Faceting in Glossy Reflections

[max@mfx]

There seems to be a problem with glossy reflections (that I've actually experienced before when using Turtle, thats part of why we switched.)

It appears on lower tessellations when using a low value for glossy reflections (<0.7). Usually it is most visible with silhouette-type lightings:



Is this how it's supposed to look and a known limitation or is it possible to correct? We rely heavily on glossy reflections for our product renders so I have to come up with a solution.

Best Regards,
Max Persson

[Morbid Angel]

best is to post the scene for analysis. There is also an option - soften edge in the reflection attributes which may help.

[olivervfx]

Is this a joke?

I mean, "i want a faceted sphere" to look right with lighting and show no faceting.?

Ofcourse a light that you face perpendicular towards the camera, will show the faces more blended together, your just lighting them brighter,
and the silhouette will ofcourse show the edges more.

Maybe i'm not understanding the problem.

If you have higher sudivs, and subdivided the object, or smoothed it you wouldn't have this problem..



O.

[Morbid Angel]

I don't think its a joke; glossy rays become more apparent when the glossy spread exceeds a certain angle. By subdividing a sphere you make the effect less, however it is still there.

[olivervfx]

It makes sense to me why it would... but ok, maybe i'm just not getting it at all,

so never mind..

[max@mfx]

Thanks for your replies, and I can tell you that this is not a joke.

What seems to happen is that the reflection engine fails to use a smoothing algorithm for the calculation of reflection angles with low glossy levels. Another fault could be that some of the glossy rays are sent inside the object when the initial rays are generated. Either way it's always there as you say Dimitry, just more or less visible.

I can't find that option you mentioned, are you referring to the stable release or a nightly?

I'll post a scene as soon as I get a minute.

/Max

[vlado]

It is a problem with the definition of smoothed normals. Remember that smoothed normals are a cheat to give smooth appearance to an object that is not actually smooth. In many cases this cheat is successful, but in many others it is not enough to hide the actual shape of the underlying geometry (f.e. the silhouette of the object is still crude).

The option that Dmitry refers to is available in SP1.

Best regards,
Vlado

[max@mfx]

But it must be possible to calculate a smooth normal (as for lambert shading) and use that as the main ray direction? Is that maybe what the option does?

I will report back as soon as I have tried the SP1.

/Max

[vlado]

Perhaps this picture will help you visualize the problem; V-Ray calculates and uses the smooth interpolated surface normal (the same one used for diffuse shading), however because the normal does not match the actual underlying geometry, some reflection rays end up being shot into the object itself, rather than away from it.

This is for the most part, not a problem; the problem is that once we get to the next triangle on the right, these reflection rays are ok to trace. This causes a sharp difference in the reflections between the two adjacent triangles, which is what you are seeing.

Best regards,
Vlado

[max@mfx]

Thanks for the clear explanation Vlado, I was thinking it was something like that.

Would it be possible to filter those inside ray hits out and let them pass right through to the other (in most cases correct) side? Maybe that solution would just cause a bunch of other problems, but it could lessen the faceting in those cases...

/Max

[max@mfx]

We are still having problems with this, and I have seen a lot of other threads about it. Currently we have to retouch all the issues caused by the glossy reflections, and we use them a lot like other users have also stated.

I never got a reply on my idea on filtering out invalid rays, or something similar...

Have you considered writing a fix for this extremely common issue? Even if it's working around the original cheat with smoothed normals..

Cheers,
Max

[Dariusz Makowski (Dadal)]

Heya

Ok I'm really lost on this issue. The sphere you show us is reasonably subdivided and what it looks like is that each face has its own smoothing normal and neither of them are relating to each other. Is as if you run automatic normal smooth algorithm that calculate normal smoothnes based of angle of faces to each other that is set to 0.0001 so that no faces match each other...

Can't you just smooth normals?

Thanks, bye.

[max@mfx]

That sphere is a standard Maya sphere with 100% smoothed normals, ie the normals are shared at each vertex. The thing that happens internally is what Vlado describes in an earlier post, his diagram is the best explanation. What I suggest is some sort of ray bias or ray discarding algorithm, that can smooth out the reflection over polygon boundaries.

[Dariusz Makowski (Dadal)]

Ok. Now I'm totally lost.

So I set up a scene. 12/12 subs on sphere/added smothing normals/added vray mtl with 0.094 glosy refl and all looks fine to me...

Am I doing something wrong here?

smothing.zip

ps I can see weird stuff happenign at 5/6/7/8/9 but above that it seems fine...

[max@mfx]

Attached is a screenshot of my trigger setup. All I did was this:

- Create sphere, standard settings
- Create place, standard settings
- Assign white VRayLightMtl to plane
- Assign VRayMtl to sphere, the following settings were made:
--- Color black
--- Reflection color white
--- Reflection glossiness 0.5
- Render settings:
--- DMC 1/100, thrs 0.005
--- Prim GI: Irradiance
--- Sec GI: Light Cache

I'm using the latest VRay for Maya 2012 SP2, 2.20.01. On OS X 10.8.


severe_artefacts.mb.zip