cutoff value

[shadevfx]

Can someone explain a little better than the docs what cutoff values on lights and shaders do? And, what the practical applications are?

We noticed that it can improve speeds dramatically, but I'm not exactly sure what or why I'm adjusting it, and when it's best.

Thanks!
Bryan

[shadevfx]

Im guessing by the lack of response, Im not the only one who doesn't fully get how, when and why to adjust this setting.

Vlado... any tips here?

Thanks
B

[vlado]

The cutoff value for materials will try to estimate the contribution of reflections and refractions from the material to the final pixel values, and if this is below the threshold, it will not trace them. If the values are too high, you can get artifacts (e.g. you will be able to see the boundary where reflections are not traced). You may also see this boundary if something very bright is reflected.

For lights, V-Ray will try to estimate the contribution of the light to the final pixel value, and if this is below the cutoff threshold, it will not trace it (will assume it's black). If the value is too high, you may get artifacts in the image (e.g. you will be able to see the boundary where the lighting is not calculated).

Best regards,
Vlado

[vlado]

Here are three examples with the material cutoff (0.0, 0.01 and 0.05 respectively). I've set the environment to twice the white point (e.g. RGB 512, 512, 512) to better show the effect.

Best regards,
Vlado

[vlado]

Here are three examples with the light cutoff (0.001, 0.01 and 0.1 respectively).

Best regards,
Vlado

[shadevfx]

Thank you Vlado, that certainly helps me get my head around it.

Bryan

[cpnichols]

Cut off is the key to many things Vray in terms of speed. If you turn all your cutoffs to 0, the rendertimes will go way up. Basically, Vray stops considering the rays based on their value rather then their distance (near/far distance) which is more efficient. As an example, IES lights used to be very very slow in Vray until cutoff was added to them. Now they are as fast as any other light. I have been rendering a scene with 500 IES lights in it, and it has been rendering very fast.

One thing to be aware is that cutoff values need to be lowered if you are using linear work flow, as the blacks cut cut off prematurely.

[Morbid Angel]

i wish cutoff would have had a falloff. Like in the third example there is a hard line where the rays terminate, not a very feasible result. A falloff would be much better.

[m_hinks]

i wish cutoff would have had a falloff. Like in the third example there is a hard line where the rays terminate, not a very feasible result. A falloff would be much better.

I thought the same thing when I read this thread

[vlado]

i wish cutoff would have had a falloff. Like in the third example there is a hard line where the rays terminate, not a very feasible result.

Since you don't get any such thing in nature, the result in not feasible either way Better looking is another topic ; in any case, the original idea of the cutoff was to be used only to extents where it would not be visible; it's original goal was not to be used as a falloff. (Just for the record, future builds of V-Ray will have a separate reflection falloff for VRayMtl).

Best regards,
Vlado

[Morbid Angel]

Since you don't get any such thing in nature

what we do is so far from nature in any case. In my opinion the more control the faster the rendering, the sooner we can all go home and enjoy some beer!

[PaavoMFX]

Here is some lights casting a highlight on a plane, the range is from 0.004 - 0.000 .
Maybe there could be a different cutoff for the highlights?
I can only use cutoff 0.000 for the lights in my scene with about 50 lights, so the rendering time is going up a lot.

[vlado]

Since the diffuse light and hilights are all done in one pass, it is not possible to have separate cutoff thresholds for them (well, it can be done, but would be twice as slow, which makes the whole thing somewhat pointless). The only thing we can do is add a slight falloff so that the boundary is not abrupt.

Best regards,
Vlado

[PaavoMFX]

I don't Know how much the cutoff will lower the rendering time, but why not make it dependent of the distance to the camera, like an animated multiplier..

[vlado]

Hm, that will cause the scene to get darker the further away it is from the camera; probably not quite what is needed. And it won't help reflections either (as reflection strength does not depend on the distance to the camera at all).

Best regards,
Vlado