Dim Distance

[TubeSmokeGuy]

Hi!

At first I want to say, that Im a big fan of your detailed and quite good understandabe documentation.

But the explaination of the dim distance feature just lets some questions open to me.

This is the text from the doc:
"
Dim distance – Specifies the distance after which the reflection rays will not be traced.

Dim fall off – Specifies the fall off radius for the dim distance.
"
------

So i wonder what comes after the dim distance has been reached?
- Black?
- Diffuse color?
- Environment map (from material slot or environment override in VRay render settings or in 3dsmax render environment tab)?
- Something different?

Can dim distance save render time? What are the cases?

Is dim distane measured in scene units according to 3dsmax "scene units setup"?

How does dim distance interact with reflection depth? f.e. An object outside the dim distance is reflected from the surface of an objects which is in the dim distance. Would it be visible in the reflection limitied by the dim distance?


So thanks in advance!

[hermit.crab]

Think of the 'Dim Distance' parameter as camera clipping for reflections (something like an area of effect of the reflections), with the distance measured from the object (with the applied VRayMtl) and units taken from the 3ds max system units. After the dim distance has been reached, the environment color/map is returned. It may reduce render time since fewer reflection rays are traced.

The Reflection Depth, on the other hand, is the maximum amount of bounces that can be traced.

Check the attached screenshot for visual clarification. The scene consists of two mirrors with a sphere between them and a camera looking towards the sphere and one of the mirrors.

[TubeSmokeGuy]

Thaks for the explaination. Just simple thinking from me is, when you have a small dim distance / dim radius, many objects which are responsible for huge amount of multiple reflection bounces, are out of range and so they dont have to be calculated in the reflections. I think this can save rendertime?

Another question i have is, where the dim fall off takes place? When i got a dim distance of 50cm and dim fall of of 15cm ... is the thim fall off then from 35cm to 50cm or from 50cm to 65cm?

[jtswfr]

What happens if Dim Distance is *not* toggled on in a material? Are the reflections measured out infinitely into space? Or is there a default value after which reflections will environment color/map rather than reflections will be returned, and Dim Distance simply *overrides* that distance?

[hermit.crab]

What happens if Dim Distance is *not* toggled on in a material? Are the reflections measured out infinitely into space? Or is there a default value after which reflections will environment color/map rather than reflections will be returned, and Dim Distance simply *overrides* that distance?

If the dim distance is off, the reflections are traced until their contribution becomes negligible.

[radvfx]

We still did not get an answer for whether dim distance saves render time. Seems like it would. Does it?
And is there a good method for calculating the difference (with, without, amount).

[JonasNöll]

We still did not get an answer for whether dim distance saves render time. Seems like it would. Does it?
And is there a good method for calculating the difference (with, without, amount).

Well that depends on the usecase. Let's say you have an object standing on a reflective surface and then you will be able to speed up the rendering by enabling a dim distance. But of course you won't get the same look. Basically the shader will switch to the environment (either the one set up in the shader or your global environment) after a the reflection of an object reached your specified distance value. From my understanding environment reflections are considerably faster to render than geometry reflections so it should speed up the rendertime.
For me it is more of an artistic option than a speedup option though. So for example you can have different shaders in the same scene reflecting different environments by setting up an individual environment for each shader and then choosing a very large Dim distance.

As you can see from my example pictures the speedup would come from emitting objects from the reflections which can result in a very different look. For example the light behind the teapot is removed in the reflection as soon as the dim falloff is no large enough. So a speedup would always be achieved only by removing stuff in your reflections and rendering the environment instead. Which will always change the look. In some usecases it would be easy to get away with it, for example if your floor is like a concrete floor with very rough reflections you probably wouldn't notice a big difference in the final result but most likely in your rendertimes.

[JonasNöll]

And here another demonstration with a very blurry floor this time where you you can see that there is a speedup happening. But it's not too big because the part of the teapot and the light on the floor is not very a huge part of the image. Also notice that the specular of the light is still being calculated on the floor, even the dim distance is set. It only affects the direct reflection of the light and not its specular. You can see that if the reflection is very blurry the difference between both is not that noticable anymore and the speedup in rendertime might justify to use the dim distance in this case.

[vlado]

The intention of the dim distance parameters is not to speed up rendering, users wanted it in order to achieve some artistic effect when rendering objects against a white background. If it saves render time, this is purely coincidental.

Best regards,
Vlado