Specular fireflies with Vray

[Steve Davy]

I have a scene with quite a lot of small details, being rendered at a relatively small resolution (940 x 529).

I'm using an image mapped dome light and four Vray rectangle lights, but no GI and nothing really special going on otherwise, simple Vray shaders etc.

Everything's looking good except I am getting some very bad specular fireflies on the edges of certain small features when the camera moves, and I can't get rid of them. I've noticed that the specular pass in Vray is often very noisy in this way.

I'm rendering using Adaptive Subdivision rather than DMC because after testing overall this seemed to give me the best looking result, with DMC making areas of the image a bit grainy.

I'm now up to 1 and 2 samples, with threshold set to 0.013, so quality settings already fairly high and render times are starting to creep up. Rendering at double rez might be an approach I'd normally try but it's not an option here because of time.

Can anyone suggest anything I can try?

I'm not sure whether light samples affect the quality of specular highlights, however they are already set to 32 on all lights, and only a couple of the lights are contributing to specular anyway.

[mark palkoski]

I have had something similar with some growing crystals, no caustics
but loads of refractions and reflections plus a video backplate.
you might want to try changing the color mapping setting in the vray
render tabs under settings off linear multiply to exponential or HSV
exponential. or maybe the clamp check box.
Though this might affect some other colors.

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Mark Palkoski

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[Steve Davy]

Thanks for the suggestions, unfortunately none of this seemed to have any effect at all.

It seems like a sampling issue, but no matter how high I crank up samples either globally or on the problem shaders, nothing fixes it.

The only thing I haven't tried is supersampling by uprezzing, but I just don't have the render resources to do that for the whole show anyway.

[mark palkoski]

If you add sample rate to your render elements. This might help in
trouble shooting.
I use this plus lighting, GI, and others.
Using the sample rate pass your looking to minimize the red/ which
indicates antialiasing working overtime.
while keeping the grain low.
with DMC the higher you set the max samples and lower threshold you
can tune to a quicker render with out

You might check the sample rate on individual vray shaders as well as lights.
Or use the spec pass as a lumakey for a color correct in Comp.
That's all I got.

[Dylan Neill]

Is there a small, very bright sun in your dome light texture? This can sometimes be the cause.

I think this is a known problem in V-Ray 2 anyway. Apparently it's better or even fixed entirely in 3. I have a render with similar problems which I'm going to try in 3 today and see if it helps. I would say if you're not on the beta then sign up and give it a try.

[matt estela]

i've only dabbled in vray, but i've found the core issues with pathtracers are similar across the board.

As Dylan said, fireflies are usually caused by lights which are too small and too intense for the render to cleanly filter. A bit of housekeeping can normally calm things down again.

First, isolate the light(s) which is causing the issue. try disabling its specular/reflection visibility. If that fixes the fireflies but kills your nice highlights, try putting a reflection card near the light, but with a more reasonable intensity (eg no more than 1).

if its from a domelight, like Dylan says, look for errant high intensity values, either blur it out or clamp them down. vray handles this stuff better than most, but in a pinch some pre-filtering can work wonders.

Another thing to try is to clamp highlights. if you're using multiple bounces, the bounced light from a specular ping, which would become a caustic highlight in the real world, will usually be very undersampled and noisy in a pathtracer, ie, fireflies. the render globals has a setting somewhere to clamp the maximum intensity secondary rays can be. in max vray3 its called 'max ray intensity', can't find a reference for maya. I assume its the same.

I also found some sketched out notes I wrote from when I was using vray, maybe they'll be useful if you haven't tried them already:

http://www.tokeru.com/mayawiki/index.php?title=Vray#Reflection_fireflies

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[Steve Davy]

Thanks for all the input. Some methodical testing turned up the culprit. The Specular Contribution of one my lights had got set to 8 instead of 0.8. Doh! How easy it is to make dumb mistakes when you've been looking at the same scene file for days...

Still, good to learn some of the stuff that's been shared anyway.

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From: ma...@tokeru.com
Date: Fri, 8 Aug 2014 09:34:31 +1000
Subject: Re: [maya_he3d] Re: Specular fireflies with Vray
To: maya...@googlegroups.com

[Keith Rogers]

Might I also suggest watching Grant Warwick's videos on mastering VRay.  Very well explained and completely debunks the common method and settings everyone uses.  Lesson one and four is free on vimeo here...

https://vimeo.com/86010717

[Steve Davy]

You might, and thanks!

Learning Vray is actually kind of FUN. So different to the cold sweat that anything related to Mental Ray used to induce.

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From: keithrog...@gmail.com
Date: Fri, 8 Aug 2014 10:05:05 +0100

[mark palkoski]

What Keith suggested (Grant Warnick's Method) is what I was quickly
and rudimentarily trying to describe.
It has helped me keep quality with out sacrificing time.
And Matt thanks for the maya wiki, have gone to it many times.

[Steve Davy]

Yeah I've seen other tutorials explaining this balancing act using the Sample Rate pass, but Grant Warwick's is the best so far.