Black Leaches on surface!

[treddie]

I ran into a problem that seems familiar to me, but it happened in an unusual way this time. A while ago, I had these black spots all over everything like little bed bugs. Turns out the problem was my scene scale. This time, however, I get these big black "leaches" on a surface I have been experimenting with. The surface was originally meant to be a simple flat disk with a normal map applied. But I could not get the depth I needed, and the bumps were too large anyway for the normal map to do justice to. So I did a number of versions with displacement-turned-to-mesh via Blender so that the height variations were a physical part of the mesh. Each version rendered correctly except this last one (please see attached image). So I don't think it's scene scale this time. If so, I would have thought all the other versions would not have rendered correctly either. In addition, none of the other meshes in this project has any problems.

The normals are all correctly oriented and verified in C4D, and there is no Normal Tag. Renders fine in C4D.

In Octane, no maps are applied...This is just a mesh with a simple glossy texture. Problem exists regardless of lighting model (Daylight, Environment, Custom Lights)

So what's up with the black leaches? Should I call an exterminator?

[face_off]

Did you check your Ray Epsilon?

[treddie]

Hot diggity! That was it. Thanks for the solution...I'm still trying to get my mind around some of the settings and how they affect a scene.

[kavorka]

If your ray epsilon fixed it, than you probably have a scale issue. Either your object is huge, or it is exporting or importing the wrong scale.

[MB]

Our firm does airports and we have had issues rendering things that size in Octane even when the models are imported in the correct units. I would appreciate some detailed explination/advice on settings in the software manual regarding rending very large models as it would help our users.

Ray Epsilon is not a well understood term and we bring new users into the Octane fold almost daily.

Thanks

Mark

[kavorka]

Make sure you have things placed near the world origin. when things are very far from the world origin, they can get those black marks, sometimes you can fix it with the ray epsilon.

For large scenes, you make need to scale it down and render it that way.

[grimm]

Please tell me if I'm wrong, but I'm pretty sure that ray epsilon controls how ray -> object self intersection distance is handled. Basically as you cast rays into a scene they hit the object meshes you have set up. Once they have hit a mesh the renderer then needs to cast other ray(s) from that point. If you just take that position and cast a new ray from it, there is a good chance that the new ray will re-intersect with the mesh. When that happens you see black pixels or spots on the mesh because the renderer thinks those rays are on the inside of the mesh and blocked from the light source. To keep this from happening you move the intersection point a tiny bit off of the surface before you cast any new rays to ensure that it won't happen. This would be affected by scale, so the larger your scene the more this becomes an issue.

I suspect that the reason for this in Octane is because Octane uses single precision numbers which can limit how big the scene can get. For really big scenes Octane has to adjust the range of values it can handle to match the scene size. To help with this Octane allows you to adjust the epsilon so you can move this offset closer or further away from the mesh. It also means that it's really important to let Octane know the scale of the scene so it can adjust how the light in the scene will act. But I'm not sure how to do that exactly in Octane. Maybe the devs can give us some more insight into this subject.

[treddie]

Great posts everybody. Thanks!

One item of note...This problem does not just happen for huge objects. It can happen to really SMALL ones, too. Which was true in this latest case of mine.

As for a 1-click solution to scaling up/down an Octane scene, everything in the scene can be passed through one last transform node, I suppose. I have a scene with 73 parts. That means scaling up 73 objects one at a time, without that extra node.

[roeland]

It can happen on objects on any scale. The ray epsilon should be smaller than the fine details in your scene. The black spots are places where the ray epsilon offset causes the ray to start behind another surface which is closer than the ray epsilon offset.

You should never need to change the scale of the scene. Changing the ray epsilon parameter should have the same effect. In your case you need to decrease it.

--
Roeland

[treddie]

Thanks for the clarification, Roeland. I think I have a good understanding of it now.