chrome reflector is set to 20, fully glossy, raytracing to 20 rays - which all works fine, but the only problem is, that many fire flies appear.
If I set raytracing to 3 rays, I haven't anymore fire flies, but it's too dark behind the glasses.

I have already hidden the glass for all my tests.
I already have put clamping to 20 and subpixel mapping on (even clamping to 1 did not solve it)

The scene is only enlightened with a sun light HDR dome light.
increasing the sampling of the dome light from 8 to 20 did not change anything.

Sampling is already at min 4 and max 20 subdivisions, threshold 0.010
DMC at 0.850 threshold 0.005 and 16 samples (there was also no difference with 32 samples)
Min shading Rate at 6 (no difference when set to 20)

GI switched ON or OFF: no difference.
Using a skylight portal (rect light) also no difference (the lamp isn't also too deep).

What else could I do to reduce those fire flies?

I don't want to blur the sharpness of reflections (which was until now the only solution).
The failure is only in the reflection pass.

In my last tests I have Min. shading rate at 20, GI off, Max Sampling at 40, threshold at 0.005

Many thanks...

[Morbid Angel]

The issue is the sun in your hdr is a small pixel which is very bright. So in terms of indirect reflection - imagine a piece of geometry which is shooting rays outwards from its normal into the world and maybe 1-2 rays hit the area of the dome light where the sun is, returning that bright speckle, while other rays miss that area (because you just need a whole lot of rays there) So a classic solution is to blur the sun quite a bit in the hdr, then use an actual vray light in the same place where the sun is and that will surely fix the problem. If you still have the problem you can send me the piece of headlight geometry and hdr demonstrating the issue and I can have a look.

[Alexx31]

Thanks, I will try that way.

Just another question:
Isn't there maybe a tool to "select" or measure only the brightest areas of reflection, to be able just to reduce there the IOR or the glossiness?
(similar like the samplerInfo node with measuring normal facing angle)

I have looked at the Maya lightInfo Node, but this seems not help anything with an dome light.

Thanks again

[Zdravko.Keremidchiev]

Maybe you can use V-Ray Light meter for that.
Also, you can smooth such speckles with lens effects as in real cameras.

[Morbid Angel]

Thanks, I will try that way.

Just another question:
Isn't there maybe a tool to "select" or measure only the brightest areas of reflection, to be able just to reduce there the IOR or the glossiness?
Thanks again

No not really. Doing what I suggested is the only way to resolve it (one that I know of anyway).

[Alexx31]

Thanks for all those inputs, I solved the issue yesterday with post-processing.
Somebody later suggested me, to make a second chrome shader, which has only reflectivity, and to take it's output, reduce it to the brightest lights and use it to drive the glossiness or IOR of the main shader.
I didn't test this workflow yet, does this sound possible?