Secondary Ray Bias - How to use?

[thomthom]

I have a model where I have some glass pieces sitting inside a chrome shape. Where the glass meets the chrome shape the faces lay right on-top of each others. When I render I get artifacts like you see on this page: http://www.spot3d.com/vray/help/150R...s_switches.htm

By default settings the Secondary Ray Bias is set to 0.001, and both the chrome and glass has 2-sided ticked.

But still I get artifacts.

If I however offset the chrome shape the glass pieces are inserted into by 0.25mm the artifacts are gone.

So I'm wondering: have I completely misunderstood this option? I thought this option you let me avoid such artifacts without offsetting the surfaces.

[Jackson_cg]

Re: Secondary Ray Bias - How to use?

I've never played around with Secondary Ray Bias, but if I understand correctly the reason you're still seeing artifacts is because the two overlapping planes have different materials applied to them. The problem described in your spot3d link seems to apply to artifacts appearing on coplanar faces which have the same material applied (I had this problem on a massive scale in a converted Archicad file in which every one of the 2000+ building groups had a perfectly coincident clone with the same material. The only way around it was to delete them one by one, but it sure speeded the skp model up afterwards).

If the coplanar faces have two different materials applied then no matter what you tell V-Ray to do, there's no way for it to decide which face and material to give priority to, so they "fight" for visibility in the render, causing not so much artifacts as random patches of material.

[thomthom]

Re: Secondary Ray Bias - How to use?

Ah! The faces need the same material for that to work. I think that's the piece I've been missing.

So for objects with different materials the only option is to slightly offset one of the surfaces, right?

[Jackson_cg]

Re: Secondary Ray Bias - How to use?

Yep. What a funny example of both of us learning something we didn't know before!

[dalomar_cg]

Re: Secondary Ray Bias - How to use?

The reason why V-Ray (and most renderers for that matter) have trouble with overlapping surfaces is that the intersections of a ray and those two surfaces are hard to sort out. Either it hits one and not the other, but a good amount of time the ray actually gets "caught" in between the two surfaces, thus rendering black.

Secondary Ray bias shifts the intersection of the ray slightly, which is usually enough to circumvent getting caught in between the surfaces. It doesn't necessarily promote one of those overlapping surfaces over the other, just prevents rays from getting stuck. This is why objects with the same material will look fine, but objects with different ones still looked F-ed up.

Be weary about setting this too high though as there is a potential for this to through off some calculations and cause artifacts such as light leaks and evil bears appearing in your rendering ;D

[Jackson_cg]

Re: Secondary Ray Bias - How to use?

Be weary about setting this too high though as there is a potential for this to through off some calculations and cause artifacts such as light leaks and evil bears appearing in your rendering ;D

THAT explains it.... finally I get can get rid of this artefact!

[thomthom]

Re: Secondary Ray Bias - How to use?

Ah. I always thought that the appearance of the evil bears was related to the amount of time left until deadline...

[dalomar_cg]

Re: Secondary Ray Bias - How to use?

Yea there's a reason why Colbert has bears as the number one threat. They pop up everywhere and always at the worst times ;D

[thomthom]

Re: Secondary Ray Bias - How to use?

They are pretty hard to explain to clients.


"Bear? ... Why...? Errh... Well... You see, the thing is, next to the site there's a forest and... errh..."

[Jackson_cg]

Re: Secondary Ray Bias - How to use?

The worst is when the render looks perfect on your monitor, you even double-check it on a couple of colleagues' machines, but then you email it to the client, they print it off on their straight-out-of-the-box uncalibrated printer and email back complaining "It's too dark, can't you turn up the brightness and send it again? Oh, and can you get rid of that evil bear?" :

Thank God for Photoshop CS4's Evil Bear Patch Tool!